46 WALTER HE APE. 



is the point where in the earlier specimen the blastoderm was 

 perforated (fig. 31), the increased size of the area being due 

 hitherto to a growth backwards. 



Fig. 12 represents the most advanced condition of the primi- 

 tive streak. The embryonic area is pyriform, and the primitive 

 streak is considerably longer than in the former specimen 

 (fig. 11), and extends relatively further along the area; it is 

 more opaque, and ends behind in a dark rounded mass or 

 knob. 



I could distinguish no primitive groove by an examination of 

 the surface, and am obliged, therefore, to rely chiefly upon sec- 

 tions to determine the relations of the growth. Near the front 

 end of the streak there is here also distinct evidence of an 

 involution of the epiblast, although there is no actual perfora- 

 tion ; and I am inclined to believe this point is identical, both 

 with the front end of the primitive streak in fig. 11, and with 

 the point where the perforation exists in the younger embryo 

 (vide fig. 31). It is a curious fact, however, that the extent 

 of the area anterior to the front end of the primitive streak 

 appears to be less in this area than in the younger one (fig. 11), 

 while the length of the primitive streak in fig. 12 is greater 

 than that in the older embryos (figs. 13 — 14). 



The presence of the involuted point at the front end of the 

 streak appears to me to favour the view that this structure 

 has not grown forwards, while the addition of the pyriform 

 hind end is an argument in favour of its backward growth. 



The reduction in size of that portion of the area anterior to 

 the primitive streak may possibly be due to curvature, but this 

 I am unable definitely to decide. 



The eventual reduction in length of the primitive streak is 

 more easily comprehensible, and is doubtless due to the widen- 

 ing out of the end knob, this structure having disappeared in 

 older embryos. 



I have frequently observed in surface views a darkly-shaded 

 spot at the front end of the primitive streak, wnich is spoken 

 of as the node of Hensen, and find that ir conesponds with the 

 spot where the three layers unite. It may also be seen some- 



