EMBRYO WITH TWENTY-EIGHT SEGMENTS. 



287 



wall of the chick. The form of the arch indicates the commencing closure of the 

 embryonic somatopleure on the ventral side, by which the body of the embryo 

 will ultimately become shut off from the underlying layers of the blastoderm. In 

 the median plane of the embryo we find the spinal cord, cut somewhat obliquely, 

 the notochord, nch, and the very large section of the aorta, Ao. The great trans- 

 verse width of the aorta is due to its approaching division to ward, the caudal end 

 of the body to form the two branches which run out to the area vasculosa and 

 are known as the omphalo-mesaraic or vitelline arteries. Before they leave the 

 body of the embryo each of these arteries gives off a branch which continues in 

 the body of the embryo not far from the notochord and close to the entoderm. 

 These branches subsequently become the allantoic arteries. On either side of 

 the spinal cord lie the secondary segments, My. A short distance from the 



SOHI. 



IV. D. . Se 



Ahs. 



Cce. Spl. Ve. nch. Ent. Spl. mes' '. 



FIG. 162. SECTION OF A CHICKEN EMBRYO WITH ABOUT TWENTY-EIGHT SEGMENTS. TRANSVERSE SERIES 



92, SECTION 356. 



Cce, Ccelom. EC, Ectoderm. Ent, Entoderm. Mes, Somatic mesoderm. mes', Splanchnic mesoderm. N, 

 Xephrotome. nch, Notochord. Stg, Segment. Som, Somatopleure. Sp.c, Spinal cord. Spl, Splanchno- 

 pleure. Ve, Blood-vessel. IV. D, Wolffian duct. X 5 diams. 



aorta on either side appear sections of two rather small blood-vessels, the cardinal 

 veins, card. Between the vein on each side and the aorta there is a little accu- 

 mulation of denser tissue. " If a series of sections is followed through, the Wolff- 

 ian duct may be traced into this condensed tissue, and when the duct is differen- 

 tiated, it will take the place of this tissue between the aorta and the vein. 



Section through the Middle Portion of the Open Intestine (Fig. 162) . Compari- 

 son of this section with the preceding is instructive as an illustration of the fact 

 that the differentiation of structures is found less advanced as we proceed toward 

 the caudal end of the embryo. In the present section the amniotic folds can 

 hardly be said to have appeared at all, although the coelom, Coe, is very wide in- 

 deed, and there is little differentiation in either the somatopleure, Som, or splanch- 



