420 THE PHYSIOLOGY OF REPRODUCTION 



and the walls of the foetal capillaries. In the syncytial tropho- 

 blast, however, the Prussian-blue test is negative (see p. 486). 



At birth the long villi are left in situ and absorbed by the 

 maternal tissues. 



HYRAX. As in the elephant, the placenta of Hyrax has been 

 studied only in isolated specimens, and its development is not 

 known. According to Assheton, 1 the trophoblast is probably 

 thickened all round the wall of the blastocyst, as in the hedge- 

 hog and Man, but there is no appearance of a decidua reflexa. 

 Maternal blood is carried directly to the fetal side of the tropho- 

 blast, where it is close to the foetal vessels, and so may provide 

 nutriment. It then trickles back through a complicated system 

 of lacunae in the trophoblast. 



The placenta is at first diffuse and later zonary. In the 

 mucosa of the placental area the glands disappear early, and 

 a great increase in the inter-glandular stroma occurs, as in 

 Rodents. 2 



RODENTIA. Among the Rodents there are variations in the 

 mode of attachment. It is centric in the rabbit, excentric in 

 the mouse and rat, and interstitial in the guinea-pig. In all 

 the ultimate form of the placenta is discoid. 



It was in Rodents that the proliferation and vascularisation 

 of the trophoblast were first described by Selenka. 3 Later 

 Duval 4 gave a fuller account of the earlier stages, and Hubrecht 

 discovered the same conditions in other orders. 



Rabbit. The fertilised ovum of the rabbit, clothed by the 

 prochorion, reaches the uterus at the beginning of the fourth 

 day after coitus. At first it has no fixed position ; but by the 



1 Assheton, lac. cit. 



2 Hubrecht (Quar. Jour, of Micr. Science, 1908) draws attention to the 

 peculiar position of Hyrax. It has many archaic peculiarities, and has 

 been placed near Kodents, elephants, and Ungulates by different authors. 

 Yet its placental characters resemble those of the hedgehog and Man. This 

 he takes as strong evidence that the type of placenta found in Hyrax, the 

 hedgehog, and Man, diverges less widely from the primitive type than the 

 placenta of Ungulates and Rodents. 



8 Selenka, Keimbldtter und Primitivorgane der Maus, 1883. 

 4 Duval, " Le Placenta des Rongeurs," Journ. de VAnat. et de la Phys., 

 1889-92. 



