REPRODUCTION 



ion 



whole period of development, is tapped, and a regular channel of 

 supply established. Oxygen is at the same time absorbed through 

 the porous shell ; but later on this respiratory function is taken over 

 by the second or allantoic circulation. In the mammal the circula- 

 tion on the umbilical vesicle is of much less consequence, for the 

 quantity of material left over after the formation of the blastoderm 

 is exceedingly small ; it is only with a few days' provision in its 

 haversack that the embryo starts out on its developmental march. 

 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 placental 

 circulation is es- 

 tablished, and 

 soon shrivel up 

 and [disappear, ; 

 as the umbilical^ 

 vesicle shrinks. 



The second 

 circulation of the 

 embryo is de- 

 veloped in con- 

 nection 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 be- 

 tween the so- 

 matic and 

 splanchn ic 

 layers of the 

 mesoderm i.e., 

 in the pleuro- 

 peritoneal cav- 

 ity and grows 

 through the um- 



FIG. 445. DIAGRAM TO ILLUSTRATE FORMATION OF 

 AMNION. 



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

 and complete the amniotic cavity ; m, mesodermic layer of 

 amnion ; B, allantois ; I, intestinal cavity of embryo ; 

 Y, yolk-sac ; h, endodermic layer ; e, ectodermic layer of 

 embryo. The embryo is the shaded portion 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. 



bilicus, carrying 



bloodvessels along with it in its mesodermic layer. Still earlier, 

 and, indeed, w r hile the embryo is being separated off from and 

 raised above the level of the rest of the blastoderm by the deepen - 

 ing of the ditch around it, the further banks of this furrow, 

 formed of ectoderm and somatic mesoderm, 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. 445). 

 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 amniotic fluid, is secreted in the cavity which it en- 

 closes ; and the embryo, loosely anchored for the rest of its intra- 

 uterine life by the umbilical cord alone, floats freely within it. The 



64 2 



