The development of the nose in the spermophile with particular reference to the part played by the primitive choanae in the formation of the nasal cavity in mammals.код для вставкиСкачать
TIIE DEVELOPMENT OF THE SOSE IN THE HPER*hC(1PHILE, WITH P m T I c : r r J m REFEREXCN TO THH PART PLAYED BY THE PRINITIVE CHOASAE IS THH FORMATION O F THE NASAL CAPITY IS MAMMALS ISTlt.Ol)lYITIOI\' Despite the vast umouiit of work that lius hecii tloiie oii the developmeiit of the noso, a review of the literature learce 11s very clec.ideclly in tho dark with rcferenco t.o certain very essential phases. It;was with a view to clarifying these stages tlist the work on which this report. ia based wt18 iiiiclertaken. Siiice the literature contdiie uo complete acc.ouiit of the development of the iiose in the speimophile, it Reems worth while to report hers the gciieral embryology of this regioii in the mammal clio~eilfor this investigation. A complete discussion, however, is beyond the scope of this paper. Seither is it possible to coiisidcr lrerc the exteiisive litcrature 011 the subject. This seems the less necessary iii view of tlic recent work of J. P. sC.Iiaeffer, in whose publicatioiis a full report is.to be had. The appeiiclecl hiMiography ia inteiidd merely as a referelice to the literature pertinent to the purticnlar phases iii which we were primarily iiiterestecl-the relationship of the primitive clioaiiac! to tho foimatioii of the nasal cavity. No pretense at a11 aiialgsis of the lateral and medial iinsa.1walls will he mntlc at thicJ time, snch o1)servatioiifi aa liavc bcoii mado int1ic:atiag that. they (lo iiot differ materially from thoee of othcr rodents. 1i T H E .\N.\TOSIIC.\I, HRVORD. V<IL :$I. WI. I MATE.HIAI. AXI) h1I:ETIIOI~S ‘I’lic material oii wliicli this report is based eoasists of n w r y complctc eollec!tion of spcimopliile emI)r;vos b per- inoplrilurs tritl~~cmiliiieRtus h.iitchel1). Thc c m b r p s arc scvtioiictl at 10 ~ 1 SO ~ p 1 mid stained wit.li hematoxyliii and cosiii. No rcvort.1 of the age is available, the stage being rccor~Icdin cro\vii-riimp leiigtli. This measurement. was takeii previous to fixation. Tiidentally, note may he madc of t.lic f w t thut tlic ntagc of development. of the individual cmbryo corrc~nponclswry elo~olyto the recorded length-a.u adlvaiiw of 0.5 mm. iii lciigtli showing a correspondiiig advance iii the formation of t.he organs. Bepresentative stages wer(! schlcrtd aiitl reconstructed by tlie Born wax-plate metliod. The f(,dlowiiigpugen iliclutlc a dwcript.ioii of the wax models of t l i c w stagen together with observatioiia and interpret.atioils luisc(1 on microscopic study a i d on ti comparisoii of tlic st t\gE!X. ‘t’lre writer ~vishcsto make grateful ack~iowledgme~it to I h.H. .J. Prontiss for helpful snggestioiis and criticisms rluriiig tlic progress of this study a i d to Dr. E.31. Mac’tc:weii who cdlcctt.(l this series of crn1)ryo.s for the tlc.partrnest. I)IS(*CSJnIOI\’ The first iiidictitioli of tlic formtltioii of tlie olfact.or;\.lipis iii tlic a.ppoaraiice of tlic iiasal placodes at ahout t Iic 3-mm. stage. These structures appear near the l a t c ~ a l mnrgiiis of tlie iiamfroiital process. I h e to tlic! growth outward of t,lic~surrounding timiic, the iiasal area is depressed t o form the nasal pits ; the primitive iiasal eavitim are formed I)p tlio growth posteriorly of tliesc pitN, as well as l y their passive tlccpeiiiiig hy t.lie coiitiiiueci growth outward of t.hc region surrouidiiig the origiinal ilalclal area. 111 its growth I)ost.oriorly, tlic floor of the primitive nasal cavity teiids to parallel tlic roof of the primitive biiccal cavity. Each nasal (ttwit.y, as it cxtciids into tlio soft. tissue of tho aasofrontal ~JI-OCWH, tleliinits 11 ltiteral portion of that prc~essfrom thc pzii’titus central mass. Thus the iiasofroiital process comes to coiisist of three parts, tlie medial iiasal process lying 1)etweeii the iiasal cavities aiid the lateral iiasal processes lateral to the iiasal cavities. In an embryo of 8 mm. (fig. 1) tlie nasal cavities are separated hy the wide medial nasal pr(~cessand bountleci laterally hy the lateral r i ~ s a lprocesses. Extentliiig inferiorly a i d slightly laterally f w m each aiiterioi. iiaris toward the primitive mouth is a distinct groove, the biicconasal groove, which is bounded metiially by the inferolateral portion of the medial nasal process (glolmlar process of His) and laterally by tho junction of the lateral iiasal aiicl maxillary processes. Posterior1:- the primitive iiasal cavity 1)ecomc.smarketlly iiarrowed iii its traiisverse diameter, appearing in cross-section as ii slit-like cavit;v. Just in front of its blind posterior extremity it exteiids inferiorly and slightly medial1;v t owarcl the primilive buccal rarity (fig. a). o n the roof of tlie primitive buccal cavity, directly opposite this 1)uccalwartl exteiisioii of thc depressioii nasalprimitive iiasal cavity, is a corrc~spo~idiiig ii*ard. 'l?hcse two dcpressioiis indicate the point where the nasal aiicl oral epithelium later abut t o form the 1)uccoiiasal membrane (Hochstetter). At this time, however, the two epithelial layers are separated by a considerable tliickiiess of mesericliymal tissue. Just lateral to the ciepression in the roof of the mouth is a slight elevation or1 the maxillary process representing the begiiiiiiiig palatc process. 0 1 1 tlic superior surface of the mandibular process tlie aiittlrior anlageii of tlie tongue appear as two lateral elevatioiis separated by a broad shallow groove. Refereiice to a model of a 9-mm. emhryo (fig.3 ) s1iom-s tlica anterior iiares still wiclely separated h;v the medial nasal process. Study of sectioiis through the anterior iiares a t this time reveals a clistiiict tliickeiiiiig of their epithelial wall, due to a piling up of the superficial cells (fig. 4). The lumiiia of the anterior iiares are, therefore, somewhat dimiriislicd i i i size, hut are still patent. The bucconasal groove is still q)parciit, but is partially o1)literatecI at its Iiuccal eritl 1 ) t~ ~ i c 30 B. I. BI!RXS fiisioii of thc musillary with the globular process, tliie fusioil progrossiiig from beliiiid foiward. Tho nasal ctl\.ib is CIIc?rotwhecl upon meclially By the bulging into it of thc septa1 wdl c!oiitail-iiiig tlic? vomoroiia.sa.1 organ. Trrcgnlarit ies are appareiit on the 1at.cl.d wall, due to the clorelopmcnt. of the t.urbinuls. I’oeteriorly, the nasal cavities, 8 s in tlie 8-mm. stage, mil bliidly. Well iii froiit of its blind extremity c w l i ctivity Iiils iiow c~xtc?iidedtoward the primitive hilccal cnrit.y ntitil, ovcr a w r y limited area, the nasal mid biiccal epitlielium are in contact. Tliis small area of abutting iirrsal aud h u c . c d q)it.hc!liurn forma tlie lnzc~coiiasalmembrane. 0 1 1 t.lie roof of the primitive mouth the positioii of this membrane is iiidicated hy a much more distinct deprcsaion t.lian in the Imrious etngc. 1 ~ t e r a to l tliis c’lepressiou the pnlutc process is now a rlisiiiiat c!lcvat.ioii 011 the maxillary process. The rnmidi1)iilar pitoc.c.ss ~ h o w ano cliangc! escept in t.lie size of the tongtie ~ i i d tlic oblit cratioii of the g:roo\-c?hctalocn the two aiilngcn. Tlio 10-m~ embryo (fig. 5 ) I)rtwiit.s some very marked c*liaiigeoover the provions atago. The medial nasal process I i m c.lecrotiaed s;omewhtlt in tliickness, so that tlic anterior iliii’es arc less wiclely scparatcd. Altllongli tlicb anterior iiares HTC distinctly patent., the lnmiiis are markcclly dec?rcancrtl in size as IL rusult of tlic fnrthor piling up of tlie surface cpitlitblium of thcii- wdls. The h!coiiasal groove has hc!cii ohlitwt i t 4 by tlie fusion of tlic maxill~ryand lateral iiasal processes with tlie medid nasal procesn. The buccoiias~lmembraiie has iiow ruptured, and commiiiiication has been csttihlishd beiween tlie primitive iiasal and biiccal cnvitics. 