EARLY ONTOGENETIC PHENOMENA IN MAMMALS. 129 
by later comparative investigations of the more primitive 
Platyrhines and Arctopitheci. Only lately Strahl (068) has 
recognised the presence of a decidua capsularis in Mycetes, 
a platyrhine monkey ! 
2. The Classificatory Value of the Placenta. 
The short account of diverse placentas in different orders 
of mammals in this and the preceding chapter can have con- 
vinced us of the inadequacy of judging about the more or 
less close agreement of these different placentas by their 
outward shape, as was done in the second half of the last 
century when the distinction of zonary, diffuse, and discoid 
placenta was first proposed, and when at the same time the 
now obsolete subdivision of the Mammalia placentalia into 
Deciduata and Indeciduata came into use. 
The discoid placenta of the mole out of which at birth 
allantoic villi are retracted like so many fingers out of a 
glove and which is further resorbed in situ; the discoid 
placenta of Galeopithecus in which at the outset enormous 
lacunee are filled with maternal blood and which at a later 
period is quite 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; all of them are most intricate and very 
variously specialised, and at the same time essentially very 
temporary productions of these different mammals, the discoid 
shape being of no value whatever when considering their 
respective affinities. 
To allow of the arrangement of the different types of 
placenta in anything like phylogenetic sequence the placen- 
tation of all living mammals should first be investigated and 
made known, and even then it will be very questionable 
whether the mutual relationship can be established to its full 
VOL, 53, PART 1.—NEW SERIES. 8) 
