216 THE CONTINUITY OF THE GERM-PLASM AS THE 



continuity of the germ-plasm could not be assumed in any instance, 

 not even in the simplest cases, where, as in Diptera, the germ-cells 

 are the first-formed products of embryonic development. For even 

 in these insects the egg possesses a specific histological character 

 which must depend upon a specifically differentiated nucleus. If 

 then two kinds of nucleoplasm are not present, we must assume that 

 in such cases the germ-plasm of the newly formed germ-cells, 

 which has passed on unchanged from the segmentation nucleus, is at 

 once transformed entirely into ovogenetic nucleoplasm, and must be 

 re-transformed into germ-plasm at a later period when the egg is 

 fully mature. We could not in any way understand why such a 

 re-transformation requires the expulsion of part of the nuclear sub- 

 stance. 



At all events, my explanation is simpler than all others, in that 

 it only assumes a single transformation of part of the germ-plasm, 

 and not the later re-transformation of ovogenetic nucleoplasm into 

 germ-plasm, which is so improbable. The ovogenetic nucleoplasm 

 must possess entirely different qualities from the germ-plasm ; and, 

 above all, it does not readily lead to division, and thus we can better 

 understand the fact, in itself so remarkable, that egg-cells do not 

 increase in number by division, when they have assumed their 

 specific structure, and are controlled by the ovogenetic nucleoplasm. 

 The tendency to nuclear division, and consequently to cell-division, 

 is not produced until changes have to a certain extent taken place 

 in the mutual relation between the two kinds of nucleoplasm 

 contained in the germ-nucleus. This change is coincident with 

 the attainment of maximum size by the body of the egg-cell. 

 Strasburger, supported by his observations on Spirogyra, concludes 

 that the stimulus towards cell-division emanates from the cell- 

 body ; but the so-called centres of attraction at the poles of the 

 nuclear spindle obviously arise under the influence of the nucleus 

 itself, even if we admit that they are entirely made up of cytoplasm. 

 But this point has not been decided upon, and we may presume 

 that the so-called ' Polkorperchen ' of the spindle (Fol) are derived 

 from the nucleus, although they are placed outside the nuclear 

 membrane l . Many points connected with this subject are still in a 



1 E. van Beneden and Boveri have recently, quite independently of each other, 

 made a more exact study of these ' Polkorperchen ' (' Centrosoma,' Boveri). They 

 show that nuclear division starts from these bodies, although the mode of origin of 

 the latter is not yet quite clear. A. W., 1888. 