1;ntcrd to the I.)iiccsl opcniirgs of the primitive clioanat! arc the ptilatc ~JTVJWMWHvrliich iiow extend from a poiiit opposite the ;iiitc~ioi: mtirgin of the primitive clioantie wcll 11egc.wdtlici I* pcwtwior limit. Tliesc processc?~prcselit iii the month cmity iii the ;~iiglcbetweeii thc toiigiie ant1 tl-ic? floor of tlic mooth, t l i c b giiigivii i i o t yet htiriiig l)(x!ir diffcreiitiatcd. At this stag(’ of cicvelopmciit tlic iianal apparatus consiete of thc tw.)primit ivc iiastil cavitics opciiiug to the cxtcrior tlirongh the tintcbrior 1 I a i w s niid ct )rnmwiicat iiig wit1i t Iio primitiw linccal ~ i1-i i ty 31 Fig. 1 Moclel of f:iw wgioii of a n 8-nitn. crii1)ryo. X 12.3. ( z i t , :interior naris; mf, iiasv-optic furrow: 0 7 t q . , I~ueeon:is:il groove; ? i i ) t p , niedial 1ias:il process; gp. glohnlar process ; I n p , 1ater:il 11:isnl process : w p , nxixillary process; niilp, niandilmlar process with tongue. Fig. 2 Lateral view, head region of iin 8-mm. rn11~ryo. X 1Z.3. an, anterior n;irir : ) I < . nas:rl ravitp ; nip., iii:ixill:iry process ; ~ 7 p m:indilml:ir , process. Fig. 3 Model of nose regiou aiid roof of nioull~,9 uim. tmlxyo. X 1‘7.3. nit, anterior n i ~ r i s ;nof, iiaso-optic furrow; h p , lateral nasal process; 17znp, medial iiasal process; p p , pol.itc prtwrss; rpni, roof primiti\e iiioiitll; him, bueconex:il nieinbranr. Fig. 4 Sketch of section through :interior narcs, !)-inin. embryo. X 12.5. nit. anterior i i a r k : cp.pZ, epitlieliuni bcing pilctl np to form epitlielial plug; m n p , medial iizisal process; P.br, floor of fowlmiin. Fig. .i Model, roof of mouth :ind nose regioii seen froin below, 1 0-mm. einbrpo. X I?..;. wtp, medial iias:il process; mi, :iirtc4or niiris; l t t p , 1:itcral nasal process; g r , ~ I O O e\ bctwccii iu;ixill;iry :iiid inecli:il iias;ll processes; i ~ i p r in:ixill:irF , process: p p , p h t e prorcss; p c k , priniitive c*lio;iri:i ; f i t g , tongue. 32 R. I. BURNS through the primitive choanae. The primitive nasal cavities are separated from each other by the thick primitive nasal septum and from the primitive buccal cavity in front of the primitive choariae by the primitive palate, a thick mass of mesodermal tissue. A s will be seen from a study of this aiid later stages, the primitive nasal cavity, from the time of the formation of the primitive choanae until the definitive iiasal and mouth cavities have been formed, always extends somewhat posceriorly beyond the posterior margin of the primitive choanae. Thus the primitive palate does not form the entire floor of the primitive nasal cavity, a portion of that floor lying behind the primitive choanae. The primitive choaiiae are formed between the lateral and medial nasal processes, these processes forming the roof of the primitive huccal cavity. The tongue is well developed, lying in the interval between the palate processes and extending only as f a r forward as the anterior margin of the primitive choanae. I n an embryo of 10.5 mm. (fig. 6): the reconstruction of xvliich is featured here with the permission of Dr. E. 31. NacEwen, the primitive choaiiae are seen to be established arid the anterior nares closed by an epithelial plug formed by tlie proliferation of their lining epithelium. The nasal cavity is represented as a cast and the floor