364 GENERATION. 



umbilical vesicle, and the allantois. The last two structures 

 are not derived from the external blastodermic layer, and 

 will be described farther on, after we have studied the full 

 development of the amnion and its relations. 



When the allantois has become the chorion, or the exter- 

 nal membrane of the ovum, having taken the place of the ex- 

 ternal layer of the amnion, the structures of the ovum are 

 the following : 1. The chorion, formed of the two layers of 

 the allantois, derived from the internal blastodermic mem- 

 brane, and penetrated by blood-vessels. 2. The umbilical 

 cord, which connects the embryon with the placental portion 

 of the chorion, and the umbilical vesicle, formed from the 

 same layers as the allantois. 3. 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 internal amniotic layer, or the 

 true amniotic membrane, is closely applied to the surface of 

 the embryon, and is continuous with the epidermis at the 

 umbilicus. It is then separated from the allantois by a layer 

 of gelatinous matter ; and in this layer, between the amnion 

 and the allantois, is embedded the umbilical vesicle. At this 

 time, the umbilical cord is short and not twisted. As devel- 

 opment advances, however, the inter-membranous gelatinous 

 matter gradually disappears ; the cavity of the amnion is en- 

 larged by the production of a liquid between its internal sur- 

 face and the embryon ; and, at about the end of the fourth 

 month, the amnion comes in contact with the internal sur- 

 face of the chorion. At this time, the embryon floats, as it 

 were, in the amniotic cavity, surrounded by the amniotic 

 fluid. The amnion forms a lining membrane for the cho- 

 rion ; by its gradual enlargement it has formed a covering for 

 the umbilical cord ; and, between it and the cord, is the atro- 

 phied umbilical vesicle. The amnion then resembles a se- 

 rous membrane, except that it is non-vascular. It is lined by 

 a single layer of pale, delicate cells of pavement-epithelium, 



