338 A. A. W. HUBREOHT. 
these conditions are more favorable than in many other 
mammals, certainly more favorable than those where the 
blastocyst develops in the uterus lumen without any further 
attachment to the maternal tissue, as in the Didelphia. The 
large amount of albumen which is present round the blasto- 
cyst in many of these (Selenka, Caldwell) may no doubt be 
interpreted as a remnant of Sauropsidian or Ornithodelphian 
arrangements, but the circulating maternal blood is certainly 
a richer source of nutritive material than an albuminiferous 
layer as the one here alluded to. From the fact that in those 
mammals that have reached the highest stage of differentiation 
the maternal circulation is most elaborately adapted to these 
nutritory functions we may, moreover, conclude to the higher 
perfection of this arrangement. It is only natural that the 
question, how this highest stage was gradually reached excites 
considerable interest. The current idea which seems to have 
taken root in modern embryological text-books is that the 
phylogenetic development of the new arrangement, which 
reached its culminating phase in the placenta of man, must 
necessarily have started from an arrangement in which the 
blastocyst, lying free in the uterus lumen, sought to attach 
itself by as many rootlets as possible to the maternal tissue, 
and that a network of maternal capillaries round these rootlets 
or villi was at the same time the starting-point for the maternal 
participation in the process of placentation. It is certain that 
we find similar primitive modes of attachment in the pig, this 
animal being considered by Turner and others to represent a 
mode of placentation that must have been transitory in the 
ancestors of all those that have at present a more concentrated, 
non-diffuse, but discoid placenta, as, for example, man. This 
is apparently confirmed by the fact that in man the chorion 
is villous all over in the very earliest stages, whereas later on 
only the circular part, known as the chorion frondosum, retains 
this character, the chorion leve losing its villi. 
Ina later chapter I will discuss this phylogenetic speculation 
at greater length. Here I must point out that the phase of 
the human blastocyst, in which it is villiferous all over, admits 
