412 THE PHYSIOLOGY OF REPRODUCTION 



descended from ancestors in which the ovum had a large supply of 

 yolk, but that, when the fertilised ovum found a new supply of food 

 in the uterus, the yolk was reduced and ultimately disappeared. At 

 the same time the' envelopes, which were developed under the 

 infliience of the vitelline contents, have been preserved and modified 

 in different ways to aid uterine nutrition,^ 



In the early stages the development proceeds, as in birds and 

 reptiles, with the gradual extension of the hypoblast round the wall 

 of the blastocyst, which thus becomes didermic. The mesoblast grows 

 out between the epi blast and hypoblast, starting at the embryonic 

 area and gradually extending for a variable distance round the wall 

 of the blastocyst. Near the embryo appears the ar-ea vasculosa, 

 in which blood-vessels and blood are developed from the cells of 

 the mesoblast, while at the same time the embryo begins to be 

 folded off from the yolk-sac by anterior and posterior folds. The 

 area gradually extends further and further round. Its outer boundary 

 is marked by the sinus terminalis which communicates with the 

 vitelline veins. The blood is brought from the dorsal aortse by a 

 series of lateral vitelline branches. These arteries break up into a 

 deeper arterial network, from which the blood is collected into the 

 sinus terminahs and the superficial venous network, and in this way 

 reaches the vitelline veins and so passes to the heart. 



During the spread of the mesoblast, it splits into an external 

 layer or somatopleur, and an internal layer or splanchnapleur. The 

 former is non-vascular and adheres to the inner aspect of the 

 trophoblast which is ectodermal, forming with it the diplo-trophohlast, 

 otherwise called the false amnion or chorion, and the splanchnopleur 

 is applied externally to the hypoblastic wall of the yolk-sac. By the 

 splitting a space is formed between the two layers. This is the extra- 

 embryonic cmlom, which thus intervenes over a larger or smaller area 

 between the diplo-trophoblast and the yolk-sac. 



'The amnion in the rabbit arises by the formation of folds of 

 extra- embryonic ectoderm, together with the somatopleur which is 

 in conjunction with it, the yolk-sac and the splanchnopleur taking 

 no part in the process.^ The extra-embryonic ccelom is continued 

 into the folds, each of which contains two layers. The folds grow 

 up over the dorsal surface of the embryo and eventually meet and 

 fuse, and when the union is complete the outer and inner layers 



' According to Hubreoht's views, the mammalian ovum is not descended from 

 the ovum of Sauropsida. 



^ In the guinea-pig and many other species of Mammals, including probably 

 man, the amniotic cavity develops as a closed sac from its inception. It is , 

 formed inside the embryonic knot, which comprises the material for both 

 amnion and embryo. For an account of the various ways in ■ftrhich the foetal 

 membranes are formed in the different animals see Jenkinson, Verteh-ate Em/- 

 hryology, Oxford, 1913 (see below, footnote, p. 490). 



