488 ON THE ZOOLOGICAL POSITION 



two groups, fox", whereas in the Anthropoid placenta the villi 

 are strongly marlied and individualised structures which project 

 freely into what is practically a continuous blood-sinus formed by 

 the confluence of the intervillous spaces, in Tarsius they are less 

 prominent structures and except at their origin from the chorion 

 cannot be said to be individualised at all, since their enclosing 

 layers of syncytial trophoblast are not individually distinct but 

 are connected with each other by anastomoses, the trophoblast 

 persisting in the form of a syncytial network, in the walls of 

 which the mesodermal villi are enclosed (PI. I. figs. 3 & 4). 



Furthermore, the intervillous spaces, except round the peri- 

 phery of the placenta,, immediately below the chorion, where they 

 have coalesced to form definite blood -sinuses of some size (PI. I. 

 figs. 2 & 3), take the form elsewhere of nariow and tortuous 

 channels, which, indeed, in the deeper paiis of the organ are 

 frequently incompletely hollowed out, many of them being more 

 or less blocked by a light-staining reticular material (PI. 1. fig. 4), 

 no doubt deiived from the breaking down of the trophoblast 

 during the formation of its lacunae, but which has not been com- 

 pletely removed, owing perhaps to the slowness of the circulation 

 in the central region of the organ. Compared with the Anthro- 

 poid placenta, that of Tarsius strikes one as being on a much 

 lower plane of functional efliciency. 



Hubrecht in one of his more recent papers (1908) has himself 

 emphasised the above- described difi'erence in the relations of the 

 villi. He writes :—" The freedom with which they float about 

 in the maternal blood is another characteristic of Man and 

 the Monkeys. In Tarsius and in the Hedgehog their arrange- 

 ment is more that of a suspension in a very delicate and at the 

 same time most intricate treliis-work formed by the trophoblast 

 cells that have become spun out into this. AVhen the connecting 

 trabecuh^e of this trellis-work are suppressed, as we see it in the 

 higher Primates, the surface available for osmotic interchange 

 .is naturall}' increased and the free movements of the villi may 

 also be considered as an advantageous circumstance." Leaving 

 aside consideration of certain other differences in detail in the 

 placentation of the two groups, e. g. in the early development, in 

 the constitution of the investing trophoblastic layer of the villi, 

 and in the occurrence of placental blood-extravasations, we may 

 deduce from the above quotation that Hubrecht regarded the 

 difference between the Tarsitts and Anthropoid placenta as one 

 of degree only and that he looked upon the former as a much 

 less perfect organ functionally than the latter. 



With these deductions, I am in agreement with the reservation 

 that I hold the degree of diffei'ence to lie such as to justify us in 

 definitely excluding Tarsius from the Anthropoid gioup. In its 

 placentation, Tarsius is clearly on the line which leads to the 

 Anthropoids, but it has failed to attain their status, and in this 

 respect, as in so many others, is a true " Halbafie," intermediate 



