TBE FCETU8. 



895 



CHAPTEE II. 



THE FCETAL ENVELOPES OF SOLIPEDS. 



We have seen, in the preceding chapter, that the vitelline membrane lined by 

 the serous vesicle forms a complete shell around the developing ovum ; and 

 we have also noted that the layers of the blastoderm, in becoming meta- 

 morphosed, constitute three membranous sacs, which contain the foetus. 

 These various membranes are described as the envelopes or annexes of the 

 foetus. There are also added to these the cord of vessels and the capillary 

 ramifications which establish the relations that exist between the mother 

 and the young creature. 



The annexes of the foetus comprise : 1, A membranous envelope exactly 

 moulded on the uterus, and known as the chorion ; 2, A second ovoid sac 

 included in the first, and directly containing the foetus : this is the amnion ; 

 3, The allantois, a membrane formed of two layers spread over the inner 

 surface of the chorion and the external face of the amnion, as well as 

 covering the parts between these two envelopes ; 4, A small pyrifovm bladder 

 constituting the umhilical vesicle ; 5, The placenta, a collection of vascular 

 tufts which graft the young creature to the mother ; 6, The umhilical cord, 

 composed of vessels that attach the foetus to the envelopes surrounding it, 

 and which ramify in the placental tufts. 



1. The Chorion. (Pig. 422.) 



The outermost envelope of the ovum, the chorion is a vast membranous 

 and perfectly-closed sac, whose shape exactly corresponds to that of the uterus. 



Fig. 422. 



EXTEEIOK OF THE CHOKIAL SAO; MAEE. 

 A, Body ; B, C, Cornua. 



It has a hodv and two cornua. The latter, when inflated, are plicated and 

 bosselated like the cffioum, and are always unequal in volume : that m which 

 the foetus is developed having by far the largest dimensions. , ^ ,^ 



The external face is studded with small red tubercles, formed by the 



