STUDIES IN MAMMALIAN EMBRYOLOGY. 313 
histological modifications in the region affected have set in. 
Among these must be noticed, in the first place, the occlusion 
of the lumina of the glands, the glandular epithelium remain- 
ing distinct in the earlier stages, but gradually disappearing. 
This occlusion affects both the mouth of the gland and the 
greater part of its course; the coiled terminations in the 
deeper parts of the mucosa are, however, much less visibly 
affected (fig. 2); whereas, close to the mesometrium the glands 
(which are here shorter) retain their normal character and 
their opening till much later (cf. figs. 2 and 37). 
Parallel with the gradual disappearance of the glands there 
is to be noticed a strong development of the vascular channels 
and capillaries in the swollen region. In the earlier stages, 
such as those of Pl. XV, fig. 2, and of Pl. XX, fig. 37, this 
development is in no way manifest by an increase in the 
number of distinct lumina, but in favorable preparations it 
may be noticed that processes preparatory to the formation of 
vascular lumina are actively going on. The smaller arterioles 
running in a centripetal direction from the muscular region 
into the mucosa send out delicate ramifications, from the ends of 
which vasifactive outgrowths (often recognisable by a different 
degree of absorption of the staining solution) penetrate in all 
directions between the proliferating interglandular connective 
tissue. These delicate ramifications are not as yet distinctly 
hollow tubes (fig. 37). In the early stages this active increase 
of vascular and vasifactive tissue does not in any way diminish 
the compactness ef the proliferating decidual region of the 
mucosa; rather the contrary. One particularity of this region 
is very easily understood from the condition under which the 
proliferation comes about, viz. the presence of a special de- 
pression on the surface of the proliferating region that faces 
the uterus lumen. This depression is not a cylindrical pit (as 
might erroneously be concluded from an isolated transverse 
section, such as the one represented in fig. 2 of Pl. XV), but 
a longitudinal groove, continued towards both ends into that 
fold of the mucosa of the unimpregnated uterus which is dia- 
metrically opposite to the mesometrium. It owes its origin to 
