300 



STUDY OF YOUNG CHICK EMBRYOS. 



line between the floor of the medullary canal and the roof of the pharynx. 

 The notochord presents a circular outline, and its large size, as compared 

 with that of the mammal, should be noted, for it is characteristic of the Saurop- 

 sida. The space between the pharynx and the ectoderm is partially filled by 

 loosely scattered mesodermic cells, mes. Underneath the head of the embryo 

 we have two layers of cells which belong respectively to the ectoderm, Ec, 

 and entoderm, En, and together constitute the so-called pro-amnion. On 

 either side, at a point corresponding roughly to the lateral boundary of the 

 head, the two layers of the pro-amnion separate widely from one another and 

 a third layer of cells appears between them, which belongs to the mesoderm 

 and surrounds the large ceelomic cavity which is characteristic of the amnio- 



Fig. 171. — Chicken Embryo with Seven Segments. Transverse Section through the Anlage of 



the Heart. 



J/./, Medullary groove. Ec, Ectoderm, mes, Mesenchyma. Am.ves, Amnio-cardiac vesicle. /'//, Pharynx. 



msth, Mesothelium. Emio, Cells to form the anlage of the endothelial heart. 



cardiac vesicles, Am.V. The letters in the figure are placed in the ccelom 

 of the right-hand vesicle. 



Section through the Posterior Division of the Fore-gut (Fig. 171). — In the em- 

 bryo from which this section was taken the medullary groove, Md, was not yet 

 closed at this point. At the upper edge of the groove traces of the differentiation 

 of the neural crest, so clearly shown in the previous figure, can be distinguished, 

 and the ectoderm of the groove at its edge passes over into the general ectoderm, 

 Ec, covering the head of the embryo. The fore-gut, Ph, the notochord, and 

 the mesoderm, mes, are all very much the same as in the previous section. 

 On the under side of the embryo the structure is quite different. The amnio- 

 cardiac vesicles, Am.ves, come very close together in the median line. They 



