The growth pattern of the cranial vault in the albino rat as measured by vital staining with alizarine red s Э.код для вставкиСкачать
T H E GROWTH PATTERN O F THE CRANIAL VAULT TN THE ALBINO RAT AS 3lEASURED BY VITAL STAINING W I T H SLIZABINE RED “ S ” MAURP MASSLER’ A N D ISAAC SCHOUR University of Illinois, College of Dentrstiy, 806 South Wood S t r w t 5 Chicngo 12, Illinois FOUR FIGURES INTRODUC’I’ION The purpose of this investigation was to study the growth patkern of the cranial vault in the albino rat. An attempt was made t o designate: ( a ) the sites of growth, (h) the mode of bony growth at these sites, (c) the chronology and ( d ) the rates of growth at the different sites. This program was made possible by the fact that alizarine red “8,” when injected as a vital dye, selectively stains actively growing bone (and dentin) (Sehour, ’36). RETIEW OF TJTTEK~’I’IJRE The gron-th of the cranial vault has been often deduced by comparison of youiig arid adult crania (Hatai, ’07 ; Krogman, ’30). I t s growth has bccn investigated only infrequently by direct espcrimental methods (Troitzky, ’33). Brasli (’34) used the indirect method of vital staining by ineaiis of madder feeding in the pig. He explained the growth of the cranial vault b7 a simple mechanism of esteriial (ectocraiiial) deposition and internal (endocranial) resorp‘Submitted in partial fulfillment of t h e requirements f o r t h e Degree of Master of Science in the G m l u a t e Schnol of the University of Illinois. 83 84 MAURY &!IASSLEK. AND ISAAC SCHOUR tion. H e ascribed to the sutures only the secondary role of adjustment. METHODS AND MATERIALS Vital injections of alizarine red “S” stain the bone, dentin and cementum growing and calcifying at the time of the injection. It is, therefore, a vital stain for all the calcifying tissues except enamel (Schour, ’36). The direct method of alizarination consists of giving multiple injcctiolis of the Pig. 1 Cranium of albino r a t virtually injected with alizarine red ‘ S ’ a t one day of age and sacrificed at 20 days of age. Dark areas, red-staincct bone growing at the time of thc injection. Light areas, bone formed between one and 20 days of age. dye at selected interval’s. After sacrifice, tlie gross specimens or ground histologic scct,ions sliow the injection effects as separate lines which can easily he measured. The indirect method can he used oiily in the young, rapidly growing animal. It consists of giving a single injection which stains all the bone present a t that time. This is so since all the bones a r c actively growing. A sufficient interval is then GEOTC‘TH O F CRANIAL VAULT O F RAT 85 allowed to elapse before the animal is sacrificcd. The new bone formed after the injection appears white (fig. 1). The indirect method of alizarination was used in this study. One hundred and thirteen albino rats were each given a single intraperitoneal injection of a 2% solution of alizarine red “ S ” a t various ages betwecn birth and 310 days and were sacrificed three to 90 days later (table 1). The dosage ranged from 75 to 100mg per kilogram of body weight. The animals were a well-standardized, modified Wistar strain bred by the Department of Physiology of the University of Illinois. The analyses were made i q o n the gross macerated wet specimens. The bones growing a t the time of thc injection were stained red by the alizarine. The new bone growing after this time was uncolored (white) (fig. 1). In this manner the sites of growth a s well as the chronology and rate of growth at these sites were made apparent. hIeasuremerits of the amount of new white bone were made from the red edge of one bone to the red edge of the next at right angles t o the growing surfaces. The measurements were made under a dissecting microscope with a caliper and were read against a stage slide divided into 10ths of a millimeter (table 1). OBRERVA TT O N 8 Periods of g r n e r a l i w d awl localiwd growth Vital injections of alizarine red ‘‘8” in rats under GO days of age resulted in a red stni&ag of all t h e honcs present a t t h a t time. Microscopic analysis (unstained ground sections) showed that this generalized staining in the young animal was due to the fact that bone growth was progressing upon all’ bony surfaces -periosteal, endosteal, iiitersutural aiid endochondrab Injecfious of thr dye i~ aninicds over 70 days of age resulted in t h e red stuuking of only crrtaiw localizrcl areas, indicating that after the 70th day of age, bony growth was confined to localized sites. 