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- 



,.^7-s- 



Fig. 54. — Transverse section through the dorsal region 

 OF AN embryo of 46 HOURS. (After Duval.) 



_ Ao, aorta. CSW, nephrostome of Wolffian body. CW, Wolffian duct. C/, 

 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 



