608 DEVELOPMENT OF THE EGG IN HUMAN SPECIES. 



CHAPTER X. 



DEVELOPMENT OF THE EGG IN THE HUMAN 

 SPECIES. FORMATION OF THE CHORION. 



Fig. 213. 



have already described, in a preceding chapter, the manner 

 in which the outer lamina of the amniotic fold becomes adherent 

 to the adjacent surface of the vitelline membrane, so as to form 

 with it but a single layer ; and in which these two membranes, thus 

 fused and united with each other, form at that time the single ex- 

 ternal investing membrane of the egg. The allantois, in its turn, 

 afterward comes in contact with the investing membrane, and lies 

 immediately beneath it, as a double vascular membranous sac. In 

 the egg of the human subject the development of the membranes, 

 though carried on essentially upon the same plan with that which 

 we have already described, undergoes, in addition, some further 

 modifications, which we shall now proceed to explain. 



The first of these peculiarities is that the allantois, after spread- 



ing out upon the inner surface of 

 the external investing membrane, 

 adheres to, and fuses with it, just 

 as the outer lamina of the amni- 

 otic fold has previously fused 

 with the vitelline membrane. At 

 the same time, the two layers be- 

 longing to the allantois itself also 

 come in contact and fuse toge- 

 ther; so that the cavity of the 

 allantois is obliterated, and instead 

 of forming a membranous sac con- 

 taining fluid, this organ is convert- 

 ed into a simple vascular membrane. 

 (Fig. 213.) This membrane, 



moreover, being, after a time, thoroughly fused and united with the 

 two which have preceded it, takes the place which was previously 



HUMAN Ov0M, about the end of the first 

 mouth ; showing formation of chorion. 1. 

 Umbilical vesicle. 2. Amnion. 3. Chorton. 