86 X A U B Y MASSLER A S D ISAAC S C H O L X TABLE 1 A m o u n t of postnatal bone growth ( i n mm) at various growth sites a t different ages NO AXIN.4LS AGIO A-r lBJECTlON AGE A T INPTIRVA~ SACRIFICE INTERhAS4L YETOPIC SUTURE SUTURE SUPERIOR MARGIN O F FORAMEN INTEROCCIPITAL I n dugs 1. 11. 111. 1V. v. 0 3 3 0.5 * * * 0 10 10 0.5 1.7-3.5 0.6 1.8 2 12 10 0.1 1.5-3.0 0.6 2.0 0.5-1.0 * 1.2 8 6 4 5 13 10 15 20 25 10 10 10 10 10 30 40 70 5 10 20 30 60 10 100 90 20 20 20 40 50 100 20 30 80 16 VI . 7 30 60 30 VJI. 10 41 41 41 71 101 131 30 60 90 VITI. 8 60 90 30 9 70 90 100 100 120 130 30 30 HO 250 310 60 IX. 0.0 0.0 x 0.2 0.7 1.2 1.3 0.0 Growth ceases at 3 days Growth ceases Growth ceases at at 10 days of age circa 20 days 0.5 0.8 1.2 1.5 1.5 0.0 Growth ceases at circa 20 days of age * Growth was so slow- t h a t the amount of new bone formation could not bc nieasurcd grossly. New bone formation was, however, diseernible. 87 GROWTH O F CRANIAL VAULT O F 1tAT TABLE 1 - contiiiued Amount of postnatal bone gromth (in mm) a t zarious growth sites at diffeTent ages S9QITTAL SUTURE I'OSTBRIOR LdMBDOIUAL b UTURE $ &::!" ANTERIOR LAMUUOIUAL ANTERIOR MARGIN O F SUTURE N A S A L BOXF FRONTONASAL SVTCRE CIRCT'MTBXPORAL SUTURE TEMpoRAL CREST * * * * * * x .. 2.5 2.6 1.5 2.6 1.7 5.0 * .. 2.5 2.7 1.8 2.8 1.3 4.8 2.0 .. 1.8 1.5 1.0 1.5 0.8 3.5 1.0 .. 0.3 0.7 1.0 1.2 1.6 1.8 0.4 0.6 0.8 1.5 1.7 0.7 1.0 0.9 1.3 1.8 0.6 0.9 1.2 1.8 9.0 1.5 2.5 4.6 6.5 9.0 0.3 0.8 0.8 .* 0.6 0.6 0.8 1.3 1.2 2.3 1.7 2.0 2.4 10.0 * 0.7 0.8 1.2 * * 2.7 .. x 1.2 1.0 1.3 1.8 * * 7.0 x .. 0.0 0.0 0.6 1.1 9.3 x 0.6 1.0 1.2 1.3 1.6 2.0 0.2 * 0.5 0.9 1.1 * * * x Growth ceases at circa io days of age * * * * * * * * x * x 0.0 0.7 0.8 0.0 Growth ceases at 20 days of age Growth ceases at 30 days of age Growth seams at Growth ceases a.t 25-30 3040 days of age days of age * 5.0 * w * x x * * * .. .. .. .. I. .. * * * * * * Growth was so slow that the amount of new bone formation could not be measured grossly. New bone formation was, however, discernible. 88 MAUItY IWASSLER A N D ISAAC SCHOUB Sites of growth Increase in cranial size Iiztersutural growth. This study revealed the sutures to be the most prolific sites of postnatal growth in the cranium. Intersutural bone growth was very rapid and quite evident from birth to 30 days of age. The rate of growth decreased markedly with age (age gradient), so that after the 30th day of life growth was confined to only certain of the sutures. Increase in skull width. Increase in the width of the slrull was accomplished by growth at the sagittaI complex of sutures. The sagittal complex consists of three segments : (a) the internasal suture between the two nasal bones of the snout, the major site of increase in nasal width; (b) the metopic or interfrontal suture between the two frontal bones which contributed t o the increase in bifrontal width; and (c) the sagittal suture proper between the parietal hones. The sagittal suture was the site mainly responsible for the increase in the maximal width of the cranial vault (figs. 1 and 2). A part of the increase in snout width was contributed by the lateral growth of the frontal process of the maxillae. This dimension is not considered in this report and the growth of the binasal width a t the internasal suture alone was studied. Increase i n skzill Zewgth. Cranial vault. Increase in the length of the cranial vault was accomplished primarily by intersutural growth at the coronal and the anterior and posterior lamhdoidal sutures. The anterior lambdoidal suture is the lambdoidal suture proper, while the posterior lambdoidal suture is an accessory suture formed bv the presence of the interparietal bone between the parietals and the occipital bones (figs.1 and 2). Snout. The frontonaeal suture and the anterior margin of the nasal bones contribute to the increase in snout length. The frontonasal suture was the most prolific site of inter- 89 GROWTH O F CRANIAL VAULT OF RBT sutural growth in the ent,ire skull', a condition to be expected of a long-snouted animal (tables 1 aiid 3 and fig. 2). Imzcrease in oault hei.ght. Increase in cranial height was accomplished by growth a t ( a ) the superior margin of foramen magnum (which, unlike man, opens posteriorly) ; SITES OF GROWTH AMOUNT FORMED Post. Lambdoidal suture _------------ Ant. Lambdoidal suture - - - ~ il Sagittal suture Ir ---/-+ -?!