. Elementary text-book of zoology, tr. and ed. by Adam Sedgwick, with the assistance of F. G. Heathcote. 106 OBGAXIZATIOX AND DEVELOPMENT OF AMMALS IX t I* a v • f- ,ric& 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 wi
. Elementary text-book of zoology, tr. and ed. by Adam Sedgwick, with the assistance of F. G. Heathcote. 106 OBGAXIZATIOX AND DEVELOPMENT OF AMMALS IX t I* a v • f- ,ric& 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 from 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 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 ayamic females peculiarly modified in the absence of organs for copulation and fertilization; and the reproductive cells aro 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 Bar
Size: 956px × 2614px
Photo credit: © Paul Fearn / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., book, bookcentury1800, bookcollectionbiodiversity, bookdecade1890
