320 A. A. W. HUBRECHT. 
Maternal blood-corpuscies can already be detected in these 
vascular spaces. Blood-spaces, like those just described, become 
both more numerous and more distinct in the following stage 
(fig. 39), and again seem to congregate in the immediate 
vicinity of the blastocyst. In this stage the trophoblast has 
reached that relative maximum of development in thickness 
that was already discussed in the foregoing chapter, and which 
is characterised by the spaces, sp., as free cavities in the thick- 
ness of its multicellular extension. The traces of the retro- 
grading glands are still present, and whereas these glands are 
encroached upon by the cellular substance of the trophoblast 
as narrated above, the vascular spaces, with their swollen 
endothelium, enter into direct communication with the tropho- 
blastic spaces just alluded to. This is partly represented in 
the lower corner to the right of the blastocyst of fig. 89, and 
was, moreover, studied in numerous sections of this stage, of 
which I have several specimens, all of them cut up in series of 
sections. The most instructive stages in the development of 
the connecting region between the blastocyst and the maternal 
tissue are those that now immediately follow, of which I have 
again numerous specimens. Of this stage figs. 40 and 41 were 
taken, the first just outside of, the second through, the blasto- 
cyst. The central space in fig. 40 is thus a space that is 
immediately contiguous with the blastocyst; its contents bathe 
the blastocyst’s wall. The scale, at which the preparation is mag- 
nified in this figure, is considerably less than that of figs. 38 and 
39, but again corresponds with that of fig. 87, and with that of 
the figs. on Pls. XXIV—XXVI. The embryo itself is in the 
phase of PI. XVI, fig. 15, the trophoblast and subjacent hypoblast 
having already bulged out to a greater size, the relative thickness 
of the former has been somewhat reduced, its intercellular spaces 
have assumed the shape of cuplike depressions between irregular 
protuberances on the outer surface, and the spaces themselves 
have fused with the maternal blood-cavities that were already 
indicated in the foregoing stage. The latter cavities are still 
in an increased degree characterised by a thickened and pro- 
liferating endothelium. This applies both to those which 
