14 Hewitt, Cytological Aspect of Parthenogenesis in Insects. 



GENERAL CONSIDERATIONS. 



In studying the work which has been done on partheno- 

 genesis in insects, certain points of similarity occurring in 

 the changes undergone by the nucleoplasm and cytoplasm 

 of the ovum are noticeable. In the majority of forms in 

 which the maturation of the fertile ovum has been studied, 

 it is found that true polar bodies are formed ; and 

 these may be either separated completely from the ovum 

 or lodged in a depression. In other cases the two polar 

 nuclei, or sometimes only one, may be contained in a 

 cytoplasmic projection at the periphery of the ovum. 

 Except in the case oi Aphis and the tychoparthenogenetic 

 ova of Lepidoptera which may, perhaps, be left out of con- 

 sideration on account of their exceptional occurrence, and 

 the usual failure of the resulting embryos to complete 

 their development, we do not find any definite polar 

 bodies formed in the parthenogenetic ova of insects. 

 Three polar nuclei are formed, but they are not extruded 

 as polar bodies.* This appears to me to have some 

 significance as regards the power which the ovum 

 possesses of developing parthenogenetically. 



The two results of fertilisation are Amphimixis and 

 Embryogenesis. In parthenogenesis the former is ex- 

 cluded, and only the latter takes place. The question 

 then arises — what is the factor in the parthenogenetic &%^ 

 which replaces the stimulus to development which is 

 brought about by the entrance of the spermatozoon ? 



The statement of Boveri (12) that 'das Centrosoma 

 ist das eigentliche Theilungsorgan der Zelle : es vermittelt 

 die Kern- und Zelltheilung' is certainly disproved by 



* In the fertile ova of a few insects it is found that the polar nuclei are 

 not extruded, but remain at the periphery of the egg 



