PLACENTATION 103 



The phenomena of placentation, according to 

 Hubrecht, are " intimately related to the higher 

 development which characterises the mammalia 

 as against the lower Vertebrates." This means 

 that the intensive nutrition of the embryo is a 

 prior condition to the development of brain 

 power, and it should be kept in mind as a 

 counterpoise to Gaskell's opinion that the brain 

 power is the primary directive influence in evolu- 

 tion. We may indeed state the case outright 

 by saying that cerebral concentration is the end 

 and not the beginning of evolution. 



The history of the mammalian placenta affords 

 many examples of convergence. Diffuse placenta- 

 tion, in which the surface of the maternal uterine 

 mucous membrane is thrown into a dense net- 

 work of folds and crypts into which correspond- 

 ing folds or villi, diffused over the entire surface 

 of the blastocyst, fit without any strong adhesion 

 to the uterine wall, has usually been regarded as 

 a primitive condition ; and the arrangement seen 

 in the horse and the pig has always been looked 

 upon as the prototype of the diffuse placenta. 

 Hubrecht, however, gives weighty reasons for 

 thinking that it is due to a secondarily simplified 

 process, or descent with simplification, in the 

 course of which the intense phagocytic activity 

 of the trophoblast or outer fcetal envelope had 

 subsided and had given place to a diffused osmotic 



