lie J. T. PATTERSON 
knob. In the whole condition the knob appeared as a conical- 
like elevation extending up from the trophoblast. The surface of 
the mass was uneven, due to the projecting embryonic cells (fig. 
36). 
In detail, the embryonic knob is composed of a central core of 
protoplasm surrounded by nuclei (fig. 7). Some of the nuclei ly- 
ing towards the free surface of the mass are completely delimited 
by the cell-membranes, but in the majority of cases that portion 
of the cell membrane which is directed towards the center of the 
mass has faded out. Some of these incomplete cells have two 
nuclei. In the region of the trophoblast the cells have lost all 
traces of membranes, but Rauber’s portion of the trophoblast re- 
tains its distinctiveness from the underlying embryonic mass. 
While the embryonic knob of the preceding blastocyst presented 
when viewed from above, a perfectly sharp contour, the knob of 
this specimen, on the contrary, showed many ray-like protusions 
extending out from the base of the cone along the trophoblast. 
Thus the picture revealed in the upper view of such a mass is that 
of a many pointed star. The spreading of the inner cell-mass 
would seem to be accomplished by the migration of the cells from 
the base. They creep out with pseudopod-like processes, and can 
frequently be seen in the sections. On the left side of the median 
section (fig. 7) one of these processes is seen pushing out along the 
under side of the trophoblast, and on the right of the same section 
another cell is in the act of beginning a similar migration. In con- 
sequence of this spreading, the diameter of the embryonic mass 
has increased to 0.090 mm., as against 0.055 mm. in blastocyst 
No. 287; and the arc on the circumference covered by it is 28°, 57’. 
Blastocyst No. 335. In many respects this is one of the most 
interesting of all the early blastocists in the collection. It is 
easily the smallest, since it measures but 0.220 mm. in diameter; 
and as compared with No. 310, is less than two-thirds the size. 
Nevertheless, the differentiation of the embryonic knob is dis- 
tinctly more advanced than ‘that of the preceding blastocyst. 
What explanation are we to offer then for this apparent disparity 
in size? Is this variation in size of the vesicles to be correlated 
with a difference in the size of the undivided egg? I think not. 
