EARLY ONTOGENETIC PHENOMENA IN MAMMALS. 141 
much specialised lateral offshoot in the line of development. 
To our picture of the eventual earliest arrangement we 
must yet add that the blastocyst itself must in that ancestral 
form have been characterised—on account of what we have so 
fully discussed in Chapter [V—by a very early local or total 
vascularisation of the trophoblast by means of a connective 
stalk which formed an ab initio connection between the 
embryonic shield and the trophoblast. No free allantois can 
have been present in the very earliest cases ; this must have 
made its appearance only gradually, probably in consequence 
of the vascularisation of the connective stalk having been tem- 
porarily overtaken by the vascularisation in the anterior four- 
fifths of the annular zone of entoderm, where blood and vascular 
tissue was being formed out of the latter (cf. p. 34). The 
vascular area on the umbilical vesicle was thus brought at an 
early period in close contiguity with the vascular maternal 
mucosa and an early omphaloidean placentation may have 
been called forth out of what had primarily been a surface 
of hematopoietic significance in the ancestors. 
At the same time the direct chorionic placentation came to 
be retarded. later, however, overtaking the precocious 
omphaloidean placentation, again it supplanted the latter in 
the later phases of development. ‘This gave rise to the first 
appearance of a free allantois. 
That the partial vascularisation of the trophoblast by 
means of a primitive connective stalk is not merely a hypo- 
thetical possibility is proved by Tarsius, which corresponds 
to a transition stage as here imagined, its blastocyst being 
moreover situated in the uterine lumen. 
The great advance which has been made by the other 
Primates (monkeys and man) is that in these the blastocyst 
becomes attached to the uterus by a more considerable 
surface, and that the resulting placenta—be it single or 
double—is not stalked as in Tarsius but sessile, whereas in the 
Anthropomorphe and in man the very considerable difference 
from Tarsius is this, that the blastocyst quite disappears 
within the maternal tissue, and is by the formation of a 
