138 J. T. CUNNINGHAM. 



Riickert's investigations were carried out principally on 

 ovaries of Pristiurus at Naples, though he also examined 

 ovaries of Scyllium and Torpedo. For fixing he employed 

 Hermann's osmic mixture, sublimate, and sublimate with 5 

 per cent, acetic. The youngest ova he describes are 28 /j. in 

 diameter. lu these eggs a membrane was not seen around the 

 germinal vesicle in material fixed with sublimate, but was 

 distinct enough in osmic preparations. The cavity of the 

 vesicle contained some small gleaming nucleoli, and a distinct 

 and still stainable chromatin reticulum. The latter consisted 

 of separate unbranched chromosomes of fairly uniform thick- 

 ness and bent into undulating curves. These formed a tangle 

 (knaiiel). In this respect the structure of the germinal 

 vesicle differed from that of the ordinary resting nucleus, and 

 approached the tangle phase of mitosis ; from which, however, 

 it was distinguished by the less compact form of the chromo- 

 somes. There was never in the growing egg a branched chro- 

 matin network like that which can be demonstrated in the 

 ordinary resting nucleus, although this period in the history 

 of the egg extends over a long period of time ; in the case of 

 eggs which ripen late in the fish's life, over several years. 



Riickert was not able to count the chromosomes at this 

 stage exactly, but estimated their number at thirty to thirty- 

 six, which was about the same as in the somatic cells of the 

 embryo of the same species. The chromosomes in the ger- 

 minal vesicle were not all of the same size, some being remark- 

 ably small and slightly stained. 



The first period of the development of the egg is from the 

 stage described above to that of eggs which are 1^ to 2 ram. 

 in diameter, in which the germinal vesicle is at its maximum 

 size, namely, mm. in diameter. The membrane of the 

 vesicle becomes thicker, and is thrown into grooves and folds, 

 which last, if examined in section, might be taken for outgrowths 

 of the vesicle. The nucleoli increase in number and size, and 

 after being at first chiefly peripheral, gradually aggregate in a 

 particular part of the vesicle, usually that towards the surface 

 of the egg. The network of the vesicle becomes almost in- 



