DEVELOPMENT OF LEPUS 231 



The amnion arises by the upgrowth of folds at the edge 

 of the embryonic plate. The hinder amniotic fold develops 

 faster than that in front, and when these folds meet, the embryo 

 is no longer at the surface of the trophoblast, but folded away 

 within it. The embryo is then enclosed in the amniotic 

 cavity, just as in the chick, and the trophoblast of the rabbit 

 corresponds to the chorion of the chick, the relations of which 

 are identical (see p. 211). (It may be mentioned that in some 

 other mammals such as the mouse, the amnion is not formed 

 quite in this way, but arises precociously, even before the 

 embryo (see p. 254). The rabbit has been chosen for de- 

 scription here because its development is so easily comparable 

 with that of the chick.) 



The mesoderm splits into somatic and splanchnic layers 

 with the ccelomic cavity between them. The splanchnic 

 layer overlies the yolk-sac. The somatic layer grows up 

 round the amnion and separates the latter from the trophoblast. 



An area vasculosa develops in the wall of the yolk-sac, and 

 the blood-vessels so formed extend as far as the sinus terminalis. 

 The lower wall of the yolk-sac is not vascularised. In some 

 mammals this lower wall of the yolk-sac with its overlying 

 trophoblast persist for some time, and absorb nourishment 

 from the walls of the uterus. In the rabbit, however, this 

 " omphaloidean " region of the trophoblast together with 

 the lower wall of the yolk-sac disappear, and the cavity of 

 the yolk-sac is then openly continuous with that of the lumen 

 of the uterus. This disappearing part of the blastocyst 

 contained neither blood-vessels nor mesoderm. 



Placenta.— Meanwhile, the upper part of the trophoblast 

 which is in contact with the wall of the uterus on the mesometric 

 side becomes much thicker, forming a syncytium (or plasmodi- 

 trophoblast). The more basal part of the trophoblast, between 

 the syncytium and the mesoderm, retains its cell-boundaries 

 (and is called the cyto-trophoblast). The allantois grows out 

 from the region of the hind gut and brings with it a covering 

 layer of mesoderm and blood-vessels. The mesoderm 

 covering the allantois fuses with the mesoderm underlying 

 the cyto-trophoblast, and the allantoic blood-vessels make 



