AVES. 



155 



In the course of further growth the area pellucida soon 

 becomes pyriform, the narrower extremity being the posterior. 

 The primitive streak (fig. 96) elongates considerably, sp^ as to 

 occupy about two-thirds of the length of the area pellucida ; but 

 its hinder end in many instances does not extend to the posterior 

 border of the area pellucida. The median line of .the primitive 

 streak becomes marked by a shallow groove, known as the 

 primitive groove. 



During these changes in external appearance there grow 

 from the sides of the primitive streak two lateral wings of 

 mesoblast cells, which gradually extend till they reach the sides 

 of the area pellucida (fig. 97). The mesoblast still remains 



FIG. 97. TRANSVERSE SECTION THROUGH THE FRONT END OF THE PRIMITIVE 



STREAK OF A BLASTODERM OF THE SAME AGE AS FIG. 96. 

 pv. primitive groove; m. mesoblast; ep. epiblast; hy. hypoblast; yh. yolk of 

 germinal wall. 



attached to the epiblast along the line of the primitive streak. 

 During this extension many sections through the primitive streak 

 give an impression of the mesoblast being involuted at the lips 

 of a fold, and so support the view above propounded, that the 

 primitive streak is the rudiment of the coalesced lips of the 

 blastopore. The hypoblast below the primitive streak is always 

 quite independent of the mesoblast above, though much more 

 closely attached to it in the median line than at the sides. The 

 part of the mesoblast, which I believe to be derived from the 

 primitive hypoblast, can generally be distinctly traced. In many 

 cases, especially at the front end of the primitive streak, it forms, 

 as in fig. 97, a distinct layer of stellate cells, quite unlike the 



