286 



STUDY OF YOUNG CHICK EMBRYOS. 



that the amniotic fold does not join its fellow, and therefore has no raphe. In 

 this condition the amnion is said to be " open." 



Section through the Anterior Portion of the Open Intestine (Fig. i6i). — In this 

 section the intestinal cavity, In, being without a ventral wall, opens directly into 

 the general entodermal cavity under the germinal area and above the yolk-mass. 

 The median plane of the embryo is still inclined to the left. The extra-embry- 

 onic somatopleure, Som, rises in two high folds, one on each side of the embryo; 

 the inner portion of each fold. Am, belongs to the amnion, the outer portion, Cho, 

 to the chorion. The splanchnopleure, Spl, passes without demarcation into the 

 wall of the intestinal cavity. In. The entoderm, Ent, of the extra-embryonic 



Som. 



Ec. instil, card. My. nch. Sp.c. card.s. b.w. Am. 



Cho. 



In. Ao. Ve, vies. Ent. Spl. 



Fig. i6i. — Section of a Chicken Embryo with about Twenty-eight Segments. Transverse Series 



92, Section 220. 

 Am, Amnion. Ao, Aorta, b.w. Body wall, card. Right posterior cardinal vein, card.s, Left cardinal vein. 

 Cho, Chorion. Ec, Ectoderm. Ent, Entoderm. In, Intestine, mes. Splanchnic mesoderm, msth, Meso- 

 thelium. My, Myotome, nch, Notochord. Som, Somatopleure. Sp.c, Spinal cord. Spl, Splanchnopleure. 

 Ve, Vein. X 5° diams. 



splanchnopleure is very thin, but where it passes into the embryonic region to- 

 ward the median line, it thickens a little. The splanchnic mesoderm is a thin 

 layer of mesothelium which, of course, bounds the coelom everywhere and can 

 be followed continuously over on to the somatopleure. The splanchnic mesen- 

 chyma is loose in texture and surrounds the large blood-vessels. The splanch- 

 nic mesoderm on either side of the intestinal groove appears quite dark, owing 

 to the condensation of the tissue. Whether this condensation is developed from 

 the mesothelium or from the mesenchyma it is very difficult to say. The soma- 

 topleure, Som, where it becomes embryonic, increases greatly in thickness and 

 forms an arch, h. w, which is the beginning of the formation of the ventral body- 



