Some further effects of an antagonistic temperature gradient upon the frog's egg.код для вставкиСкачать
SOME FURTHER E F F E C T S O F AN ANTAGONISTIC TEMPKRATURE GRADIENT UPON THE FROG'S EGG N. A. TAZELAAR, J. S. HUXLEY, AND G . H. DE BEER Zoiilogicnl Department, K i n g '8 College, London, England FOUR TEXT PIOURES A N D ONE P ~ A ~ I(FOUR E FIQURES) This preliminary note deals briefly with a set of experiments carried out in Easter, 1929. Similar experiments were carried out in Easter, 1927, frogs' eggs being exposed to temperature gradients which were adjuvant ( apico-basal) , antagonistic (baso-apical), and side to side in relation to the egg polarity (Dean, Shaw, and Tazelaar, '28). The apparatus was similar to that used previously and consisted of two silver discs fused on to shallow copper plates which were supplied with inlet and outlet tubes through which a flow of water a t the required temperature could pass. The eggs were placed between the silver plates together with small slips of glass of a definite thickness; these slips kept the silver discs a certain distance apart and prevented the eggs from being crushed. The temperature of the cold water was about 6°C. and of the warm about 245°C. The actual temperature difference between the two poles of the eggs could not be measured, but was probably considerable. The chief difference between these and the earlier experiments was the greater length of time for which the eggs were under experimental conditions in the later experiments. In 1927 the longest period was eleven hours, whereas in 1929 the eggs were between the plates for at least twenty-four hours and very often for longer. 111 most cases where the eggs were under experimental conditions for only twenty-four hours, the same type of result 1 THE -4NATOMICAL RECORD, VOL. OCTOBER, 1930 47, NO. 1 2 M. A. TAZELAAR, J. S. HUXLEY, AND G. R. DE BEER was obtained as in previous experiments, namely, the whole early development (as judged by cell size) of the eggs exposed to an adjuvant temperature gradient was more advanced than in those e-posed to an antagonistic temperature gradient, even though all other conditions, and therefore the mean temperatures of the two sets of eggs, were similar. This even applied to the yolk cells of the former series, though these had actually been chilled. I n experiments where the period was longer, however, practically all those treated with an adjuvant temperature gradient died, gastrulation appeared t o be impossible, and large masses of yolk were thrust out. F o r this reason, in this short paper we shall deal almost entirely with those treated with an antagonistic temperature gradient ; those treated with a side-to-side temperature gradient will be dealt with in a later paper. The experimental treatment of most of the eggs was begun at the two-celled stage. After experimental conditions lasting, in most cases, for about thirty-two hours, the eggs treated with an antagonistic temperature gradient were at various stages of gastrulation. Usually the dorsal, ventral, and lateral lips had formed, so that a large rim of overgrowing tissue was present which enclosed a fairly extensive area of yolk cells. In the case of a batch of eggs treated for the same period and at the same temperature with an adjuvant temperature gradient, only the dorsal lip was well formed, but it was situated in a more ventral position and the area of yolk cells showing was often smaller. The eggs were then placed in normal conditions. After about twenty-four hours, practically all those treated with an adjuvant temperature gradient were dead. Among the very few survivors, gastrulation was attempted, and during this process masses of yolk were extruded. I n the case of those exposed to an antagonistic temperature gradient, however, a very interesting feature was invariably present in the shape of a peculiar ring-shaped groove which appeared above the equator (fig. A). This feature was not obtained in previous experiments. I t ~ 7 a salso found to be present to a much less TEMPERATURE GRADIENTS AND FROG DEVELOPMENT 3 marked extent in fertilized eggs which were left in a cold chamber for some days where the temperature was below 4°C. The groove, which was not necessarily complete, slowly disappeared in all cases after the eggs were allowed to continue their development under normal conditions, but in the most pronounced cases the animal pole became much wrinkled. The eggs which were left in the cold chamber can be compared with eggs exposed t o an antagonistic temperature gradient, by assuming that since the animal pole is more susceptible than the vegetative pole, the cell division rate at that region will be comparatively more affected than at the more inert vegetative pole, and hence a condition approximating to an antagonistic temperature gradient would be obtained. A similar condition was also produced by Vogt ('22) in Triton embryos in which a portion of the roof of the blastula was removed. An explanation of this occurrence was suggested by Vogt and elaborated by one of LIS (G. R. de Beer, '27), namely, that the future ring of overgrowth is by these processes (removal of blastula roof or subjection to antagonistic gradients) drawn up above the equator ; when gastrulation starts, this rim attempts to grow over and at the same time to decrease its diameter, and this results in a constriction above the equator. This would imply a separation of the process of invagination from that of epiboly. This explanation might possibly account for cases in the temperature-gradient