Chap. VI.] GENERAL EMBRYOLOGY. 



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yolk-sac continually shrinking as its contents are absorbed into, 

 and minister to the growth of, the embryo. But owing to the 

 shrinkage of the yolk-sac (y. s.) the sub-serous space (s. s. c.) 

 gradually extends round the yolk to the lower pole of the egg. 

 And as it does so the serous' membrane is split off from it, and 

 forms a complete layer beneath the vitelline membrane (which 

 is not represented in the ^figure, but- closely adheres to the 

 serous membrane). 



Another important structure, the allantois, has still to be 

 described. From the hinder-end of the digestive tube there 

 grows out, even before the aninion folds have met, a hollow 

 outgrowth. It is lined with hypoblast, but covered externally 

 by a layer of splanchnic mesoblast. It spreads out (Fig. 38, v.) 

 as a flat sac lying over the dorsal and posterior, part of the 

 embryo between the true amnion (omitted in.tke fig.) and the 

 serous membrane. In the fowl a rich network of blood-vessels 

 is developed in it, and it performs the functions of embryonic 

 respiration. Furthermore, in the later stages of embryonic 

 growth it extends outwards and downwards until its lower folds 

 meet below, overlap, and fuse together, thus enclosing the 

 albumen in a space lying between the allantois and the yolk-sac, 

 which has been termed the placental sac. Simple villi developed 

 on the walls of this sac absorb the enclosed albumen partly 

 into the blood-vessels of the yolk-sac, partly into those of the 

 allantois. In the rabbit (Fig. 38, vi.) its mesoblastic layer 

 unites and fuses with the serous membrane. The umbilical 

 vesicle, which has become flattened out (umb.), also fuses with 

 the serous membrane. These two membranes, which become 

 practically continuous, are known as the chorion. That in the 

 dorsal region, formed from the allantois, is called true chorion, 

 that in the ventral region, formed out of the flattened umbilical 

 vesicle, is called false chorion. From the outer surface of the 

 true chorion finger-like processes (villi) grow out and fit into 

 crypts or depressions formed in the wall of the uterus of the 

 mother. Both walls become highly vascular, and nutritive fluid 

 passes from the mother to the foetus, each villus being provided 

 with an artery, a vein, and a capillary plexus connecting the 

 two. There is no transfusion of blood from the mother to the 

 offspring, but a diffusion of nutritive material from the mother, 



