238 



THE CHICK. 



bation, this opacity becomes lengthened out into a linear band, 

 the primitive streak, which, starting from near the centre of 

 the blastoderm, extends backwards across the area pellucida 

 towards its margin. The increase in length of the primitive 

 streak is effected almost entirely by growth backwards of its 

 hinder end, the anterior end lengthening very little, if at all. 



The area pellucida grows more rapidly in its posterior than 

 in its anterior part, and from about the fifteenth hour becomes 

 pyriform in outline. The primitive streak keeps pace with the 

 growth of the area pellucida : and about the twentieth hour, when 

 the area pellucida is markedly pyriform in shape (Fig. 107, AD), 



AD 



FIG. 107. A diagrammatic figure of the blastoderm of a Hen's Egg about the 

 twentieth hour of incubation. (In part after Duval.) x 8. 



AD, area pellnckla : the part left white consists of epiblast and hypoblast alone ; in 

 the hinder part of the pyriform area, covered by the light shading, niesoblast is present 

 as well. AK, area opaca. M, dotted line indicating the boundary of the mesoblast. 

 ]STP, neural plate, the first commencement of the central nervous system. PS, primitive 

 streak. 



the primitive streak, PS, forms a well-defined opaque band 

 stretching about two-thirds of the way across the area pellucida. 

 The anterior end of the primitive streak is sharply denned ; the 

 posterior end is less distinct, is often irregularly bent, and 

 usually dies away a short distance before reaching the edge of 

 the area pellucida. A shallow median furrow, the primitive 

 groove, runs along the whole length of the primitive streak. 



Transverse sections of the blastoderm (Fig. 108) show that 

 the primitive streak is formed by proliferation of cells from the 

 under surface of the epiblast, in the median line. The cells 

 grow downwards as a solid keel, which spreads out right and 

 left as a horizontal sheet of cells, PS ; these are spherical in 



