EMBRYO WITH SEVEN SEGMENTS. 



299 



the fore-gut the anterior portion from the posterior portion which overlies the 

 eoelom. This ccelom is the anlage of the pericardial cavity. The anterior 

 division of the fore-gut forms the pharynx proper. It ends blindly in front. 

 The opening of the fore-gut into the general entodermic cavity, Ach, is termed 

 the fovea cardiaca, fo. At the posterior end of the embryo we have a thickened 

 mass of cells constituting the primitive streak, Pr.s. The line on the under 

 side of the figure represents the entoderm, and the space underneath it is a 

 portion of the primitive entodermic cavity. 



Study of Transverse Sections. — Section through the Anterior Part of the 

 Head (Fig. 170). — The head lies free and is covered by a well-defined layer of 

 ectoderm, Ec, which on the dorsal side is continuous with the walls of the 



rich 



Fig. 170. — Chick Embryo with Seven Segments. Transverse Section of the Head. 

 iii/i, Xotochord. 67, Ganglionic or neural crest, md. Wall of medullary tube, mes, Mesoderm. Ph, Pharynx. 

 Et, Ectoderm. En, Entoderm. Am. /", Amnio-cardiac vesicle. 



medullary canal, md, which is here quite wide, corresponding to the level of 

 the future hind-brain. Where the outer ectoderm joins the wall of the medul- 

 lary canal there is an accumulation of cells, Gl, readily distinguishable from 

 the adjacent ectoderm and the medullary wall proper. These cells constitute 

 the so-called neural crest, and represent the beginning of the formation of the 

 true ganglionic cells. On the ventral side we have the very widely distended 

 pharynx, Ph, which has the characteristic crescent shape in transverse sec- 

 tions. It is lined by entoderm which forms a distinct continuous layer, is very 

 thin on the dorsal side and thicker on the ventral side, and on the dorsal side 

 is bent upward so as to form a median longitudinal groove. The base of this 

 groove touches the notochord, rich, which lies closely packed in the median 



