2o8 COXTINUITY OF THE GERM-PLASM AS THE [IV. 



The sexual differentiation of the germ-cells is not material to 

 the question we arc now considering; the important point is 

 to ascertain whether here, at the very origin of heteroplastid 

 organisms, the germ-cells, sexuall}' differentiated or not, arise 

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

 substance of the parent germ-cell, during embryonic develop- 

 ment, is from the first separated into somatic and germ-cells. 

 The former interpretation would support Nageli's view, the 

 latter would support my own. But Kirchner ^ distinctly states 

 that the germ-cells of Volvox are differentiated during em- 

 brj'onic development, that is, before the escape of the young 

 heteroplastid organism from the egg-capsule. We cannot 

 therefore imagine that the phj-letic development o^ the first 

 heteroplastid organism took place in a manner different from 

 that which I have previousl}^ advocated on theoretical grounds, 

 before this striking instance occurred to me. The germ-plasm 

 (nucleoplasm) of some homoplastid organism (similar to Pan- 

 (iorina) must have become modified in molecular structure 

 during the course of phjdogeny, 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 Volvox seems to 

 affc)rd distinct evidence that in the phj^letic origin of the 

 heteroplastid groups, somatic cells were not, as Nageli sup- 

 poses, 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 Pan- 

 dorina. Thus the continuity of the germ-plasm is established at 

 least for the beginning of the phyletic series of development. 



The fact, already often mentioned, that in most higher or- 

 ganisms the separation of germ-cells takes place later, and 

 often very late, at the end of the whole ontogeny, proves that 

 the time at which this separation of the two kinds of cells took 

 place must have been gradually changed. In this respect the 

 well-established instances of early separation are of great 



' Compare Butsclili in Bronn's ' Klasscn und Ordnungen des Thier- 

 reichs,' Bd. I. p. 777. 