of this cavity is seen to parallel the roof of the primitive mouth. The position of the primitive dioanae with reierence to the buccal portion of the Iiypophysia is t o he noted. This embryo represents a stage at tlie time of the establishmeiit of the primitive choanae wild these openings are seen to be, at this time, nearer the aiiterior nares than to the buccal portion of the hypophysis. The 14-mm. embryo (fig. 7 ) is chosen a s representing a tlecidcd advance in the tlevclopment of the nasal and mouth structures. The anterior nares remain completely closed hy their epithelial plugs, their position being indicated ext ernally by a marked depression of the plugs. That portion of the iiasal cavity lying immediately posterior to the plug is a transrerselp narrowed slit; its roof is somewhat lower and its floor somewliat higher thaii the roof and floor of the cavity 1)ropcr. I’ohte~iorly,the c + o i i t o i i i +of’ the cavity coilforms i l l general to that of tlie earlier stages, tlie w-alls yrcsenting more marked irregularities tluc to aclvaiice in developmcnt of thc tni+i~ials,cite. Tlic prirniti\-p (*hoiniaearc i m v mark- 34 B. I. RWRNS edly elongated anteroposteriurly tis n remlt of t:lie graclual rewssiori c.)f tlic floor of the primitiw iiasal cavity posterioi* to the c*hoaiial ttpWtllYclS. Wliilc the posterior margiiis of t I i c ~t~liom.iacai:c ciir1*ieiI posterioi*ly by this reccssioii, tlicre is i i ~ ) l ) ~ ~ clit.tle, ~ I l t lif~ iiiiy, c.steiit anteriorly of thc anterior iiitii*giiis c.)f t1w clio;iiitic!. This 1att.er ftwt is evic'lcnt in that tlic rintwior margins of the dioauae remain approximately o1q)osite t lit, iiiitcrior miirgiiis of tlie palato prowsses from t l i v timcr of tlicir first appc!arancti. Tho primitire palatc!, limvm-~r,g r o w antcriorlp togethcr with the otlier striictiirw, that tlic c'listuiicw hetaeeii tlitr anterior margiiie of the c.lroniit~ctiiitl tlw t ~ i i t w i ~ .iiar~!s )i* is iiit!rerrncd. Tliiis the migratioii postwiorly of tlicb primitiw cltotriiao spoken of in the litc~i~tiirc (,J. 1'. Scliaeifcr) is redly a recession of the floor of t I i c primitive nasal cavity (posterior mnrgiii of primitivc t h i ~ i ~ t wplus ) a growth anteriorly of tho iiose aucl moiith striictnres in the dovelopmai t of tlic face. !lkpulatc pro+ wsw i i o \ v extend well linlr;llally in tlie angle betwceii t l i ~ t o i r g i i o ~ i i dthe inferior gingiva, occiipyiiig practically tlic cwtirc primitive biiccal (bucconasal) cavity between t l i e totigii(L ~ i i c lgingiva. Tlie toiiguc! is iiow n v-ell-derelopcd orgmi occ~npyi~ig that part of tlic primitive huccal carit' lwt\v(wt the palate processes niid extending well forwnid i i i front of tlie miterior margin of the primitive chonnao. A l(i-mni. embryo (fig. 8 ) s1iows marked enlai*gemcnt of t l i e ziiiteroposterior and rertictd (liltmeters of the primitive tltixill cavities, while t l i c b trtiiisvcwe tlitimetcr rcmaiiis rehit ivcly slight. The uiitcrior IiHrtiti arc still closcd by I I i c h cq)itlieliul pliigs. That portioii o f tlic cta.vity immetliatcly l)ontci*ioi*to the pliig is \wry small, an ctompa.roc1to the mon. I)ostcai-ior portion, i i i hoth its verti(d t i n d ti-aiisversc &amv t c w . Tlic primitiw clioaiiac arc! iiicreasetl in tlicir aiitcro~)os:tc~i*ior tlinmetcw, tliic to fnrtlicr recession of the floor o f tlic! primitive nasal cavities. Tlic p a l ~ t eprocesses Iiavc! i i o n . twtutcd i n d a l l y from tlicir Literti1 lingual position and IIHVC comc in eo!.itac*t. witli. cadi vther aud begun to fuse ill tlio rnitlliiic! nl)o\-ci tlrc toiigic.. 