DESCRIPTION OF THE FIGURES. 27 



eight weeks' embryos. ISTote that while the yolk sac has 

 remained almost stationary, the absorbing area has dimin- 

 ished, while the capacity of the allantois (all.) has greatly 

 increased. The villi (c.v.) extending from the allantois 

 towards the yolk sac are numerous, long and slender, but 

 still devoid of blood-vessels. Indications of the coming 

 external vascular villi occur in the form of minute dots 

 (t.t.) over the surface of the embryonic sac. The yolk- 

 sac and allantoic stalks have already united at their inner 

 or proximal ends. 

 Fig. 7. Kepresents an eight-weeks' (56 days) horse embryo sus- 

 pended in the amniotic cavity, and bathed by the amniotic 

 fluid. The embryo is connected with the yolk sac (y.s.) 

 and the allantois (only part of which is shown) by rela- 

 tively large blood-vessels. These vessels and their investing 

 tissues form the proximal part of the umbilical cord (sta.). 

 The yolk sac, though having still as great a capacity as 

 formerly, is folded longitudinally, so as to occupy a com- 

 paratively small space. Note that the absorbing area is 

 now very small, and especially that numerous simple villi 

 now project from the surface of the embryonic sac. Each 

 villus consists of a vascular allantoic core, and a thin capsule 

 derived from the embryonic sac. The villi are represented on 

 too large a scale, but the embryo and yolk sac are natural size. 

 7 a . Kepresents three villi from an eight weeks' embryo. 

 7". A single villus from a sixteen-weeks' embryo, and 

 7''. A single villus from a twenty-four-weeks' embryo — all thirty 

 times natural size. 



The villi fit into pits or moulds in the lining of the uterus. 

 As the foetal blood circulates through the villi it comes 

 sufficiently near the maternal blood circulating through the 

 wall of the complex pits or moulds to admit of an exchange 

 of fluids and gases— the foetal blood transfers its excess of 

 carbonic acid to the maternal blood, while the maternal 

 transfers to the foetal blood some of its oxygen, and also 

 fluids containing all the ingredients required for the develop- 

 ment and growth of the embryo. There is, however, no 

 actual mixing of the maternal and foetal blood. At birth 

 the foetal villi are simply withdrawn from the uterine pits. 

 The villi and pits together are generally spoken of as the 

 Placenta. 



