FERTILIZATION IN PLANTS AND UNICELLULAR ORGANISMS 333 



dition, by only going through a part of the processes of maturation 

 which are related to the subsequent amphimixis, and by thus retaining 

 its own centrosome. Nothing is more instructive in this connexion 

 than the cases we have already briefly discussed of facultative or 

 occasional parthenogenesis. We have seen that in some insects, for 

 instance in the silk-moths, there are sometimes, among thousands 

 of unfertilized eggs, a few that develop little caterpillars. If we 

 examine a large number of such unfertilized eggs we not infrequently 

 find among them several which, although they have not gone through 

 the whole course of development, have at least gone through the 

 earlier stages, and others which may have advanced somewhat further 

 and then come to a standstill ; in short, we can see that several of 

 these eggs were capable of parthenogenetic development, although in 

 varying degrees. 



The cause of this parthenogenetic capacity has not as yet been 

 definitely determined by observation, but we shall hardly go wrong 

 if we seek it in the fact that the centrosphere of the ovum does not 

 always perish immediately and completely during maturation, and 

 may persist, rarely in its integrity, but sometimes in a weakened state. 

 Future observations will probably reveal some differences in the size 

 or aster- forming power of the centrospheres of such eggs ; in any case 

 it is of the greatest interest that stimuli of various kinds — mechanical 

 or chemical — can strengthen the disappearing centrosphere of the 

 ovum, although as yet we are far from being able to say how this 

 comes about. 



The experiments already mentioned of Tichomiroff, Loeb, and 

 Winkler give us at least an indication how we must picture to our- 

 selves the origin of parthenogenesis, namely, through the fact that 

 the breaking up of the apparatus for division, introduced for the sake 

 of compelling amphimixis, is prevented. Minute changes in the 

 chemistry of the ovum, similar to those caused artificially in the ova 

 of the sea-urchin by the introduction of an infinitesimal quantity of 

 chloride of magnesium (Loeb), in the ovum of the silk-moth by 

 friction or by sulphuric acid (Tichomiroff), or in the sea-urchin ovum 

 by an extract of the sperm of the same animal (H. Winkler), will 

 efiect this modification, and normal parthenogenesis is induced. 



For the ovum, therefore, amphimixis is certainly not a life- 

 renewing or rejuvenating factor; it only appears as such because 

 the process has in the course of nature been made compulsory by 

 making the two uniting cells each incapable of developing by itself. 

 As we have seen, this is true also of the sperm-cell, for although it 

 contains a centrosphere, and would be capable of division as far as 



