1128 



REPRODUCTION; 



umbilicus, carrying bloodvessels along with it in its mesodermic layer. 

 Still earlier, and, indeed, while the embryo is being separated oft" 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. 485). 

 The superficial 

 layer of the pouch 

 Fig. 485 Diagram to illustrate Formation of Amnion and j s called the false 

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

 to coalesce and complete the amniotic cavity; m, meso- D l en( j s 'with the 

 dermic layer of amnion; B, allantois; I, intestinal cavity t u ft ec j chorion or 

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



dermic layer of embryo. The embryo is the shaded por- ' >m . m 

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

 head region. No attempt is made to delineate its actual Ovum, 

 form. The mesoderm is represented by the interrupted layer persists as 

 line. the true amnion ; a 



liquid, the amniotic 



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 foetus, 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 

 or transudation. But something is contributed by the foetus in the 

 form of renal, and perhaps of skin, secretions. The fluid is poor in 

 solids. Its maximum content of protein, reached during the first half 

 of pregnancy, is only 0-7 per cent. Later on it diminishes, and at full 

 term is only one-tenth of this amount. The specific gravity is 1006 to 

 1009. Its osmotic concentration, as measured by the depression of the 

 freezing-point, is less than that of the mother's blood-serum. 



The allantois, growing out at the umbilicus, in the manner described, 

 insinuates itself between the true and false amnion, and soon blends 

 with the latter. For a time the secretion of the primitive kidneys 

 continues to be poured into the cavity of the allantois, so that it serves 



