664 LECTURE XXIII. 



instances of this. In such a case we can only assume that 

 some of the adult somatic . cells of the leaf have returned to 

 the embryonic condition. 



We will now discuss the significance of the processes 

 which accompany the development of reproductive cells, and 

 which lead to this result, that the original nuclear substance 

 of the mother-cell does not all go to form the nuclear sub- 

 stance of the one or more reproductive cells to which it gives 

 rise, but that a portion of the original nuclear substance is, at 

 some stage or other, excluded from taking part in the formative 

 process. We have seen that, in the case of spores, the pro- 

 cess is a comparatively simple one, consisting merely in the 

 extrusion of a portion of the nuclear substance of the mother- 

 cell, whereas, in the case of gametes, it is more complicated, 

 involving at least nuclear division, and not uncommonly cell- 

 division. 



With regard to the extrusion of the paranucleolus from 

 the spore-mother cell, we have no direct evidence to prove 

 that it is essential to the differentiation of the spores as re- 

 productive cells, but the peculiarity of the process is emi- 

 nently suggestive. Strasburger suggests that the extrusion 

 of the paranucleolus is the expression of the return of the 

 cell to the embryonic condition ; but, if we consider that 

 spores are specialised reproductive cells, differing from the 

 somatic embryonic cells of the plant in that they add neither 

 to the tissues nor the organs of the parent, but develope into 

 distinct organisms, we are led to conclude that the extrusion 

 of the paranucleolus, if it means anything, means more than 

 this. The significance of the process is probably this, that it 

 marks the differentiation of a reproductive from a somatic 

 cell. We may put the case in this way, that if the extrusion 

 of the paranucleolus did not take place, the division of the 

 spore on germination would lead to the production, not of the 

 body of a new organism, but of mere repetitions of itself. It 

 may be that the extrusion of the paranucleolus is the ex- 

 pression of the elimination of what Weismann terms the 

 histogenic nucleoplasm. 



Passing now to the consideration of the significance of the 