experiments in which gastrulation is not very f a r advanced, but does not seem applicable to cases where the blastopore is quite small and gastrulation has apparently proceeded fairly normally. On cutting sections of these forms, it was noticed that the circular groove-like invagination was internally always at the same level as the top of the yolk (fig. B) and sometimes there was a marked ingrowth of ectoderm over the yolk. It is possible that the temperature was in some cases too high o r too low (in the case of those left in the cold chamber), thus having a deleterious effect upon the yolk and causing it to become more inert than usual. If, on invagination, the yolk remained immovable, 4 M. A. TAZELhAR, J. S. HUXLEY, AND G . R. DEBEER owing to its inability to divide normally and perhaps also owing to the cooled apical region retarding the later process of development, then the archenteron would be prevented from extending up as far as usual into the yolk and invagination would remain incomplete. The downgrowth of tissue from the animal pole would thus be hindered and the extra growth would naturally be pushed in at the position of weakest resistance; it would thus be forced in at the level of the top of the yolk, hence the formation of the peculiar ringshaped groove. In a few cases the tissue did apparently grow down past the yolk in the region of the dorsal lip, and here the extra down-growing tissue is present as a fold in the dorsal lip, but on the side away from the lip the ring is present at the level of the yolk. Later stages of these forms differed chiefly according to the degree of closure of the blastopore. Where the blastopore remained widely open, the embryo became much deformed. The most normally developing embryos of this experiment were those in which the blastopore nearly closed. I n many of these forms, even where the blastopore did apparently close, the head was very small and there was a deficiency of iiervous material. The brain and sense organs were often fairly well formed, but abnormally small (fig. C). Microcephaly was also obtained in the earlier experiments as the result of antagonistic gradients during segmentation, but never in any degree at all approximating that found in some of these present cases. The forms in which the blastopore did not close were interesting t o study. As the embryo lengthened so the blastopore hecame elongated, and a t the time of the formation of the neural folds these were always found to be lying on either side of the unclosed blastopore, thus giving rise to an extreme condition of spina bifida (fig. D). Behind the anterior fold which represents the rudiment of the brain there is a deep invagination which, since it produces gill slits from its walls, must represent the foregut (fig. 1). I n a stage twenty-four hours older, the foregut is more differentiated, TEMPERATURE GRADIENTS A N D FROG DEVELOPMENT 5 the liver diverticulum is present, and the stomodaeal invagination is beginning to form. Lying in front of the posterior border of the blastopore is a smaller invagination. The tail, by the time its somites are well formed, is always bent up and forward over the yolk (figs. 2 and 3). In still older embryos the yolk is still showing, although the neural folds have become somewhat drawn together. The external f o r m Fig. 1 Cainera-lueida drawing of a longitudinal section through about the middle of an embryo treated in thc same way as the embryo shown in plate 1, figure 1). a h . , anterior limit of brain; u.v., anterior invagination; b., brain; y., yolk mass. of the head is sometimes well developed to a degree which is surprising after one has seen the extreme condition of spina bificla through which the embryo has passed in earlier stages. The stomodaeal invagination is now complete and opens into the foregut invagination. Thus in these forms there is invariably a connection established between the mouth in the normal position and an aperture on the dorsal surface just behind the brain (fig. 3) and this connection persists in later embryos up to the stage a t which death occurs, usually when 6 M. A. TAZELAAR, J. 8. HUXLEY, AND G . R. DEBEER the body length is about 4 to 5 mm. The posterior invagination is more developed (fig. 3), and on examining serial sections the two pronephric ducts are found to open into it, hence the invagination must represent the cloaca. The original invagination here must represent the hindgut and proctodaeum, from which the cloaca is later developed. (No separate proctodaeal invagination is required, since the original invagination aperture never closes.) Thus the anus in this case is at the posterior end of the yolky mass and in front of the tail. The development of the alimentary canal in these abnormal forms is interesting, since it arises by two or possibly three independent invaginations : the anterior invagination, the foregut, and the posterior invagination, the hindgut and proctodaeum. These are undoubtedly quite independent, while it is possible that the stomodaeal invagination is dependent upon the presence of the foregut. We have here a good example of independent differentiation of parts of the alimentary canal which through experimental conditions have become isolated from each other. I n all these forms treated with an antagonistic temperature gradient, the nervous system is reduced in size and in some markedly so. This can probably be accounted for in two distinct ways. First, the area of the presumptive nervous material may possibly be reduced by the application of a low temperature to the animal pole, thus preventing the cells from dividing as rapidly and to such an extent as under normal conditions. The thickness of the blastocoele roof previously mentioned in these specimens seems to bear out this possibility. There is further the point that, if the anterior end of the dorsal, foregut invagination be taken as repreFig. 