The germ-cells. 679 



In the same memoir Balfour also describes a twofold mode of 

 formation of "permanent ova" from primitive ones. The one of these 

 methods is essentially that set forth by Semper. The other is character- 

 ised by the formation of nests of nuclei and the breaking-down of 

 cell-boundaries. The nuclei of the polynuclear mass increase in size 

 and form delicate vesicles (? vacuoles) filled with fluid. Other changes 

 take place, for an account of which the reader may be referred to 

 the original memoir. 



On reading Balfour's description the writer was so struck with 

 the similarity of these changes to those termed by Flemming ('85) 

 "chromatolysis", to those witnessed in the degeneration of the "mega- 

 spheres" (i. e. primary germ-cells), and to those depicted and described 

 concerning the dying germ-cells of Plate 44, that an examination of 

 later ScyUium- embryos was resolved upon. Balfour's figures un- 

 fortunately throw no light upon the matter. 



The examination of later "embryos" (39 — 70 mm) amply confirmed 

 the suspicion. From this it presently became clear, that in both male 

 and female specimens of Scyllium canicula^ the form studied by 

 Balfour, secondary germ-cells do degenerate with chromatolysis in 

 testis or ovary in very late phases, long after the sex is established. 

 Balfour's description refers to examples of 7 cm in length: the 

 writer has witnessed the degeneration of such cells in specimens of 

 39, 45, 60 and 70 mm. This degeneration was erroneously described 

 by Balfour as a second form of secondary germ-cell-formation. 



Similar degenerative phenomena have been put on record by 

 Flemming in Mammals and Amphibia and by 0. Hertv^ig in Ascaris. 

 There is, therefore, nothing remarkable in the occurrence of this de- 

 generation of germ-cells in Scyllium. The one novel point is, that 

 the process may take place at any period of the life-history, for it 

 is now known to happen in the very earliest phases of the devel- 

 opment, in young and in older embryos, and in sexually immature 



and mature organisms. 



A further conclusion is, that there is in Elasmobranchs one 

 mode, and one only, of the formation of secondary germ-cells and 

 follicles, to wit, that originally described by Semper, and confirmed 

 by Balfour. 



XV. Germ-Cells, Germi)lasni, and Somatic Cells. 



It is a far cry from the fishes to the hydroid polypes or Hydro- 

 medusae. But it is largely owing to Weismann's researches on the 



