FERTILIZATION 165 



matozob'n. One such explanation states that the spermatozoon im- 

 parts a peculiar mode of motion to the egg, leaving it to science to find 

 out what this mysterious motion is. Other authors say that the egg is 

 comparable to a watch which cannot go unless a spermatozoon enters, 

 leaving it to science to find out the wheels and the spring in the egg, and 

 the relation of the spermatozoon to this mechanism. Others again say 

 that the spermatozoon exercises a stimulus, forgetting, however, to 

 tell us what is the nature of the stimulus. The list of such expla- 

 nations might be continued, but they all show the same characteristic; 

 namely, that an explanation by phrases or words is offered where 

 an explanation by facts is wanted. Instead of devoting any time to 

 this kind of metaphysics, we shall consider some of the facts of par- 

 thenogenesis. 



, The oldest and best-known case of parthenogenesis is that of 

 plant lice (Aphides}. When the temperature and moisture are suffi- 

 ciently high, the Aphides reproduce themselves parthenogenetically. 

 Males do not exist under such circumstances. This condition can be 

 maintained for years, possibly indefinitely. Similar cases of partheno- 

 genesis seem to occur in Daphnia. A remarkable case of parthenoge- 

 netic development occurs among bees (and possibly among social wasps), 

 where, according to Dzierzon, the male bees originate from unfertilized 

 eggs, while the female (queens and workers) originate from fertilized 

 eggs. The queen pairs only once and the sperm is carried in a recep- 

 tacle. When an egg passes the duct without any sperm coming from 

 the duct, it remains unfertilized. Dzierzon found that old queens lay 

 only eggs from which male bees develop, and the examination of the 

 receptacle showed that in such cases the receptacle was free from sperm. 

 It was, moreover, observed that the workers, whose rudimentary sexual 

 organs exclude copulation, occasionally lay eggs from which, however, 

 only male bees originate. The observations of Dzierzon were confirmed 

 and enlarged upon by Siebold, Leuckart and very recently by Petrun- 

 kewitsch. 



In such cases of parthenogenesis the development of the egg is not 

 called forth by a spermatozoon, but by another, at present, unknown 

 condition. More recently the fact has been established that eggs, which 

 naturally develop only when a spermatozoon enters, can be caused to 

 develop artificially by certain physical and chemical means. In 1886 

 Tichomiroff published the fact that the unfertilized eggs of silkworm, 

 Bombyx mori, can be caused to develop by rubbing them gently with a 

 brush, or by putting them for a short time into concentrated sulphuric 

 acid. Siebold had already mentioned, and Nussbaum confirmed his 

 observation, that a small number of such eggs develop without these 



