EARLY ONTUGENETIC PHENOMENA IN MAMMALS. 103 
mucosa, the maternal epithelium being eroded just where 
the trophoblast is in contaet with it, and finally becoming 
embedded in this mucosa, the mouth of the pit in the 
maternal tissue being at the same time closed by extrava- 
sating blood and by cell proliferation. There can be no 
doubt that in these three early stages the action of the 
trophoblast cells, both towards the maternal epithelial and 
subepithelial tissues, is strongly corrosive (perhaps chemically) 
or phagocytic (cf. ’89, Pl. 22a, 23, figs. 39a, 41). Nor that in 
the now following stages (during which the trophoblast shows 
a very extensive proliferation all around the whole blastocyst) 
it eats its way yet further into the tissues of the maternal 
trophospongia,' and locally destroys the endothelium of fine 
capillaries which then shed the blood they contain into the 
lacunar spaces of this trophoblastic proliferation. These 
lacunar spaces are not of an irregular sponge-like shape. 
When seen in transverse section they are disposed (as cup- 
shaped arcades filled with maternal blood) round the internal 
cavity contained in the blastocyst (Figs. 36 and 37), which, as 
soon as the entoderm has come to line the inner surface of the 
trophoblast, will have become the cavity of the umbilical vesicle. 
Into this cavity the maternal blood which circulates in the 
lacunee of the trophoblast can now with great facility give off 
such substances as may be selected by the trophoblast cells 
that form the separating wall between the maternal blood and 
the cavity of the umbilical vesicle. Between the contents of 
this cavity and the trophoblast cells there is only a thin 
layer of endoderm cells. Later the extra embryonic vascular 
+ The name trophospongia, which I introduced nineteen years ago (’89), 
is here taken in the sense in which I have used it since 1899 (’99, p. 350), 
and in which my pupil Resink (02) has applied it to the hedgehog. It 
indicates maternal cell-proliferation, specially intended for the fixation of the 
blastocyst, and shows a different histological evolution in different genera 
(Sorex, Lepus, Tupaja, Tarsius, etc.). In the hedgehog I formerly called this 
proliferation the decidual swelling (’89, p.311). For the hedgehog the amount 
of foetal trophoblast is thus seen to be yet more considerable than I dared to 
suppose originally, when I mistook part of the embryonic trophoblast for 
maternal trophospongia. 
