Variations in susceptibility of amphibian ova to the x-rays at different stages of development.код для вставкиСкачать
\7BRIXTIONS I N SCSCEPTIBILITY O F A X P H I B I A N OYA TO THE X-RAYS AT DIFFERENT STAGES O F DEVELOPNENT. BY CHARLES RUSSELL BARDEEX'. Universit of Wiscotisin. I n experiments with living organisms, it is necessary to take into account variations in internal conditions and in the environment, as well as the nature of the specific influence brought to bear upon the organisms. Thus toad or frog ova are much more readily injured when over-ripe at the time of fertilization than when fertilized at the normal period, and, as a rule, are hardier in moderately cool weather than in hot. Owing to variable factors of this kind, the results of exposure of sex-cells to x-rays of a given intensity for a &-en period of time are not uniform in detail in a series of experiments, although the general results are fairly uniform. There is, moreover, great individual variability in susceptibility in any given lot of sex-cells, so that the percentage of organisms affected as well as the extent of the abnormalities of development o€ the organisms affected must be taken into account in estimating the effects of the rays. Exposure of females, in which the eggs are still ovarian, will prevent the ripening of the eggs. The eggs remain indefinitely in the ovaries and are not capable of artificial fertilization. Exposure of the sperm or of ripe ova to intense x-rays for a considerable period of time seems not markedly to affect the power of fertilization. The earlier stages of cleavage in fertilized ova, when one or both sex-cells have previously been exposed, are apparently nearly normal, but if the exposure has been sufFiciently severe, abnormality in development of a considerable percentage of eggs appears at the time of gastrulation. Gastrulation may be markedly interfered with, being either interrupted at an early period or so modified that spina-bifida or similar abnormalities result. Jf the effcct is less (163) 16.4 Charles R. Ihirdeeii. severe, thc a1)iioriiialities Lcgin to appcar at thc tiine of the differentiatioii of tlic aliineiitnry canal a i d the nciiral tube, either or both of which may 1 ) ijLilornlally ~ fornicd. IYhcn the action of the rays is lws ninrkctl, the abnoriiialitics appear in one or inore parts of the orgiiiisin at a still later priod. The vasciilar sptciii rnay fail t o tlcvc.lop, or various abnormalities may appear i l l aiiy of the newly-forniiiig organs. I\'hcn tlic effects are slight, tlic deforirlitics iiiay not appear until tlic Inrvn: arc ~velliicl\.anced aiid thcn inerrly in n sinall pcrcentagc of the l ~ r v ; ~ Thus, . for instarwe, a hiiitl 1c.g may fail to dcvrlop in sornc individiials. Rspowrc of A fcrtile fcnialo toad for one hoiir and tlie subseqiwi~t r d m y expcritnents fcrtilixaticm of the (ygs with norinnl sperm c i ~ n ~ in rtiost of tlic eggs to dcvc~lopmarked abnormalities, many of thein of t l w sl)iiia-l)ifida typc. Xt.vcvtholcss, a fcw intlivitliials develol)ed iiito normal tadpoles. IYlicn it €cinale frog was exp,md for an hour and fiftceii minn ttbs, and the eggs wcrc siibsequc~ntlpfcrtiliml, at the end of seven days all thc ova mere abnorinal i n form. Esposiirc of silcrmatozoa for a corrcsponding period leads to esscritially similar wsnlts. I n onc expcrimcnt with toads in whicali thc sperm was exposed for one how, about 88 per cent of nornial ova fcrtilixerl by the sperm developed into tadpolcs iibnorlnnl in forin, while, mhrn the slwrrn was exposed on(?hour and a quarter, all the o r a fertilized, in oiir instancc OVPT 2,;O spec*iinens, were abnornial at the end of fire days. I n oiie cispeiiiricnt in which toad sperm was exposed f o r one-hdf hour to the x-rays, only 1.2 per ccnt of the eggs developed normally, h i t in other instances the pcrcmtage was greater than this. Seyeral experiments were made to test the action of the x-rays on fertilized ova at diflerent stages of dcvclopment. I n onc series of experiments, from a single female toad eggs were rcmored, fcrtilized with fresh spcrni and then exposed at successive intervals for one-half hour to tlic x-rays. Of thosc csposed diiriiig the first half hoiir after fertilization, 23.3 per cent were iindergoing apparentlp normal development at tlie cnd of two ~vceks. Diiring this. period of csposiire the sccoiitl polar body is given off. Of those exposed during the second half hoiir, 17.3 per ecnt were apparently normal Susceptibility of Amphibian Ova to S-Rays. 165 i n development at the end of two weeks. Of those exposed during the third half hour, 6-8 per cent were normal at the end of two weeks. During this period of exposure the male and female pronuclei were approaching one another. Of those exposed during the fourth half hour, but 4.8 per cent were normal in appearance at the end of two weeks. None of those exposed for half an hour during the subsequent stages of cleavage up to the 64th to 128th cell stage developed normally. I n later cleavage stages, however, exposure had a less marked effect. Thus of eggs exposed thirteen honrs after fertilization for half an hour, at this time i n a state of advanced cleavage, 64.2 per cent were apparently normal in development at the end of two weeks. Eggs exposed between thirteen and twentyfour hours after fertilization, for half an hour at a time, varied in the percentage of those developing normally from GO to 80 per cent. Eggs exposed for one-half hour periods after the first twenty-five honrs, that is after the blastopore was closed, practically all developed normally. Exposure of the eggs after closure of the blastopore and of young larvae for several hours to the x-rays failed to prodiire any noticeable abnormalities of form. I n other series of experiments a considerably smaller percentage than in the series cited of eggs exposed for one-half hour during the first hour and a half after fertilization developed normally, but otherwise the results corresponded with those giren. These experirncnts show conclusively that both the male and tl!r female sex-cells may be so altered by the x-rays as to give rise to the formation of monstrous forms. The susceptibility of the male and female sex-cells is approximately eqml, although the abnormalities appear earlier in development and are greater when the ova are exposed. After fertilization until cleavage begins, the ova at first appear to be no more susceptible than the sex-cells before fertilization. During the earlier stages of cleavage the siisceptibility of the eggs to the s-rays is markedly increased, but during the later stages of cleavage before closure of the blastopore the susceptibility of the eggs becomes much less, and after the blastopore is closed the power of the x-rays to influence derelopment becomes strikingly reduced. The period of greatest susceptibility is the period dnring yhich there is the most rapid production of nuclear material.