86 'DETERMINANTS' VERSUS 



so-called 'polar bodies'), the effect of which is to leave the 

 egg-nucleus with only half its usual number of chromosomes. A 

 reducing division occurs also during the formation of spermatozoa, 

 though at an earlier stage — the result being that the nucleus of the 

 spermatozoon, as it enters the egg, also has only half the number 

 of chromosomes peculiar to the species. The fusion of the two 

 nuclei during the act of foecundation restores to the egg its normal 

 number of these bodies. 



It becomes a matter of much interest to ascertain, therefore, 

 what occurs when parthenogenesis takes place. Do the ' reducing ' 

 divisions occur as usual in the egg which develops without male 

 influence, or does such an egg retain its full number of chromo- 

 somes ? This interesting point has been settled by the researches 

 of Weismann and others for some animals, and he is thus enabled 

 to say that "those eggs of Aphides and Daphnids which are 

 wholly parthenogenetic retain the full number of chromosomes of 

 their species," owing to only one 'polar body' being extruded. 

 Why, in different species, there should be different numbers of 

 chromosomes ; and why in a developing egg, in one way or 

 another, it seems necessary that the full number of chromosomes 

 should always be produced, still remain absolute enigmas ; though 

 a similar confession has to be made in regard to many of the 

 phenomena of karyokinesis generally. 



In this connection other facts of extreme interest must be 

 mentioned. In some silk-moths and hawk-moths, and in quite a 

 number of other Lepidoptera, it sometimes happens that out 

 of a large number of unfertilised eggs a few will develop and 

 produce caterpillars. But, as Weismann says, these facts gain 

 " increased importance through the investigations of the Russian 

 naturalist, Tichomiroff, who succeeded in considerably increasing 

 the number of unfertilised eggs that developed by gently rubbing 

 them with a paint-brush, or by dipping them for a little in dilute 

 sulphuric acid. It is thus possible to make eggs which would not 

 ordinarily develop without being fertilised, capable of partheno- 

 genetic development by means of a chemical stimulus. This 

 sounds almost incredible, but it is beyond a doubt, and it is still 

 further corroborated by the fact that Prof. Jacques Loeb has 

 succeeded in inciting the eggs of a sea-urchin to parthogenetic 

 development by means of a chemical stimulus. When he added 

 to the sea water in which the eggs were laid a certain quantity 

 of chloride of magnesium the ova developed, and not only went 