'l'liis fiisioii begins H slioi-t SO distance bcliiiicl the anterior margin of tlie primitive clioanae and progresses posteriorly. Thus tlie anterior margins of the palate processes arc free in tliis stage, forming the posterior houiitiarp of a cwmmuiiic*atioii1,etwecn tlie iiasal aiicl month cavities. This commnnication is the c a i d of Steiisoii, the anterior bouiidary of ~vliicliis the medial nasal process at its jimctiirc in this region with the lateral nasal process. Tlw aiiterior margilt of the primitive choauae becomes, tliercfore, the anterior 1)ouiidarp of tlic cannl of Stcnson, a i d the ca~ial of Stenson may be designated as that portion of tlie primitive choana in front of tlie fusing palate processes. Fusion of tlie primitive iiasal scptum with the palate processes has not 1)cgun a t this time. With tlie rotation aiid fusion of tlie palate processes just described, a portion of the primitire huccal (hncconasal) cavity is incorporated iiito the nasal cavity. ‘l‘liis portion of tlie iiasal ctivit:T deri\-ed from the primitive buccal cavity is, h o w w c r , R small portion, in so far a s its vertical diameter is conccrnccl ; a larger portion of tlie primitive lniccal cavity is occupied by tlie palate processes themsclrw. Witli the union of tlie palate proccsses we m q speak of a clcfiiiit i r e huccal cavity aiitl a tlefiiiitive iiasal cavity. The latter is composed of the two primitive nasal cavities and the portion derived from the primitive buccal cavity. Eacli primitir-e choana iiow communicates alm-e witli its corresponding primitive iiasal cavit).. Iiifer.iorl~-, e ~ c lclioaiia i communicates in front of the fusing palate processes (definitive palate) with the ciefiiiitive hnccal cavity, arid Iwliind the anterior margin of the definitive palate Tvitli that portion of the tlefiiiitire iiasal cavity incorporated from tlie primitive buccal cavity as described above. The defiiiitire nasal cavity posterior to tlie primitive choaiiac is now seen to coiisist of two portions, a superior portion derived from tlic primitive iiasal cavity and an inferior portion derived from tlitl primitivc buc*c+alcavity. These two portions are separated by that portion of the floor of tlic primitive nasal cavity not yet obliteratccl 1))- the posterior migration of tlie postcrior margin of the primitii-c choaiia. l’hc two primitire nasal cavities arc 3; B. I. BVRXS sepilratccl 1.))- the primitive septum aiid, since the septum litis iiot yet. fiieecl with the palate, the t ~ 7 oprimitive cavitics communicat.e throngh tlie lmccal derivative of the nasal cavity b? way of their primitive clioanae. (:loincident with the rotation of tlie ptilate processes, there is ti (lepressioii of the floor of the month which permits the toiignc t.o free itself from its l.)osition hetween the palate processes aiid take a positioii iiifwior to them. Ti1 takiiig this posit.ioii the toiigue ilecrcwm iii its verticnl diameter am1 spreads laterally to ocoupg that portion of tlic buccal cavity foimerly occupied by llw lingliillly placcd palate processes. The mecha.iiism by v*liicIi thc! toirgiic is freed from its poeitioii between. the palate processes has rweived considerable attentioil in the literature and u iiumher of theories have been advancetl in explanation ( Keibcl : Iiiimaii ’Embryolog~~-lieibcl and Xa.11). In obs(~viiig the various stager; t.he writcr lias I)eoii imprc*ssc.dwith two factors wliicli it seems must iiifliieiicc this change aiid \rill Iar& cxplain it.. First, the gc?ncrnl depressioii of the floor of t.licmaiitli tcids to pull the tongue from betwccw the palat-c: ~I-OCCRSCN. St~coiid,the growt.h vertically of tlic masillary p~*o(:c.ss kccps pace with the geiie~alvcrtical gro\vth and tcwls to lift tlic palate procclrses over t IN toligiie. h i . t m emhryo of l(i.5 mm. certaiii iioteworthy atlvanc-cs tire trppareiit (fig. 9). The anterior narcs remuhi (*loset11)y thc qitlielial pliigs a i d the nasal tiiiil month cavitice, which may i i o w he spokcii of as t.lie clcfiiiit.i\-eiiasal aid mouth cavities. still commiinicate hi froiit. of tlic deliiiitivo ptilato tliroagli thc? c - ~ i i n l ot‘ Stciisoii. ‘I’Iw primitive iiastil septum lins ~ I - I ) W I I inferiorly and. just. posterior to tlic canal of Qtciimi, has hcgiiii to fnsc*with t.lic1 clefiiiitivc pnlatc in tlic mi(l1iiie. I\-itli the fnsioii of tiit? scpt.nm i i i i t l p~1nl.et.he c*nllill of Stcqisoi~ Iwcointbs ti Y-shtq)ctl C*UIIMI, tlre iiifcirior limb of tlw T opc.iiiiig iiitc.) the tlefiiiitivo l)iit*calcavity i n i d each siiperiw liml) into its rcsyc?ctive iiasttl cwitg, tlic two 112iad cavities hiiig ~ I O W pii*ilyscparatcil By the bcgiiiiiii~gfusion of t l i ~acptnm with the pttlate. ‘l’lie latertil portioiiw of the palate I)rocesscs hai-c. i i m v cst(*iitIctI forwartt to fiisv \\.it11 tlie medial irttsal prowss 'I'HIC I)E\'ELOPM'ENT OF THE NOSE 3i at its junct.urc with the lateral iiasal process a i d thus come to form the lateral limits of the canal of Stenson, decreasing its diameter. Posterior to tho canal of Stenson, t.hat.porticm 38 B. 1. BVHSS of tlie clefiiiitivc iiasal cavity derived from the primit ire nasal cctviticrr and that derived from the primitive mouth carity &I1 cornmanicate through tlie posterior part of t.lic primitivc ( h ~ i i t i e . Posterior to tho primit.ive clioanae the primitive litisti1 ~ i i t l~i(?(!til l portions of t.he nasal cavity are scparatccl. i i x iii t Iic prc4oiis stage, by the recediiig floor of the primitive iias;lti.l(vwi.ty,in which, at. this time, cart.iluge lias appearccl. ‘l’lic~19-mm. embryo (fig. 10) show some veiy important chiigc+si i i the parta of particdar interest. Thc? palato proc( w s w have fused tlirougliont thcir entire extent. Fnsioii hetwcc!ii (lit. niitcrior part of tlic tlefiiiit~ivepalatcb tincl the mcvlitil iicisnl process (anterior margins of the primit.irc c*liotiii;ic) liae also wciirrccl, with t.hc consequent olditeiation of t l i e caiial of Steiisoii, in so far as the soft tissues arc? coneeriwd. 13oiiy foramina remain in this region between ilic mw?rilli~i*y mid premasilliiry portions of t.he harcl palntc. They IIPP t l i r foramina of Stciisoii and Scnrpa and trinismit tli-r. ttii torior palat.iiie vessels a d nervcs. The canal of Stcnsou is ~ ~ o i n ~ : ~ lclosed c t d y in all embryos cxamhcd at t.liis stagc. Tlic poetcrior margins of the primitive clioanae remain practically as in tlie previous stage. Between the point of closure of tliv cniial of Stenson and this posterior margin tlic primit i n ! dioaiiac remaiii 31s elongated slit-like comm~uiicatioiiu I)etwccn t lie primitivc nasal aud primit.ive baccal portions of the clefiiiitiw iiasal cavities. The fiiaioii of the septmm and palate is iiow practically complete, so t.hat tlic two iiasal cvwitics are separate. Thc line of closiire of the canal of Stciiwii contains remiiuiits of epitlieliiun at this time (fig. 11, H). \i dcfinitc depression is prescnt on t.he nasal side at ttic! i y p r limit of this line of fusion. Tlie anterior nares are still closed by the epit.liclia1plugs, in which, however, erideiice of tlcgenertitioii may be semi in the marked depression of tlic plng (~st.ci:iidlymid, lwieatlr this depression, a large mxi of liquct’rwtion (fig. 11,A ) . I n tlic. floor of the primitire nasal c-tivitg, which rcrnaiiis bctwcen tlie primitive iiasal aiid huccal portions of the defiiiitisc nasal cavity and forms tlic posterior Imiiii(1tii-y of the primitiw cl~oaiia,the cartilage of the carti- lagiiioiis iiasal capsule lias foi-met1ii clcfiriitc traiisrcrse lamina extending from tlic