200 CONTINUITY OF THE GERM-PLASM AS THE [IV. 



veloped animal body ? It is true that we speak of certain cells 

 as being ' of embryonic character,' and only recently Kolliker ^ 

 has given a list of such cells, among which he includes osteo- 

 blasts, cartilage cells, lymph corpuscles, and connective tissue 

 corpuscles : but even if these cells really deserve such a 

 designation, no explanation of the formation of germ-cells is 

 afforded, for the idioplasm of the latter must be widely different 

 from that of the former. 



It is an error to suppose that we gain any further insight into 

 the formation of germ-cells by referring to these cells of so- 

 called ' embryonic character,' which are contained in the body 

 of the mature organism. It is of course well known that many 

 cells are characterized by very sharply defined histological 

 differentiation, while others are but slightly differentiated ; but 

 it is as difficult to imagine that germ-cells can arise from the 

 \ latter as from the former. Both classes of cells contain idio- 

 plasm with a structure different from that which is contained 

 in the germ-cell, and we have no right to assume that any 

 y* i of them can form germ-cells until it is proved that somatic 

 idioplasm is capable of undergoing re-transformation into 

 germ-idioplasm. 



The same argument applies to the cells of the embryo itself, 

 and it therefore follows that those instances of early separation 

 of sexual from somatic cells, upon which I have often insisted 

 as indicating the continuity of the germ-plasm, do not now 

 appear to be of such conclusive importance as at the time 

 when we were not sure about the localization of the idioplasm 

 in the nuclei. In the great majority of cases the germ-cells are 

 not separated at the beginning of embryonic development, but 

 only in some one of the later stages. A single exception is 

 found in the pole-cells (' Polzellen 'j of Diptera, as was shown 

 many years ago by Robin ^ and myself^. These are the first 

 cells formed in the ^%g^ and according to the later observations 

 of Metschnikoff* and Balbiani ^, they become the sexual glands 

 of the embryo. Here therefore the germ-plasm maintains a 



^ Kolliker, ' Die Bedeutung der Zellkerne,' etc. ; Zeitschr. f. wiss. 

 Zool. Bd. XLII. 



^ ' Compt. rend.' Tom. LIV. p. 150. 

 ^ ' Ent\vicklung der Dipteren.' Leipzig, 1864. 

 * 'Zeitschr. f. wiss. Zool.' Bd. XVI. p. 389 (1866). 

 ^ ' Compt. rend.' Nov. 13, 1882. 



