184 



THE PRIMITIVE STREAK. 



extension of the hypoblast round the inner side of the vesicle. The 

 jblastodermic vesicle is therefore formed of three areas, (1) the em- 

 jbryonic area with three layers : this area is placed where the blasto- 

 jpore was originally situated. (2) The ring around the embryonic 

 [area where the walls of the vesicle are formed of epiblast and 

 I hypoblast. (3) The area beyond this again where the vesicle is 

 [formed of epiblast only\ 



The changes which next take place begin with the formation of 

 a primitive streak, homologous with, and in most respects similar to, 

 the primitive streak in Birds. The formation of the streak is pre- 

 ceded by that of a clear spot near the middle of the blastoderm, 

 forming the nodal point, of Hensen. This spot subsequently consti- 

 tutes the front end of the primitive streak. 



The history of the primitive streak was first worked out in a 

 satisfactory manner by Hensen (No. 182), from whom however I 

 differ in admitting the existence of a certain part of the mesoblast 

 before its appearance. 



Early on the seventh day the embr_yonic area becomes pyriform, 

 and at its posterior and narrower end a primitive streak makes its 

 appearance, which is due to a proliferation of rounded cells from the 

 epiblast. At the time when this proliferation commences the layer 

 of hypoblastic mesoblast is present, especially just in front of, and at 

 the sides of, the anterior part of the streak ; but no mesoblast is 

 found in the anterior part of the embryonic area. These features 

 are shewn in fig. 138 A and B. The mesoblast derived from the 

 proliferation of the epiblast soon joins the mesoblast already present ; 

 though in many sections it seems possible to trace a separation 

 between the two parts (fig. 139 B) of the mesoblast. 



During the seventh day the primitive streak becomes a more 

 pronounced structure, the mesoblast in its neighbourhood increases 

 in quantity, while an axial groove — the primitive groove — is formed 



Fig. 139. Two tbansverse sections through the embryonic area of an 



EMBRYO EaBBIT OF SEVEN DAYS. 



The embryo has nearly the structure represented in fig. 140. 



A. is taken through the anterior part of the embryonic area. It represents about 

 half the breadth of the area, and there is no trace of a medullary groove or of the 

 mesoblast. 



B. is taken through the posterior part of the primitive streak. 



ep. epiblast; hy. hypoblast. 



■^ Schiifer describes the blastodermic vesicle of the cat as being throughout in a bi- 

 laminar condition before the formation of a definite primitive streak or of the mesoblast. 