---At birth Coronal suture-l---Metopic suture Fig. 2 Diagram indicating the sites and amount of growth in the cranial vault of the albino rat. (b) the interoccipital suture lying between the supra and superior portions of the occipital bone; aiid ( c ) the circumtemporal suture. The interoccipital suture in man begins to close a t three months in utero aiid is present oiily a s a cleft or fissure at birth. I n the albino rat, this suture was a site of active bone deposition until about 20 days of age. 90 MAUFLY MASSLER AND ISAAC S C H O C R I n the animal under 30 days of age, the anterior and posterior lambdoidal sutures joined laterallr and met with the interoccipital and circumtemporal sutures a t the posterior lateral or mastoid frontanelle above the tympanic bulla. The interoccipital suture was therefore continuous anteriorly with the circumtemporal suture and posteriorly with the superior margin of foramen magnum. These three areas, the free margin of the occipital bone forming the superior margin of foramen magnum, the interoccipital suture and the circumtemporal suture thus formed a continuous site of growth at the side and back of the cranial vault. This complex ran paralclel to the dorsal aspect of the cranial vault and growth at these sites contributed the major portion of the increase in vault height. Increase in thickness of rault bones Growth om ectocranial and endocranial s t i r f o w s . Increase in the thickness of the vault bones was accomplished by the deposition of hone upon the wto- and endocranial surfaces. The red staining was readily seen upon both the ecto- and endocranial surfaces in all animals ZPSS thaiz 70 days of agqc. The red staining was especially vivid upon the endocranial surface. Sections cut through the vault and examined under low power showed a virid staining of the endocranial snrface of the inner table and of the diploe. There was only mild staining on the ectocranial surface of the outer table. These findings do not agree with those of Brash ('34) w110 found, in the pig, new bone growth on the ectocranial surface with resorptions of the endocranial surface. After the 70th day of age, the red staininq was confined to only certain areas on the ectocranial surfaces: the area over the frontal sinuses ; the frontonasal suture ; the temoral crest and circumtemporal. suture complex ; the nuchal area of muscular attachments and the zygomatic arch. These sites of growth persisted up to (and probably beyond) 300 days of age. G R O W T H O F CRAR-IAL VAULT O F IlAT Rates a i d chroieology 91 of growth The absolute rates of growth a t the various sites of growth studied a re tabulated in table 1. I n all cases the rate of growth was most rapid a t birth and decreased markedly with age (age gradient). The relati~t:rates of growth at the different sntures are perhaps of greater interest than a re the absolute rates since these a r e responsible f o r the changes in proportions that characterize development. Sagittal complex; (&wrease in cmnial t d t h ) . F rom table 1 it can be seen that the sagittal complex of sntures grew very slowly and only for a short period of time. Growth at the internasal. and the interfrontal ( o r metopic) portions was completed b p the 3rd and 10th days of age, respectively (tables 1 and 2). The sagittal suture proper grew a little more rapidly and f o r a longer time, growth beinq completed at this site by the 20th dav of age (tables 1 and 2). Since the sagittal compkx of sutures contributed to skull width, it can be sccn that nasal width was completed by the third