356 
ZOOLOGY: HEGNER AND RUSSELL 
follows that the myelin formation is also a function of age. A glance 
at the graphs in Chart 1, and at column (4) in Table 3, will show that 
the most active production of myelin, as indicated by the rapid loss in 
the percentage of water, occurs early, i.e., during the first twentieth 
of the life span in both the rat and man. 
1 Lowrey, L. G., Aizat. Rec, 7, 143-168 (1913). 
2 Donaldson, H. H., /. Comp. Neur. Psychol, 20, 119-144 (1910). 
3 Donaldson, H. H., Ibid., 21, 129-137 (1911). 
4 Donaldson, H. H., Ibid., 21, 139-145 (1911). 
5 Donaldson, H. H., Ibid., 21, 161-176 (1911). 
6 Donaldson, H. H., The Rat, Mem. Wistar Inst. Anat. Biol., Philadelphia, No. 6 (1915). 
7 Koch, W. and Mann, S. A., Arch. Neur. Psychiat. (Mott), 4, 201-204 (1909). 
8 Weisbach, A., Med. Jahrhucher 16, Nos. 4 & 5, 1-76 (1868). 
9 Donaldson, H. H., The Rat, Mem. Wistar Inst, Philadelphia, No. 6 (1915). 
10 De Regibus, C, Torino, Atti. R. Acc. Med. 6, 323-328 (1884). 
" Koch, W. and Koch, M. L., /. Biol. Chem., 15, No. 3, 423-448 (1913). 
DIFFERENTIAL MITOSES IN THE GERM-CELL CYCLE OF 
DINEUTES NIGRIOR 
By R. W. Hegner and C. P. Russell 
ZOOLOGICAL LABORATORY, UNIVERSITY OF MICHIGAN 
Received by the Academy, May 26, 1916 
One of the most interesting and important periods in the germ-celJ 
cycle of certain insects is that during which oogonia give rise to nurse 
cells and oocytes. While such a period does not occur in certain in- 
sects, such as the paedogenetic fly, Miastor, in which the nurse cells 
are of mesodermal origin (Kahle, 1908; Hegner, 1914), perhaps in the 
majority of the members of this class, the growth of the egg is preceded 
by the formation of nurse cells from which the oocyte derives most of 
its contents. 
Many investigators have studied the origin and history of the cellular 
elements within the ovaries of insects, but in only one family, the Dytis- 
cidae, have clearly defined visible differences been discovered between 
the nurse cells and the oocytes at the time of their origin from the same 
mother cells. In the diving beetle, Dytiscus marginalis, Giardina (1901) 
discovered true differential mitoses which result in the derivation of 
one oocyte and fifteen nurse cells from each ultimate oogonium. In this 
case there is a series of four mitoses during each of which one cell divides 
unequally; the larger daughter cell is characterized by the presence of 
an extra-nuclear 'chromatic ring' and leads to the formation of the 
oocyte; the other gives rise only to nurse cells. Giardina supposes 
that this peculiar chromatic ring consists of part of the chromatin 
