22 Hewitt, Cytological Aspect of Parthenogenesis in Insects. 



female larvae first, and so by their deaths the percentage of 

 males produced will be increased. Strasburger (87) for 

 plants. Beard (7) and Lenhossek (57) all support the idea 

 on theoretical grounds that sex is already present in the 

 germ. 



Absence of fertilisation, that is, development by 

 parthenogenesis, may produce different sexes in closely 

 allied forms, as in the saw-flies and in other Hymenoptera. 

 It is very probable that in the ovum we have only one sex ; 

 this may differ in allied forms, but it is characteristic of 

 the species. The production of the opposite sex is 

 determined by other factors, chief of which is fertilisation. 

 What other factors influence sex remains to be deter- 

 mined. I believe that nutrition does in some forms 

 influence the production of sex, for I fail to see otherwise 

 how a sexual generation is produced in the Aphides, as 

 shown by Kyber (55), by the diminution of sap in their 

 food, caused by the tissues becoming woody, in the summer 

 months, that is when everything which man can observe 

 is favourable for the continued production of partheno- 

 genetic females, except nutrition. 



The Chromosomes. 



The facts which we have at hand relating to the 

 number, division, and role of these bodies in cytological 

 phenomena are almost beyond comprehension. That 

 the chromosomes should have attracted so much atten- 

 tion is not surprising, as they are the chief visible factors 

 of cell division and reproduction which can be dealt with. 

 The rapid progress which has been made in the study of 

 these bodies has been due very largel}' to the fact that 

 they are considered to be the bearers of hereditary 

 characters, so it has been thought that by a study of 



