EARLY DEVELOPMENT OF THE CHICK. 



75 



the primitive folds are the elevations flanking the depression. The ectoderm 

 is thickened perceptibly for some distance on both sides of the groove, thus 

 forming the early neural plate. Beyond the neural plate the non-neural 

 ectoderm extends laterally to the edge of the blastoderm, in fact forming its 

 margin. The entoderm is a thin layer which extends laterally until it merges 

 with the yolk to form the germ wall. The cavity beneath is the archenteron, 

 extending from the germ wall on one side to that on the opposite side. The 

 mesoderm at this time is not an extensive layer, for it constitutes only a por- 

 tion of the mass of the primitive streak and extends laterally only a short 

 distance between the other two layers as scattered irregular cells. 



Primitive groove and folds 



Ectoderm 



Ectoderm 



Mesoderm 

 Entoderm 



FIG. 47. Transverse sections of blastoderm of chick (21 hours' incubation). Hertwig. 



a, Section through primitive groove, posterior to Hensen's node. 



b, Section through Hensen's node. 



\/A transverse section through the primitive pit shows essentially the same 

 structural arrangement as in the streak farther caudally (Fig. 47, B). In 

 some birds the pit opens into the archenteron, but not in the chick. The 

 region of the primitive knot also shows the same arrangement, the knot 

 itself being an elevation just in front of the pit. Caudally the primitive 

 groove becomes more shallow and finally disappears, the caudal end of the 

 streak broadening out as the primitive plate. 



The morphological significance of the primitive streak is a question 

 which has not yet been unequivocally answered. It is generally agreed, but 

 not universally, that the streak is the homologue of the blastopore in the 

 lower animals on the ground that all three germ layers are fused as 

 they are in the lip of the blastopore, that it marks the caudal end of the 



