STUDIES IN MAMMALIAN EMBRYOLOGY. 305 
peripheral surface of the extra-embryonic celom that is not 
taken up by the placenta. The region is indicated by a thin 
orange line in the diagrams, figs. 36, 35, and 34; it is repre- 
sented from the actual preparation in figs. 44, 48, and 45. It is 
there seen that the constituent cells of this thin somatic layer 
—one cell thick—assume a more cubic shape in these later 
phases, and that at the same time a homogeneous layer is 
developed between it and the trophoblast, this homogeneous 
layer increasing in thickness till the period of birth. It is a cha- 
racteristic feature in the later stages of the foetal membranes. 
I find the homogeneous layer, with the somatic mesoblast 
cells loosely attached to it, even in an after-birth that was 
encountered post partum in the uterus. Fig. 48 gives the first 
indication of these changes in the layer of somatic mesoblast. 
Fig. 45 has reference to the much later stage, when the layers 
external to it, which are still easily comparable and recognisable 
in fig. 48 (Tr., Trs., sl.), have extended and have become 
membranaceous. 
We have now to consider the phenomena of the allantois 
entering into connection with that part of the trophoblast 
which we have termed the allantoidean trophoblast, and which 
is situated antimesometrically.!. The process preludes with the 
metamorphosis of the trophoblast above described. This meta- 
morphosis, although leading to a quite similar result as in the 
region of the omphaloidean trophoblast, offers a few points that 
deserve special mention. 
In the first place, the early stages of the allantoidean tropho- 
blast are somewhat less distinct than those of the omphaloidean, 
because of its origin out of the cell material that overcaps the 
germinal area. These cells undergo a great amount of stretch- 
ing during the early developmental phases (see fig. 205 and 
fig. 51). And only in favorable cases can the allantoidean 
trophoblast be as distinctly made out as it was in the prepara- 
tion of fig. 52, in which it may be said to have attained to the 
1 Even in the rare cases that came to my notice of twins and triplets 
having the yolk-cavity in common, the several embryos were all thus situated 
that they took up the antimesometrical surface, 
