206 



STUDY OF YOUNG CHICK EMBRYOS. 



erately thick, but it is somewhat thinner near the median dorsal line. On either 

 side of the pharynx the mesodermal layer, mes, is very thick and stands out con- 

 spicuously, owing to its dark staining. Above the pharynx it thins out and passes 

 over on to the somatopleure,', Som, and so on to the amnion, Am. On the ventral 

 side of jthe pharynx the mesodermal layer passes over into the muscular wall of 



m.ht. 



FIG. 156. SECTION OF CHICK EMBRYO WITH ABOUT TWENTY-EIGHT SEGMENTS. TRANSVERSE SERIES 9?, 



SECTION 144. 



Am, Am', Amnion. Ao, Aorta. Au, Cardiac auricle. Cho, Chorion. Coe, Ccelom, D.C, Duct of Cuyier. 

 EC, Ectoderm. Endo, Endothelial heart, mes, Mesoderm. m.ht, Muscular heart. My', Primitive segment. 

 Ph, Pharynx. Raph, Raphe of amnion. Som, Somatopleure. Sp.c, Spinal cord. Spl, Splanchnopleure. 

 Ven, Ventricle of heart. X 50 diams. 



the heart, m.ht. The heart itself is very large; it has two tubes, the endothelial, 

 endo, and the muscular, m.ht, which are very distinct. The endothelial cavity is 

 very large. It is especially expanded immediately underneath the pharynx to form 

 the auricular end of the heart, which receives the veins. Throughout a large 

 part of the auricle the cndothelium is closely fitted against the muscular wall. 

 Farthei ventralward me double heart-tube bends tr the right of the embryo to 

 form the ventricular limb, Ven, in whiph the epithelial cavity is also enlarged. 



