GEMM^. 



[ 287 ] 



GENERATIONS. 



vaiiable in the appearance of its pinnate 

 subdivisions; both spores and tetiaspores 

 are found on the ramules, the former in favel- 

 lidia immersed in sna ollen ramules. 



BiBL. Harvey, Brit. Mar. Alg. p. 137. 

 pi. 17 B, Phyc. Brit. pi. 53. 



GEMMiE. — This term is apphed to those 

 cellular structures formed in Flowerless 

 Plants, which become detached, and repro- 

 duce the individual independently of the 

 spores. They correspond to the buds of the 

 Flowering Plants and the gonidia of the 

 Thallophytes. They present themselves in 

 various forms, generally either as minute 

 green bodies or as bulbils. The gemmae are 

 especially remarkable in Marchantia. 

 They occur more commonly in the Mosses 

 and Hepaticae than in the Ferns and higher 

 Crvptogamia. 



GENERATIONS, alternation of. 

 — The general plan upon which the repro- 

 duction of animals is effected, viz. that of 

 sexes, involving the action of the spermatic 

 secretion upon the ova, and the subsequent 

 series of changes ultimately giving rise to 

 new individuals resembling the parents, is in 

 some instances departed from, and the em- 

 bryos of certain animals after their escape 

 from the ova, do not become directly deve- 

 loped into individuals resembling the parents, 

 but produce a new, larval kind of being, 

 which produces generations of the same 

 larval or other kinds, the last of which re- 

 semble the original parents. 



While, therefore, in animals reproduced 

 by the ordinaiy sexual process, the new 

 individuals resemble each other, or differ 

 only in sex ; in those which produce these 

 alternate or intermediate generations, the 

 new individuals differ from the parents and 

 even from each other, until the last of the 

 series returns to the state of the first parents. 

 This mode of reproduction has received the 

 above name, from the alternation of the lar- 

 val generations with the ordinary sexual form. 



Many instances of this process are men- 

 tioned under the heads of the Classes, &c. 

 in which they occur; as under Acaleph^, 

 Aphis, Entozoa, T^nia, &c. Thus, for 

 instance, in the Acalephae, the ciliated embryo 

 (PI. 40. fig. 6) produced by the ordinary 

 sexual process, becomes fixed (fig. 7) and 

 passes into the state of an asexual polype 

 (fig. 8) ; it then reproduces new individuals 

 from gemmae and stolons (fig. 9), idtimately 

 becoming segmented (fig. 10), and produ- 

 cing new individuals which resemble the 

 sexual parents. The intermediate or nvirse 



forms are those represented in figs. 7-10. 

 Again, in Tcenia, the Cysticercus or Echino- 

 coccus forms the nurse, producing new indi- 

 viduals by gemmation ; these when reaching 

 the alimentaiy canal becoming transformed 

 into T(Bnice Mdth sexual organs. 



But the alternation of generations, or a 

 modification of it, also occurs in animals in 

 which sexes are not known to exist, as in some 

 Infusoria. In these, the ordinary plan of 

 reproduction by division and gemmation is 

 departed from, and an animal differing from 

 the parent or a nurse form, resembling or 

 identical with Acineta and Actinophrys, is 

 produced, which give rise to embryos subse- 

 quently growing into the parent form. But 

 in these instances the nurse form is the 

 result of a kind of metamorphosis, rather 

 than of generation. 



The phaenomena designated by the phrase 

 alternation of generations are also strikingly 

 exemplified in the vegetable kingdom ; but 

 the conditions are very complicated, and the 

 analogies with those occurring in animals 

 somewhat difficult to trace. The Mosses, 

 Hepaticaceae and Ferns afford very clear 

 analogies to the Medusae, and others admit 

 of being made out; but it appears to us that 

 Steenstrup and others have confounded 

 various distinct points in the parallel drawn 

 between the alternation of generations of 

 animals and the metamorphoses (commonly 

 so-called) of plants. We will endeavour to 

 give a summary of the general facts con- 

 nected with the doctrine. 



1. All animals and plants reproduced by 

 a sexual process (and there is reason to 

 believe that this will ultimately be found 

 universal), originate from a simple cell, and 

 undergo a series of changes, in the course of 

 their development to the complete forai 

 endowed with sexual organs, in which they 

 assume forms analogous to animals (or 

 plants) belonging to classes of lower 

 (simpler) organization. 



2. In the highest animals, the metamor- 

 phoses are intra-uterine, as in most of the Mam- 

 malia ; in the lower animals these metamor- 

 phoses are in part or wholly extra-uterine. In 

 the higher plants the changes are partly mfra- 

 uterine {i. e. the embryo has already become 

 a leafy axis within the ovary, but it becomes 

 perfected into the sexual form subsequently), 

 in the lower partly or wholly extra-uterine. 



3. The lower animals and all plants are 

 capable of an asexual or vegetative repro- 

 duction, by the isolation and separation of a 

 portion of their substance. 



