1084 



REPRODUCTION 



And the vitelline vessels deriving their further supply of food and 

 oxygen from the tissues of the mother in contact with the ovum cease 

 to be of use as soon as the second and more perfect placenta! circulation 

 is established, and soon shrivel up and disappear, as the umbilical 

 vesicle shrinks. 



The second circulation of the embryo is developed in connection with 

 a remarkable offshoot from the hind-gut called the allantois^ which, 

 before the fifth day in the chick and during the second week in man, 

 pushes its way out between the somatic and splanchnic layers of the 

 mesoderm i.e., in the pleuro-peritoneal cavity and grows through the 

 umbilicus carrying bloodvessels along with it in its mesodermic layer. 

 Still earlier, and, indeed, while the embryo is being separated off from 



and raised above 

 the level of the rest 

 of the blastoderm 

 by the deepening 

 of the ditch around 

 it, the further 

 banks of this fur- 

 row, formed of 

 ectoderm and 

 somatic mesoderm, 

 have risen up on 

 every side, and, 

 growing over the 

 back of the em- 

 bryo, have finally 

 coalesced and en- 

 closed it in a 

 double- walled 

 pouch (Fig. 462). 

 The superficial 

 layer of the pouch 

 is called the false 

 amnion ; it soon 

 blends with the 

 tufted chorion or 

 common outer 

 envelope of the 

 ovum. The inner 

 layer persists as 

 the true amnion ; a 

 liquid, the amniotio 



fluid, is secreted in the cavity which it encloses ; and the embryo, loosely 

 anchored for the rest of its intra -uterine life by the umbilical cord alone, 

 floats freely within it. The amniotic fluid acts as a water-jacket or 

 cushion, to break the force of the inevitable shocks and jars transmitted 

 from the mother to the foetus and from the foetus to the mother. To 

 some extent, in addition, it may serve as a nutritive fluid, for substances 

 can pass from the blood of the mother into the amniotic fluid, and the 

 amniotic fluid can be swallowed by the foetus. This is shown by the 

 fact that sodium sulphindigotate, when injected into the maternal 

 circulation, is found in the amniotic fluid and in the alimentary canal 

 of the fcetus, although not in any of the foetal tissues. Fine lanugo 

 hairs from the foetal skin have also been found in the meconium. 



The precise origin and manner of formation of the amniotic fluid 

 have not been settled. It is probably in the main a maternal secretion 



Fig. 462. Diagram to illustrate Formation of Amnion and 

 Allantois. A, cavity of true amnion; F, F', folds about 

 to coalesce and complete the amniotic cavity; m, meso- 

 dermic layer of amnion; B, allantois; I, intestinal cavity 

 of embryo; Y, yolk-sac; h, endodermic layer; e, ecto- 

 dermic layer of embryo. The embryo is the shaded por- 

 tion in the middle of the figure. E is placed over the 

 head region. No attempt is made to delineate its actual 

 form. The mesoderm is represented by the interrupted 

 line. 



