392 THE PHYSIOLOGY OF REPRODUCTION 



absorbed, and at the same time the yolk-sac contains a charac- 

 teristic yellowish-green, glassy coagulum with granules in it. 

 Later the mucosal cushions disappear and the adjacent tropho- 

 blast thins (see p. 454). 



In the mole a simple yolk-sac placenta persists throughout 

 pregnancy (Robinson ^). Unhke the hedgehog and the shrew, 

 in which the gland lumina are plugged by the trophoblastic 

 syncytium, there is in the mole a copious glandular secretion 

 containing degenerated cells, which is absorbed by the tropho- 

 blast (see p. 456). 



Twpaia javanica differs from the other Insectivora in having 

 a temporary yolk-sac placenta formed in the same situation as 

 the allantoic placenta subsequently occupies (see p. 458). The 

 same occurs in the bat (p. 462). 



Primates. — In monkeys, old- and new-world, there is no 

 decidua reflexa, and a portion of the trophoblast is in contact 

 with the uterine fluids. But even in Selenka's earUest specimens 

 of monkeys and apes, the yolk-sac is a smaU, closed sac attached 

 to the ventral surface of the embryonic area, and is entirely 

 separated from the trophoblast. The embryonic area is con- 

 nected with the inner surface of the chorion by a short stalk of 

 mesoderm, in which the vessels run. 



In the youngest human ovum yet examined, the yolk-sac is 

 also a small, closed vesicle, separated from the trophoblast by a 

 single thick layer of mesoblast (Fig. 86). The sphtting of the 

 mesoblast occurs very early, eyen before the appearance of the 

 primitive streak, and the ccelom spreads round the whole circum- 

 ference of the ovum. The earhest vessels appear on the under 

 surface of the sac, and gradually extend over its upper pole, 

 until the whole sphere is covered by a vascular network. The 

 vessels are in direct continuity with vessels which develop in 

 the connecting-stalk (see p. 463), and through them with the 

 vessels of the chorion by a vascular loop, the sinus ensiformis of 

 Eternod (Bryce ^). This communication appears to exist before 

 any vessels appear in the embryo itself. From the third 

 week onwards, saccular dilatations of the entodermal lining of 



' Robinson, Hunterian Leot., toe. cit. 



2 See Quain's Anat., vol. i., Part I., 1908. 



