464 THE PHYSIOLOGY OF REPRODUCTION 



the embryonic circulation has found the shortest route towards 

 the trophoblast by way of the 'ventral stalk,' trophoblastic 

 lacunae, with their profusion of maternal blood, which have 

 been there from the very earhest periods of development, are 

 exquisitely situated for rendering this new adaptation highly 

 advantageous. And while in the ancestral forms of the Primates 

 both yolk-sac and allantois largely drew upon the trophoblastic 

 source, these embryonic organs come to be dispensed with to a 

 very great extent in their more highly developed descendants 

 who come to use that trophoblastic source along a more direct, 

 a shorter, and an earher estabhshed route." 



Fig. 120. — Median longitudinal section of an early human ovum, 0"4 mm. in 

 length. (From Quain's Anatomy, Longmans.) 



e.ec, embryonic ectoderm ; ch., chorion ; ec, ectoderm ; mes., mesoderm ; 

 all., allantois ; c.s., connecting stalk ; a., amnion ; y.s., yolk-sac. 



In old-world monkeys there is no decidua capsularis. The 

 trophoblast thickens over two discoid areas on the blastocyst, 

 and the thickenings form a primary placenta on the dorsal 

 surface, and a smaller secondary placenta on the opposite 

 aspect. Hence two groups of chorionic villi are developed. 



No unattached blastocyst has yet been obtained. In the 

 youngest specimen of an old-world monkey, Semnopithecus. 

 nasicus, the ovum was attached to the surface of the uterus 

 by large villous processes with mesoblastic cores at the bases. 

 The trophoblast consisted of two layers, the cytoblast, which 

 was much thickened at the tips of the mesoblastic cores, and, 

 externally to it, a syncytium which was blended at the apices 

 with maternal decidua. Over the non-viUous chorion, syncytium 



