80 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY, 
rated 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 Ilyanassa, 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 Zellteilung," 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 differentiation cannot in the least shake confidence in 
this demonstration 
2. GASTRULATION. 
In addition to the problems béaring directly upon cleavage, the plan 
of the present work included a study of some of the later morphogenetic 
processes, affecting masses of cells and leading to the differentiation of 
organs, in order to determine the relation of cleavage to these. Of these 
processes, gastrulution and the ensuing invagination of ectodermio 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 descriptive 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 others; the result is à gradual inward migration of the ventral cella. 
