2B8 



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 al most entirel y by growth backwards of its 

 hind er en d, t he anterior end lengthening very little, if at all. 



The area pellacida grows mor e rapidly in its posterior th an 

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

 pvri form in outli ne. 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), 



Fig. 10/. -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 pellucida : the part left white consists of epiblast and hypoblast alone ; in 

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

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

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

 streak. 



th e primitive streak, P S, 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 sharjDly defined ; 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 s hallow median furrow, the prim itive 

 groove, runs a long the whole length of the primitive strea k. 



Tra nsverse sections of the bla stoderm (Fig. 108) show that 

 the pri mitive streak is for med by p roliferation 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 



