304 THE EVOLUTION THEOKY 



as arising from sexually differentiated individuals (females), and from 

 germ- cells (true ova), but brought about by the agency of individuals 

 of only one sex, the female. These parthenogenetic eggs emancipate 

 themselves, so to speak, from the law that was previously regarded as 

 without exception, that all ova require fertilization to enable them to 

 develop. That this law admits of many exceptions is now universally 

 admitted ; thus in the small family of water-fleas (Daphnids) there are 

 even two kinds of eggs, the summer and winter eggs we have already 

 mentioned, which are produced by the same female, and yet the 

 former kind develop without fertilization, while the latter require to 

 be fertilized before they can develop. 



It was obviously important to learn the state of affairs in regard 

 to reducing divisions in parthenogenetic ova, to find out whether here 

 also, three, or, in some circumstances, two polar bodies were formed, 

 and whether the second polar division reduced the number of chromo- 

 somes to half. If the theory previously advanced as to the impor- 

 tance of the chromatin, and especially of the reducing effect of the 

 second maturing division be correct, we should expect the second 

 division to be wanting in parthenogenetic eggs, since otherwise the 

 number of chromosomes would be reduced to half in each generation, 

 and would thus gradually disappear or sink to one. 



Having directed my attention to this problem, I succeeded iu 

 establishing for a Daphnid, Polyphemus, that the second polar 

 division does not occur, and that only one polar body is formed. 

 Blochmann found the same in the parthenogenetic eggs of plant-lice 

 or Aphides, in which, moreover, the eggs requiring fertilization 

 exhibit, like the winter eggs of Daphnids, two polar divisions. It 

 was thus established that at least those eggs of Aphides and Daphnids 

 which are wholly parthenogenetic retain the full number of chromo- 

 somes of their species, as is represented in the diagram. Fig. 78. 

 When parthenogenesis set in the polar divisions were limited to one, 

 and that this could happen justifies us in concluding a posteriori 

 that it could have happened also in the case of ova which required 

 fertilization if that had been necessary or even merely indifferent. 

 The polar divisions are thus not mere 'vestigial' processes; they 

 have an immediate significance, and it lies in the reduction of the 

 number of chromosomes. 



But I must make a reservation here ; it is not universally true 

 of parthenogenetic eggs, that maturation takes place without the 

 second polar division. The first exception was observed in the salt- 

 water crustacean, Artemia salina. In this case only one polar body 

 is actually extruded and the number of chromosomes remains normal. 



