* POLYEMBRYONIC DEVELOPMENT IN TATUSIA 573 
The explanation is rather to be sought in the condition of the troph- 
oblast. If the median sections of blastocysts Nos. 310 and 335, 
which have been photographed at the same magnification (figs. 36, 
38) be compared, it is at once evident that, while the trophoblastic 
cells in 335 are unchanged, remaining relatively thick, as they 
must have been at the close of cell division, those of 310 are very 
much attenuated. The nuclei have become widely separated by 
the stretching of the wall, which in turn has become so thin in 
places that it appears as a delicate line in the photograph. Evi- 
dently this enormous increase of the blastocyst is the result of the 
accumulation of fluid within the cavity of the vesicle, and the con- 
sequent stretching of the wall. Itis a fact worthy of note that one 
never finds the trophoblastic cells undergoing division while the 
blastocyst is free within the uterine cavity. The conclusion to 
which we must come is that the accumulation of fluid in the cavity 
of the vesicle must, at least in some cases, go on quite independ- 
ently of the differentiation of the embryonic mass. 
Turning to the detailed structure of the embryonic region one 
finds that the inner cell-mass is flattened out until it has become a 
lens-shaped structure measuring 0.024 by 0.115 mm., and cover- 
ing an arc on the circumference equal,to 63°, 1’. There has also 
been some increase in the number of embryonic cells, the knob 
now showing 166 cells. 
The point of greatest significance and interest concerns the two 
types of cells of which the embryonic mass is composed. In the 
two preceding blastocysts there were two sizes of nuclei observable 
and here not only does this same disparity still exist, but there is 
also a corresponding difference in the cells themselves. Thus 
there are clearly two types of cells, a large cell in which the nucleus 
is large, and a smaller cell with a comparatively small nucleus. In 
this particular specimen the small cells have a general distribution 
throughout the mass, and appear either singly, or in groups of two 
ormore. The ratio of the small cells to the larger is about as one is 
to three. Thus in the median section there are fourteen large cells 
and five smaller ones. The latter are in three groups: a single 
cell lying on the surface at the extreme left (fig. 8 a), a pair situ- 
ated some distance to the right of this (b and c), and a second pair 
