230 THE PHYSIOLOGY OF REPRODUCTION 



Pakthenogenesis, Natural and Aktificial^ 



, The fact that the ova of various kinds of organisms are capable 

 under certain circumstances of segmenting and developing into new 

 individuals without the intervention of male germ-cells, has been 

 already referred to. In such animals as the Aphidse this method 

 of reproduction appears to be called forth by certain conditions of 

 temperature and moisture, In other forms of life the necessary 

 factors for the occurrence of parthenogenesis are not so evident, but 

 the fact of its existence has been known from early times. 



In many animals parthenogenesis has been observed to occur 

 occasionally, although it may never have become a confirmed 

 physiological habit. The silkworm moth {Bombyx mori) affords 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 partheno- 

 genetic in nature. 



Tichomiroff ^ showed that the unfertilised eggs of the silkworm 

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

 genetic 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 O'l per cent, solution of sulphate of strychnine. Not 

 long afterwards Mead ^ observed that the eggs of the marine Annelid, 

 Chmtopterus, 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. 



^ For ehemistry of artificial parthenogenesis, see p. 313. 



^ Janosik, " Die Atrophie der Follikel, etc.," Arch. f. Mikr. Anat., vol. xlviii., . 

 1896. i 



3 Tichomiroff, " Die kiinstliche Parthenogenese bei Insekten," Arch. f. Anat. 

 V Phys., Phys. Abth., Suppl., 1886. 



* Perez, "Des Effets des Actions m6caniques sur le D^veloppement des 

 (Eufs non-f6cond6s, etc.," Proch-Verhaiix de la 8oc. des Sciences de Bordeaux, 

 1896-97. 



'' Hertwig (E.), "Ueber Befruchtung und Conjugation," Verhandl. der 

 Veutsch. Zool. (ieseUsch., 1-892. 



° Mead, Lectures delivered at Woods HoU, Boston, 1898. 



