PRIMITIVE STREAK TO SOMITE FORMATION 



39 



area pellucida 



entoderm 

 cell in mitosis ^ area of 



entoderm mdifferent cells 



I J High power thru primitive streak at region (a) T^^ High power thru edge of germ wall 



on section C. at region (b) on section C. 



Hensen's node primitive 



neural plate | ^primitive pit ridge 



■im. 



yolk 



mesoderm •'.''t*f 



extent of primitive gut and of area pellucida 



Fig. 13. — Sections of 18-hour chick. The location of each section is indicated 

 by a line drawn on a small outline sketch of an entire embryo of corresponding 

 age. The letters affixed to the lines indicating the location of the sections 

 correspond with the letters designating the section diagrams. Each germ 

 layer is represented by a different conventional scheme: ectoderm by vertical 

 hatching; entoderm by fine stippling backed by a single line; and the cells of 

 the mesoderm which at this stage do not form a coherent layer, by heavy angular 

 dots. 



A , diagram of transverse section through notochord ; B, diagram of transverse 

 section through primitive pit; C, diagram of transverse section through primitive 

 streak; D, drawing showing cellular structure in primitive streak region; E, 

 drawing showing cellular structure at inner margin of germ wall; F, diagram 

 of median longitudinal section passing through notochord and primitive streak. 



