THE PROCESS OF FERTILIZATION 



307 



case in Arfemia, unite again afterwards; they remain separate, and 

 the numljer of chromosomes — there are sixteen of them — is thereby 

 reduced to half in the segmentation nucleus. But this is not all, for 

 before embryonic development has begun the normal number can be 

 again seen in the segmentation nucleus ; the chromosomes must there- 

 fore have doubled their v.uniler by division u'ithin the iiudeuB. 



It is probable that something similar takes place in the cases of 

 exceptional parthenogenesis which have long V)een known, but this 

 point has not yet been sufficiently investigated. Nevertheless I cannot 

 pass them over, as they are instructive from another point of view. 



In some silk-moths (Bombycidge) and hawk-moths (Sphingidte), 

 especially in the silk-moth 

 proper [Bombycc inori)^ in 

 Liparis dispar, and in 

 quite a number of other 

 Lepidoptera, it sometimes 

 happens that, out of a 

 large number of unfer- 

 tilized eggs, a few will 

 develop and produce 

 caterpillars. This is in- 

 teresting enough, but it 

 gains increased importance 

 through the investigations 

 of the Russian naturalist, 

 TichomirofF, who suc- 

 ceeded in considerably in- 

 creasino- the number of 

 unfertilized eggs that de- 

 veloped b}^ 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 fertilized, capable of parthenogenetic 

 development by means of mechanical or 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 parthenogenetic 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 through the process of segmenta- 

 tion, but even reached the stage of the quaint easel-like Pluteus larva. 

 Quite recently Hans Winkler has made the interesting observation 



U 3 



Fro. 79. Tlie two maturation divisions of the 

 'drone eggs' (unfertilized eggs) of the Bee, after 

 Petrunkewitsch. E^p i, the first directive spindle. 

 A- r and k 2, the two daughter-nuclei of the same. 

 Ixsp 2. the second directive spindle, k 3 and k 4, the 

 two daughter-nuclei. In the next stage k 2 and k 3 

 unite to form the primitive sex-nucleus. Highly 

 magnified. 



