182 STUDY OF YOUNG CHICK EMBRYOS. 



cord, Sp.c. In this specimen the open medullary groove, Md, begins at the level of 

 the fourth segment. The outline, a, of the free portion of the head is sharply marked, 

 as is also the outline, Ph, of the pharynx or fore-gut, which opens at the fovea 

 cardiaca into the general sub-germinal entodermic cavity. The omphalo-mesaraic 

 veins, V.om, can be traced peripherally to their junctions with the vascular net- 

 work of the area pellucida, and if the embryo b,e viewed by its ventral surface, the 

 two veins can be seen to unite, beneath the fore-gut, with the caudad end of the 

 heart. The notochord can be seen under the mid- and hind-brain, as a narrow 

 median band which is slightly irregular, and also very clearly, nch, underneath 

 the rhomboidal sinus; it fades out at its caudal extremity where it merges into the 

 undiffer'entiated tissue of the primitive streak. A band of cells, G, can be seen on 

 either side of the head, extending tailward from the optic vesicle. This band is 

 usually designated as part of the ganglionic crest, but its origin and fate have 

 not yet been satisfactorily elucidated. 



Longitudinal Section of a Chick. In order to facilitate the study of the trans- 

 verse sections of this stage, figure 132 is inserted, which is a nearly median longi- 

 tudinal section. In consequence of the head end, H, having grown .forward above 

 the pro-amnion, pro.a, it has become free on all sides, and at the same time the 

 entodermal cavity has been carried forward with the head, making the so-called 

 fore-gut of English authors. This fore-gut is the anlage of the pharynx, the oeso- 

 phagus, and the stomach. Underneath the posterior portion of the fore-gut there 

 has appeared in the mesoderm'a ccelomic cavity, />,' which serves as the connection 

 across the median line with the amnio-cardiac vesicles just described in surface 

 views. We can, therefore, distinguish in the fore-gut the anterior portion from the 

 posterior portion which overlies the ccelom. This coelom is the anlage of the peri- 

 cardial 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 foiea 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. Attention should be directed, first, to the three 

 germ-layers, their composition and their rdles in the production of organs; second, 

 to the exact topographical relations of the various organic anlages, because these 

 relations are fundamental and determine the anatomical dispositions in the adult. 

 Before beginning the detailed study of the sections, the student should have a clear 

 conception of the manner in which the free head of the embryo merges into the 

 embryonic body and germinative area. Fifteen figures represent as many cross- 

 sections of an embryo chick with eight fully formed segments, and the ninth seg- 

 ment beginning. The drawings are uniformly magnified 100 diameters. There are 

 interpolated figures 132, 138, 139, 147, 149 from other embryos to illustrate certain 

 details with higher magnifications. 



