ANIMAL PARTHENOGENESIS 41 



reproduce the species, but if the process took place rather 

 earlier in the life-history, before the embryonic period had 

 closed, we should class it as budding, or call it embryonic 

 fission. This does happen in some embryos of parasitic 

 Hymenoptera, as has been shown by Marchal x and Silvestri, 2 

 and it does not seem to differ fundamentally from the psedo- 

 genesis of Cecidomyia. So that, although some cases of 

 alternation of generations are commonly described as consisting 

 in reproduction alternately by budding and by a sexual process, 

 and others by alternation of parthenogenesis and bisexual 

 reproduction, yet it is not clear that any true distinction can be 

 drawn between them. 



The most frequent form of natural parthenogenesis is that 

 which alternates with a bisexual process, and in this class 

 we get every stage between cases where the parthenogenetic 

 and bisexual generations do not differ in any character except 

 their manner of reproduction, and those in which two or more 

 wholly different generations are produced in the life-cycle. 



In the rotifer Hydatina, for example, it seems that the 

 difference between the females which will produce eggs that 

 must be fertilised, and those which reproduce partheno- 

 genetically, depends on whether the young female has copulated 

 with a male or not. 3 In one case she produces large eggs with 

 much yolk which must be fertilised, and after a resting period 

 develop into females, and in the other she produces much 

 smaller eggs which are not fertilised and yield males. Another 

 class of female, not differing in appearance, produces eggs of an 

 intermediate type, which are not fertilised, and produce females. 



In the Daphniidae also there is no morphological difference 

 between the bisexual and parthenogenetic generations ; in fact, 

 the same female may produce parthenogenetically either females, 

 males, or both, and may at another time lay eggs requiring 

 fertilisation. 



The number of parthenogenetic generations which normally 

 intervene between one conjugation and the next varies largely 

 in different species of Daphniidae, but there is considerable 

 evidence 4 to show that the kind of egg produced by some 



1 Arch. Zoo. Exp. et Gen. (4) ii. p. 257. 



2 Attn. Scuola Agric. Portici, vi. 1906, p. 1. 



3 Lensen, La Cellule, xiv. 1898, p. 421. 



4 Issakowitsch, Biol. Centralbl. xxv. 1905, p. 529. 



