472 ARTHUR ROEINSON AND RICHARD ASSHETON. 



groove being raised slightly into ridges. The same feature in 

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



Fig. 16 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 and mesoblast. It is altogether 

 ventral to the archenteron, so that there can be no fusion with 

 true hypoblast. There is no fusion either with the yolk-cells. 

 The surface is grooved, as in the sections anterior to 

 the anus. 



The fusion of layers continues through six more sections ; 

 that is to say, there are about twenty sections ventral to the 

 anus, in which there is fusion of epiblast and mesoblast ; in 

 other words, the anus is a perforation through the 

 primitive streak. 



Comparison of the '^Primitive Streak^^ of the Frog 

 with the Primitive Streak of the Chick. 



We have now described the history of the blastopore or 

 anus of Rusconi from the time of its first formation until it 

 has been reduced to a comparatively minute passage between 

 the archenteron and exterior. We have described this change 

 as being due to the concrescence of the lower lips of the 

 blastopore, as shown diagrammatically in figs. 10, 18, 19, 

 whereby there is produced a median streak characterised by 

 the fusion of layers, along the surface of which lies a groove, 

 stretching from the ventral lip of the remaining blastopore 

 ventralwards as far as the original extension of the blastopore 