cartilaginous septum to tlie latcral cartilage (fig. 11,B ) , This c~artilagiiionslamina lias appareritly Fig. 11 Sketches tlirougli the now region of :I 10-111ni.cnil~ryo. dl)ovc, through nntcrior n:iris. 13clon, i m i r scptnm. X 1%;. t i ( ' , nns:ll r n d y ; 7 1 q . nrrn of 1icjucf;iotion i n epitlirllal plug ; cp.pZ, cpil1irli:~l plug in nriterior naris; sc, wptul w r t i h g r ; f, floor of primitive ii:isnl m r i t y ; e, cxrtil:rge extending into floor of primitive 11 11 e;i\ity; ~ . t p bone , in mnsi1l:ii \ portioii of p:ll:itc: pinrp, bone in prein:ixill:iry portion of p:il:iir ; f'p, epitlielinl remn:uit :it point of c.losure o f canal of Htenson. Fig. 1 2 Shctcdli of s:igitt:il srction tlirougli riglit nnsial cavity of n 2j-inni. einby 0. x 1 5 5 . . r,c,e, portions of c:trtil:rgiaous iiasal enpsule; t , t , l . turbinates ; f , floor of pyimiti.ce n;isal c : t v i t ~forniiiig :L ti:usvrise srptuin lJct\r*ern this or tion of tlic clefinitire m s a l c:trit> ant1 that portion derived from t h e primitix-e buccal cavitv; bc, knsilnr plate of c.:irtil:igc; f7>r, floor of forebrain; /I!!, hucral portion of 11ppn~1li;vsis ; pnt r p , I ) o i i e of p r r n ~ ~ ~ x i l l poi ~ i rtion y of pa h t r ; tttrp, bone of ninxillnr? portion of p:il:itv; hp, 1torizont:il portioii of p:il:itr 1)oiir. retarded, for tlie timc heiig, the recession of the posterior margin of the primitive elioaiia, the rccesioii having coiitinuetl steadily until the appearance of this cartilage in ahout the l4-mm. st age, b'igiire 12 represeiits a sagittal section tlirough the right iias;irll ca\-ity of a 25-mm. embryo aiicl is presented here to w l l attention to cwtaiii features at this relatively late stage. I t will Iw seen that the canal of Steiisoii is entirely obliterated iilicl t h a t 1 1 0 c~~itlicliuiri is iiow pi'eseiit i i i its line of closurc. 'l'liti ti11 tei*ioi*nares ai*e agaiii hecomiiig patent, due to the tlcg~~iic.i*atio~i of tlic epithelial plugs with which the?- were f o i m ~ r 1 filled. ~The eai-tilagiiions iiasal capsule is shown in its ixAlatioii t o t l i e iiasal wall. T l i c floor of the primitive tiasti1 c#ai.ity p o ~ t c ~ i . i otol ~tlici pi-imitivc elioaiiw is still preseiit N K I voilt:iiiis t l i c h will-formed lamiiia of cartilage to which atteiitioii was called iii tlie previous stages. Xo statemciit as t o tlic~ultimate fate of this portioii of the floor of the primitive iitisnl cni-it?-caii be made at this time. Tlie cartilage contaiiietl i i i it is prohahly j(tenlien1 with what has lneeii called the post o i * i o i * tixiisvthiw stytimi (JIwi(1). Kefei*elice t o figiii-c 12 chstiil)lisi1(bs,wyq)i*oximately,tlic positioii of the canal of Steiisoil i i i tliv cvir1ic.r stiiges-a point hetwecii the boiie of tlici I ~ l i i s i l l 1.)n a 1 i ( 1 ~)remaxilliiry palate. S I' S4 M A It Y its c w t i i i t i d f'cnt i i i w tlie clevelopmciit of tlie nos^ in tllc slwrrnopliilc. ( 1 0 ~ s iiot differ from tliat of other mammals iii which it has 1)ec.~tlcsci*ihetl. Tlie primitive nasal cavities clcvelop iis a pair of i~igrowtl~s from the region of the olfac111 t 0i.y ]'l;lcoclt~s. .\ s1ioi.t clisttiiicc iii f i w i i t of its hliiitl posterior extremity c w + l i c4;tvit.t- grows towaixl tlic primitive 1)accal eavit)- (8-mm. twil)ryo). 1,ater it liws esteiidecl toward the buccal cavity at tliis 1)oiiit iiiitil it 1-emaiits separated from the buccal cavity 1)). i i tliiii rnem1)raiic~wliich cwiisists only of the buccal a l i t l ir;is;il ~ ~ ) i t I i ~ 4 i i r rrl'liese i, a h t t i i i g cpitlielial layers form w h t is k i i o w i i a h tho 1)iiecoiiasal membrane. 