18 J. T. PATTEKSON 



c. The third division 



The third set of divisions produces the eight-celled stage, all 

 four cells dividing about at the same time. There is nothing c.on- 

 stant about the arrangement of the spindles -in preparation for 

 this division, and hence we find a great variety of cleavage figures 

 in eight-celled stages. 



Some time before the division is completed, the polar nucleus 

 becomes active and undergoes two divisions. It forms a large 

 spindle which lies at )ight angles to the major axis of the egg 

 (text fig. 1, 0). The two nuclei produced by this division are 

 shown in figure 39, P. These two polar nuclei quickly divide 

 to produce four, which in turn form spindles. A case of this 

 kind is seen in text figure 1, P, in which two of the spindles are 

 in side view and two in polar view. The first two polar-nuclear 

 divisions and the formation of the spindles for the third all occur 

 before the eight-celled stage is reached. During the latter stage 

 the eight polar nuclei, produced by the third division, very soon 

 divide again to form sixteen nuclei (fig. 19). 



It is during the eight-celled stage that another remarkable 

 change also takes place in the polar cap. The protoplasm of 

 this region gradually moves down along the sides of the eight 

 embryonic cells (fig. 19), and finally encloses them by a thin layer 

 (fig. 20). The layer thus surrounding the embryonic cells is 

 destined to play an important role in the formation of the poly- 

 germ. It is in a way comparable to the trophamnios of Ageni- 

 aspis,- but we shall refer to it as the polar region or membrane. 

 In later stages the polar nuclei from the anterior portion of the 

 polar region also move down alongside the embryonic cells, so 

 that the polar membrane becomes nucleated. 



d. The fourth division 



In the fourth division all of the embryonic cells divide, ex- 

 cept the two which contain the nucleolar substance. The result 

 is the production of a fourteen- instead of the typical sixteen- 

 celled stage (fig. 21). In the four-celled stage the nucleolus 

 breaks down and its content spreads around the nucleus (fig. 



