146 OUTLINES OP EVOLUTIONARY BIOLOGY 



takes place in spite of the fact that reduction of the nucleus of 

 the ovum has occurred in maturation and has nofc been made 

 good by union with the nucleus of a spermatozoon. 



In cases of parthenogenesis it is clear that the developing 

 organism is provided only with maternal chromosomes, but we are 

 now also acquainted with cases which form the exact converse to 

 this, only paternal chromosomes being present. Such cases may 

 arise when an ovum is artificially enucleated, so that all the 

 maternal chromosomes are removed, and then fertilized by a 

 spermatozoon. It has actually been found possible in this way to 

 induce the development of enucleated eggs, and the phenomenon, 

 our knowledge of which we owe mainly to Delage, is known as 

 merogony. Although it is obvious that, as a result either of 

 artificial parthenogenesis or of merogony, the developing organism 

 may start life with only half the normal number of chromosomes, 

 it is possible that, as maintained by Delage, this number may be 

 subsequently doubled in some way. 



It cannot therefore be the union of male and female pronuclei 

 that furnishes the stimulus to development ; this union, or 

 amphimixis as Weismann terms it, has another significance, and, 

 as we shall see later on, is most probably connected with the 

 transmission of inherited characters from parent to offspring. 



Boveri, followed by other observers, has put forward the view 

 that the unfertilized egg cannot develop because the centrosome, 

 which is to be regarded as the dynamical centre of the cell, has 

 been eliminated during the process of maturation. It is the 

 spermatozoon that, in the act of fertilization, brings with it 

 the centrosome upon the activity of which the cell-divisions of the 

 fertilized egg depend. This is probably perfectly true in cases 

 of normal fertilization and development in animals, but the view 

 that the centrosome of the spermatozoon supplies the essential 

 stimulus to development seems to be hopelessly negatived by the 

 phenomena of parthenogenesis, in which new centrosomes 

 undoubtedly arise in unfertilized eggs. 



It seems therefore as if we were for the present thrown back 

 upon Loeb's hypothesis of a chemical stimulus, which he main- 

 tains for normal fertilization as well as for artificial partheno- 

 genesis. In the former case the necessary chemical substances 

 are supposed to be brought in by the spermatozoon. Loeb 

 believes that there are two of these substances. One, which he 

 terms a lysin, is supposed to bring about the formation on the 



