THE FOSTUS. 1017 



2. The foetus being isolated, the chorion is studied with regard to shape and external 

 aspect. To scl- the relations of its internal face with the other annexes, we may resort to a 

 plan which gives good results with the foetus of Ruminants. This consists in tilling tlie 

 allantois and amnion by small openings with diflferent-coloured fluids. The limits of the two 

 sacs can then be very well made out through the cliorion, while their reciprocal relations can 

 also be observed. With Solipeds, the allantoid cavity must necessarily be opened ; the mem- 

 brane of that name is partially separated from the inner surface of the chorion by blowing in 

 air along the principal divisions of the vessels of the cord, by means of a straw or blow-tube. 



3. To complete the study of the amnion and the allantois of Ruminants, it is best to blow 

 in air beneath the chorion, after removing the contents. The chorion is then carefully taken 

 away, in breaking through — by means of the fingers and forceps — the laminal tissue uniting 

 these two membranes; in the same way are destroyed the fasciculi which bind the allantois 

 to the amnion. In Solipeds, notliing is more easy than to inflate the amnion, and to study 

 its shape and relations. The chorial layer of the allantois is demonstrated as already de- 

 scribed ; a similar procedure is employed to show the existence of the amniotic layer, which is 

 more easily done than with the other. 



4. The vessels of the umbilical cord and placenta should be injected, and to make an 

 injection penetrate satisfactorily, tlie placental vessels, with the foetus and chorion, should be 

 placed in tepid water for some time. 



The annexes of the fcetus comprise : 1. A membranous envelope exactly 



Fig. 554. 



DIAGRAM OF THE FCETAL MEMBRANES OF A MAMMAL. 



pc. Zona pellucida with villi, and sz, the remainder of the extra-embryonal somatopleure con- 

 stituting the chorion; am, amnion; AC, cavity of amnion; UV, umbilical vesicle; al, allantois; 

 ALC, cavity of the allantois ; e, ectoderm, or outer layer of the embryo (epiblast) ; m, mesoblast 

 (or middle layer) ; H, endoderm of the embryo (alimentary canal). 



moulded on the uterus, and known as the chorion. 2. A second ovoid sac in- 

 cluded in the first, and containing the foetus — the amnion. 3. The allantois— o. 

 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 pyriform bladder constituting the umbilical vesicle. 

 5. The placenta — a collection of vascular tufts which graft the young creature 

 to the mother. 6. The umbilical cordy composed of vessels that attach the foetus 



