ALTERNATION OF GENERATIONS. 215 



SUMMARY. 



1. The fact that successive generations may be markedly different was 

 observed by the poet Chamisso, and first made precise by the zoologist 

 Steenstrup. 



2. A fixed asexual hydroid buds off and liberates locomotor sexual 

 swimming-bells, whose fertilised ova give rise again to hydroids. Asexual 

 and sexual generations alternate. 



3. The offspring of the liver-fluke forms from certain cells in its body 

 a numerous progeny ; these repeat the same process several times ; the last 

 generation grow into the sexual liver-flukes. Reproduction by special 

 cells like precocious undifferentiated ova, alternates with reproduction by 

 ordinary fertilised ova. So too the vegetative sexless " fern-plant " gives 

 rise to special cells like parthenogenetic egg-cells, which develop into an 

 inconspicuous sexual prothallus. From the fertilised egg-cell of the latter 

 the " fern-plant " arises. 



4. These two different kinds of alternations (§ 2 and § 3) may be com- 

 bined in a more complicated manner. 



5. In some flies precocious parthenogenetic reproduction alternates with 

 the normal sexual reproduction of the adults. 



6. In many insects and crustaceans, parthenogenetic reproduction alter- 

 nates with the normal sexual process. There may be one or many inter- 

 vening parthenogenetic generations. 



7. A hermaphrodite threadworm parasitic in the frog fertilises its own 

 eggs, which develop into free-living males and females, from the fertilised 

 ova of which the hermaphrodite parasites again arise. Here there is an 

 alternation of sexual generations. 



8. In animals these alternations occur from sponges up to tunicates. 



9. In plants they occur in a\gve and fungi, are almost constant in ferns 

 and mosses, but are inconspicuous in higher plants. 



10. The problem of heredity is somewhat complicated by such alter- 

 nations. 



11. Alternation of generations is but a rhythm between a relatively 

 anabolic and katabolic preponderance. 



12. The origin has varied considerably in different cases. 



LITERATURE. 



See the general works already cited ; also, Steenstrup " On the Alternation 

 of Generations," transl. Ray Soc, 1845 ; Owen's "Parthenogenesis," 

 &c., 1849; Hceckel's " Generelle Morphologic," 1866; Weismann, 

 A., Die Entstehung der Sexualzellen bei den Ilydromedusen, Jena, 

 1883 ; and Papers on Heredity, Translation, Oxford, 1889 ; Vines' 

 article "Reproduction — Vegetable," Ency. Brit.; and the ordinary 

 Text-books of Zoology and Botany. 



