196 



STUDY OF YOUNG CHICK EMBRYOS. 



tions at different levels through the open groove of a slightly younger chick. In the 

 first, A, the groove is quite deep and the young primitive segment is shown. At 

 the edge of the groove its thick walls pass over continuously, but quite abruptly, 

 into the general ectoderm, EC, covering the embryo. Close under the median line 

 of the medullary groove appears an oval section of the notochord, nch. The ento- 

 derm, En, is quite thin and somewhat irregular, as is shown in all of the sections. 

 In B the medullary groove is wide open and quite shallow, the notochord is much 

 larger and extends from the floor of the medullary , groove to the entoderm and 

 occupies in part a deep notch in the medullary wall. The notochord prevents the 



Md.gr. 





FIG. 148. THREE SECTIONS OF A CHICK EMBRYO WITH EIGHT SEGMENTS. SERIES 642. 



A. Section 366, through the cephalic end of the primitive groove. B. Section 400, through the middle of the 

 primitive groove. C. Section 424, through the caudal end of the primitive groove. 



extension of the mesoderm across the median line. In C the medullary groove is 

 fading out and merging into the beginning of the primitive streak, which forms 

 a large mass of cells in the median line in which the boundaries between the germ- 

 layers cannot be determined. Laterally this mass of tissue passes over into per- 

 fectly distinct germ-layers, of 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 -possible intervening position. The nuclei are much 

 crowded, there being but little protopjasm. No distinct cell boundaries can be 

 made out. The nuclei are further remarkable on account of their very conspicu- 

 ous nucleoli. 



