PRIMITIVE STREAK: ORIGIN OF THE MESOBLAST. 471 



below the hypoblast is called the subgerminal cavity, and 

 corresponds to the archenteron of the frog. 



After incubation has begun, the blastoderm spreads 

 rapidly whilst retaining its circular shape. By the end of the 



Fig. 81. Vertical section of the blastoderm and adjacent parts of the 

 yolk of a Hen's Egg at the time of laying, but before the commence- 

 ment of incubation. The anterior edge of the blastoderm is to 

 the right, the posterior edge to the left side of the figure. (After 

 Duval.) x 25. 



BV, subgerminal cavity. E, epiblast. H, hypoblast. N', nucleus in yolk 

 round which a cell will be formed later. Y, yolk. ZF, formative cell. ZL, 

 lower-layer cells. 



first day it is 20 mm. in diameter ; by the end of the second 

 it has spread about half-way round the egg. Complete 

 enclosure of the yolk is not effected until the seventeenth 

 day. 



The Primitive Streak : Origin of the Mesoblast. 



Shortly after the beginning of incubation an opaque band 

 becomes visible in the posterior part of the pellucid area, 

 which soon acquires an elongated appearance and is known 

 as the primitive streak. It grows rapidly backwards, and 

 the pellucid area elongates simultaneously, so that the 

 primitive streak always lies entirely in that area, which 

 thus assumes an oval and then a pear-shaped outline. 

 The appearance of the blastoderm at the twentieth hour is 

 seen in fig. 82. 



As the primitive streak elongates a fine transparent line 

 may be seen running down its centre. This appearance is 

 caused by a groove, the primitive groove, which traverses the 



