84 A. A. W.. HUBRECHT. 
ventral mesoblast at very early stages. On the surface of 
the umbilical vesicle of both man, monkeys, and Tarsius a 
very intricate net of bulky blood-vessels is developed. To a 
certain extent these blood-vessels may contribute to bring 
about exchanges between the fluids contained in the cavities 
both of the umbilical vesicle and the extra-embryonic ccelom 
outside of it (which is shut off from the exterior by the 
diplotrophoblast) and between the embryonic blood. It is, 
however, not known whether these fluids contain nutri- 
ment that might be of use to the developing embryo, as 
is the nutriment contained in the yolk-sac, say, of Sauropsida. 
Nor is it known whether the uterine lumen of T'arsius contains 
anything resembling uterine milk which could be transferred 
by a special action of the trophoblast cells inside the blasto- 
eyst, and from thence by the blood-vessels of the umbilical 
vesicle be transported towards the embryo. And also for the 
catarrhine monkeys are we yet in the dark as to whether the 
annular zone of non-proliferating trophoblast, which separates 
(see Fig. 132) the dorsal from the ventral placenta is utilised 
in the same direction, and whether in that case the vascular 
net of their umbilical vesicle would effectuate osmotic absorp- 
tion and transportation of materials that had passed from 
the uterine lumen into the extra-embryonic ccelom of the 
blastocyst. For man and the anthropomorphe such a process 
would be in any case excluded, their blastocyst being enclosed 
in a decidua reflexa, and not possessing the ring-like zone 
just mentioned for lower monkeys. 
Under these circumstances considerable doubt must be 
entertained as.to the efficiency of the vascular area of the 
umbilical vesicle for nutritive or respiratory purposes among 
the Primates. A suggestion which I published some 
years ago (Hubrecht, ’99), and which has been independ- 
ently brought forward by Saxer (96) and Spee (’96) should 
then be considered more closely, viz. that this very fine- 
meshed net of considerable calibre has, in the first place, a 
hematopoietic significance.! There is no reason for wonder 
! Tt should be borne in mind that the bone-marrow as focus of hemato- 
