PLACENTATION 105 



hog Gymnura, and rather less completely in the 

 Chiroptera (bats). 



Similarly the discoidal form of the placenta 

 gives no indication of affinity between the widely 

 different mammals which present it. It is, as 

 Hubrecht says, a temporary production, the dis- 

 coidal shape being of no value when considering 

 questions of affinity. He instances the discoid 

 placenta of the mole out of which the allantoic 

 villi are withdrawn at birth like the fingers 

 out of a glove, the young when born being 

 " enveloped in the allantois with the fully extracted 

 villi forming a woolly covering to that foetal in- 

 volucrum" ; the discoid placenta of Galeopithecus, 

 containing lacunae filled with maternal blood and 

 imbedded in the uterine wall ; " the discoid 

 placenta of the rabbit and of Tarsius which, 

 when full grown, is attached to the mother by a 

 stalk of much smaller diameter than the placenta 

 itself; the discoid placenta of the hedgehog and 

 of man, the latter with its loose and floating 

 villi as against the dense trellis -work of villi 

 and trophoblast in the former." 1 



Before leaving the subject of the nutrition of 

 the young of vertebrates, one more matter may 

 receive attention, namely, the secretion of milk 

 that takes place in the mammary glands from 

 which the mammalia take their name. According 



1 A. A. W. Hubrecht, "Early Ontogenetic Phenomena in 

 Mammals," Quart. Journ. Micr. St., vol. liii., 1908, see pp. 70, 113, 

 129, 142, etc. 



