128 OBGANIZATION AND DEVELOPMENT OF ANIMALS IN GENEBAL. 



(Phylloxera), in that one or more (winged and apterous) female 

 generations are characterised by parthenogenetic reproduction, and 

 consist only of oviparous females ; while the generation of females, 

 which lay fertilised eggs, appears with the males only at certain 

 times of the year, and can be distinguished by their small size, and 

 by the reduction of their mouth parts and digestive apparatus. 



Such forms of heterogamy lead back apparently to alternations 

 of generations, especially when the parthenogenetic generations 

 present, in the structure of their generative apparatus, essential 

 differences from the females which copulate. 



The plant-lice and gall-flies afford instances of this. The repro- 

 ductive processes of these animals, on the authority of Steen- 

 strup and V. Siebold, were regarded for a long time as instances of 

 alternations of generations, until the view, which was supported by 

 the reproductive processes of the allied bark-lice, that they came 

 under the head of heterogamy, received general assent. According 

 to this view, the viviparous forms of the plant-lice (Aphides) are 

 merely modified females adapted for parthenogenetic reproduction, 

 and their reproductive gland is nothing more than the modified 

 ovary. There are also cases of heterogamy in which, in the partheno- 

 genetic generations, the development of the egg commences in the 

 ovary of the larva, reproduction being shifted back into larval life. 

 This form of heterogamy, which resembles alternations of genera- 

 tions, was shown to occur in the larva of Cecidomyia (Miastor) by 

 Nic. Wagner and by 0. Grimm in the larva of a species of Chiro- 

 nomus, and is to be looked upon as a case of precocious development 

 of the egg in the parthenogenetic generation. The morphologically 

 undeveloped larva has acquired the power of reproducing itself by 

 means of its rudimentary ovary, a phenomenon which, following the 

 proposal of C. E. v. Baer, has been designated Pcedogenesis. 



If the reproductive gland of the Cecidomyia larva be looked upon 

 as a germ-gland, and the cells contained in it as germ cells or spores, 

 the reproduction of the Cecidomyia falls into the category of alterna- 

 tions of generations. But the idea involved in the term " spore " is 

 borrowed from the vegetable kingdom, and there is no reason for 

 looking upon these or any other structures in the Metazoa as spores. 

 The above explanation, therefore, is untenable. The reproductive 

 cells in the Metazoa, which have been regarded in this light, have 

 much more probably originated from masses of cells which represent 

 the rudiment of the ovary, and which are usually visible in early 

 stages of development. 



