316 



Embryogenesis: Progressive Differentiation 



ectodermal structures, whereas material 

 arovmd the vegetal pole becomes incorporated 

 in entodermal structures. A question im- 

 mediately arises as to whether the develop- 

 mental capacities of animal and vegetal 

 halves differ from the earliest stages or 

 whether these differences arise progressively. 

 To date operations have not been possible 

 on ovarian eggs, but certain experiments on 

 uncleaved eggs provide some information 

 bearing on this question. 



EXTENT OF SPECIFICATION OF 



CYTOPLASM ENCOUNTERED IN 



UNCLEAVED EGGS 



Unfertilized eggs of the nemertine, Cere- 

 bratulus, can be separated into animal and 

 vegetal halves or into meridional halves. 

 Horstadius ('37a, p. 335) sixmmarizes his 

 studies on the developmental capacities of 

 such halves as follows: "Our isolation of 

 animal and vegetative halves prior to fertili- 

 zation confirms the resvilts of Wilson ('03) 

 and Yatsu ('10), — that any fragment of the 

 unfertilized egg may develop into a pilid- 

 ium."* However, evidence from defect ex- 

 periments on the egg of Cerebratulus indi- 

 cates that cytoplasmic specification increases 

 progressively in the vmcleaved egg from 

 fertilization vip to the first cleavage (Yatsu, 

 '04) . Non-nvicleated regions of the cytoplasm 

 were removed at each of the following stages: 

 before dissolution of the germinal vesicle, 

 at the metaphase of the first meiotic division, 

 at the period of conjugation of the egg- and 

 snerm-nuclei, and after constriction of the 

 first cleavage had appeared. For the first 

 three developmental stages listed above the 

 percentages of normal pilidia decreased pro- 

 gressively as follows: 85.7 per cent, 52.3 per 

 cent and 24 per cent; too few operations were 

 carried out at the fourth stage to provide 

 significant percentage values. Yatsu inter- 

 preted these results as demonstrating that 

 there must take place some progressive 

 changes in the general make-un of the ee:g 

 during the period extending from th'^ time 

 of dissolution of the p-errninal vpsicle to 

 the imion of the germ nuclei, in other words, 

 that cvtoplasmic localization has been pro- 

 gressively established. + 



* Similarly animal and vegetal halves, as well 

 as meridional halves of mifertilized eggs of the 

 ascidian Ascidiella scnbra. can gastrulate and de- 

 velop into remarkably normal tadpole larvae, 

 although there are many exceptions to this state- 

 ment (Dalcq. '38). 



•f Centrifugation experiments on uncleaved eggs 

 of the mollusks Physa ancillaria and Lymnaea cata- 



By contrast, isolated animal halves and 

 vegetal halves of vmfertilized eggs of other 

 invertebrates develop into larvae which dif- 

 fer from each other and from normal larvae 

 in certain characteristic ways. For example, 

 Horstadius ('28, '37b) has demonstrated that 

 isolated animal halves of unfertilized sea 

 urchin eggs (Paracentrotus, Arbacia) de- 

 velop into blastulae with enlarged apical 

 tufts and that these blastulae fail to gastru- 

 late or form a skeleton, whereas isolated 

 vegetal halves of the same eggs gastrulate 

 and form a skeleton and develop in some 

 cases into quite normal plutei, although 

 usually they develop into larvae with an 

 ovoid body shape, no mouth, and a mal- 

 formed skeleton. By contrast, meridional 

 halves of unfertilized sea urchin eggs de- 

 veloped into more or less typical plutei. J 

 Similarly Wilson ('04a) has demonstrated 

 characteristic differences in development of 

 isolated animal and vegetal halves of un- 

 fertilized eggs of the mollvisk, Dentalium, 

 whereas such differences appear to be lacking 

 in isolated meridional halves. He states (p. 

 69), "Fertilized fragments of the vmseg- 

 mented unfertilized egg, obtained by hori- 



scopium (Conklin, '10) and Physa heterostropha 

 (Clement, '38) seemed to indicate that the effects of 

 centrifuging are least injurious just after the com- 

 pletion of maturation and most injurious just before 

 the first cleavage. Again it was suggested that this 

 might indicate increasing differentiation of the egg 

 with time prior to the first cleavage. However, 

 Raven and Bretschneider ('42) obtained no definite 

 increase in injurious effects with time in their 

 centrifugation experiments on Limnaea stagnalis; 

 thus their experiments lend no support to the hy- 

 pothesis of a progressively increasing differentiation 

 of ooplasm between maturation and first cleavage; 

 they feel that their results are more critical since 

 their material was examined histologically whereas 

 that of Conklin and Clement was not. 



t Contrary to the above results, Taylor and Ten- 

 nent ('24, p. 205), using eggs of the sea urchin 

 Lytechinus (Toxopneustes) variegatus, find that 

 "From many pairs of both horizontal and vertical 

 sections we obtained blastulae with mesenchyme, 

 normal gastrulae with triradiate skeletal spicules, 

 and plutei which, except for size, could not be dis- 

 tinguished from those developing from normally 

 developing entire eggs." Tennent, Taylor and 

 Whitaker ('29, p. 4) stated, ". . . the results showed 

 that both halves of the egg, no matter what the 

 plane of section, might form normal identical 

 larvae. The animal halves, like the vegetal halves, 

 produced larvae with mesenchyme and an archen- 

 teron." Horstadius ('37b) feels strongly that such 

 results must have been due to some error in orienta- 

 tion of the cut or to a rotation of the egg during cut- 

 ting, and his arguments are convincing. 



