420 M. Leuckart on the Reproduction of Bark-lice, 



On the other hand, however, it cannot be denied that many 

 considerations come in the way of the assumption that the two 

 kind-j of reproductive bodies occurring in the Aphides are both 

 of the nature of ova. 



I will not dwell too much upon the fact that, according to 

 this supposition, the Aphides would produce two kinds of eggs. 

 We are acquainted with similar facts in other animals, especially 

 the Daphnice and Rotifera*, the reproduction of which might 

 also be adduced in favour of this view, inasmuch as, according 

 to the investigations of Lubbock (Phil. Trans. 1857, i. p. 98) 

 and Cohn (Zeitschrift fiir Wiss. Zool. 1858, p. 284), the animals 

 in question also possess the faculty of Parthenogenesis. It is 

 true that the two kinds of eggs of the above-mentioned animals 

 are by no means so strikingly different as the eggs and germ- 

 cells of the Aphides; but we must also admit that the peculiari- 

 ties of the latter (judging from my observations upon the ova of 

 the Cestoid worms and their development) by no means over- 

 step the bounds of the empirically established limits of egg- 

 formation, remarkably as these peculiarities, on the other hand, 

 approach to the nature and destiny of unmistakeable germ-cells. 

 (Generationswechsel und Parthenogenese bei den Insekten, p. 20.) 



It appears to me to be of far more importance, that the germ- 

 granules of the Aphides are evidently not calculated for any 

 fecundation. I have taken a previous opportunity of pointing 

 out this distinction (Generationswechsel und Parthenogenese, 

 p. 110). At that time it appeared to me sufficiently great, not- 

 withstanding any similarity to Parthenogenesis, to cause me to 

 regard the reproduction of the Aphides as an alternation of 

 generations. An egg which excludes all fecundation still ap- 

 pears to me to be a somewhat problematical structure ; but this 

 furnishes no sufficient reason for denying the possibility of such 

 eggs. Glaus here refers to the eggs of the worker bees, which 

 would also never be fecundated, and we might cite other cases, 

 and especially our Bark-lice, in which even the organization of 

 the sexual passages, in the same way as in the viviparous Aphides, 

 appears to betray the absence of a sexual intercourse. But all 

 these cases only present limited analogies, inasmuch as the 

 obstacles to fecundation in them (as, indeed, Glaus admits) con- 

 sist only in external, more or less accidental conditions, and are 

 by no means caused, as in the Aphides, by the nature of the 

 germinal product. 



* The Freshwater Bryozoa can scarcely be adiluced here, as, according 

 to Alhnan (Monogr. of the Freshwater Polyzoa, p. 37), the so-called 

 winter-eggs of these animals are not eggs at all, but structures of very 

 different organization and develo[)ment, which the author regards as 

 asexual reproductive bodies (statoblasts). 



