332 ECONOMIC IMrORTANCE OF NEHtlATODES . 



Ill connection with the stiiicture of the male leproductive 

 organs, there has been much controversy. Stone and Smith, 

 working- in 1898 on H. radicicnla, came to the conclusion that 

 the male gonads consisted of a pair of testes. E. A. Bessey 

 (1911), on the other hand, shares the view with Atkinson 

 (1889) that there is only a single testis in H. radlcicohi, and 

 states that a double testis in the male is one of the charac- 

 teristics of H. schachtii, the sugar-beet gall-worm of Europe. 

 So far, all efforts on the part of the author to procure 

 specimens of H. schachtii parasitic in South Africa have been 

 unsuccessful. Among the parasites prevalent in South Africa 

 a number of adult males have been carefully examined, and in 

 these only a single testis was evident, but in others taken 

 from the same infested material, and so presumably of the 

 same species, two well-developed separate testes were 

 distinctly visible. The question consequently arises regarding 

 the validity of two separate species of the genus Heterodera. 

 It seems possible that the presence of two testes in the male 

 may be a variation of comparatively large magnitude and of 

 not infrequent occurrence, or it may be that two testes are 

 developed and one atrophies or unites with its fellow at a 

 certain stage in its development so as to appear single. Other 

 characters stated to be peculiar to one or other species also 

 seem to be inconstant when tested with parasites from different 

 sources. 



The biological and bionomical activities of Heferodeva 

 still require further investigation, in order that more 

 successful attempts may be made to deal effectively with this 

 pest. Problems such as the absence of the parasites from 

 plots of ground which have been badly infested during 

 previous seasons and have undergone no special treatment 

 to exterminate the parasites, while ordinarily a plot of ground 

 once infested may remain so for a large number of years, 

 seem at present inexplicable. The explanation will probably 

 come when more definite infonnation on the resistant stages 

 of the life cycle, whereby the parasite can withstand adverse 

 climatic conditions, and whereby their distribution is affected, 

 is acquired. 



Concluding Remarks, with Special Eeference to South 



Africa. 



The value of a study of the i^ematodes. both from the 

 purely scientific view-point as well as from the position that 

 the group occupies in relation to agriculture and disease, 

 cannot be over-emrdiasised. Erom the " pure science " aspect, 

 much research has already been accomplished, and the results 

 that have been derived therefrom have been of paramount 

 importance^as an instance, our first insight into the highly 

 romplicated process of karyokinesis, or division of the nuclear 

 chromatin wliicli precedes the fertilisation of the ovum, was 

 obtained from the study of the eggs of Ascaris, the common 

 round-wonn of man. On tlie other hand, a great deal still 



