THE MAMMALIA 303 



Diagrams illustrating the development of the blastocyst and formation of the 

 placenta in Mammalia. A, a blastocyst at the end of segmentation; B, an 

 older blastocyst, in which a cavity has appeared to one side of the inner 

 mass of cells ; C, a later stage, showing the formation of the trjiger and growth 

 of the yolk epithelium round the yolk cavity ; D, formation of lacun£E in the 

 trager and commencement of the embryo ; £, further development of the trager, 

 the mesoblast has split and the amnion and extra-embryonic coelom are 

 formed ; J^, longitudinal section of uterus, showing the position of the 

 embryo in a pit in the uterine wall. G, longitudinal section of a later stage, 

 showing the obliteration of the old lumen and formation of a new lumen in the 

 uterus. «//, allantois ; «?«, amniotic cavity ; ca?, embryonic coslom ; ^c, epiblast; 

 eec, extra embryonic coelom ; e^f embryonic knob ; ejn, embryo ; ^/, uterine 

 glands ; Ay, hypoblast ; i»i, inner mass of cells ; /ac, lacunas in trager ; 

 /«!, original lumen of uterus ; /«2, secondary lumen of uterus ; nc/t, notochord ; 

 K^, neural groove ; s^m, thickened sub-mucous layer of uterus ; tr, trager ; 

 trOy tropholDlast ; yk, yolk-sac ; y^.e, yolk epithelium. In all the figures the 

 trophoblast is shaded with dots, and the embryonic mesoblast is represented 

 in black. 



as that which appeared at an earlier stage in the embryonic 

 knob. The space between the somatopleur and splanchnopleur 

 is of course the coelomic cavity, and it is evident that the 

 coelom is continued ouiside the embryo as a wide space 

 extending all round the amnion. As development proceeds, 

 the splanchnopleur with hypoblast, and in a lesser degree the 

 somatopleur with epiblast, are folded inwards in front, behind, 

 and at the sides of the embryonic area, so that the embryo is 

 raised up above the yolk-sac and projects into the amniotic 

 cavity, as shown in fig. 75, G. While these changes are in 

 progress great changes have also taken place in the maternal 

 tissues. The uterine wall is greatly thickened, chiefly by the 

 growth of the submucous layer, which becomes very vascular. 

 The uterine epithelium is entirely disintegrated and the walls 

 of the maternal blood-vessels in the neighbourhood of the 

 embryo break down, allowing the maternal blood to escape 

 into the now spacious lacunae in the trager. The last named 

 organ has become closely adherent to the walls of the uterine 

 pit in which the embryo lies, and thus we arrive at a stage 

 in which the embryo is nourished by the maternal blood circu- 

 lating freely in an embryonic organ — viz. the trager. 



In the next stage the embryo is folded off from the yolk- 

 sac, and many of its more important organs have made 

 their appearance. The gut has been established by the 

 infolding of the spanchnopleur with its lining of hypoblast, 

 but is still, and for a long time remains, in free communication 

 with the cavity of the yolk-sac by a hollow stalk. As soon- 

 as the hind-gut is established, a ventral outgrowth is formed 

 from it, corresponding in position to the cloacal bladder of 



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