EMBRYO WITH TWENTY -EIGHT SEGMENTS. 





211 



inclined to the left. The extra-embryonic 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 ento- 

 derm, Ent, of the extra-embryonic splanchnopleure is very thin, but where it passes 

 into the embryonic region toward the median line, it thickens a little. The splanch- 

 nic mesoderm is a thin layer of mesothelium which, of course, bounds the ccelom 

 everywhere and can be followed continuously over on to the somatopleure. The 

 splanchnic mesenchyma is loose in texture and surrounds the large blood-vessels. 

 The splanchnic mesoderm on either side of the intestinal groove appears quite 

 dark, owing to the condensation of the tissue. Whether this condensation is devel- 

 oped from the mesothelium or from the mesenchyma it is very difficult to say. 



Som. 



N. Seg. Sp.c. W.D. EC. Mes. 



Cat. 



mes' . 



FIG. 160. SECTION OF A CHICK EMBRYO WITH ABOUT TWENTY-EIGHT -SEGMENTS. TRANSVERSE SERIES 92. 



SECTION 356. 



Cue, Coelom. EC, Ectoderm. Ent, Entoderm. Mes, Somatic mesoderm. mes', Splanchnic mesoderm. A', 

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

 pleure. Ve, Bloocl- vessel. W.D, Wolffian duct. X 50 diams. 



The somatopleure, Som, where it becomes embryonic, increases greatly in thickness 

 and forms an arch, b.w, which is the beginning of the formation of the ventral 

 body-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 em- 

 bryo 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 

 transverse width of the aorta is due to its approaching division toward 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 somites, My. A short distance from the aorta. 



