The Development of Scyphomedusse. 237 



AURELIA FlAVIDULA 



Oogenesis 



Sections of the ovary of breeding medusae showed that young 

 germ cells were still quite abundant, as well as quite a consid- 

 erable number of older eggs, though the majority of the latter had 

 been discharged. The eggs in the ovary secure their nourishment 

 through the germinal epithelium to which they remain attached 

 until mature ; no absorption of other cells takes place. 



In Tubularia crocea it was found (G. T. Hargitt, 1909) that the 

 stages immediately following the last oogonial division were of 

 great interest for it appeared probable that a synapsis occurred at 

 that time. Attention given to similar stages in Aurelia gave the 

 following results. The last oogonial division, which is mitotic, 

 gave rise to small oocytes in which the chromatin arrangement is 

 not easily made out. There appears to be a spireme more or less 

 massed together and concentrated toward one side of the nucleus 

 and perhaps arranged in loops; no instance was found, however, 

 in which this could be certainl}^ determined. In older oocytes 

 (fig, 25) the chromatin was in a thread for only a short time, but 

 all trace of a polar arrangement (if any existed) had been lost. 

 The spireme disappears soon after growth begins and does not 

 again reappear, in the eggs within the ovary, nor does a synapsis 

 occur during the growth of the egg, for the stages of this period 

 are sufficiently abundant to make it highly improbable that 

 such a condition, if present, would be overlooked. One may only 

 conclude then that if a synapsis occurs in Aurelia it probably 

 takes place in the oocyte just before growth begins. 



The changes in the chromatin of the germinal vesicle during 

 the growth period involve, (1) the disappearance of the spireme, 

 already noted, and the formation of a reticulum; and (2) the con- 

 densation of the chromatin near the end of the period and the for- 

 mation of the chromosomes. The reticulum in the early growth 

 period (fig. 25) is wide meshed and the chromatin is mostly assem- 

 bled into a few large, rather dense, flocculent masses. In older 



