EMBRYO WITH TWENTY-EIGHT SEGMENTS. 



285 



rather at the expense of the ectoderm of the amnion than at 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. 



Section through the Omphalo-mesaraic Veins (Fig. i6o). — This section is 

 intermediate in structure between figure 159 and figure 161, here described. 



Om S. 



Om.D. 



Fig. 160. — Section of Chick Embryo with about Twenty-eight Segments. Transverse Series 92, 



Section 179. 



Am, Amnion. Ao, Aorta, card. Cardinal vein. Cho, Chorion. Ca, Coelom. Ent, Entoderm. In, Intestine. 

 nisth, Mesothelium. Aly, Muscle plate. 7ich, Notochord. Om.D, Right omphalo-mesaraic vein. Om.S, 

 Left omphalo-mesaraic vein. Som, Somatopleure. Sp.c, Spinal cord. Spl, Splanchnopleure. X 5° diams. 



We are now beyond the region of the heart and liver. The cavity of the intes- 

 tine is open on the ventral side, so that the walls of the intestine pass over 

 directly into the extra-embryonic splanchnopleure, Spl, in which are lodged 

 the very wide omphalo-mesaraic veins, Om. D and Om. S, which are enter- 

 ing the body of the embryo to run forward past the liver anlage (Fig. 159) 

 to join the posterior or venous end of the heart. It will also be noticed 



