No. 611] GENESIS OF ORGANIZATION OF INSECT EGG 647 



ules, thus differing in content from the blastoderm cells. 

 They differ also from blastoderm cells in size, being con- 

 siderably larger. This is probably due at first to the in- 

 clusion of the pole-disc granules, but later a greater dif- 

 ference in size is brought about by the failure of the germ 

 cells to divide as rapidly as do the blastoderm cells. 



An early origin of the primordial germ cells in a simi- 

 lar manner has been described in a number of insects be- 

 sides Coleoptera, especially in the midge, Chirouomus,'' 

 where these cells are derived from one of the cleavage 

 nuclei at the four-cell stage; in the pedogenetic larva of 

 the fly, Miastor,^ where one of the first eight cleavage 

 nuclei becomes the nucleus of the primordial germ cell, in 

 the fly, Callipliora,^^ and in parasitic Hymenoptera.^^ 



5. The Formation of the Ovaries 

 In chrysomelid beetles, CMronomus, Miastor and cer- 

 tain other species of insects, the primordial germ cells 

 undergo a multiplication period shortly after they are 

 formed. This is followed by a period during which they 

 become lodged within the embryo- citlior l>y tlic shifting 

 of the surrounding tissues or by Miiurnlion <>r l»y both 

 these processes. At this time also they Ix'coinc >c'i)arated 

 into two groups; in ehrysomelid beetles each giou]) ap- 

 l^ears to contain tliiity two germ cells; in Miastor each 

 consists of four germ colls. One group becomes located 

 on either side of the embryo and later gives rise to one 

 half of the germ glands. The sex of the individual can 

 be determined by the morphology of the germ glands be- 

 fore the young hatches. 



The further history of the germ cells in female insects 

 is in general as follows. From each ]iriniitivc ovary a 

 number of ovarian tubules arise cacli (Muitainiug many 

 germ cells (oogoiiia) which have un<lergoiie a iiniltiplica- 

 tion period. The oogonia finally cea>e divi.liiig and the 



