206 . University of California Publications in Zoology. [ VOL - 4 



(ectodermal portion), probably indicating more rapid cell divi- 

 sion in this region. 



Numerous large round cells, many of them in mitosis, are to 

 be seen near or at the surface of both the ectodermal and endo- 

 dermal portions of the blastosphere. These cells are further 

 distinguished from the remaining cells by having different stain- 

 ing properties of the cytoplasm. One of them is shown in fig. 18, 

 g. c. None were noticed earlier. 



Bateson found a greater difference between the ectodermal 

 and endodermal portions of the blastosphere than I have just 

 described. He did not, however, notice the presence of the round 

 cells, above mentioned, which are very conspicuous in this and 

 later stages of D. pusiUus. 



Gastrula. The gastrula is formed by the invagination of the 

 endodermal portion of the blastosphere, and elongation in the 

 direction of a line passing through the center of the blastopore 

 and opposite pole. As elongation proceeds the endoderm ap- 

 proaches the ectoderm until the two layers meet. At the same 

 time the blastopore is closing. 



Fig. 19, pi. 7, shows the early stage of gastrulation before 

 elongation. It will be noticed that the endoderm is somewhat 

 thinner than the ectoderm, and that there is less difference in the 

 histological character of the cells than in the stage just preceding 

 invagination. The rounded ends of the endodermal cells and 

 indications of their being amoeboid, as described by Bateson '84 

 (p. 213), I was unable to find. 



Fig. 20 is a longitudinal section through the blastopore at the 

 last stage of gastrulation. Here the blastopore is about closed, 

 and the two layers are adjacent. The cells of the region of the 

 blastopore are irregular and in rapid division as indicated by the 

 numerous nuclei. The ectodermal part of the blastoporic rim is 

 already in process of fusion. Later the endodermal part fuses, 

 and thus the blastopore is completely closed. The ectoderm and 

 endoderm of this region remain coalescent for a time (fig. 21), 

 but finally separate (fig. 22, pi. 7). 



In this manner the gastrula passes into what Bateson (p. 215) 

 calls a " two-walled cylinder," the outside wall of which is the 

 ectoderm, and the inside the endoderm. With the exception 



