EMBRYO WITH TWENTY-FOUR SEGMENTS. 



197 



Sections through the Primitive Groove (Fig. 148). In all of these we find merely the 

 three germ-layers, which are all united in the median line with the axial band of cells, 

 constituting the primitive streak. The ectoderm has the deep furrow which toward 

 the head runs into the medullary groove. Caudad, the primitive groove widens 

 out and is gradually lost. The thinning out of the mesoderm should also be no- 

 ticed, as the series is followed in the caudad direction. Under a higher power the 

 character and arrangement of the cells comes out more clearly (Fig. 149). 



FIG. 149. CHICK EMBRYO WITH SEVEN SEGMENTS. TRANSVERSE SECTION ACROSS THE PRIMITIVE GROOVE. 

 EC, Ectoderm, mes. Mesoderm. Ent, Entoderm. Pr.g, Primitive groove. The large black dots represent 



yolk-grains. X 230 diams. 



Embryo Chick with about Twenty-four Segments and Three Gill-clefts (about 



Forty-six Hours' 1 Incubation). 



The following description will apply almost equally well to embryos with from 

 twenty-six to twenty-nine segments. 



Examination in toto. The specimen as a whole, as in the fresh state, has a 

 grayish tint when viewed by transmitted light. As soon as it is hardened the 

 opacity of all the tissues is greatly increased. In the center of the germinal area 

 is the very conspicuous area pellucida, which is somewhat pear-shaped. The por- 

 tion around the anterior end of the embryo (Fig. 150, A.p) is very wide. In the 

 center of the area vasculosa appears the embryo, the head end of which is twisted 

 over so that the left side of the head lies against the yolk. This twisting of the 

 neck- and head so that they become asymmetrical in position is very characteristic 

 of birds. Below the .head and somewhat to the right may be seen the tubular 

 heart, Ht, which, in the fresh specimen, pulsates regularly. Around the area pellu- 

 cida comes the dark area opaca, in which we readily distinguish the outer boundary 

 or terminal sinus of the area vasculosa. In this there is already a well-developed 

 network of blood-vessels through which the blood is circulating, being driven by 

 the heart. The blood moves out from the embryo by two large vessels, A.vi, 

 which lie symmetrically, the vitelline or omphalo-mesaraic arteries. These arteries 

 arise from the dorsal aorta of the embryo and pass out to the area vasculosa, 

 over which they ramify. -The blood returns to the heart by means of a network 

 of small vessels, across the posterior part of the area pellucida; the network close 

 to the embryo fuses into two larger short trunks, one each side. The two trunks 



