VII PARTHENOGENESIS 265 



about which there is some interest ; differences which 

 have been observed seem to depend on this, that 

 Apus continues growing as long as the pond in which 

 it lives does not dry up, and hence the eggs which 

 hatch soonest give the largest-sized progeny. In his 

 tabular statements Siebold gives measurements of the 

 specimens examined by him at different times from 

 various localities. 



A few words must be said here upon the very 

 extraordinary history of the ovum of Apus made out 

 by Siebold, the structures being identical, whether 

 the female examined belonged to a parthenogenetic 

 or digenetic brood. The essential female organs of 

 reproduction in Apus may be roughly described as 

 two large tubes placed on either side of the alimentary 

 canal, opening externally at the posterior end, and 

 giving off towards the other end primary and second- 

 ary branches. On the ends of these short secondary 

 branches are situated the egg follicles. Four cells 

 appear in each egg follicle in a very early stage of its 

 development, and one of these takes on more rapid 

 growth becoming the egg -cell whilst the others 

 disappear as deutoplasmogen or vitellogenous cells ; 

 the egg then acquires some size and a red colour, and 

 has a visible germinal vesicle. But such eggs are 

 much smaller than the eggs observable in the main 

 stem of the ovarian tube, and this appears to be the 

 very startling explanation. The eggs escape from 

 their follicles as a matter of course, and pass along 

 the canal leading from it to a primary branch of the 

 ovarian tube, and there two and sometimes three of 



