VI] 



POLAR DIVISIONS OF INSECTS 



81 



the polar and spermatocyte divisions, it may be mentioned 

 that very unequal spermatocyte divisions occur in the Bee, 

 Aphids and Pediculus. In these cases one centrosome is 

 drawn out at the tip of the finger-like process of the cell, 

 and in the telophase of the mitosis this process is con- 

 stricted off, much as a polar body is separated from an 

 egg-cell. 



In insects and some other animals the polar divisions are 

 of a somewhat different type from that described above, 

 though retaining the same essential features. The difference 



gg$ 



FIG. 9. Stages in the maturation of the egg of the Sawfly Nematus. 

 First polar mitosis, second polar mitosis, and close of second mitosis with 

 egg-nucleus below and three polar nuclei, of which the two inner unite. 

 (From the writer's preparations.) 



consists in the fact that the polar nuclei are not extruded 

 from the egg, but remain within it and usually degenerate 

 in a short time. The first maturation division begins as 

 described above, but the outer pole of the spindle is not 

 quite at the egg margin, and the mitosis gives rise to two 

 groups of chromosomes, one close to the surface and one 

 rather deeper. Each group may form itself into a nucleus, or 

 in some insects the second division follows so rapidly that 

 this does not occur, and two new spindles are formed im- 

 mediately, lying end to end perpendicular to the egg surface, 

 with the two groups of chromosomes from the first division 



D. c. 



