342 THE EVOLUTION THEORY 



in a ' quickening ' of the ovum, we may quite well speak of a quicken- 

 ing by fertilization in another sense, if we mean the impulse to 

 embryonic development, for this is really supplied by the entrance 

 of the sperm -nucleus with its centrosphere into the ovum. But even 

 this impulse can, under certain circumstances, be given in another 

 way, and certainly the awakening of it is not the end of fertilization, 

 but only the condition without which the end, the union of two kinds 

 of nuclear substance, could not be attained. There is no indication 

 whatever that this ' quickening ' of the ovum would be necessary for 

 any other reason except that the ovum was previously made incapable 

 of development. There would be no ' fertilization ' were not the 

 mingling of hereditary substances of fundamental importance for 

 the organic world. 



Moreover, an ovum, or a fragment of an ovum, may also develop 

 of itself, having only one of the sex-nuclei, and the union of the 

 hereditary substance of two cells is therefore not indispensable for 

 the mere production of a new individual. 



AVhat has been observed in regard to fragments of ova is 

 particularly interesting in this connexion. Ernst Ziegler first suc- 

 ceeded in halving a newly fertilized sea-urchin ovum, so that one half 

 contained the female and the other the male pronucleus. The latter 

 alone contained a centrosphere, and developed a blastula larva, 

 Delage carried these experiments further, and cut an unfertilized but 

 mature sea-urchin ovum into pieces, and then ' fertilized ' the non- 

 nucleated pieces with spermatozoa. These pieces developed and 

 yielded young larvae of the relevant species ; so it is clearly seen that 

 even a piece of mature ovum-protoplasm may undergo embryonic 

 development, provided that a nucleus furnished with a dividing- 

 apparatus penetrates into it, Unfortunately it is technically im- 

 possible to cut such a non-nucleated and then fertilized fragment 

 of ovum so that one half shall contain the male nucleus the other its 

 centrosphere. Even without this experimentum crucis we may say 

 that the half with the male nucleus would not multiply by division, 

 and that the other probably would, though it would not go through 

 the regular course of segmentation processes, because the hereditary 

 substance absolutely necessary for these was wanting. 



But these and similar experiments prove something more, namely, 

 that the nuclei of the sperm- cell and egg-cell do not, as was formerly 

 believed, stand in a primary and essential contrast to each other, 

 which may be described as male and female, but that both are alike 

 in their deeper essence, and may replace each other. They only 

 differ from each other as far as the cells to which they belong differ, 