2 Camera-lucida drawing of a longitudinal section thraugh an embryo treated in the same way as abovr, hut allowed t o develop under normal conditions for two days longer. Fig. 3 Longitudinal section of same embryo through stomodaeum. h., brain; f., foregut invagination; g., gill pouches; h., heart; Z., liver; o., olfactory invagination; p . , posterior invagination; s., ston~odaeum;t., tail, bent upward and forward o w r yolk. TEMPERATUEE GRADIENTS AND FHOO T>EVELOI’MENT 7 senting the position of the dorsal lip-as seems inevitablethe distance between this point and the anterior end of the brain is often considerably less than is to be expected from 8 M. A. TAZELAAB, J. 8. HCXLEY, A N D G. R. DE BEER the researches of Vogt, accordiiig to which it should comprise 80" of the egg's circumference. This presumably can bc accounted for by the failure of the archeiiteroii of these specimens to penetrate a s f a r animalward in the yolk mass a s in normal devclopmcnt. Siiice tlie presence of archeiitcroii roof appears, accordiiig to tlie work of Spemaiin aiid his pupils, to be the filial and chief determiiier of medullary plate in tlic. a b c 'd Fig. 4 To show the modificd invaginatory proems in t h e limited forward extension of medullary plate in tlie spina-bifida cases. a and b, a f t e r Vogt (Roux Archiv. Entwnieeh., 1929). Median srctions through eggs of Bombinator showing: a, the prospective nenral-plate inaterial (stippled) in the blastula stage ; b, formation of the arclrenterori and the position of the neural plate. e aid d represent diagranis of what has apparently oecurred in the experimental casey. R shows the riiig-shaped groove in sec*tioii and t h e thin roof of the blastocoele. d shows the limited extciit of both aiitwior and posterior invaginatioiis. Tlic stippled area represents tlie approximato extent of the niedullary plate (see t e x t ) . A ., archenteron; B., blastorode ; D.L., doma1 lip ; L4nt., anterior invaginatioil : Post., Iwsterior invagination ; JJ., ring-shaped groove in section; Y., yolk plug. T E M P E R A T U R E G R A D I E N T S A N D FROG D E V E L O P M E N T 9 superjacent ectoderm, we should expect that in these cases the aiiterior limit of the brain should fall short of that of the presumptive medullary plate. SUNMARY Frog eggs were treated with temperature gradients for long periods (twenty-four to forty-eight hours). Previous results showing that eggs heated at the animal pole were more advanced, even as regards segmentation of the vegetative cells, than those kept at the same mean temperature, but heated vegetatively (antagonistic gradient), were confirmed. Long-continued antagonistic gradients caused the following effects in gastrula stages : 1)A ring-shaped groove encircling the egg at the level of the top of the yolk. 2) A very thin roof to the blastocoele. 3 ) I n later development strikingly microcephalic embryos were produced. If the closure of the blastopore is nearly complete, no other abnormality is noted. 4) Often, however, the blastopore remains widely open, producing extreme spina bifida. I n these forms the foregut invagination is found dorsally just behind the brain; the proctodaeum, quite separate, dorsally just in front of the tail. The stomodaeum is later formed in the normal position, thus establishing a communication from the mouth to the dorsal surface just behind the brain. 5) I n these forms it appears also that, probably owing t o the incomplete extent of invagination, the most anterior section of the presumptive medullary plate does not become converted into actual medullary plate ; thus the anterior end of the brain is in a different positioii, relative to the original egg axes, from that which it occupies in normal embryos. LITERATURE CITED BEER,G. R. 1927 Mechanics of vertebrate development. Biol. Reviews, vol. 2, no. 2, March. DEAN,I. Id., M. E. SHAW,AND M. A. ‘PAZELAAR 1928 The effert O f a temperature gradient on the early development of the frog. Brit. Journ. Ex”. Biol., JUIW. VOOT,W. 1922 Ver. Aiiat. Ges. Versamml., Bd. 31, S. 53. 1922 Deut. med. Urocli., Bd. 48, R. 926. DE PLATE 1 EXPLANATION OF FIGURES A (Kc5.) Effect of ail antagonistic. temperature gradient on tlie eightcelled stage for forty-eight hours. This egg was the least affected of the batch, since the blastopore is nearly closed. Note clearly defined ringshaped groove. B (Kal 17/4.) Section through egg in same batch as above, but removed after only thirty-two hours. This egg was apparently more affected than the above. Note thin roof of blastocoele (collapsed in preparation), marked ringshaped groove, and small archenteron. C Drawing of embryo treated with an antagonistic temperature gradient for thirty-two hours, a few hours after fertilization, and then allowed t o develop under normal conditions f o r seven days. Note extremely small head. I) (Kald 18/4.) Drawing of embryo treated with antagonistic temperature from eight-celled stage for thirty-two hours and then allowed t o develop under normal conditions for forty hours. Note large, elongated blastopore. a.v., anterior invagination; b., anterior neural fold representing brain region; n., lateral neural folds on either side of the blastopore.