No. 611] GENESIS OF ORGANIZATION OF INSECT EGG 657 



more or less evenly scattered since they cannot be distin- 

 ^ished in later stages in the germ-cell cycle. 



3. Diifcrenhnl Duisions dunuq th< ronnattou ol Xiuse 

 Cells 



There is no evidence of any definite localization of sub- 

 stances or physiological processes in the i)rimordial germ 

 cells when formed, nor do these cells exhibit recognizable 

 polarity or symmetry of 

 any kind. As desciil)cd 

 in preceding pages, they 

 multiply; migrate into or 

 are enveloped by the tis- 

 sues of the embryo; sepa- 

 rate into two groups from 

 which the ovaries on cither 

 side of the body arise ; 

 and then pass throusrh an- 

 other period of multipli- i^/ 

 cation. This brings them \^ 

 to the stage just preceding v 



the growth period. At A- pP, 



this time phenomena occur 



in the ovaries of certain , „J„'_, " J ui-r i- ■ In!'! 'its*""? 



species of insects that oompanymg nurse cells (no. </r germ- 



have a direct bearing upon ^ 

 our problem; these are concerned principally with the dif- 

 ferentiation of oocytes and nurse cells. In Miastor the 

 nurse cells are mesodermal in origin, and a irroup of nurse 

 cells and one oocyte become enclosed within a sheath of 

 epithelial cells (Fig. 8). the ooc\ te increa^- m m/c 

 it elongates, and then for tlic iii'st tunc in its In-torv ex- 

 hibits recognizal>le polai-itv: tlu^ nntcrior end nd loinnii^ 

 the group of nurse cells, ]^)lantv mnv. Iiowfvcr. ]i;no 

 been present from the time the ]>iimo7(lml mini ((P 

 fir=;t formed, correspon<lino: to tliat of tin tM'diti] . 

 The gOTminal \e^icle -oon become^ eoceiitiic, luit \\}i<'tlKr 

 or not tI^^ indicate- tliat bilateial ^^mmetI\ Iia-^ aNo been 



