STUDIES IN MAMMALIAN EMBRYOLOGY. 329 
has been entered upon, and when the layer of stretched cells 
(sl.) has developed (see pp. 300, 306) between trophospongia 
and trophoblast (figs. 44, 53, 54), the trophospongia may be said 
to have passed its culmination stage in so far as henceforth the 
greater portion of it undergoes a retrogressive metamorphosis. 
This goes parallel with the retrogressive phases of the tropho- 
blast described on p. 304, and affects the region which we may 
call the omphaloidean trophosphere. The general phases of this 
retrogression are represented in diagrams 33—386, a special 
feature in fig. 45. I will not follow these retrogressive phases 
in any further detail, although very numerous preparations 
are at my disposal. I will only note that a strong flow of 
maternal blood to the incriminated portions of the tropho- 
spongia precedes and coincides with this metamorphosis, and 
that finally, after having passed through this phase of conges- 
tion and retrogression, the membranaceous stage of the 
trophosphere and decidua reflexa characteristic of the later 
phases of pregnancy is reached. 
We must now give a rapid glance at the allantoidean tropho- 
spongia, and then turn our attention to the outer decidual 
layers and the rest of the mucosa. The allantoidean tropho- 
spongia retains, in the later stages of pregnancy, the same 
thickness it had in the earlier stages, or may even be said to 
become thinner if we compare stages figs. 35 and 86 with 32 
and 38. But what gives it the appearance of considerable re- 
duction in thickness is the vast increase of the trophoblastic 
tissue and of the allantoidean mesoblast enclosed in it. It is 
thus, especially by comparison, that it may be said to become 
thinner as pregnancy advances (cf. figs. 54—57). At the 
same time certain of the blood-spaces it contains considerably 
increase in size, and can in sections be detected with the naked 
eye. The distinction between the deeper trophospongian layers 
and the deciduofracts is retained in the allantoidean (or 
placental, as we may also call it) trophospongia to the last, 
only the larger deciduofracts (dd., fig. 57) are much less well 
preserved in sections than are the smaller, deeper cells. As 
will be seen in the following chapter 1 am inclined to ascribe 
