ALTERNATE GENERATION. 937 



ently unfertilized pseudova^ or germinal cells. This happens, for ex- 

 ample, in certain Entozoa, and also in the Aphis, or plant-louse. 

 Sometimes these two modes of reproduction alternate, in different gen- 

 erations, more or less regularly. In such cases, the form of the ani- 

 mals produced from the true ova, or the first generation, differs, in 

 some respects, from that of the parent, especially in being nonsexual ; 

 whilst the offspring of these, or the second generation, derived from 

 pseudova, may either resemble the original parents, or may produce, 

 nonsexually, a third generation, or several generations, the last of these 

 producing animals, which are sexual and resemble the parents. This 

 is named propagation by alternate generation. In it, a female parent 

 animal produces ova, which are duly fertilized. The embryos, or 

 larvce, developed from these, are, at no time, like the mother ; they 

 grow, and then develop, in their interior, either a single individual, 

 which becomes like the parent ; or they may, by external division, or 

 external or internal gemmation, produce many such ; or they may 

 form, either at once, or in succession, a series of young, derived from 

 unfertilized pseudova, which at once, or after two, three, or more gen- 

 erations, ultimately produce animals similar to the first parents. These 

 again, like those parents, propagate sexually. The intermediate gen- 

 eration, or generations, of nonsexual proliferous larva?, have been called 

 by Steenstrup, nurses, to distinguish them from true mothers. 



This development by alternate generation never occurs in the Ver- 

 tebrata, and only rarely in the higher Non-vertebrata. Amongst the 

 Mollusca, no proper example of alternate generation has yet been met 

 with ; but it is almost constant in the Molluscoida. Amongst the 

 Arthropodous Annulosa, it has been observed in but one Crustacean, 

 Daphnia, and in only a few Insects, such as the Aphides, but not in 

 the Arachnida or Myriapoda. The Aphides present a remarkable 

 example of this alternation : in the hot season, they multiply rapidly 

 by successions of internal generations of pseudova; but as the tem- 

 perature is lowered in the autumn, males and females appear, and 

 development by ova ensues. In early spring, these ova again produce 

 viviparous individuals, which multiply by pseudova, and, after many 

 generations, towards the approach of winter, sexual Aphides once 

 more appear. This alternate generation likewise occurs in many An- 

 nelida, its asexual phase then constituting the so-called fission, as in 

 Nemertes, Nais, and others. It is common also, and occurs in all 

 degrees in the Annuloida, as in the Scolecide, the Trematode, and the 

 Cestode parasitic worms, in the Rotifera, and the Echinodermata, and 

 also, generally, in the Coelenterata, and in many Protozoa. Amongst 

 the Coelenterata, and others, the form which is evolved from the fer- 

 tilized ovum is named the scolex ; the compound forms arising from the 

 budding or fission of the scolex are named strobila, and the perfect 

 animals, again exhibiting true reproductive organs, are named pro- 

 glottides. In the Sponges, sexual reproductive organs have been seen, 

 giving rise to bodies like ova, in which a spongilla is developed. These 

 alternate with the gemmules. In the unicellular Protozoon, besides 

 fission, the so-called nucleus and nucleolus, or double nuclei, are 