day and miii;mal bifrontal width was virtually completed hp the 10th d a r , while the maximal cranial width (or bitemporal-crest distance) was attained by the 20th day. Coronal a i d larnhcloidal sutures (increase i n c r a n i a l k n g t h ) . The sutures which contributed to the increase in cranial length grew more rapidly and for a longer period of time than did the sutures which contributed to the growth in width. The coronal and lambdoidal sutures not only grew a t a more rapid rate than did the sagittal sutures, but growth at these sites persisted f o r a longer time -until the 40th day of age. Maximal vault width was virtually completed by the 20th day of life while the increase in vault length ~ - a snot completed until the 40th day. I n addition, growth in length was accomplished a t tliree sites (coronal, anterior and posterior l~ambdoidalsutiires), while growth in width occurred a t only a sincle site (sagittal sutures). These thrcc factors (greater rate of growth, longer duration, larger number of growth 92 M A U R Y MASSLER A N D ISAAC S C H O U R sites) readily explained how the round head of the new-born rat became the elongated tennis racket-shaped head of the adult rat (fig. 3 ) . Frolztoiaasnl suture and anterior m a r g i ~of t k e WLSXZE bories ( i m w a s e i j z nasa.Z length). Growth a t the anterior and posterior (frontonasal) margins of the nasal bones was very rapid and resulted in a marked and rapid increase in the TABLE 2 Sites and chronology of poslnatal growth of bone in the cranial vault and snout of the albino rat AGE A T WHICH GROWTH CEASES 1. lnerease in width IT. - Sagittal coniplex of sutures 1. Internasal suture 2. Metopic (interfrontal) suture 3. Sagittal suture lnerease in length (A) 3 days 10 days 20 days Cranial vault 1. Coronal suture 2. Anterior lambdoidal suture 3. Posterior lambdoidal suture (B) Circa 30 days 30-40 days 25-30 days Snout 1. Anterior margin of nasal bone 2. Frontonasal suture Circa 70 days Active after 300 days 111. Increase in height - 1. Superior margin of foramen magnum 2. Interorcipital suture 3. Cireumtemporal suture Circa 20 days Circa 20 days Active after 300 days IT. Increase in thickness of vault bones 1. Eetocranial surfare (generalized) 2. Endocranial surface (generalized) 60-70 days Active after 300 days 93 GROWTH O F CRAXIAL VAGLT OF RAT length of the nasal bones. Growth at the sutural margin was much more rapid and continued f o r a longer period of time than a t the free anterior margin. This bone lends itself to an interesting and provocative study of the rate and mode of bone growth a t a free and sutural margin. 1 DAY LO DAYS 30 DAYS 6 0 DAYS 100 DAYS W 2 0 0 DAYS Fig. 3 Drawings illustrating the growth o f the eraiiial vault in the albino rat from one to 200 days. The frontonasal suture was the site of most rapid growth in this study (table 1). Its growth continued until the 300th day of life (possibly longer), while the free anterior margin of the nasal bones ceased its growth by about the 70th day. The rapid increase in the length of the nasal bones gave to 94 MAURY MASSLER A N D ISAAC SCHOUR this animal its typical long-snouted appearance, a characteristic which progressed with age. The increase in nasal length occurred more rapidly and earlier in the male than in the female (Hatai, '07). The snout was quite small in the newborn, while the cranium was relatively large. This relationship is typical of most mammals, including man, I n the rat, however, the snout grew at a relatively more rapid rate than did the cranium. R.lzdo- and ectocranial bogte deposition. It was not possible to measure the rate of endo- and ectocranial surface deposition of bone in the rat because of the thinness of the bones and the difficulty in preparing adequate ground sections. This phase of the problem will be analyzed by vital staining of the crania of rhesus monkeys. Growth potential. It is interesting and perhaps significant to notice the close correlation betwen the rate of growth at a given site and its chronology. Growth tcnds t o persist for a longer period of time at those sites where the initial rate of growth was more rapid. It