III.] THE PRIMITIVE STKEAK. 55 



In addition to the cells clearly derived i'voni the 

 epiblast, there are certain other cells (Fig. 15), closely 

 adjoining the hypoblast; these are derivatives of the 

 cells, interposed between the epiblast and hypoblast, 

 which gave rise to the appearance of the embryonic 

 shield during the previous stage. In our opinion these 

 cells also have a share in forming the future meso- 

 blast. 



It thus appears that the primitive streak is essen- 

 tially a linear proliferation of epiblast cells ; the cells 

 produced being destined to give rise to the mesoblast. 

 This proliferation first commences at the hinder end of 

 the area pellucida, and thence proceeds forwards. 



While the primitive streak is being estabhshed, the 

 epiblast becomes two or more rows of cells deep in the 

 region of the area pellucida. 



Soon after this, the hitherto circular pellucid area 

 becomes oval (the opaque area remaining circular). The 

 oval is, with remarkable regularity, so placed that its 

 long axis forms a right angle, or very nearly a right 

 angle, vsdth the long axis of the egg itself Its narrow 

 end corresponds with the future hiud end of the embryo. 

 If an egg be placed with its broad end to the right hand 

 of the observer, the head of the embryo wiU in nearly 

 all cases be found pointing away from him. 



The 12tli to the 16th hour. The primitive streak 

 at its first appearance is shadowy and ill-defined; gradu- 

 ally however it becomes more distinct; and during the 

 same period the pellucid area rapidly increases in size, 

 and from being oval becomes pear-shaped (Fig. 16). The 

 primitive streak grows even more rapidly than the 

 pellucid area; so that by the 16th hour it is not only 



