The Chorion 355 



and urea aud is, consequently, similar in character to the amniotic 

 fluid, except that no meconium can reach this cavity since only 

 the urinary tract communicates with it. 



In the fetus the urinary secretions may pass either backward 

 through the urethra into the amniotic cavity or forward through 

 the urachus into the allantoic cavity and, consequently, through 

 this tract the two cavities communicate throughout intra-uterine 

 existence. In the human embryo there is virtually no allantoic 

 sac except for a very brief time early in pregnancy, the walls of 

 the allantois soon coming in immediate contact. 



Collectively, these membranes, with the addition of the vesti- 

 gial remains of the vitelline sac, constitute the fetal membranes, 

 or after-birth. 



The Chorion. 



The external or vascular layer of the allantois, with the ex- 

 ternal or false amnion, constitutes the permanent or allantoic 

 chorion. In the brief interval elapsing between the formation of 

 the amnion and the allantois, the external or false anniion per- 

 forms temporarily the functions of the placenta by throwing out 

 placental tufts which acquire intimate attachments with the 

 muco.sa of the uterus, maintain the attachment and fixation 

 of the embryonic ma.ss in its position and provide an avenue for 

 nutrition and excretion. Later, when the vascular layer of the 

 allantois spreads over the inner surface of the false amnion and 

 fuses with it, the allantoic tufts push out into the existing 

 amniotic structures, acquire intimate relations with the uterine 

 mucosa and lead finally to the displacement or effacement of the 

 amniotic chorion. 



The form, extent aud relations of the allantois in different 

 animals present the greatest variability. In the mare, as indicated 

 in Figs. 74 and 75, the allantois grows completely around the 

 amnion and constitutes a complete double envelop, except for the 

 area occupied by the vestigial yolk sac. It thus completely 

 separates the amnion from the false amnion or amniotic chorion, 

 its sac being filled with the allantoic fluid. Its internal surface 

 is lined with endobla.st from the enteric canal, while its exterior 

 layer is of splanchnopleuric mesoblast. A study of Fig. 75 shows 

 that the somatopleuric mesoblast, or outer layer of the true 

 amnion, is in contact with the splanchnopleuric mesoblast of 



