284 



STUDY OF YOUNG CHICK EMBRYOS. 



the septum transversum is formed also upon the right side of the body of the em- 

 bryo. The mesothelium between the upper division of the eoelom, Cce, and the 

 sides of the entodermal canal is very much thickened and deeply stained. On 

 either side of the very large median aorta, Ao, and just above the ccelom, appear 



nch 



Ao. 



card. 



Soul 



raph 



Che 



m si h. 



m. hi. 



Ent. 



Fig. 159. — Section of Chick Embryo with about Twenty-eight Segments. Transverse Series 92, 



Section 165. 



Am, Amnion. Ao, Aorta, card, Cardinal vein. Cho, Chorion. Ca\ Ca', Crclom. Ec, Ectoderm, en . 

 Endothelial heart. Ent, Entoderm. Li, Liver, rites. Mesoderm, m.hl, Muscular heart, mslh, Meso- 

 thelium Afv, Primitive segment, nc/i, Notochord. raph, Raphe of amnion. St. l\ Sinus venosus of 

 heart. Som, Somatopleure. Sp c. Spinal cord. Spl, Splanchnopleure. I'e, Vein. )", Accumulation of 

 mesodermic tissue about the omphalo-mesaraic vein. • 50 diams. 



the right and left posterior cardinal veins, card. Concerning the foetal envelopes 

 little need be said, except to call attention to the large raphe, raph, of the amnion, 

 which is now a rather conspicuous ectodermal thickening and seems to be formed 



