342 A. A. W. HUBRECHT. 
trophoblast tissue to be overfilled with blood, this outer, 
stretched layer will at the same time have reached its maximum 
of tension, and thereby the small openings in it, through which 
‘the blood passes from the trophospongia into the trophoblastic 
lacune (figs. 44, 53) become stretched, i.e. smaller. Sup- 
posing the flow of blood to be thus diminished the tension 
might decrease, and the return to a less overfilled stage be 
brought about. It is easily understood that such a regulating 
apparatus would be most wanted at the time the trophoblast 
undergoes its sudden increase. It is actually in this phase 
that we find the layer s/. best developed. In the ripe placenta 
it leaves only faint traces (fig. 57). 
It must for the present remain a point of controversy whether 
the cells of the layer s/. are derived from maternal tropho- 
spongian or from embryonic trophoblast cells (cf. p. 300). They 
are situated on the boundary line between the two at a stage that 
these two have come to resemble each other in a very confusing 
way, and so I am not as yet justified in giving a definite deci- 
sion. However, I incline towards accepting their trophoblastic 
origin. Fig. 57 will show that certain preparations tend to 
favour this view, but I must repeat that I wish to postpone 
definite judgment on this point. Towards the end of pregnancy 
(fig. 57) the placenta has become a gigantic sponge, in which 
maternal blood circulates. I cannot decide whether this circu- 
lation is slower than that in the placenta of the Primates, where 
the lacunz are more continuous In the hedgehog they are in 
every direction traversed by trophoblastic tissue, in conse- 
quence of which the allantoic villi are not freely suspended in 
the lacunee, nor only attached to the serotina by some of: their 
longer tips, as is the case in the human subject, but are every- 
where sustained and surrounded by the cellular network and 
the meshes of the trophoblast. Reduce this trophoblastic net- 
work of the hedgehog’s placenta and the aspect will imme- 
diately resemble a section of the human placenta much more 
closely.1. The branching of the allantoic villi is more elaborate 
1 A comparison of fig. 57 with Sedgwick Minot’s cut No. 35, on p. 424 of 
