THE DEVELOPMENT OF THE MOLE, 131 



ninth and tenth protovertebrse as a series of branches, after 

 which the aortse immediately become reduced to very minute 

 proportions. 



The venous system, which is barely distinguishable in Stage 

 H, is very slightly developed in Stage J. The vitelline veins 

 run in the converging folds of the splanchnopleure to the 

 posterior end of the heart on a level with the second and third 

 protovertebrse. 



The only veins in the trunk of the embryo are two slightly 

 developed anterior cardinal veins which are situated on the 

 outer edge of the anterior protovertebrse (fig. 47, a. c. v.). 

 They communicate with the ductus Cuvieri where the vitelline 

 veins run into the heart between the second and third proto- 

 vertebrse, and run forwards as far as the first protovertebra. 



Traces of a posterior cardinal vein may be seen for some 

 little distance behind the ductus Cuvieri ; but as a vessel it 

 exists only for a few sections, and is situated at the point 

 where the somatopleure commences to turn upwards to form 

 the amnion. 



Thus it may be observed the arterial system is in a far more 

 advanced condition than is the venous system in the body of 

 the embryo. 



The Structure of the Heart. — In Stage e the heart merely 

 consists of a small tube in the thickened splanchnic mesoblast 

 on either side, in front of the protovertebrse (fig. 14) . Then 

 (Stage f) the thickened portion is bulged outwards into the 

 body cavity and splits up into two layers. The outer layer 

 bounding the body cavity forms the wall of the heart itself, the 

 inner the flattened epithelial lining of the cavity of the heart. 

 The space between these two layers increases and in Stage h 

 (fig. 25) is considerable. In this figure the epithelial layer is 

 connected with the outer layer of the heart by long proto- 

 plasmic processes stretching from cell to cell across the space. 

 In Stage j the wall of the heart has increased in size more in 

 proportion than has the inner epithelial layer. The latter is 

 now an elongated bag within the space contained by the outer 

 wall and connected with the latter by marvellously delicate 



