24 J. T. PATTERSON 



4. FORMATION OF THE POLYGERM 



The first steps leading to the organization of the polygerm can 

 be observed as early as the 220- to 225-celled stage (fig. 50). As 

 we have already noted, the initial step consists in the differen- 

 tiation of the embryonic cells into two classes. Certain blas- 

 tomeres become transformed into spindle-shaped cells, while 

 others, retaining their polygonal shape, become arranged into 

 groups. The latter constitute the definitive or true embryonic 

 cells. The spindle-shaped cells become drawn out into long 

 processes, which assist in dividing up the egg into its primary 

 divisions. The cells adjacent to the true embryonic cells tend 

 to fuse together. Their intervening walls soon disappear, and 

 thus there is formed about the group of embryonic cells a 

 nucleated membrane (fig. 51, I.M.C.). The entire structure 

 thus formed constitutes a primary mass of the polygerm. 



These changes occur between forty and fifty hours after the 

 egg is deposited. During this time, and for the next few hours, 

 both kinds of cells multiply rapidly. By the time 500 cells are 

 produced the polygerm is well advanced in its organization. 

 Such a stage is shown in figure 54. In this preparation the 

 primary masses are not especially well defined, for many of 

 the embryonic cells are shrunken and loosely arranged, due in 

 part to poor fixation. In some places the cells adjacent to the 

 true embryonic cells have already formed a nucleated membrane 

 (fig. 54, I.M.C.). Protoplasmic strands from these cells are 

 seen extending throughout the egg, in between the formative 

 primary masses (fig. 54, P.S.). 



The next change which takes place in the organization of the 

 polygerm is the lengthening of the egg along its major axis 

 (fig. 55). There also occurs at the same time a change in the 

 staining reaction of the true embryonic cells. They take a very 

 much deeper stain than do the adjacent nuclei and their cyto- 

 plasm (fig. 55, I.M.C.). 



Figure 52 is a detailed drawing of an oblique section of a young 

 polygerm seventy-two and one-half hours old. The details of 

 structure are remarkably clear, making the matter of interpre- 



