FERTILISATION 217 



occasionally, although it may never have become a confirmed 

 physiological habit. The silkworm moth (Bombyx mori) afiords 

 an example of this phenomenon. In the higher animals also it 

 has been shown that unfertilised eggs may very abnormally start 

 to segment without any obvious source of stimulus. Janosik ^ 

 has recorded segmentation in the ovarian ova of Mammals, but 

 it is doubtful whether such cases should be regarded as truly 

 parthenogenetic in nature. 



Tichomiroff ^ showed that the unfertihsed eggs of the silk- 

 worm moth, which, as just mentioned, is occasionally partheno- 

 genetic, can be caused to develop in greatly increased numbers 

 by rubbing them lightly with a brush, or by dipping them for 

 about two minutes in strong sulphuric acid, and then washing 

 them. Perez ^ subsequently made some similar observations, 

 noting also that normal parthenogenetic development was 

 commonest in those individuals which were most robust. 



Richard Hertwig * was the first to show that if the ova of 

 various Echinoderms are treated with certain reagents, and 

 then restored to normal sea-water, they will frequently display 

 signs of segmentation. The particular reagent originally 

 employed by Hertwig was a 0-1 per cent, solution of sulphate of 

 strychnine. Not long afterwards Mead ^ observed that the eggs 

 of the marine Annehd, Chwtopterns, which ordinarily become 

 mature only after the entrance of the spermatozoa, could be 

 induced to throw out their polar bodies by adding potassium 

 chloride to the sea-water in which they were placed. 



Morgan ' found that an addition of sodium chloride to sea- 

 water containing ova of sea-urchins caused these to form astro- 

 spheres, while, if the ova were afterwards transferred to ordinary 



' Janosik, " Die Atrophie der Follikel," &c., Arch. f. Mikr. Anat., 

 vol. xlviii., 1896. 



' Tichomiroff, " Die kiinstliche Parfchenogenese bei Insekten," Arch, 

 f. Anat. in Phys., Phys. Abth., Suppl., 1886. 



' Perez, "Des Effets des Actions m^caniques sur le Developpement des 

 QDufs non-fteond^," &o., Proces-Verbaux de la Soc. des Sciences de Bordeaux, 

 1896-97. 



* Hertwig, " Ueber Befruchtung und Conjugation," Verhandl. der Deutsch. 

 Zool. Oeeelkch., 1892. 



' Mead, lectures delivered at Wood's Holl, Boston, 1898. 



' Morgan, "The Action of Salt Solutions on the Unfertilised and Fertilised 

 Ova of Arbacia," &c., Arch.f. Entwick.-Mech., vol. iii., 1896, and vol. viii., 1899. 



