14 J. T. PATTEESON 



During the process of fertilization or in the corresponding 

 period of the unfertiUzed egg, the nucleolus gradually moves 

 down from its original position near the middle of the egg 

 (figs. 1 to 5, No.) to the side of the cleavage nucleus (fig. 10). 



3. THE CLEAVAGE STAGES 



a. The first division 



In respect to cleavage, the egg of polyembryonic insects differs 

 from that of the typical insect egg, in that the cleavage nuclei 

 are from the first accompanied by cytoplasmic segmentation. 

 Another point of interest is the fact that the course of develop- 

 ment is in nowise modified by fertilization. The history of 

 cleavage, as well as that of the polygerm, is the same in fertilized 

 and unfertilized eggs. At least, one can detect no difference. 

 The first segmentation spmdle, which is organized about the 

 cleavage nucleus, is devoid of asters at its poles (fig. 11). It 

 takes a position at the extreme posterior end of the egg, with its 

 long axis approximately at right angles to the long axis of the 

 egg (fig. 12). The chromatin divides in a typical manner. The 

 chromosomes soon move to the opposite ends of the spindle, and 

 remain connected for some time by a series of curved interzonal 

 fibers (fig. 13). The daughter nuclei are then reorganized and 

 move in opposite directions, finally coming to rest just inside 

 the cell membrane (fig. 14). The cytoplasmic division follows. 

 It starts as a furrow extending around the posterior end and in 

 a plane practically coinciding with the median longitudinal plane 

 of the egg. Each end of the furrow passes upward for a distance 

 equivalent to a third or a fourth of the length of the egg proper, 

 and then curves to the right and to the left, each branch finally 

 reaching the side of the egg at a point near its middle (figs. 15, 

 16, 37). 



By this manner of division the egg protoplasm is divided into 

 three parts, of which the two at the posterior end are the true 

 embryonic cells or blastomeres. The third, or anterior part, 

 is the polar region or cap, and this contains the polar nucleus. 

 It includes slightly more than one-third the volume of the entire 



