210 RE GENERA TION 



adult animal. So far as the question of descent enters the problem, 

 we can infer with some degree of probability that the groups in ques- 

 tion may have come from a common group in which the egg divided 

 in much the same way as we find it dividing at the present time. As 

 a formal hypothesis this view meets with no serious difficulty, since a 

 chain of forms, or a continuous living substance, connects the present 

 animals with those living in the past ; and we may assume that the 

 same factors peculiar to the egg of the ancestors are still present in 

 the eggs of their descendants. This sort of explanation gives us no 

 causal knowledge of the way in which the egg divides, nor does it 

 preclude the possibility of new changes coming in that may entirely 

 alter the form of the cleavage. Moreover, since we are dealing with 

 a question of historical probability only, we cannot be certain that the 

 same type of cleavage may not have arisen quite independently in 

 each group. 



The argument in favor of the gastrula stage also representing an 

 ancestral larval stage may be admitted as a remote possibility, but 

 on evidence even far less satisfactory than that for the similarities of 

 cleavage being accounted for by a common descent. That this gas- 

 trula was ever an adult form we have no means of deciding, even as 

 a matter of probability, and even if this could be made plausible it by 

 no means follows that such an adult stage would become an embryonic 

 stage of later forms. Consequently that part of the germ-layer theory 

 that rests on such a supposed connection cannot be looked upon as 

 much more than a fiction. 



But even granting that there is an historical, embryonic l connec- 

 tion, its small importance for the scientific problems connected with 

 embryonic development, and budding and regeneration has been 

 shown by a number of recent discoveries, and nowhere more clearly 

 than in the cases of the formation of new individuals by budding. 

 As an example may be cited the method of development of the 

 ascidian from the egg, and by means of buds. The work of Kowa- 

 levsky, Delia Valle, Seeliger, and Van Beneden on the budding pro- 

 cess of ascidians showed that there are some discrepancies between 

 the bud development and the embryonic development. The more 

 recent papers of Hjort, Oka, Pizon, Salensky, Lefevre, and others 

 have shown very clearly that the germ-layer theory is inapplicable to 

 the bud development in this group. The bud arises as a double- 

 walled tube, or rather a tube within a tube, with a space between. 

 The outer tube comes in all cases from the ectoderm of the animal ; 

 the inner tube has a different origin in different species. In perophora, 

 didemnum, and clavellina, the inner tube comes from endoderm ; in 

 botryllus it arises from the ectoderm of the larval peribranchial or 



1 That is, one not depending on inheritance through adult forms. 



