CH. LVIII.] 



THE F(ETAL MEMBRANES. 



821 



Fig. 636. Human em- 

 bryo of fifth week 

 with umbilical vesi- 

 cle ; about natural 

 size. (Dalton.) The 

 human umbilical ve- 

 sicle never exceeds 

 the size of a small 

 pea. 



- 637. Diagram of 

 fecundated egg- a, 

 umbilical vesicle ; b, 

 amniotic cavity ; c, 

 allantois. (Dalton.) 



the cavity outside the amnion, between it and the false amnion, 

 is continuous with the pleuro-peritoueal cavity at the umbilicus. 

 This cavity is not entirely obliterated even 

 at birth, and contains a small quantity of 

 fluid, which is discharged during parturition 

 either before, or at the 

 same time as the amnio- t> 



tic fluid. 



Allantois. Into 

 this space the allantois 

 sprouts out, its forma- 

 tion commencing during 

 the development of the 

 amnion. 



Growing out from the 

 hinder portion of the in- 

 testinal canal (c, fig. 637), 

 with which it communi- 

 cates, the allantois is at first a solid pear-shaped mass of splanch- 

 nopleur ; it becomes vesicular by the projection into it of a 

 hollow outgrowth of hypoblast. The 

 hypoblast, however, does not extend very 

 far. The mesoblastic part of the allantois 

 very soon becomes vascular, and insinuates 

 itself between the amniotic folds, just de- 

 scribed. It unites with the outer of the 

 two folds (false amnion), which has itself, 

 as before said, become one with the re- 

 mains of external investing membrane of 

 the egg. As it grows, the allantois be- 

 comes exceedingly vascular ; in birds it 

 envelops the whole embryo taking up 

 vessels to the outer investing membrane 

 of the egg, and lining the inner surface of 

 the shell with a vascular membrane, by 

 these means affording an extensive surface 

 in which the blood may be aerated. In 

 the human subject and in other mam- 

 malia, the vessels carried by the allan- 

 tois are ultimately distributed only to 

 a special part of the false amnion, where, 

 by interlacement with the vascular system 

 of the mother, the placenta is developed. 



In mammalia, as the visceral laminae close in the abdominal 



. 638. - Fecundated egg 

 with allantois nearly 

 complete, a, inner layer 

 of amniotic fold ; '. outer 

 layer of ditto ; c, point 

 where the amniotic folds 

 come in contact. The 

 allantois is seen pene- 

 trating between the outer 

 and inner layers of the 

 amniotic folds. This 

 figure, which represents 

 only the amniotic folds 

 and the parts within 

 them, should be com- 

 pared with tiffs. 629, 630. 

 in which will be found 

 the structures external 

 to these folds. (Dalton.) 



