THE PROCESS OF FERTILIZATION 303 



permanent nature, which only apparently break up in the nuclear 

 reticulum, but in reality persist, the fact of reduction would point in 

 this direction. For if they were not permanent structures and distinct 

 from one another, and if their number depended solely on the quantity 

 of chromatin which the nucleus contains, the reduction in number might 

 be secured if the chromosomes in the growing egg and sperm-cells 

 increased in size more slowly than the cell-body and the other parts of 

 the cell. But from the fact that the reduction takes place not in this 

 simple way, but, in sperm-cells and in ova which require to be 

 fertilized, only through cell-division and a specific mode of nuclear 

 division, we may conclude that it cannot happen otherwise, that 

 chromosomes are not mere aggregates of organic substance, but organs 

 whose number can only be reduced by the extrusion of some of them 

 from the cell. 



It is true that there are ova in which the process of reduction 

 does not follow the course we have described, but the exceptions only 

 serve to confirm our view of the reducing significance of the polar 

 divisions, and of their persistence because of the necessity for 

 reduction. 



As far back as the middle of the nineteenth century it was 

 known that in various animals the eggs develop without fertilization. 

 This reproduction by ' parthenogenesis ' was first established with 

 certainty by the German bee-keeper Dzierzon in 1845, and then 

 scientifically corroborated by Rudolph Leuckart and C. Th. von 

 Siebold. Although parthenogenesis was at first observed only in a 

 few groups of the animal kingdom, in bees and some nocturnal 

 Lepidoptera (Psychidse and Tineidae), it has become more and more 

 apparent in the course of years that this ' virgin reproduction ' is by 

 no means a rare form of reproduction, and that it occurs regularly and 

 normally in many cases, especially in the very diverse groups of the 

 great series of Arthropoda. Thus among insects it is found in certain 

 saw-flies, gall-flies, ichneumon-flies, in the honey bee, and in common 

 wasps, and it is particularly widespread among plant-lice (Aphides) 

 such as the vine-aphis (Phylloxera), whose prodigious multiplication 

 in a short time depends partly on the fact that all the generations, 

 with the exception of one, consist only of females with a partheno- 

 genetic mode of reproduction. 



Among the lower Crustaceans also parthenogenesis plays a large 

 r61e, and in many species it even occurs as the sole mode of reproduc- 

 tion, but more often — as is also the case among insects — it occurs 

 alternately with bi-sexual reproduction. For parthenogenesis must not 

 be regarded as asexual reproduction, but rather as unisexual, that is. 



