STUDIES IN MAMMALIAN EMBRYOLOGY. 299 
portion has set in. As soon as the dise of embryonic epiblast 
is actually split off no confusion between “ epiblast,” s. str., 
and ‘ trophoblast ” is any more to be feared. 
With the further development of the hedgehog’s embryo we 
may distinguish that portion of the trophoblast against which 
. the vitelline circulation is applied from the mediodorsal portion 
against which it is not. The former I will call omphaloidean, 
the latter allantoidean, trophoblast; their distinction by these 
two adjectives being merely proposed for convenient reference 
to the different regions of the trophoblast. On Pl. XVI the 
letters tr. (o.) and ¢7. (a.) are used to distinguish them. In 
the early stages figured on Pl. XVII, figs. 26, 27; Pl. XXII, 
fig. 89, and in the diagrams 29 and 80, the hypoblast is closely 
applied against the trophoblast. Their relative aspect and 
position is represented more in detail in figs. 41 and 42. Soon 
the mesoblast is inserted between them (cf. diagrams 31 and 
32), the splanchnic portion developing blood-vessels and blood 
(fig. 43, m. sp.), the somatic portion being an attenuated 
membranous cell-layer (fig. 48, m. som., 51 and 52), and 
forming, together with the trophoblast, the diplotrophoblast. 
Even in the region where the formation of the extra-em- 
bryonic coelom (“‘ausserembryonale Leibeshéhle”) has not 
extended, because the somatic and splanchnic layers have not 
separated, the presence of a separate thin cell-layer of somatic 
mesoblast can yet be distinctly made out (cf. diagram 32 with 
fig. 43). But at the same time it can always be demonstrated 
that this somatic layer is a very insignificant structure when 
compared to the more massive trophoblast, the cells of which, 
instead of being flattened as are the somatic mesoblast cells, 
are more bulky with distinct granular protoplasm. Com- 
parison of figs. 43 and 52 shows that this applies as well to the 
allantoidean as to the omphaloidean diplotrophoblast. 
This striking difference between somatic mesoblast and 
trophoblast becomes still more accentuated in the next de- 
velopmental phases, represented by diagrams 33, &c. We 
then see that the villiferous trophoblast of figs. 20, 43,51, and 
52, undergoes a series of very important changes. ‘The cells 
