902 



GENERATION. 



bilical vesicle, formed from the same layers as the allantois. 8. The amnion, which is the 

 internal layer of the amniotic fold, persisting throughout the whole of foetal life. 4. The 

 embryon itself. 



During the early stages of development of the umbilical vesicle and the allantois, the 



FIG. 291. Five diagrammatic representations of the formation of the membranes in the mammalia. (Kolliker.) 



FIG. 1 : , a', external layer of the blastoderm; d, vitelline membrane; d', villi on the vitelline membrane ; i, inter- 

 nal layer of the blastoderm ; m, m', middle layer. 



FIG. 2: a', external layer of the amnion; d, d', vitelline membrane; e, embryon; dt*, umbilical vesicle; vl, *, ss, 

 folds of the amnion; dd, m', st, internal layer of the blastoderm ; dd, connection of the embryon with the um- 

 bilical vesicle. 



FIG. 3: d, d', vitelline membrane; vl, internal amniotic layer; e, embryon; ah. amniotic cavity; sh, s 7i, external 

 amniotic layer ; a m, space between the two layers of the amnion ; d d, internal layer of the blastoderm ;df, * t, i, 

 walls of the umbilical vesicle ; d g, omphalo-mesenteric canal; d s, cavity of the umbilical vesicle ; a I, first ap- 

 pearance of the allantois. 



FIG. 4: s/t, external layer of the amnion ; ss, villi of the external layer of the amnion, which has now become the 

 chorion, the yitellirie membrane having 1 disappeared; /*//-, am, internal layer of the amnion; e, embryon; ah. 

 amniotic cavity : d fl, omphalo-mesenteric canal ; d s, cavity of the umbilical vesicle ; a I, allantois ; r, space be- 

 tween the two layers of the amnion. 



FIG. 5: ch,8h,ch, a I, allantois (which has now become the chorion, the external amniotic layer having disap- 

 peared), with its villi; am, amnion; as, amniotic covering: of the umbilical cord; r, space between the amnion 

 and the allantois; aft, amniotic cavity; d s, umbilical vesicle; d g, omphalo-mesenteric canal. 



