188 



THE THIRD DAY. 

 FIG. 65. 



[CHAP. 



SO 



SECTION THROUGH THE DORSAL REGION OF AN EMBRYO CHICK 

 AT THE END OF THE THIRD DAY. 



Am. amnion. m. p. muscle-plate. C. V. cardinal vein. Ao. dorsal 

 aorta. The section passes through the point where the 

 dorsal aorta is just commencing to divide into two branches. 

 Ch. notochord. W. d. Wolman duct. W. b. commencing 

 differentiation of the mesoblast cells to form the Wolman 

 body. ep. epiblast. SO. somatopleure. Sp. splaiichno- 

 pleure. hy. hypoblast. The section passes through the 

 point where the digestive canal communicates with the yolk- 

 sac, and is consequently still open below. 



This section should be compared with the section through 

 the dorsal region of an embryo at the commencement of the third 



