PART II 



CHAPTER XIII 

 OLIGOENTOMATA AND APTILOTA 



COLLEMBOLA 



The Springtail {Isotoma cinerea Nic.) 



Isotoma cinerea is a grayish species, not exceeding 1.5 mm. in length, 

 generally distributed in northern Asia, Europe, and the United States. 

 It occurs in colonies under recently loosened bark of trees and logs. 

 The eggs are sperical, measuring 0.15 mm. in diameter. The nucleus 

 lies near the center of the egg (Fig. 63) surrounded by yolk which contains 

 yolk globules of varying sizes. 



Segmentation of the eggs, as described by 

 PhiUptschenko (1912), is total and at first 

 equal (Figs. 64, 65), the cells dividing mitoti- 

 cally. Each blastomere contains a nucleus, 

 imbedded in a plasma mass which in turn is 

 covered by the yolk. At the 16-cell stage the 

 regularity of the divisions is lost, some of the 

 blastomeres sinking below the surface and 

 thus producing a morula. By the time the 

 64-cell stage is reached, the plasma mass with 

 the enclosed nucleus no longer occupies the 

 center of the blastomere but has moved to its 

 surface. The nucleate plasma masses of the 

 peripheral blastomeres move to the outer 

 surface of the egg (Fig. 65), forming thus a blastoderm in subsequent 

 cleavage stages (Fig. 66), The blastoderm is completed by additional 

 cells migrating to the surface and possibly also by the mitotic division of 

 cells that have already reached the periphery. Cells that have remained 

 behind after completion of the blastoderm (Fig. 67) are the yolk cells, or 

 vitellophags (yc), and also a small mass of contiguous cells, the germ cells 

 (go), which at this stage are not yet morphologically distinguishable from 

 the yolk cells. The segregation of the germ cells occurs possibly as early 

 as the 16-cell stage. It is possible also that some yolk cells may have 

 migrated back into the yolk from the periphery. 



165 



Fig. 63. — Isotoma. Egg. 



