242 THE EMBRYOLOGY OF THE HONEY BEE 



quite completely, so that a prolonged discussion would be out of 

 place here. 



A migration of cells from the germ band into the yolk was first 

 claimed by Graber (1871, 1878), and later (i888a) described 

 more completely in the beetle Melolontha. Similar cells were also 

 observed by Korotneff (1885) in Gryllotalpa, and Wheeler (1889) 

 in Leptinotarsa (Doryphora). Voeltzkow (1889) and Noack 

 (1901) have found that in the muscids also cells enter the yolk 

 from the blastoderm. According to Noack some of these sec- 

 ondary yolk cells form true yolk cells as in the bee, while others 

 degenerate. Mecznikow (1866) and Will (1888) mention the 

 migration of cells into the yolk from the posterior pole of the egg 

 in the aphids, but the relation of these to the secondary yolk cells 

 of other insects is not clear. In the Lepidoptera, Schwarze 

 (1899) states that in Lasiocampa cells detach themselves from 

 the anterior end of the mesoderm and wander off into the yolk. 

 Later they leave the yolk and enter the body cavity becoming blood 

 cells. Schwangart (1904), states that the greater part of the 

 anterior mesenteron rudiment becomes resolved into its component 

 cells ; these enter the yolk and become concerned in its absorption. 

 Later they contribute to the formation of the mesenteron. Hey- 

 mons (1895) nas paid special attention to the secondary yolk cells 

 in Forficula, Gryllotalpa and Gryllus, and which he names 

 "paracytes." These cells are first formed from the lateral por- 

 tions of the blastoderm, but may later be seen arising from all 

 parts of the germ band, especially the mesoderm. The one un- 

 mistakable characteristic, distinguishing these cells from 'normal 

 embryonic cells consists, according to Heymons, in "the dis- 

 solution of the nucleus, and particularly in an extremely charac- 

 teristic separation of the chromatin from the other substances 

 contained in the nucleus, especially the nucleolus." In other 

 insects true "paracytes" have apparently thus far been observed 

 only by Schwarze (1899) in Lasiocampa and by Friederichs 

 (1906) in Meloe. The relation of the paracytes to yolk cells of 

 secondary origin observed in other insects is at present uncertain. 



Friederichs (1906) has made a especially intensive study of the 

 cell elements of the yolk in certain beetles. Some of his con- 

 clusions regarding the primary yolk cells have already been men- 



