262 J. THOS. PATTERSON. 



subgerminal cavity a distinct ectoderm can be recognized, and 

 just beneath this ectoderm are cells mingled with yolk granules 

 in which periblastic nuclei are present. As the overgrowth pro- 

 ceeds over the yolk, followed by the zone of junction, it is evi- 

 dent that that portion of the germ-wall between the zone of junc- 

 tion and the subgerminal cavity (SG), that is, the yolk zone (F), 

 will continue to increase in width. But the cavity, due to the 

 liquefaction of yolk, is also increasing in width, but at a slower 

 rate. In this widening of the cavity there are left around its 

 margin cells which were previously embedded in the yolk. These 

 cells form the beginning of the yolk-sac entoderm, and when the 

 invaginated entoderm has spread over the subgerminal cavity its 

 free margin becomes continuous with that of the yolk-sac ento- 

 derm. The last place for this union to occur is in the anterior 

 region of the cavity. 



Within this region of the cavity are found many large yolk 

 masses (M), in some cases so numerous as to cause an elevation 

 of the ectoderm, especially in later stages. Against the under 

 surface of this layer are crowded a few remaining segmentation 

 cells {-X). These are the cells which Gasser ('82) has mistaken 

 for wandering entoderm cells and Nowack ('02) for wandering 

 ectoderm cells. At the extreme right of this figure the free end 

 of the invaginated entoderm can be seen (Fig. 7, L). 



Fig. 8 is a portion of the posterior half of a longitudinal me- 

 dian section (see Fig. 6). In the posterior region is the thickened 

 rim, or dorsal lip of the blastopore [R). x From its rounded ap- 

 pearance one might infer that the posterior margin is still rolling 

 under to form the entoderm, but at this stage it is more correct 

 to regard the entoderm as arising as a forward growth from the 

 inner edge of the thickened rim. At the place of origin (U) the 

 entoderm is very thick, but gradually thins out anteriorly. The 

 blastopore (B) is a shallow passage extending inward from the 

 exterior to become continuous with the archenteric cavity {AC), 

 which is that portion of the subgerminal cavity covered by the 

 entoderm. Within the archenteric cavity are two large yolk 

 masses (M) which have just risen out of the yolk. Below the 



1 The ventral lip is represented by the yolk lying immediately beneath the 

 blastopore. 



