Development of the Second Day 159 



tributed to the vascular area in that region, as 

 there is, as yet, no vessel connecting the pos- 

 terior part of the sinus terminalis with the 

 vitelline veins. These veins run parallel to 

 and a little in front of the vitelline arteries. 

 The course of the circulation at this time may 

 be understood from Fig. 56, which is of a some- 

 what later period. 



The Wolffian duct, during this period, con- 

 tinues to elongate both anteriorly and pos- 



FIG. 54. TRANSVERSE SECTION THROUGH THE DORSAL REGION 

 OF AN EMBRYO OF 46 HOURS. (After Duval.) 



Ao, aorta. CSW, nephrostome of Wolffian body. CIV, Wolffian duct, of, 

 alimentary canal. 



teriorly, and its posterior end becomes free 

 from the mesoblast and lies in the space 

 between the ectoblast and mesoblast. A small 

 lumen now appears near its middle point and 

 gradually extends both forwards and back- 

 wards. At a later period (on the fourth day) 

 the duct opens into the cloaca. 



The Wolffian body makes its first appearance 



