428 WALTER HE APE. 



The Primitive Streak Mesoblast. 



The primitive streak originally appears at the hind end of 

 an area similar to the one represented in fig. 10, its presence 

 being shown in surface view by a slight opacity. 



Fig. 31 is a longitudinal section through such an area, along 

 the middle line. The anterior portion of the area consists of a 

 layer of columnar epiblast, and a somewhat flattened layer of 

 hypoblast : at the hind end, however, a passage perforates the 

 blastoderm and surrounding it the epiblast and hypoblast be- 

 come continuous with one another, forming the wall of the 

 perforation. The opening is wider below than above, and 

 owing, I believe, to the curved condition of this specimen, was 

 not visible from the surface. The whole length of the area 

 is not drawn in the figure, and the portion anterior to the spot 

 at which the reference letters ep. are placed, was bent back, and 

 underlay the hinder portion of the area. 



The cells forming the wall of the passage give rise to the 

 first mesoblast cells, which are thus derived from epiblast and 

 hypoblast conjointly; they extend in front and laterally for a 

 short distance only as a thin sheet lying free between the 

 two primary layers, while posteriorly they form a thicker layer 

 and are united with the e])iblast in the middle line. 



From this point the primitive streak extends backwards, the 

 embryonic area itself enlarging in that direction. 



Figs. 11 and 12 are surface views of two areas, in which 

 the primitive streak represented by the dark shading is well 

 defined. In the former, which is the younger of the two, the 

 opaque band extends about half way across the oval area, 

 spreading out behind into two short horns ; and down the 

 centre of the band a lighter streak may be seen, which is 

 caused by a groove in the epiblast, and is the well-known 

 primitive groove. 



At the front end of the primitive groove there is distinct 

 evidence in section of the involution of the epiblast, although 

 no actual perforation of the blastoderm exists. This I consider 



