Generation and Development 389 



3 feet in length, and the amniotic portion is twisted very 

 much like the strands of a rope.* 



We have traced the development of the ovum from the 

 period of impregnation to the development of the primary 

 circulation and investing membranes. It is not our inten- 

 tion to follow out the development of the various parts of 

 the body, until the foetus is completely formed, as ample 

 information on these points can be obtained in works on 

 anatomy and embryology. t We will here only mention the 

 peculiarities of the foetal circulation, as these are of physio- 

 logical interest. 



Foetal Circulation. — The blood of the foetus, returning from 

 the placenta in a purified condition, passes by means of the 

 umbilical vein to the umbilicus, then turns and runs along 

 the floor of the abdomen, reaches the liver, and opens into 

 the vena porta ; from the union of the umbilical and portal 

 veins, a single vessel results, which breaks up in the liver, 

 and the blood passing through this gland, reaches, in course 

 of time, che posterior vena cava through the medium of the 

 hepatic veins. So that in the foetal circulation of the horse 

 the whole of the blood first passes through the liver. (See 

 Fig. 50.) 



In ruminants the umbilical vein on reaching the liver 

 joins the vena porta, from which a canal — the ductus 

 venosus — takes a portion of the blood direct to the posterior 

 vena cava ; the other portion, traversing the liver through 

 the hepatic veins, eventually reaches the posterior vena 

 cava; from here in both animals it passes to the right 

 auricle of the heart, where it meets with the blood which 

 has been circulating through the head, neck, and fore ex- 

 tremities. Here occurs a remarkable change characteristic 

 of the foetal circulation in all animals : that portion of blood 

 which reached the right auricle by the posterior cava divides ; 



* To Fleming's ' Veterinary Obstetrics ' I am indebted for the 

 account of the masterly foreign researches on the formation of the 

 membranes in solipeds and ruminants. 



f The development of the ovum and the various parts is fully 

 described in Chauveau's 'Anatomy' (Fleming), and in Fleming's 

 ' Veterinary Obstetrics.' 



