THE DESCENT OF THE PRIMATES 15 



There is no interlocking of maternal and foetal 

 surfaces. There is no loose attachment between 

 two extensive vascular expansions. But there 

 is a very perfect, sharply defined, and compli- 

 cated organ by means of which the foetus 

 anchors itself, so to say, into the maternal tissue. 

 This organ is called the placenta. At the time 

 of its very early origin, when the young Tarsius 

 has only just started on its development, this 

 placenta is seen in its true nature as an embry- 

 onic proliferation. The small embryonic vesicle 

 may be said to scoop out a circular spot of the 

 mother's tissue and then and there to attach 

 itself most firmly, more firmly than a leech or a 

 bloodhound, to the inner surface of its mother's 

 womb. The blood which under other circum- 

 stances would flow from a wound thus made, 

 is carefully stored and conducted by the pro- 

 liferated embryonic tissues which after some 

 time succeed in establishing a very complex 

 spongelike, cavernous structure of purely embry- 

 onic derivation, in the cavities of which maternal 

 blood freely circulates. The sohd meshwork, on 

 the other hand, eventually carries very fine and 

 very numerous embryonic blood-vessels, which 

 then find themselves bathed in maternal blood. 

 The way in which this result has been attained 



