DEVELOPMENT OF PARACOPIDOSOMOPSIS 21 



large cell (marked X) is embedded in the polar cap. In some 

 eggs two or even more such cells may be fomid. The first im- 

 pression one gains on examining such preparations is that these 

 have been organized about polar nuclei from the polar protoplasm. 

 But a detailed study of several eggs showing a similar condition 

 has convinced me that these cells have separated from the main 

 mass of embryonic cells and have pressed up into the plastic 

 polar region. A very clear case, which supports this interpreta- 

 tion, is shown in figure 49. The two large cells (X) have their 

 upper portions embedded in the polar cap, while their under 

 surfaces are still connected with the other embryonic cells by 

 means of protoplasmic strands. 



In stages yomiger than this one it is not unusual to find several 

 large blast omeres Ijdng in contact with the under surface of the 

 polar cap (figs. 45, 47). Silvestri ('06) has noted a similar group 

 of cells in the egg of Litomastix, and attributes to them an 

 important significance; but it seems more reasonable to suppose 

 that they owe their large size to their proximity to the polar cap 

 which undoubtedly serves as a nutritive organ to the growing 

 embryonic cells. 



Figure 50 is the final morula-Uke stage that we need consider. 

 It has 221 cells which form a sohd spherical mass. Some of the 

 cells are spindle-shaped, others are polygonal. The latter class 

 is frequently grouped together (fig. 50, F). In one region of 

 the egg a group of polygonal cells has become transformed into 

 a nest or cyst, in which the core consists of several cells surrounded 

 by a layer formed by the fusion of spindle-shaped cells. The 

 central group is made up of the true or definitive embryonic cells 

 (fig. 51, D.E.C.). The outer layer becomes syncytial in charac- 

 ter (fig. 51, I.M. C), and finally forms the inner membrane 

 of the primary mass and then derivatives in the polygerm. 



In this stage the polar membrane is of equal thickness about the 

 entire embryonic mass, and its nuclei are fairly evenly distributed. 

 At certain points mesenchyme cells, derived from the host 

 tissue, adhere to its outer surface (figs. 49, 50, M.C.). In most 

 eggs these mesenchyme cells are isolated, although in one case 

 they formed a membrane over about hah the circumference. 



