Gametogenesis, Fertilization and Parthenogenesis 



179 



Chargaff ('42). He finds equal rates of for- 

 mation of free and bound phosphatide 

 and somewhat higher initial formation of 

 vitellin. 



The results of the various experiments 

 strongly support the view that these com- 

 povmds are formed elsewhere in the body 

 and transported through the plasma to the 

 growing oocyte. In the case of the phos- 

 phatides the main site of synthesis appears 

 to be the liver. 



To what extent this situation may hold 

 for the various substances that the oocyte 



and estimate that the nuclei are 512-ploid 

 when they have reached full size. On the 

 basis of this and other considerations 

 Painter ('40, '45a, b) proposes that one of 

 the chief functions of the nurse cells is to 

 supply large quantities of nucleoprotein ma- 

 terial to the egg so that it may be readily 

 available for the formation of chromo- 

 somes in the period of rapid division that fol- 

 lows fertilization. For those species of ani- 

 mals whose oocytes are not associated with 

 nurse cells Painter suggests that the large 

 germinal vesicle assumes this function. He 



Fig. 53. Oocyte and nurse cells in the leech Pisciola. Groups of four or five "oogonia" are set free into the 

 lumen of the ovary and divide into a mass of about 50 cells (A) of which only one, as a rule, becomes an oocyte 

 (B), while the remainder function as nurse cells, contributing to the growth of the oocyte (C) and finally de- 

 generating. (From Jorgensen, '13.) 



accumulates is not known. However, it 

 would not be too surprising if the oocyte 

 were fovmd to be incapable of synthesizing 

 many or most of its stored materials. Pre- 

 sumably the same enzymes that function in 

 synthesis also operate in dissociation, so the 

 lack or inactivation of various enzymes in 

 the oocyte may permit it to perform its 

 storage function more efficiently. 



Nurse Cells and Nucleoprotein Absorption. 

 Another example of an important constitu- 

 ent of the yolk that appears to be largely 

 supplied to the oocyte in rather complete 

 form is nucleoprotein. This is illustrated in 

 oocytes of various species of animals that 

 are associated during their growth with 

 large nurse cells. The nurse cells at first 

 increase in size and then, as the oocyte 

 approaches full size, the contents of the 

 nurse cells are absorbed by the egg (see 

 Fig. 53). During the early stages the nvxclei 

 of the nurse cells increase considerably in 

 volume. Painter and Reindorp ('39) have 

 described a series of endomitotic cycles oc- 

 curring during the growth phase of the 

 nurse cell nuclei of Drosophila melanogaster 



infers that endomitosis also occurs in the 

 germinal vesicle and upon breakdown of 

 the latter at the time of the first maturation 

 division a considerable amount of nucleo- 

 protein is set free in the cytoplasm. This 

 view is opposed by Ris ('45), who interprets 

 the "lampbrush" chromosomes of the ger- 

 minal vesicle as typical diplotene chromo- 

 somes in which there is a great longitudinal 

 growth of the chromonemata and in which 

 the apparent side branches do not represent 

 additional chromosomal material but sim- 

 ply major coils of laterally separated strands. 

 It is, however, of interest to note that the 

 oocyte nucleus remains relatively small in 

 species that have nurse cells, whereas it 

 forms the relatively large germinal vesicle 

 in species that are not provided with such 

 cells. 



Along with their general studies of the 

 nucleic acid metabolism of various kinds 

 of cells, Caspersson and Schultz ('38, '40), 

 Schultz ('41), and Brachet ('47) have con- 

 tributed some interesting observations and 

 experiments relating to the formation of 

 nucleoproteins during oogenesis. Caspersson 



