80 bulletin: museum of comparative zoology. 



ratetl blastomeres give origin to parts of the embryo only. The most 

 complete and satisfactory cases are those of ctenophores as described by 

 Chun ('92) and confirmed by Driesch und Morgan ('95), and of the 

 gasteropod Ilyan.issa, by Crampton ('96). 



It is difficult to conceive how a more complete demonstration could be 

 possible, that cleavage is accompanied in many cases by differentiations 

 which are not expressed by the phrase "reine Zelltcilung," and that 

 these differentiations are of the utmost significance for the future 

 development of the organism. Any amount of evidence that in other 

 cases there is no diffei'entiation cannot in the least shake confidence in 

 this demonstration. 



2. Gastrclatiox. 



In addition to the problems bearing directly upon cleavage, the plan 

 of the present work included a study of some of the later morphogenetic 

 processes, affecting musses of cells and leading to the differentiation of 

 organs, in order to determine the relation of cleavage to these. Of these 

 processes, gastrulation and the ensuing invagination of ectodermic cells 

 to form the pharynx (Zelinka, '91) were studied. These are in reality 

 parts of a single process, so that they may be treated of together under 

 the title of Gastrulation. 



In regard to the relation of cleavage to gastrulation, the result is 

 evident from the account given in the desci'iptive portion of this paper. 

 No separation of the two processes is possible ; gastrulation is an 

 accompaniment and a consequence of cleavage. At the passage from the 

 four-cell stage to the eight-cell stage begins a displacement of the 

 blastomeres; this displacement, or "rotation," continues in later cleav- 

 ages in the same direction, and is still in operation at the latest stage 

 examined, when it is no longer possible to follow the development cell 

 by cell. As one of the phases of this displacement during cleavage, 

 the large ventral cell of quadrant D gradually moves inward, followed 

 later by a similar transference of the ventral cells of the other quadrants 

 to the inside. The entire process has been followed step by step in the 

 descriptive portion of the paper, so that it is not necessary to go into 

 details here. In its general features the process is as follows. As the 

 ectodermal cells begin to pass into the karyokinetic condition, they 

 withdraw their more internal parts and increase in surface extent. The 

 egg" as a whole retains its form and size, so that the withdrawal of the 

 internal parts of one cell necessitates an inward movement on the part 

 of othei'Sj the result is a gradual inward migration of the ventral cells. 



