REPRODUCTION AND HEREDITY 299 



ning of the development of most animal ova, we are yet able to 

 force many of them by artificial means, often of a very crude 

 kind, to proceed to fission and parthenogenetic development. 



Considerable sensation was created in the 'eighties by the 

 discovery made by Tichomirow, showing that it is possible to 

 bring about a development of the ova of the silkworm, which 

 otherwise stand regularly in need of impregnation, by brushing 

 them, or rubbing them between cloth. The same result was 

 produced by a brief immersion of the ova in concentrated sul- 

 phuric or hydrochloric acid. Chemicals, therefore, which under 

 other conditions destroy life are here used to awaken life, a 

 mystery which even to-day has been only incompletely solved. 

 Others made experiments with other animal ova and different 

 means, and obtained similar result. The most important suc- 

 cesses in these experiments were achieved by J. Loeb who 

 * succeeded in producing complete larvae out of unfertilized ova 

 of different Echinoderms. 



How can we explain this process of artificial parthenogenesis, 

 and how can we bring it in harmony with our hypothesis of the 

 action of the centrosomes during cell-division ? As Morgan and 

 afterwards others have shown, the development of the ova 

 taking place during artificial parthenogenesis exhibits the same 

 phenomena as during normal fertilization. In the egg-plasm 

 appear minute granules resembling centrosomes which become 

 the starting point of a radiating figure, arrange themselves in the 

 manner known to us, and apparently regulate the division. We 

 may, therefore, assume that the centrosome which during fertili- 

 zation is introduced into the ovum by the spermatozoon, forms 

 itself here independently under the influence of certain chemical 

 substances and thus gives the impetus to division. It must 

 however, be understood that this is more a description of what 

 we see than an actual explanation of how the centrosome acts : 

 on that point we are only able to form a very imperfect 

 conception. 



As the ova, which were forced into artificial parthenogenesis, 

 had already passed a ' maturing-division,' their nucleus, as well 

 as that of all their descendants, contains only the reduced number 

 of chromosomes. Thus this process is at once perceived as 

 something unnatural, and is at the same time a new proof that 



