CHAP. X.] THE DEVELOPMENT OF THE CAT. 327 



soft tissue called the decidua, within which the young ovum imbeds 

 itself, and from which it at first derives its nutriment. 



As the ovum grows, however, one portion of the decidua thickens 

 and becomes highly vascular. This is called the decidua .serofina 

 (Fig. 149, »i), and it forms a ring round that part of the uterine 

 cornu in which the ovum lies. The adjacent part of the rapidly- 

 enlarging ovum also becomes specially supplied with blood-vessels 

 from the embryo contained within it, through the intervention of its 

 already-mentioned allantois. Processes from the vascular ring of 

 the choiion (Fig. 147, f) pass into recesses in the vascular ring of 

 the uterus — the serotina decidua, and the two parts form an in- 

 separable interlacement called the placenta (Fig. 147, j;/). The 

 maternal and embryonic blood-vessels, however, nowhere actually 

 communicate, and therefore no intermixture takes place between the 

 blood of the embryo and the blood of the mother. An abundant 

 gaseous interchange, however, is effected between them, the blood 

 of the embryo taking oxygen from, and gi'V'ing off carbonic acid gas 

 into, the maternal blood. Thus the placenta is a temporary and 

 indirect breathing organ for the embryo, which can breathe in no 

 other way, lying, as it does, enclosed in fluid. 



As has been already said, the allantois, with its blood-vessels — 

 called umhilical arteries and teins — stretches itself forth into and 

 across the space within the ovum, till it arrives at the inner surface 

 of the chorion, where it expands, wrapping round the whole embryo 

 with its amnion, carrying its vessels to the vascular portion of the 

 chorion, and so contributing to form the placenta. The junction 

 once effected, the arteries and veins connecting the foetus with this 

 part of the chorion rapidly enlarge. The umbilical arteries convey 

 the impure blood of the foetus to the placenta, where it is purified, 

 as before said, and nourished by the influence of the maternal 

 blood (the two fluids having but thin membrane between them), 

 and is then returned to the embryo by the umbilical veins, which 

 proceed back along the allantois from the placenta. Meantime, 

 while the embryo is thus enclosed in its amnion and nourished by 

 its allantois, the splanchnopleure grows in on all sides in the way 

 before mentioned, so as almost to separate the embryo from the 

 remnant of the yelk in its sac — the umbilical vesicle. The stalk 

 which connects this vesicle with the intestinal cavity of the embryo is 

 short but slender. It is called the riteUine duct or ductus omphalo- 

 entcricus. The umbilical vesicle remains of rather large size as a 

 transversely-elongated sac produced into two horn-like prolonga- 

 tions. It lies between the amnion and the allantois. 



The structure which connects the embryo or fcetus with the 

 placenta is called the umhilical cord. This is made up mainly of 

 the narrow part of the allantois (consisting of the embryonic or 

 foetal arteries and veins with connective tissue) with the \Titelline 

 duct, the whole being bound round and enclosed by a fold of the 

 amnion extending down round it, and being thence reflected over 

 the fcetus (Fig. 147, vc). The umbilical cord is the sole channel 