would appear that different sites enjoy different growth potentialities and that a greater amount of growth energy at a given site is expressed at a faster rate and over a longer period of time. Sex differences Differences in the rates of growth at the various sites in the male and female could be distinguished by the 30th day of life but did not become prominent until after puberty (60 days). Tlie female grew at a slower rate than did the niale, and the various sites of growth terminated at a slightly earlier age. The female skull was therefore similar to a male skull- of an earlier chronologic age level. While the absolute difference between the male and female at a given site was quite small, the cum4lative effect could be readily seen in the smalsler size and relatively immature proportions of the female cranium after 100 days (fig. 4). Whether the slower rate of growth in the female cranium was the result of a steeper gradient of growth, a smaller GROWTH O F CRANIAL VA4ULT O F RAT 95 growth potential, o r both, could not be determined by our methods. This question (whether sex differeiices in growth stem from ldifferences in growth potential or growth gradients) is fuiidamental to the underst,anding of tlic phcnoiiicnon of growth. 130 DAYS FEMALE MALE Fig 4 Drawinga illustrating differences in the cranial vaults of female and male albino rtas a t 130 days of age. DISCUSSION The growth of the cranial' vault is intimately connected with the growth of the brain and follows the same type of neural growth curve. Some texts assume that the cranial cavity enlarges by external surface deposition and internal resorptions. A simple mechanism of apposition arid resorption such as suggested by Brash ( ' 3 4 ) can hardly account for the great velocity of cranial growth during infancy. Similarly, aberrations in cranial development such as occur in hydrocephalus and in premature synostoses cannot be understood or explained by such a simple mechanism. This study emphasizes the primary role of the sutures in cranial development. 96 MAU€LY MASSLRR A N D ISAAC SCHOUR Vital staining of bone with alizarine red “ S ” shows that there are, in general, two periods of skeletal growth: 1. A period of generalized bony growth from birth to about 60 days of age, which was characterized by the active deposition of bone upon all bony surfaces. The rate of growth a t different sites did vary, however (table 1). This period is characterized by a very rapid increase in the size of the cranial vault with only minor changes in proportions. The latter are duc to differences in the rate of growth a t tlie different sites. 2. A period of localiwd growth after ‘70 days of age characterized by the fact that bonc deposition occurs only a t certain sites. These sites were, in general, the same that showed the most rapid rate of growth during the previous period. Since growth is confined to localized areas, the predominant characteristic of this period is a marked change iii the proportions of the cranial vault with only a relatively slight increase in size (fig. 3). The rat skull is relatively round at birth (fig. 3). As growth continues, the increase in vault length is more rapid and extends over a longer period of time than does the increase in vault width. By the 30th day of life, therefore, the head becomes markedly elongated. As growth continues from the 30th to the 60th day of life, further elongation occurs (fig. 3). After the 60th day of life when the period of generalized ectocranial growth is completed, growth is confined to only a few localized areas. The most prominent site of red staining after 60 days is over the frontal sinuses. These areas correspond to the supraorbital ridges of the anthropoids and apparently serve the same purpose -to permit enlargement of the frontal sinuses by resorption from within. I n addition, the areas of muscular attachments continue to grow during the period of localized growth. The temporal crest becomes prominent after 60 days of age and continues to grow throughout the normal life-span of the animal. This growth is more prominent and rapid in the male than in the female. G R O W T H O F