ON 'I'HE PROCESS OF GASTRULATION IN CHELONIA. 235 



have represented in Fig«. 10 and 12. Fig. 10 is part of a section 

 like Fig. 9 from a more lateral part of the blastoderm than tliat in the 

 latter figure. Beginning from the upper surface, we can easily recognise 

 the epiblast and the lower layer of cells lying immediately below. 

 Under these two layers, there is a rather thick stratum of spherical 

 yolk-globules. AYe then come to a crowd of cells which are the cells 

 in question. Some of these are large and full of yolk-granules ; while 

 others are smaller and formed of vacuolated protoplasm. The size of 

 their nuclei is tolerably unitbrm — being about .016 mm. in length. 

 In Fig. V2h are shown more distinctly three of these cells from 

 another region. One of them is full of yolk-granules, as is also the 

 unusually large cell shown in Fig. I2a. Another is partly full, with 

 an area of granular protoplasm around the nucleus. The third, of 

 which only one-half is seen — having no doubt been cut in two in the 

 process of microtomizing — has no yolk-globules, but is formed of 

 vacuolated protoplasm. Below this stratum of cells there is a layer 

 of closely packed fine granules which represents some liquid coagu- 

 lated in the course of hardening. Below this, we come to the thick 

 bed of yolk. The globules are here larger than in the upper layer. 

 The conclusion seems to me almost inevitable that the cells above 

 described take up and digest yolk globules and that the stratum of 

 liquid on the edge of which they are found is produced as the 

 result of their digestion. This liquid stratum has probably a genetic 

 relation with the large subgerminal liquid cavity found belovv the 

 blastoderm a day or two later in the course of development. So 

 much seems tolerably clear ; but whether the cells have for their sole end 

 the digestion and preparation of yolk globules for the nutrition of 

 other cells, or whether they themselves are to form s(jme integral 

 parts of the growing embryo, I am unable to decide. 



Cells of the second kind found in the yolk never occur together 



