328 EVOLUTION AND GENETICS 



producing them, and the conditions which have been found to 

 play a part in the behaviour of these things. 



Individuality of the Gametes. In most cases of sexual repro- 

 duction the function of each gamete is wholly complementary to 

 that of the other. The cytology of these cells suggests that if 

 the nucleus is, as supposed, the essential structure for the develop- 

 ment of hereditary characters, the presence of a complete haploid 

 set of chromosomes would enable either kind of gamete to develop 

 into a complete organism. The fact that the cytoplasm is necessary 

 for the differentiation of the somatic structures implies a limitation 

 which actually does prevent the development of male gametes 

 alone, but inheritance under certain experimental conditions and 

 the inheritance of sex in special life cycles show that each gamete 

 is fundamentally and potentially an individual. 



The fertilization of enucleate eggs of sea urchins by the sperms 

 of other species and their subsequent development of the characters 

 of the male parent shows that the male germ cell is equal to the 

 female except in its reduced and specialized cytoplasm. This 

 must be regarded as incidental to the conditions of reproduction, 

 and not as a fundamental difference in reproductive power. It is, 

 of course, very effective in limiting the capacity of highly developed 

 male gametes. 



Parthenogenesis. Development of the unfertilized ovum, or 

 parthenogenesis, occurs normally in many insects and other 

 Arthropoda, in some worms and possibly in other phjda. Arti- 

 ficial parthenogenesis has been brought about by the application 

 of various stimuli such as unusual chemical contacts and mechani- 

 cal manipulation. The production of males would naturally result 

 from parthenogenesis when this is the heterogametic sex, l^ut in 

 some cases females are produced. 



Parthenogenesis in the Honey-Bee. Parthenogenesis here is very 

 well known because of the economic importance of the species. It 

 has been proved that all drones, or male honey-bees, are produced 

 from unfertilized eggs, while the queens and workers, both females, 

 are developed from fertilized eggs. A fertilized egg can develop into 

 either queen or worker according to the food received by the grow- 

 ing larva and queen breeders have produced intermediate individ- 

 uals. Some of these are said to have mated and deposited fertile 

 eggs, although scarcely distinguishable from the workers, which 

 never mate. 



