POLYEMBRYONIC DEVELOPMENT IN TATUSIA 591 
entire core of the ectodermal sphere becomes honeycombed by 
these vacuoles, which later unite. In blastocyst No. 332 the 
union of the vacuoles has already made considerable progress 
(figs. 49, 50). Eventually there is produced a large distinct cav- 
ity within the ectodermal mass, thus transforming it into a true 
vesicle. 
I obtained one vesicle which clearly represents a further advance 
in the progress of vacuolization, and yet one in which the com- 
pleted stage of the vesicle has not been attained. Unfortunately 
the specimen became slightly crushed in the course of transporta- 
tion from the field to the laboratory, after it had been fixed and 
partially hardened. I therefore deem it unsafe to base any def- 
inite conclusions upon its structures; but simply give a photo- 
graph of one of the sections (fig. 53), which, in a measure at least 
can be understood after we have considered a normal specimen of 
a little later stage. 
In figure 51 is shown a section of a stage at the completion of 
the ectodermic vesicle. The specimen consists of the following 
structures: (1) an outer layer of trophoblast, which on the lower 
side has become modified into the primitive placenta; (2) an in- 
complete entodermal sac which is connected laterally with the 
trophoblast; and (3) an ectodermic vesicle. 
The trophoblast is composed of a single layer on the upper or 
free surface (fig. 18), but towards the base it thickens into two 
layers of cells, especially on the right side of the figure. The tro- 
phoblast, now in contact with the mucosa, has greatly extended 
its area, as compared with that of specimens Nos. 316 and 332. 
Since it is intended to devote an entire chapter to the subject of 
placentation, we shall not give here any further consideration 
to the lower layer of trophoblast, which is of course concerned 
with placenta formation. 
The entodermal sac is, as already stated, incomplete, remaining 
open on the side turned towards the mucosa. The cells at the 
point where the layer turns out to join the trophoblast are rela- 
tively thick, while those of that portion of the entoderm which 
passes over the ectodermal vesicle have undergone no important 
change. In the extraembryonic cavity are found a few scatter- 
