AND AFFINITIES OF TARSIUS. 487 



own wliicli clearly mark it oft" from tlie Anthropoid placenta. 

 Although it foreshadows the latter in the most unmistakeable 

 fashion, it has failed to attain the same level of structural and 

 functional d iflerentiation . 



The mature j^lacenta, of Tarsius, viewed as a whole (cf. fig. 1, 

 PI. I., where it is seen in section) appeal's as a massive knob- 

 shaped or rather cone-shaped structure, measuring, in n)y specimen, 

 10x11 mm. in diameter, which projects freely into the uterine 

 lumen and is attached to the very thin uterine wall in the apical 

 region of the uterine horn by a very short stalk through which 

 the maternal vessels ^iiass. Its distal, slightly concave siirface is 

 clothed by the chorion which marginally is, on the oiie hand, 

 reflected down to invest the remainder of the free surface, right 

 <lown to the stalk, and on the other, is continued on as non- 

 placental chorion, a very thin non-vascular membrane, with whose ■ 

 inner surface the amnion lies in close apposition. In its cone- 

 shaped form and freely projecting character, it contrasts with the 

 sessile, cake-like, discoidal placenta characteristic of the Anthro- 

 poids. These features are dependent on the fact that the placenta 

 develops in relation to a localised knob-like thickening of the 

 subepitlielial tissue of the uterus, the decidual swelling or tropho- 

 spongia. To this, attachment is eftected in the first instance, and 

 it later on projects and serves as an axis round which the cone- 

 .shaped placenta develops. In the process it undergoes progressive 

 degeneration, and only a I'emnant of it is preserved in the stalk- 

 region of the completed placenta. Its presence in the developing 

 placenta no doubt conditions the appearance of the extensive 

 l)lood-extravasation which is sucli a marked feature in the ripe 

 organ (fig. 2). No such conspicuous, localised, decidual swelling- 

 has so far been described in the placental development of any 

 Anthropoid. 



The Tarsius placenta agrees with that of the Anthropoids in 

 that it is deciduate and of the hsemochorial type, i. e., the func- 

 tional placenta consists, except for the maternal blood present in 

 it, exclusively of foetal tissue and the maternal blood circulates 

 through lacunar spaces bounded solely by the foetal trophoblast. 

 This same type of placenta, however, occui'S also in such diverse 

 orders as the Rodentia, Cheiroptera, Insectivora, and Xenarthra, 

 so that this similarity does not carry us very far. A more impor- 

 tant agreement, from our present point of view, lies in the fact 

 that the functional placenta comes to be established as the result 

 of the outg'owth from the mesoderm of the chorion of more or 

 less massive sprouts which grow into the syncytial layer formed 

 by the trophoblast and which branch abundantly to form charac- 

 teristic dendritic villi, in which the foetal vessels are situated. 

 At the same time, the trophoblast provides, apparently in a some- 

 what different fashion in the two groups, an enclosing layer 

 round each villus (including all its branches), whilst its blood - 

 filled lacunae extend so as to form a system of intervillous blood- 

 spaces. Biit there is an important difl^erence in the villi in the 



