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 liere, 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 role 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 



