FROM LAYING TO FORMATION OF FIRST SOMITE 75 



section passes through the anterior end of the primitive streak; 

 the ectoderm is greatly thickened (primitive knot); the base- 

 ment membrane is interrupted below, and the lowermost cells 

 are becoming loose. The third section is through a more pos- 

 terior portion of the primitive streak. The proliferation from 

 the ectoderm is more extensive, the cells are looser and are begin- 



Ent'-- 





Fig. 38. — Transverse sections through a very short primitive streak of the 

 chick. Incubated 17^ hours; no head-process. 



A. Through the anterior end of the primitive streak (primitive knot). 

 Mesodermal cells are being proliferated from the ectodermal thickening; 

 some are scattered between the two primary germ layers. The entoderm 

 shows no proliferation, though some mesoderm cells are adhering to it. 



B. Fourteen sections posterior to A. (Entire length of the primitive 

 streak is 80 sections.) The mesoblast wings are forming; the primitive 

 groove and primitive folds are indicated. The entoderm is free from the 

 mesoderm. 



Ect. Ectoderm. Ent., Entoderm. Mes., Mesoderm, pr. f., Primitive 

 fold. pr. gr. Primitive groove, pr. kn., Primitive knot. 



ning to spread out laterally. The entoderm is a continuous 

 membrane without any connection with the primitive streak, 

 and there are no cells between ectoderm and entoderm save those 

 derived from the primitive streak. 



Figs. 38 A and B show the structure of the primitive streak 



