M. Leuckart on the Reproduction of Bark-lice. 419 



The decision of this question is intimately connected with our 

 opinion as to the nature of the reproduction occurring in the 

 so-called nurses ; it depends upon whether we regard this as an 

 asexual reproduction or not. 



The preliminary question that naturally first presents itself 

 here is, where are we to seek in general for the distinguishing 

 characteristics of sexual and asexual reproduction. If we indi- 

 cate that reproduction alone as sexual in which a cooperation of 

 two kinds of reproductive matter (in other words, a fecundation) 

 takes place, there remains, of course, no ground for bringing the 

 alternation of generations in the Aphides into the question at all. 

 But then, to be consistent, we must refer Parthenogenesis to 

 asexual propagation, as indeed is done by Radlkofer ('Ueber das 

 Verhaltniss der Parthenogenesis zu den anderen Fortpflanzungs- 

 arten,^ 1858). Whether this view will some day find general 

 acceptance, I do not know ; but to me it appears to be rather 

 bold to regard the same substratum, an q^^, sometimes as a 

 sexual, and sometimes, just according to circumstances, as an 

 asexual reproductive material*. In my opinion, it is always the 

 same — always the product of the same (sexual) activity, whether 

 the cycle of conditions under which it is developed into a new 

 creature be closed by the access of semen, or without this. 

 Wherever we have to do with an egg, there also, in my opinion, 

 sexual reproduction always takes place. 



It appears to me, therefore, that it is less the occurrence of a 

 fecundation than the nature of the developing substratum that 

 must guide us in the assumption of a sexual or asexual propa- 

 gation. 



In the case now especially before us, there would also be the 

 question whether the germ-corpuscles of the viviparous Aphides 

 can be regarded as eggs. 



That these germ -corpuscles are cells like the eggs, and indeed 

 cells which become converted into the embryo in a manner ana- 

 logous to that of the eggs, can no more be doubted, after the 

 recent investigations, than the morphological relations of the 

 germ-tubes and ovaries in which the reproductive matters in 

 question originate. It is even possible that future investigations 

 may demonstrate an essential accordance in the mode of produc- 

 tion of these two kinds of structures. All this must incline us, 

 to a certain extent, to regard the germ- cells and ova of the 

 Aphides as morphologically identical structures. 



* The criterion of sexual and asexual reproduction put forward by Radl- 

 kofer, namely the idiotypic or zelotypic nature of the product, cannot 

 suffice here (as indeed in other cases in the alternation of generations with 

 larval nurses); for the product of parthenogenesis furnishes, e.g. in Chermes 

 Abietis, not (zelotypic) copies of the parents, but individuals of a different 

 and independent original development (idiotypes). 



27* 



