210 



STUDY OF YOUNG CHICK EMBRYOS. 



ward, it will be found that at this stage further back the septum transversum is 

 formed also upon the right side of the body of the embryo. The mesothelium 

 between the upper division of the ccelom, 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 the right and left posterior 

 cardinal veins, card. Concerning the fetal 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 rather at the expense 

 of. the ectoderm of the amnion than of that of the chorion. Such an ectodermal 

 raphe is very characteristic of birds; it has in the chick a considerable extent and 

 therefore appears in many successive sections of the series. 



Som. EC. msth. card. My. nch. Sp.c. card.s. b.w. Am. Cho. 



In. Ao. Ve. mes. Ent. 



FIG. 159. SECTION OF A CHICK EMBRYO WITH ABOUT TWENTY-EIGHT SEGMENTS. TRANSVERSE SERIES 92, 



SECTION 220. 

 . 

 Am, Amnion. . 1<- JX, jody-wall. card, Right posterior cardinal vein, card.s, Left cardinal vein. 



i. EC, Ectoderm. Ent, Entoderm. In, Intestine, mes, Splanchnic mesoderm. msth, Meso- 

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

 Ve, Vein. X 50 diams. 



Section through the Omphalo-mesaraic Veins (Fig. 158). This section is inter- 

 mediate in structure between figure 157 and figure 159, here described. We are 

 now beyond the region of the heart and liver. The cavity of the intestine is open 

 on the ventral side, so thai the walls of the intestine pass over directly into the 

 extra-embryonic Splanchnopleure, Spl, in which are lodged the verv^Vide omphalo- 

 mesaraic veins, Om.D and Om.S, which are entering the body of the embryo 

 to run forward past the liver anlage (Fig. 157) to join the posterior or venous end 

 of the heart. It will also be noticed 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. I5o/. In this 

 section the intestinal cavity , 7, being without a ventral wail, opens directly 

 the general emi^t- mal cavity under the germinal area and above the yolk-mu^ 

 (compare the diagrams .Figs. 29 and 45). The median plane of the embryo is still 