CKhNIAL VAULT O F BAT 97 The growth of the snout at the frontonasal suture contributes markedly to the changes i n proportions that characterize maturity. From a round-headed, small-snouted animal at birth, the r a t becomes a mesoceplialic animal, with a medium-sized snout. By the 60th day the snout hegins to assume major proportions. It continues to increase in relative size after the 60th day, whereas the cranium remains static. BP the 100th day of life, the r a t head assiimes the proportions and shape of a tennis racket. Growth of the snout in the male is more vigorous and contiiiues for a longer period than in the female. Modes of growth Bone growtli at the ectocranial and endocranial surfaces (inner and outer tables) occurred by simple, surface deposition of bone, one layer upon the other. This mode of grow& produced a compact type of bone composed of circumferential lamellae. The diploe (spongy bone) were formed by concomitant resorptions from within, as were the frontal sinuses. The intersutural growth of bone was very interesting from many aspects. A given sutural line began (before birth) as the smooth periphery of a bone. As growth continued at the free margin of the bone, it made contact with the periphery of the adjacent bone and thereafter these margins became serrated and interdigitated. It is well establsished that under the influence of tension, a given osteogenic layer will deposit a trabecular type of bone instead of simple circumferential lamella. This modc of bone growth is well8illustrated in the alveolar process of the erupting tooth. A gomphosis type of attachment similar to that present around tlie tooth root exists at the actively growing suture. The two ends of the hones are joined together hy the peg arid hole arrangement of the bony trabecnlae and thc whole held firmly in place by strands of fibrous connective tissue. The osteogenic layer upon the endo- and ectocranial surfaces is continued between the sutures. This arrangement 9s MATTRY ML4SSLElt AND ISA4AC SCHOUR permits slight mobility at the sutural junction and rapid deposition of bone at the same time. The rapidly expanding brain encased within the cranial vault exerts a definite internal pressure upon the bones of the vault. This pressure is translated into a tension force upon the intersutural periosteum. The intersutural ostegenic layers under the influence of this tension form ft trabecular type of bone instead of the simple circumferential layers of bone seen upon the ecto- and endocranial surfaaes. As growth continues at the free margins uiider such tension, the trabecnlations become more and more apparent and gross (fig. 3 ) . TABLE 3 Correlation between the height of sirtural serrations and the chronology of growth at that suture i n the albino rat AGE 4T W H I C H GROWTH CEASES 1. Internasal siitnre 2. Metopic suture 3. Sagittal suture 4. Interoccipital suture 5 . Posterior lanibdoidal suture 6. Anterior lambcloidal suture 7. Coronal suture 8. Cireumtemporal suture 9. Frontonasal suture - Circa 3 10 20 Circa 20 25-30 3040 30 Actire a t 300 Active a t 300 days days days days days claps days days days HEIGHT O F SIRRATTONS IN ADULT MALES AGE 2 2 0 DAYS 0.0 mm 0.0 mm 0.0-0.3 nini 0.0-0.2 m m 0.0-0.6 111111 0.4-0.7 111111 0.8-1.2 min (overlapped) 1.0-2.0 nun The serrations which characterize the sutures are therefore gross manifestations of this mode of tralnecular growth. This postulate is supported by the work of Troitzky ('32). When complete or partial ablation of a given suture was performed so that there could be no intersutural tension, the free margin of the parietal bone grew as a smooth line. When the bone again made fibrous contact with its neighboring bones a i d was once more under a tension force, serrations and interdigitations again blecarne manifest. Examination of the serratioiis at the different sutures further supports this hypothesis. Sutures which closed early G R O W T H O F CRANIAL VAULT O F RAT 99 were not serrated, while those a t which grou7th occurred for a longer period of time were highly serrated (table 3). The correlation between the age at which growth ceases at a given suture and the height of the serrations was good (table 3). SUMMARY The growth pattern of the cranial vault was investigated by means of vital staining with alizariiie red “ S ” in 113 rats from birth to 300 days of age. 1. There a r e two periods of growth in the skeletal system of the rat. I n the first, there is a rapid deposition of bone upon all bony surfaces. I n the second period (aft,er 60 days of age), growth is localized to certain definite sites. 