212 



STUDY OF YOUNG CHICK EMBRYOS. 



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

 car d. Between the vein on each side and the aorta there is a little accumulation 

 of denser tissue. If a series of sections is followed through, the Wolffian duct 

 may be traced into this condensed tissue, and when the duct is differentiated, it 

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



Section through the Middle Portion of the Open Intestine (Fig. 160). 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 ccelom, Cce, is very wide in- 

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

 nopleure, Spl, between the embryonic and extra-embryonic regions. The ento- 



Som. Cce. Cce.' 



nch. 



In. Ent. 



FIG. 161. SECTION OF A CHICK EMBRYO WITH TWENTY-EIGHT SEGMENTS. TRANSVERSE SERIES 92, SECTION 419. 

 Cce, Ccelom. Cce' , Diverticulum of the ccelom. Ent, Entoderm. In, Intestinal cavity, mes, Mesoderm. nch, 



Notochord. Som, Somatopleure. Sp.c, Spinal cord. Spl, Splanchnopleure. S.z, Segmental zone. X 50 



diams. 



derm is a little thicker in the embryo than in the extra-embryonic territory. A 

 similar difference may be observed in the ectoderm. The embryonic mesoderm in 

 both somatopleure and splanchnopleure is considerably more developed ' and much 

 denser than in the extra-embryonic parts. The axial structures of the embryo 

 namely, the spinal cord, Sp.c, and notochord, nch are about the same as further 

 forward, but the mesoderm is much less advanced than further headward as is 

 evidenced by the small amount of mesenchyma above the axial structures and 

 by the slight differentiation of the mesothelium. The condition of the segments 

 and their relations to the somatic and splanchnic mesoderm are closely similar to 

 those represented in figure 46. Each somite consists of a larger part, Seg, of 

 rounded outline, close to the medullary tube, and of a narrower part, the nephro- 

 tome, N, which connects the inner portion of the somite with the lateral meso- 

 derm. The secondary somite consists of a distinctly marked wall which extends 

 around underneath the ectoderm and against the side of the medullary tube, and 

 of a thick inferior wall which fills up also the center of the somite. Between the 

 nephrotome and the entoderm are small blood-vessels, Ve. 



Section through the Posterior Portion of the Open /; Fig. 161). This 



