304 STUDY OF YOUNG CHICK EMBRYOS. 



which the middle or mesoderm, mes, is by far the most voluminous. The walls 

 of the medullary groove are crowded with nuclei which lie at every possible level, 

 some close to the inner, others close to the outer surface, and also in every possi- 

 ble intervening position. The nuclei are much crowded, there being but little 

 protoplasm. No distinct cell boundaries can be made out. The nuclei are 

 further remarkable on account of their very conspicuous nucleoli. 



Section through the Primitive Groove (Fig. 174).^ — The section passes through 

 the anterior region of the groove. Underneath the groove, which appears in 

 the section as a shallow notch, Pr.g, the germ-layers are fused with one another 

 and show no recognizable boundary; laterally, however, they become entirely 

 distinct. The ectoderm, Ec, is quite thick, representing, presumably, the stage 

 of the embryonic shield (compare page 62). The mesoderm, mes, is of about 

 the same thickness as the ectoderm, but its cells are far less compactly crowded. 

 In the median line it also merges without boundary into the tissue of the primi- 

 tive streak. The entoderm, Ent, is very thin indeed and makes a striking con- 

 trast with the appearance in the same layer in. the region of the area opaca 

 (compare Fig. 35). 