2. The growth of the cranial vault occuix primarily by a rapid deposition of bone at the approximating margins of the bones which comprise the various cranial sutures. Tho mode of growth (by trabeculation) and the rapid rate a r e apparently a response to the rapidly expanding encased brain. Sutural serrations are a gross manifestation of the mode of trabecular growth. 3. The sutures a r e so arranged in relation to one another as to perinit each group to contribute to the increase in skull width, skull length and cranial height. The sites concerned with growth in each dimension a r e tabulated in table 1. 4. Deposition of bone also occurs upon the endo- and ectocranial surfaces to increase the thickness of the vault bone. Concomitant internal resorptions form the diploe and frontal sinn ses. 5. The changes in the proportions of tlie cranial vault and snout are determined by ( a ) the number of growth sites in a given direction, ( b ) tlie rate of bone deposition at a given site and ( c ) the length of time over which growth at that site is actiye. Skull length increases inore rapidly than ~1;ullwidth hecause a11 three factors are greater in the direction of length over width. 100 MA4CC’RYMASSLER AND ISAAC SCHOUR The sites of growth and their chronology as well as the rates of growth at each site for the different ages are indicated in table 1. 6. The order in which growth ceases at the various sutures and the re1,ation to the height of the suture serrations are indicated in table 3. Ririasal width is completed by the 3rd day; minimal bifrontal width by the 10th day; maximal cranial width is attained by the 20th day. Increase in vault length is virtuallv completed by the 40th day, while snout length continues to increase to (and possibly beyond) 3 0 days. 7. The female cranial vault grows at a slcightly slower rate at the various growth sites than does the male, giving to the female skull the appearance of a less mature male skull. It is not known whether this is due to a smaller growth potential or a greater gradient of growth. REFERENCES BRASH,J. C. 1934 Some problems on the growth and developmental mechanics of bone. Edinburgh Medical J., 41: N. S. IV. GRERNE,E. 1935 Anatomy of the rat. Trans. h i . Phil. Soc., 97, new series. H B N D E L S Y A N , 31. B., A N D E. F. GORDON 1930 Growth and bone changes in rats injer,ted with anterior pituitarF extract. J. Pharmacol. and Exp. Therap., 38: 349-362. HATAI,S. 1907 Studies on the variation and correlation of skull measurements in both sexes of mature albino rats. Am. J. Anat., Y : 423-442. K R W M S N , W. M. 1930 The problem of growth changes in the face and skull as viewed from a comparative study of anthropoids and man. Dental Cosmos, 76: 624-630. SCIIOT~R, I. 1936 Measurements of bone growth by alizarine injections. Pror. Soc. Exp. Biol. and Med., 3 4 : 140-141. SCEOUR, I., M. M. HOFFMAN AND B. G. SARNAT 1941 Vital staining of growing bones and teeth with alizarine red “ S.” J. Den. Res. 20 ( 5 ) : 411-418. SCIEOUR, I., AND M. MASSTAIR 1940 Postnatal craniofacial and skeletal dereloprnent in the albino rat and the Mararus rhesus monkey a s demonstrated by vital in,jections of aliznrine red “8.” Anat. Rec., 76: no. 2, suppl. no. 2, Feb. 24, 984. TODD,T. W., ..“D D. Mr. LYONS 1924 Endocranial suture closure: I. Adult males of white stock. Am. J. Phys. Anthrop., 7: 325-384. 1925 Ectocranial suture closure: IT. Adult males of white stock. Ibid., 8: 2 3 4 6 . G R O W T H O F CBANIAL VAULT O F RAT TROITZKY, 101 VON WL. 1932 Zur F r a g e der Formbildung ded Sehadeldaches (Experimentelle Unterscburig der Schadeldathnlhte und der damit verbundenen Erscheinungen) Zeits. f . iMorphol. and Anthropol., SO : 504-530.