100 ARTHUR ROBINSON, M.D., AND RICHARD ASSHETON, M.A. 



In fact, in the model from which this figure was partly drawn the anus 

 had just perforated. In the sections, however, about to be described 

 as representing this stage the anus has not quite perforated. On 

 either side of the primitive groove a distinct ridge is visible. 



The Bracket (PS) in the figure includes the whole of the primitive 

 streak from end to end. 



The sections 14, 15, 16, 17, are horizontal sections taken along the 

 lines 14, 15, 16, 17, in fig. 23, and are, therefore, taken at right angles 

 to the longitudinal axis of the primitive streak. 



Fig. 14 is a camera drawing of a section through the blastopore 

 (line 14, fig. 23), which is nearly closed in. 



In this section the epiblast, mesoblast, and hypoblast are seen to be 

 all fused at the lips of the blastopore. It may be noted that the meso- 

 blast, close to the lip of the blastopore, seems to be divided into two 

 layers : a very dense compact layer (ME') next the epiblast, in which 

 the cells are so closely packed as to render it quite impossible to draw 

 each cell by camera ; while inwardly the mesoblast-cells (ME") are 

 much looser, and are more deeply pigmented and easy traceable by 

 help of the camera. In fact, one is almost inclined to say that the 

 former are mesoblast-cells of epiblastic origin, while the latter are 

 mesoblastic cells of bypoblastic origin. 



Fig. 15 is a section taken out about midway between the blastopore 

 and the anal pit ; that is to say, across the middle of the primitive 

 streak. This section is directly comparable to that just described, 

 except that the lips of the blastopore have met and fused, as described in 

 the earlier part of this paper. The surface is distinctly grooved, the 

 edges of the groove being raised slightly into ridges. The same feature 

 in the mesoblast may be noticed here as in fig. 14. 



Fig. 1 6 is taken through the anal pit. 



Here the primitive groove is very much deepened, so that not more 

 than two or three cells prevent the completion of the anal perforation. 

 This pit is deeply pigmented, and heavy deposits of pigment line the 

 few intervening cells between exterior and archenteron. All three 

 layers are still fused. 



Fig. 17 represents a section taken below the anal pit; that is to 

 say, it was the thirteenth section after the last one figured, fig. 16. In 

 this, although behind the anus, there is undoubted fusion of epiblast 



