870 



A MANUAL OF PHYSIOLOGY 



perfect placental 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 off-shoot 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 mesoblast, i.e., in the pleuro-peritoneal cavity, and 

 grows through the umbilicus, carrying bloodvessels along with it in 

 its mesoblastic 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 furrow, formed of epiblast and somatic mesoblast, have risen 

 up on every side and, growing over the back of the embryo, have 

 finally coalesced and enclosed it in a double-walled pouch (Fig. 335). 

 The superficial layer of the pouch is called \hzfalse 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 



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 trans- 

 mitted from the 

 mother to the 



A, cavity of true amnion ; F, F', folds about to coalesce and foetUS and from 



complete the amniotic cavity ; m, mesoblastic layer of amnion ; *u~ frpf-nc rn trf 



B, allantois ; I, intestinal cavity of embryo ; Y, yolk-sac ; A, i^LUb *-V ine 



hypoblastic layer ; e, epiblastic layer of embryo. The embryo mother, 



is the shaded portion in the middle of the figure. E is placed The allantois. 



over the head region. No attempt is made to delineate its actual arr . m : nn ~ tll . I 



form. The mesoblast is represented by the interrupted line. growing OUt at 



the umbilicus, m 



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 in part as an excretory organ, while in the 

 bird it also performs the function of respiration; and in the mammal 

 both food and oxygen are carried by its vessels to the foetus during 

 the greater part of intra-uterine life. But later on the outgrowth 



335- DIAGRAM TO ILLUSTRATE FORMATION OF 

 AMNION. 



