FCETAL NUTRITION: THE PLACENTA 



491 



Hubrecht 1 says : " Once the embryonic circulation has found the 

 shortest route towards the trophoblast by way of the ' ventral stalk,' 

 trophoblastic lacunse, with their profusion of maternal blood, which 

 have been there from the very earliest 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 

 earlier established route." 



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



FIG. 147. Median longitudinal section of an early human ovum, 0'4 mm. in 

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



e.ec.j Embryonic ectoderm ; c/i., chorion ; ec., ectoderm ; mes., mesoderm ; 

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



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 nasims, 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-villous 

 chorion syncytium was absent (Selenka). Spaces, which are in 

 direct communication with maternal capillaries, are present in the 

 syncytium. The most notable characteristic in the decidua is 

 the presence of a glandular secretion in the embryotrophe. In the 



1 See Robinson's Hunterian Lectures, Journ. of Anat. and Phys., vol. 

 xxxviii., 1904. 



