126 THE THIRD DAY. [CHAP. 



FIG. 41. 



SECTION THROUGH THE DORSAL REGION OF AN EMBRYO AT THE COMMENCEMENT 

 OF THE THIRD DAY. 



M. C. medullary canal. Ck. notochord. p. v. protovertebra composed of an 



investment of columnar cells enclosing a central nucleus of rounded cells. 



w. d. Wolffian duct, which has commenced to travel down from the dorsal 



surface of the mesoblast. A. o. dorsal aorta of right side. g. e. germinal 



epithelium ; an epithelium of columnar cells lining the upper end of thepleuro- 



peritoneal cavity, and which is related to the formation of Miiller's duct and 



of the ovary. S. o. somatopleure. s. p. splanchnopleure. 



The splanchnopleure is very little folded in, the embryonic sac being in this 



middle region widely open to the yolk below. The somatopleure is much more 



folded in. At a little distance outside the protovertebra is a ridge with 



thickened mesoblast, the Wolffian ridge, marking the line along which the limbs 



will be developed. Beyond this ridge the somatopleure suddenly descends to 



form the body- wall (of the abdomen) ; it then ascends, and after a fold, probably 



due to the action of the chromic acid> forms an angular projection at about the 



level of the protovertebra. This projection is the lateral fold of the amnion, 



the section having been taken just at that point between the head and tail where 



the amnion is, on the third day, least developed. 



Beyond the amniotic fold the somatopleure and splanchnopleure come into 

 apposition, arid still farther out completely coalesce. It will be observed that 

 the pleuroperitoneal space already reaches laterally far beyond the limits of 

 the embryo itself. 



In the splanchnopleure is seen at V a section of a large branch of the 

 omphalo-mesaraic trunk. 



The shading of the general mesoblast is diagrammatic. 



The width of the cavity of the neural tube is unusually great. 



