372 AMPHIMIXIS OR ESSENTIA I MEANING OF [XII. 



Platner found that such parthenogenetic eggs, Hke those which 

 require fertihzation, expel two primary polar bodies. 



From this it is clear that parthenogenesis is possible, even 

 when the quantity of germ-plasm in the ^%^ has been reduced 

 to half. Rolph, in his day, attributed parthenogenesis to better 

 nourishment ; Strasburger surmised, in adapting these thoughts 

 to the significance of nuclear substance, which had in the mean- 

 time been recognized, that ' favourable conditions of nutrition 

 counterbalanced the deficiency of nuclear idioplasm.' He as- 

 sumed that the nucleoplasm was reduced to half, even in 

 parthenogenetic eggs, and that ' the egg-nucleus after its reduc- 

 tion to half was unable to initiate the processes of develop- 

 ment in the cell-body.' It was in these very cases of exceptional 

 parthenogenesis in single ova that I expressed the definite 

 opinion that the difference between eggs which are capable of 

 parthenogenetic development and those which are not, must be 

 quantitative and not qualitative ^. I concluded from the facts 

 connected with exceptional parthenogenesis, that a certain 

 amount of germ-plasm must be contained in the egg-nucleus if 

 it is to be in the position of entering upon embryogeny, and of 

 completing it, and that, in these exceptional cases of partheno- 

 genetic development, the germ-plasm in the ^g^, after having 

 been reduced to half its normal amount, possesses, in some 

 unusual way, the power of increasing to double. I am well 

 aware that many facts subsequently discovered appear to be 

 opposed to this suggestion, but I beheve they only appear to be 

 so. For example, my views as to the two varieties of A scan's 

 megalocephala might be cited in opposition ; of these varieties 

 one possesses two idants in the segmentation nucleus, the other 

 four. We might conclude from this that the amount of nuclear 

 matter does not control entrance upon development, but some 

 other cause,— perhaps those * spheres of attraction ' and the 

 central-bodies which E. van Beneden discovered lying in them, 

 and which Boveri has called the centrosomata. I do not dispute 

 the significance of these remarkable bodies in relation to the 

 commencement of nuclear division, but do we know whence 

 they come, and whether they are not perhaps, on their part, 

 controlled by the nuclear idioplasm (germ-plasm) ? 



^ ' Continuity of Germ- plasm.' Jena, 1885, p. 90. Translated as the 

 fourth essay ; see Vol. I. p. 231. 



