REPRODUCTION. 39 



phenomena observed may admit of explanation otherwise than 

 as an instance of parthenogenesis strictly so called. 



The Aphides, or plant-lice, which are so commonly found 

 parasitic upon plants, are seen towards the close of autumn to 

 consist of male and female individuals. By the sexual union 

 of these true ova are produced, which remain dormant through 

 the winter. At the approach of spring these ova are hatched ; 

 but instead of giving birth to a number of males and females, 

 all the young are of one kind, variously regarded as neuters, 

 virgin females, or hermaphrodites. Whatever their true nature 

 may be, these individuals produce, viviparously, a brood of 

 young which resemble themselves ; and this second generation, 

 in like manner, produces a third, and so the process may be 

 repeated, for as many as ten or more generations, throughout 

 the summer. When the autumn comes on, however, the vivi- 

 parous Aphides produce in exactly the same manner a final 

 brood, but this, instead of being composed entirely of similar 

 individuals, is made up of males and females. Sexual union 

 now takes place, and ova are produced and fecundated in the 

 ordinary manner. 



The viviparous Aphides are either wingless or winged ; and 

 the number of young produced is so great, that it has been 

 calculated that a single Aphis might in this way be, during 

 the summer months, and by the time the tenth generation was 

 reached, the progenitor of no less than one quintillion of indi- 

 viduals. Each viviparous Aphis possesses an ovary, which 

 only differs from that of the fertile females in being without 

 certain secondary adjuncts (the colleterial gland and sperma- 

 theca). This " pseudovarium " produces egg-like bodies or 

 " pseudova," which are directly developed into young Aphides 

 the latter being thus produced by " the individualisation of 

 previously organised tissue." 



The differences between the " pseudova " and true ova are 

 in no way anatomical, but are wholly physiological ; and the 

 decision involved in the viviparous reproduction of the Aphides 

 turns simply upon the question as to whether the viviparous 

 individuals possess, in addition to the pseudovarium, a testis, 

 or whether male organs are absent. Most observers maintain 

 that the viviparous Aphides are wholly destitute of male organs 

 of reproduction, in which case the phenomena just described 

 can only be explained as an example of parthenogenesis. On 

 the other hand, Balbiani maintains that the viviparous Aphides 

 are really hermaphrodite, in which case, of course, the pheno- 

 mena are of a much less abnormal character. 



In the second case of alleged parthenogenesis which we are 