'I'he mcmbl.alle riipI 11 rcw iit iil)oiit tlic 10-mm. stage aiitl the primitive clioartae arc tliiis cstal)lislictl. At their first appearance the primitive c.liow1i;ie a r c located iiearei to tlie anterior nares tliari to tlic 1 ) t i w : i I Imi'tioii of tho h)-popliysis. This statement is mntl(, M S locatiiig the choaiiae somewhat more clefinitel:- tliaii that given for mail by Schaeffer iis ‘(well iii adraiice of the buccal pituitary outgrowth in the roof of tlie moutli.” At about the time of the formation of the primitive choanae, there is evidenced a piling up of the epithelium lining the antci*ior iiares, resulting iii a climiiiutioii aiicl filially iii complete occlusion of the aiiterior nares by a11 epithelial plug. ‘i’liis plug persists until the closure of tlic canal of Stenson, when it shows signs of disintegration, aiid is f i ~ i a ltlestroyetl l~ aiid the lumeii of each anterior naris reestablished. Following the rupture of tlic bucconasal membranes, tlic primitive ehoaiiae thus formed elongate anteroposteriorly, cluc to a gradual recession of their posterior margins, i.c.., the floor of the primitive nasal carities posterior to the primitive clioanao. The tiiiterior margins of tlie primitive clioaiiae maintain their original relative positioii, any allparelit posterior migration of them heiiig ratlier a growtli outwwd of tlie face structures. The reccssioii of the flool. of tlie primitive nasal cavity (posterior margin of primitire choaiia) continues until retarded by the derclopment witliiii it of a lamiiia of cartilage derived from the cartilagiiious nasal capsule. This floor remains, in tlie latest stage studied f o r this report, as a transverse septum separating the posterioi* portion of tlie primitive nasal-cavity derivative of the clefiiiitive iiasal cavity from the buccal derivative of tliat carit?. The ultimate fate of this septum has not beeii estaBlishec1. C‘oiiicident with the above changes, tlie palate processes of the maxillary process grow medially and iiiferiorly in tlic angle between the toiignc and tlie floor of the primitive mouth. As a result of tlie general depression of the floor of the mouth aiitl the iiicrease in height of the maxillary processes, thfb tongue is freed from its position between tlie palate processea. It theii hecomes flattened vertically ancl spreads out in the floor of the mouth to occupy that portion of tlie mouth cavity foi.mer1~-occupied by the palate processes. The latter grow mediallj- ahore tlic tongue aizd fuse in the midline. TIiis rotation ant1 fnsioii of tlie palate processes iiicorporatc 21 small portioii of We primitive buccal i?a\-it.yinto the definitive iiiisul cavity. The f irsioii hetwceii t.he palate proccsaes begins a short distance post crior to their anterior limits and progresses poatcriorly. There remains botween their anterior limit and tho medial nasal process (anterior margiii of the primitkc vlioaiine) a communication between tlie tlcfiiiitive buccal aiitl iiasal cavities, tho caital of Stelison. This canal is later closed 1)y the fiisioit of the medial nasal process a i d the palate prwesxes. When t . 1 Bone ~ of tho palate develops, the 1wi.v foramina of Stensoil aiid Scarpa are formed at this point Iwtweeii the maxillary and pi-emasillarp portions of tlie Iiard rjaltrte. Tho primitive choanae do not migrate posteriorly to become? thi? permaimit: chonnae ; their posterior margins map n1igrat.c posteriorly to hcomo tlie upper margins of the pe~maiiciitchousae. Furtltcr work is necessary to decide this point, whicli tlepends 011 the fat.c of 1-hc remiiai!t of tlie floor of the primitive nasal cavity formiiig the posterior hii~idary of the primitive choanrrc. Tlic clet.ermiaatioii of the fate of this portion of the floor of the primitive naad cavity invol\-cs ;I study of tlic cartilab~iiousiia~rilcwity.