ANIMAL PARTHENOGENESIS 



By LEONARD DONCASTER, M.A. 

 Lecturer on Zoology at the University of Birmingham 



Of the numerous problems which are at the present time 

 awaiting solution at the hands of biologists, none are more 

 fundamental than those connected with sexual reproduction. 

 The questions which cluster around this subject touch almost 

 all departments of biology ; they include heredity, the origin 

 of variation, the causes of cell-division, the function of the 

 chromosomes, centrosomes, and other nuclear or cell structures, 

 as well as the function and origin of sex, and probably the 

 actual physical or chemical forces upon which life itself depends. 



One method of attempting the solution of these problems 

 consists in comparing cases of normal sexual reproduction with 

 those in which it is partially or wholly suppressed, that is 

 to say, in comparing the processes of reproduction consequent 

 on fertilisation with those observed when conjugation of male 

 and female elements does not take place. Such cases come 

 under the general description of parthenogenesis, which may be 

 defined as reproduction by means of a germ-cell which has not 

 undergone conjugation. 



Cases of parthenogenesis may be classified in various ways ; 

 we may first divide them into those which occur naturally 

 as part of the normal life-history of the species, and those 

 which occur only under artificial stimuli (artificial partheno- 

 genesis), but it is not certain whether there is any really natural 

 line of distinction between the two. Then we find that 

 parthenogenesis graduates naturally into such forms of asexual 

 reproduction as budding, which, however, are not as a rule 

 included under the term, although it is difficult to define one 

 without including the other. For example, it is not easy to 

 say that the production of internal buds in the trematodes, 

 sponges, or Polyzoa differs fundamentally from reproduction by 

 unfertilised eggs in an Aphid, or from the " pedogenesis" of 

 some of the Cecidomyids. In this latter case genital cells are 

 produced in the larva of a dipterous fly, which develop and 



40 



