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. 160). — This section is 

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



Sp.c. 



Cho. 



Am 



So 11 1. 



Out S 



Om.D. 



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



Section 179. 



Am, Amnion. .Jo, Aorta, card, Cardinal vein. Cho, Chorion. Ca, Coelom. Ent, Entoderm. /;/, Intestine. 

 tnsth, Mesothelium. ,1/V, Muscle plate, nth, Notochord. Om.D, Right omphalo-mesaraic vein. Vm.S, 

 Left omphalo-mesaraic vein. Som, Somatopleure. Sp.c, Spinal cord. Spl, Splanchnopleure. . 50 clijms. 



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 



