204 THE CONTINUITY OF THE GERM-PLASM AS THE 



heteroplastid condition, and the separation into somatic and repro- 

 ductive cells, have taken place. In Volvox (Fig. Ill) the spherical 

 colony consists of two kinds of cells, viz. of very numerous small cili- 

 ated cells, and of a much smaller number of large germ-cells without 

 cilia. The latter alone possess the power of producing a new colony, 

 and this takes place by the asexual and sexual methods alternately : 

 in the latter a typical fertilization of large egg-cells by small sper- 

 matozoa occurs. The sexual differentiation of the germ-cells is not 

 material to the question we are now considering ; the important 

 point is to ascertain whether here, at the very origin of heteroplastid 

 organisms, the germ-cells, sexually differentiated or not, arise from 

 the somatic cells at the end of ontogeny, or whether the substance of 

 the parent germ-cell, during embryonic development, is from thefrst 

 separated into somatic and germ-cells. The former interpretation 

 would support Nageli's view, the latter would support my own. But 

 Kirchner 1 distinctly states that the germ-cells of Volvox are differ- 

 entiated during embryonic development, that is, before the escape of 

 the young heteroplastid organism from the egg-capsule. We cannot 

 therefore imagine that the phyletic development of the first hetero- 

 plastid organism took place in a manner different from that 

 which I have previously advocated on theoretical grounds, before 

 this striking instance occurred to me. The germ-plasm (nucleo- 

 plasm) of some homoplastid organism (similar to Pandorina) must 

 have become modified in molecular structure during the course of 

 phylogeny, so that the colony of cells produced by its division was 

 no longer made up of identical units, but of two different kinds. 

 After this separation, the germ-cells alone retained the power of 

 reproduction possessed by all the parent cells, while the rest only 

 retained the power of producing similar cells by division. Thus 

 Tolvox seems to afford distinct evidence that in the phyletic 

 origin of the heteroplastid groups, somatic cells were not, as Nageli 

 supposes, intercalated between the mother germ-cell and the daughter 

 germ-cells in each ontogeny, but that the somatic cells arose 

 directly from the former, with which they were previously identical, 

 as they are even now in the case of Pandorina. Thus the con- 

 tinuity of the germ-plasm is established at least for the beginning 

 of the phyletic series of development. 



' * Compare Biitschli in Bronn's ' Klassen und Ordnungen des Thierreichs,' Bd. I. 

 P- 777- 



