FCETAL NUTRITION: THE PLACENTA 



491 



Huljveclit 1 saj's : "Once the eiiil)V}'<iiiic eirculatioii has found the 

 shortest route towards the tropholilast hy way of the ' \'entral stalk,' 

 tiopholilastic lacuiue, with their profusion of maternal Wood, which 

 have lieen there from the very earliest periods of development, are 

 exquisitely situated f"r rendering this new adaptation higldy 

 advantageous. And while in the ancestral forms of the I'runates 

 li(itli yolk-sac and allantois largely dre\\' u])on the tnipholilastic 

 source, these embryonic organs eorne to lie dispensed with t<i a \'ery 

 great extent iu their miJie highh' de\eloped descendants who come to 

 use that trophoblastic source along a more direct, a shorter, and an 

 earlier established i-oute." 



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



Fig. 14i, — Median longitudinal section of an early human ovum, 0'4 mm. in 

 lengtli. ( h^'oni (^>ir<ii/i'x Aniitoini/, Longmans.) 



«.(•<■., Embryonic ecfcodei'm ; r/;., chorion ; «'., ectoderm ; /«".<., mesoderm ; 

 i(]I., allantois ; ''..<., connecting stalk ; <r., amnion ; //..<., yolk-sac. 



tiTiphoblast thickens o\-(n- two discoid areas on tlic blastocyst, and 

 the thickenings form a ]>rimary placenta on tlie dorsal surface, and a 

 smaller secondary placenta on the opposite aspect. Hence two gi-ou])s 

 (if chorionic villi are de\elo]jed. 



No unattaclied l)lastocyst has yet lieen olitainetl. In the youngest 

 specimen of an old-world monkey, Sr/njidpit/icriis iia.^lciis, the, o\'um 

 was attached to the surface of the uterus liy large \-illous processes 

 with mesolilastic cores at tlie bases. The tropholilast consisted of 

 two layers, the cytolilast, \vhich was nnich thickened at the tips 

 of the mesoldastic cores, and, externally to it, a syncytium which was 

 lilended at the apices with maternal decidua. 0\'er the non-\'illou.s 

 chorion syncytiiun was aljsent (Selenka). Spaces, which . are in 

 direct conununicatif)n with nraternal capillaries, are present in tlie 

 s\-ncvtiuni. The nu.ist notalile chai-acteristic in the decidua is 

 the piresence of a glandular secretion in the endiryotro]ilie. In the 



' See Robinson's Hunterian Ijectures, Jmn-n. of 

 .\.\.\viii., 1904. 



Ainlt. iljlll I'lll/.s., \-ol. 



