POLYEMBRYONIC DEVELOPMENT IN TATUSIA 581 
tinues until the maximum thickness is reached in such specimens as 
No. 296 (fig. 10), when the embryonic spot again begins to spread 
and continues to increase in diameter until finally the mass is re- 
duced to about two cells in depth (fig. 11). 
This process of migration of the entodermal cells to the lower 
surface of the embryonic knob and their subsequent peripheral 
movement along the inner surface of the trophoblast must take 
place by amoeboid activity. Indeed, the evidence for this con- 
clusion is irresistible. If one examine in the living condition a 
stage somewhat more advanced than No. 296, one finds that 
numerous pseudopod-like processes are radiating out in all direc- 
tions from the embryonic spot. In many instances the connection 
of these processes with some of the outlying entodermal cells is 
clearly discernible. 
In order that a photographic record of this phenomenon might 
be made, glycerin jelly mounts were prepared of several unstained 
blastocysts which exhibited it. A photograph of such a prepara- 
tion is shown in figure 40. In the enlarged view of the embryonic 
spot of the same specimen (fig. 39), the pseudopodia are particu- 
larly clear and striking. The high power of the microscope was 
focused so as to bring the entoderm as sharply as possible into 
view and while the rather thick ectoderm of the central area ob- 
scured the true condition of the entoderm here, yet the periph- 
eral portions are brought out sharply. A small area of these 
anastomosing processes from the lower border of the embryonic 
area is sketched in figure 13. Some of the pseudopodia have 
sharply pointed ends, others have rather blunt ends, and still 
others have flattened terminations. The processes from two or 
more cells frequently anastomose and form a fenestrated struc- 
ture. In sections also the pseudopodia can be demonstrated, 
especially when the section happens to cut one of them length- 
wise (fig. 14). However, it is from the study of the living mate- 
rial and glycerin jelly preparations that one obtains the most con- 
vincing evidence of these pseudopodia, and of the réle they play 
in the formation and migration of the entoderm. 
The segregation of the entodermal cells from the ectodermic 
mass is completed by the time the vesicle has attained a diameter 
