THE EGG, FERTILIZATION, MATURATION, AND CLEAVAGE 27 



fatty yolk of the tenthredinid egg is formed from the Golgi vacuoles of 

 the oocyte and nurse cell, Payne (1932) in a study of the oogenesis of a 

 number of species of insects representing six orders found no evidence 

 that the Golgi bodies are concerned either directly or indirectly in either 

 yolk or fat formation but that they appear to be the intermediate prod- 

 ucts of metabolism. Vacuoles and Golgi bodies he considered separate, 

 independent structures. There is a wide divergence of opinion regarding 

 the source of both fat and yolk as the works of Gatenby, Woodger, 

 Harvey, Hirschler, Ludford, Nath, King, and others attest. Payne con- 

 cluded that since yolk and fats have no relationships to other visible 

 substances in the cell, the assumption is that they arise independently 

 in the cytoplasm, the nuclei playing no visible part in this synthesis. 

 Neither did he find indications in the insect eggs studied of the trans- 

 formation of mitochondria (chondriosomes) into fats, yolk, fibers, or any 

 other substances or structures. 



In the eggs of many animals a special inclusion, associated with the 

 primordial germ cells, was named the " Keimbahnplasma " by Hasper 

 (1911), or the "Keimxbahn-" or the "germ-track determinant" by later 

 workers. Other names that have been applied to this inclusion are 

 "oosome," "posterior granular plate," "germ-hne determinant," ''germi- 

 nal cytoplasm," "pole disk," "germ plasm," and "polar granules." The 

 term "oosome," first used by Silvestri, because of its simplicity and 

 because its adoption will cause no confusion, we shall use in this text. 

 The substance is characterized by its position near the posterior pole of 

 the egg, its affinity for stains, and its frequently disk- or saucer-like shape 

 (Fig. 6, os). The oosome has been demonstrated in the eggs of a number 

 of insects as well as in those of Sagitta, Cladocera, Sphaerium, and other 

 invertebrates. 



After fertilization the cleavage cells, as they arise, start their migration 

 toward the periphery of the egg. Of the nuclei that reach the posterior 

 pole of the egg one enters the oosome which then breaks up, either in 

 granular form or in fragments, the particles entering the cytoplasmic 

 envelope of the nucleus. The nucleus that enters the oosome becomes 

 the primordial germ cell. With some insects several nuclei enter the 

 oosome, in which case thej^ all develop into germ cells. The fact that 

 in a number of species of animals the oosome in its association with the 

 germ cells can be traced from the time of the segregation of the germ cells 

 until they are lodged in the gonads gave rise to several of the names 

 appUed to the substance. Sachtleben (1918) found that in Chironomus 

 the oosome may be traced from the unfertilized egg through to the growth 

 period of spermatocyte and oocyte of the first order during postembryonic 

 development. Continuity, however, of the oosome through two gener- 

 ations does not exist, the substance being formed anew in the egg. The 



