BY WILLIAM A. IIASWELL. 101 



In brief, I contend that the cavity represented in my figs. 1-4 

 is in all cases the same thing, viz., the segmentation-cavity, and 

 that Hoffmann's fig. 4, of Taf. ii. corresponds, or should correspond 

 with my fig. 3. This is a phase which was apparently not 

 represented in the specimens at Balfour's disposal. 



The blastoderm now increases somewhat in diameter, though 

 .still remaining very thick anteriorly. The segmentation-cavity has 

 somewhat increased in size, and extends under the entire blasto- 

 derm. At its postei'ior end, just below and somewhat in front of 

 the posterior limit of the blastoderm, appears a collection of cells 

 of irregular shape, some of which are evidently being formed from 

 the parablast of the floor of the cavity. Only a very few simi- 

 lar cells occur further back. This accumulation of cells, which 

 extends all round the posterior margin, forms the starting point 

 in the formation of the parablast endoderm. 



The next change of importance (Fig. -5) is the arching upwards 

 of the posterior portion of the blastoderm, so that where it passes 

 into the parablast it becomes for a short distance vertical, and 

 soon inclined forwards, forming the embryonic rim, which extends 

 round the entire posterior margin. At the same time the accu- 

 mulation of cells at the posterior end greatly increases and 

 becomes extended backwards as a thin layer (yolk endoderm) over 

 the entire floor of the segmentation-cavity. These cells now send off 

 processes which apparently join the processes of neighbouring cells, 

 so that the whole comes to form a reticulum, in the meshes of 

 which are to be recognised masses of the coagulum from the fluid 

 of the segmentation-cavity. Similar cells extend backwards as a 

 thin irregular layer immediately below the blastoderm in the roof 

 of the segmentation-cavity. The blastoderm has now become 

 considerably thicker, but still remains thickest towards the 

 anterior end. 



The embryonic rim now b3coiii3s more strongly inflected (Fig. 6), 

 and the blastoderm becomes greatly extended anteriorly, at the 

 same time becoming thinned out. The segmentation-cavity extends 

 pari passu with the extension of the blastoderm, but becomes 

 extremely shallow. The parablast endoderm extends over its floor. 



