STUDIES IN MAMMALIAN EMBRYOLOGY. 325 
of the tissue with which these regions are in immediate con- 
tact. In a stage following upon that of fig. 42, and thus coin- 
cident with the first preparatory processes of the formation of 
the mesoblast in the embryo a difference in size of some of the 
trophospongia-cells, which was already indicated in the stage of 
fig. 42, has become very marked and at the same time localised. 
Cells of larger size are found amongst the general mass of 
smaller ones, but in the peripheral strata of the trophospongia 
they have become especially numerous. Not only is the body 
of the cell larger than that of the majority of the trophospongia- 
cells, but also the nucleus is of comparatively gigantic dimen- 
sions. Sometimes I find these larger cells of the outer layer 
closely packed, the boundary lines assuming a polygonal shape, 
as they often do in the rest of the trophospongia (figs. 43 and 
44), But generally the outer layer is seen to be less compact 
than the inner denser ones, and in the prepared sections the 
large cells have then often an irregular multipolar or ragged 
aspect, which may be due to the mode of preparation, but 
which must still be borne in mind whew we shall come to 
discuss the significance of this layer of cells. 
Besides their larger nucleus these larger cells often contain 
granules in their protoplasm which stain very strongly with 
carmine, and in many of them I have been able to detect cor- 
puscles embedded in the protoplasm which I could not do 
otherwise but identify with nuclei of decidual cells, situated 
outside the boundary line of the trophospongia. The more so 
as certain of these decidual cells are seen to undergo retro- 
gressive phases (fig. 47). The nucleus of these larger cells 
is sometimes constricted, and in a few cases I have observed 
direct division of the nucleus without intervening karyokinetic 
stages. In the preliminary notice referred to in a former 
chapter (p. 293) I have proposed to name these large cells 
deciduofracts. The propriety—physiologically speaking 
—of this name will be discussed in a following chapter. I 
will here only notice that their protoplasm has very often (in 
the preparations in balsam) the aspect as if it were gorged with 
or formed of material of red blood-corpuscles. The latter being 
