804 



GENERATION. 



structures of the ovum are the following : 1. The chorion, formed of the 

 two layers of the allantois and penetrated by blood-vessels. 2. The umbili- 



--ch 



sh 



dg 



ch 



FIG. 295. Five diagrammatic representations of the formation of the membranes in the mammalia 



(Kolliker). 



1 : a, a', epiblast ; d, vitelline membrane ; d', villi on the vitelline membrane ; i, hypoblast ; m, m', 

 mesoblast. 



2 : a', external layer of the amnion ; d, d', vitelline membrane ; e, embryon ; d! s, umbilical vesicle ; v I, 

 k s, s s, folds of the amnion ; d d, m'. s t, hypoblast ; d d, connection of the embryon with the um- 

 bilical vesicle. 



3 : d, d', vitelline membrane ; vl, internal amniotic layer ; e, embryon ; ah, amniotic cavity ; sh,sh, 

 external amniotic layer ; a m, space between the two layers of the amnion ; d d, hypoblast ; d/, s t, 

 i, walls of the umbilical vesicle ; d g, omphalo-mesenteric canal ; d s, cavity of the umbilical vesicle ; 

 a I, first appearance of the allantois. 



4 : s h, external layer of the amnion ; s z, villi of the external layer of the amnion, which has now be- 

 come the chorion, the vitelline membrane having disappeared ; h h, a m, internal layer of the am- 

 nion ; e, embryon ; a h, amniotic cavity ; d g, omphalo-mesenteric canal ; d s, cavity of the umbilical 

 vesicle ; a Z, allantois ; r, space between the two layers of the amnion. 



5 : c h, s A, c h, a I, allantois (which has now become the chorion, the external amniotic layer having dis- 

 appeared), with its villi ; a m, amnion ; a s, amniotic covering of the umbilical cord ; r, space between 

 the amnion and the allantois ; ah, amniotic cavity ; ds, umbilical vesicle ; dg, omphalo-mesenteric 

 canal. 



