ALTERNATION OF GENERATIONS. 199 



SUMMARY. 



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

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

 Steenstrup. 



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

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

 sexual generations alternate. 



III. 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 

 fertilized 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 fertilized egg-cell of the latter the "fern-plant" arises. 



IV. These two different kinds of alternations (II. and III.) may be combined 

 in a more complicated manner. 



V. In some flies precocious. parthenogenetic reproduction alternates with the 

 normal sexual reproduction of the adults. 



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

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

 parthenogenetic generations. 



VII. A hermaphrodite threadworm parasitic in the frog fertilizes its own 

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

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

 of sexual generations. 



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



IX. In plants they occur in algae and fungi, are almost constant in ferns and 

 mosses, but are inconspicuous in higher plants. 



X. The problem of heredity is somewhat complicated by such alternations. 



XI. Alternation of generations is but a rhythm between a relatively anabolic 

 and katabolic preponderance. 



XII. 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; Haeckel's " Generelle Morphologie," 1866; Weisrnann, A., Die 

 Entstehung der Sexualzellen bei den Hydromedusen, Jena, 1883 ; and 

 Papers on Heredity, Translation. Oxford, 1889; Vines' article " Reoroduc- 

 tion — Vegetable, " Ency. Brit.; and die ordinary Text-books of Zoology 

 and Botanv. 



