120 BULLETIN OF THE 



than the disintegration of the nucleus, until the process of gastrulation 

 is completed. Such cases are not as common, however, as others, where 

 there is to be found in the cleavage cavity material which appears as 

 though it had resulted from the disintegration of similar cells. This 

 material has a spongy or vacuolated appearance, and contains faintly 

 staining bodies or granules similar to those found in the ectodermic 

 cells ; it does not possess definitely circumscribed boundaries ; on the 

 contrary, it fills the cleavage cavity more or less completely, but is not 

 of uniform density throughout. The fact that this material is not homo- 

 geneous, and that it contains granules, etc., prevents the conclusion that 

 it has been produced as a simple secretion into the cleavage cavity, 

 although it may have been formed in part by such a process. The fre- 

 quent association of this material with ingression cells in the same spe- 

 cimen (Plate II. Fig. 8), and the lack of other ways of accounting for 

 its presence, lead me to believe that it is produced by the disintegration 

 which I have suggested. 



There is another peculiarit}'^ of the development which I believe to 

 be connected with this process of nuclear disintegration. It is this : 

 after having once entered the cleavage cavity the immigrating cells seem 

 to lose their power of division, and consequently do not become more 

 nimierous, while the cells composing the blastospheric wall undergo 

 repeated divisions, as is shown by their increased number and dimin- 

 ished size. 



The number of these immigrating cells is small, usually only one or 

 two, very rarely more than three, so that I have not been successful in 

 finding the " Verbindungsglieder " connecting the conditions shown by 

 Goette ('87, Taf. I.) in his Figures 5 and 6, w^hich Clans ('90, p. 4) re- 

 garded as essential to the substantiation of Goette's view of the method 

 of gastrulation. 



Reference has been made to the fact that in some cases the ingrowing 

 cells persist both during and after the process of invagination. In the 

 latter case, they are to be found in the coelenteron rather than in the 

 cleavage cavity. Figure 11 (Plate II.) is drawn from such a specimen. 

 Figures 9 and 10 represent two sections of one individual in which the 

 invagination is not completed, and furnish a hint as to the process by 

 which the cells pass into the coelenteron from the cleavage cavity. The 

 entoderm being composed of less closely fitting cells than the ectoderm, 

 doubtless admits the passage of the large immigrated cells through it 

 more readily than the latter would (Plate II. Fig. 9). The immigrated 

 cell is of course passive in this process. Since it is prevented by the 



