vn] YOLK, NURSE CELLS 107 



presumably reach the egg by osmosis or by transference 

 from the surrounding cells. In some animals it has been 

 observed (e.g. in Medusae, Echinoderms and Tunicates by 

 SCHAXEL) that during the early stages of the yolk-deposition 

 chromatin is emitted from the nucleus into the cytoplasm, 

 and it is supposed that this chromatin plays an important 

 part in the formation of the yolk and other substances of 

 the egg. It should be mentioned, however, that other 

 observers have interpreted somewhat similar appearances 

 in the opposite way, and regard the chromatin granules 

 seen in the cytoplasm around the nucleus at various stages 

 of cell-differentiation as chromatin which is being formed, 

 and which will then be absorbed into the nucleus. It has 

 also been maintained that SCHAXEL'S granules are not 

 chromatin, but are mitochondrial in nature, and appear, no 

 doubt in connection with the nucleus, at periods of special 

 metabolic activity, for the process only takes place just before 

 substances are deposited in the cytoplasm, as for example in 

 the growing oocyte before the yolk appears, and in the 

 Echinoid mesenchyme when the calcareous spicules are 

 about to develop. 



sAl though the production of yolk without any special 

 external mechanism is not rare, a large number of animals 

 have definite organs or methods for the supply of the yolk 

 to the growing oocyte. One of the commonest of these is 

 by accessory cells generally known as nurse cells, which are 

 often derived from oogonia indistinguishable at first from 

 those which will grow up into true oocytes. These nurse 

 cells supply nourishment to the growing eggs in various 

 ways. A frequent method is by means of an egg-follicle^ 

 consisting of an envelope of cells enclosing the oocyte. At 

 first these cells are not conspicuously different from the 

 oocyte itself, especially when they arise from oogonia, but 



