106 ORGANIZATION AND DEVELOPMENT OF ANIMALS IN GENEKAL. 



sexual elements has made its appearance. When the male elements, 

 and with them the necessity of fertilization, are absent, and when, at 

 the same time, the organ which produces the germ cells possesses, in 

 its full development, a structure similar to that of an ovary, it 

 becomes very difficult to distinguish whether we have to do with 

 a pseudovary (germ-gland), and with an animal which reproduces 

 asexually ; or with an ovary and a true female, whose eggs possess 

 the capacity of developing spontaneously. It is 

 only a comparison with the sexual form of the 

 animal which makes the distinction possible. To 

 take the case of the Plant-lice or Aphides ; in 

 these animals we find a generation of viviparous 

 individuals, easily distinguishable from the true 

 oviparous females, which copulate and lay eggs. 

 They resemble the latter in the fact that they 

 are provided with a similar reproductive gland, 

 constructed upon the ovarian type ; but they differ 

 fi-om them in this important peculiarity, that 

 they are without organs for copulation and ferti- 

 lization (in correspondence with the absence of 

 the male animal) (fig. 99). The reproductive cells 

 of the organs known as pseudovaries ha.ve an 

 origin precisely similar to that of eggs in the ova- 

 ries, and only differ from ova in the very early 

 commencement of the embryonic development. 

 The viviparous individuals will therefore be more 

 correctly regarded as agamic females peculiarly 

 modified in the absence of organs for copulation 

 and fertilization ; and the reproductive cells are 

 by no means to be relegated to the category of 

 germ-cells (as formerly was done by Steenstrup). 

 We must therefore speak of the reproductive pro- 

 cesses in the Aphides as being sexual and partheno- 

 genetic and not sexual and asexual. A comparison 

 of the mode of reproduction of the Bark-lice with 

 that of the Aphides, especially of the species Pem- 

 phigus terebinthi, puts the correctness of this 

 supposition beyond the sphere of doubt. 

 A similar condition is found in the viviparous larva of Cecidomyia. 

 Here the rudiment of the generative glands very early assumes a 

 structure resembling that of the ovary, and produces a number of 



\-Tl 



FIG. 100. Vivipa- 

 rous Cecidomyia 

 (Miastor) larva 

 (after Al. Pagen- 

 stecher). Tl, 

 Daughter larvae 

 developed from the 

 rudimentary 

 ovary. 



