96 MARINE BIOLOGICAL LABORATORY. 



itself that the epiboUc gastrulation which is found in 

 these two somewhat widely separated groups can hardly 

 have arisen independently, and that its occurrence is 

 due to its having been the mode of gastrulation in the 

 ancestors of both groups. It is also to be found in 

 members of other groups, such as the Annelida, having 

 been first described by Kowalewsky in Euaxes, and it 

 occurs more or less typically in many Polychoetous 

 forms. I endeavored to express this idea in the follow- 

 ing statement : " The modes of segmentation of the 

 Platyhelminths, Annelida, Mollusca, and Molluscoidea, 

 can be referred to a common type, indicating that the 

 ovum (so to speak) in all these groups has been derived 

 from an ovum possessing a considerable amount of 

 nutritive yolk aggregated more or less completely at 

 one pole." A necessary corollary of such a proposition 

 I also stated as follows : *' The regular and equal segmen- 

 tation which occurs in certain forms in several of these 

 groups cannot be considered the original mode, but has 

 been secondarily brought about by the loss of a food-yolk 

 originally present." 



It would not be suitable in a lecture of this kind to 

 review the various accounts of segmentation in these 

 groups, but I would simply point out that if this idea is 

 correct it follows that epibolic gastrulation is more 

 primitive than embolic, the latter having been derived 

 from the former by the loss of food-yolk. Can epibole 

 have arisen from immigration .'* It can be regarded as 

 a process of migration, the migrating cells being special- 

 ized very rarely in the development, having stored up 

 within them a large amount of yolk. The protoplasm 

 separates more or less from the large yolk spherules, 



