GENERATIONS. 



[ 352 ] 



GENERATIONS. 



ENTOZOA, TJENIA, &c. Thus, 

 for instance, in the Acalephae, the ciliated 

 embryo (PI. 49. tig. 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), 

 ultimately becoming segmented (fig. 10), 

 and producing new individuals which re- 

 semble the sexual parents. The interme- 

 diate or nurse forms are those represented 

 in figs. 7-10. Again, in Tcenia, the Cysti- 

 cercus or Echinococcus forms the nurse, pro- 

 ducing new individuals by gemmation, these, 

 on reaching the alimentary canal, becoming 

 transformed into Tcenicp with 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 and Rhizopods. In these, 

 the ordinary plan of reproduction by di- 

 vision and gemmation is departed from, 

 and an animal differing from the parent, or 

 a nurse-form resembling or identical with 

 Acineta andActinophrys, is produced, which 

 gives rise to embryos subsequently growing 

 into the parent form. But in these in- 

 stances the nurse- form is the result of a kind 

 of metamorphosis, rather than of generation. 



In the Aphidae, the process is exhibited 

 by the repeated formation of viviparous 

 broods of beings, from apparently female 

 insects, without impregnation, the final 

 progeny being sexual. This is called par- 

 thenogenesis. 



The phenomena designated by the phrase 

 alternation of generations are also strikingly 

 exemplified in the vegetable kirgdom ; but 

 the conditions are very complicated, and the 

 analogies with those occurring in animals 

 somewhat diificult to trace. The Mosses, 

 Hepaticae, 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 va- 

 rious 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 

 connected 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 proto- 

 plast or cell, and undergo a series of changes, 

 in the course of their development to the 

 complete form 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 

 Mammalia ; in the lower animals these 

 metamorphoses are in part or wholly extra- 

 uterine. In the higher plants the changes 

 are partly intra-uterine (t. 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. 



4. Many animals and all plants are ca- 

 pable of being multiplied by this vegetative 

 reproduction in their intermediate stages of 

 extra-uterine development ; and in such cases 

 the reproduction, fissiparous, gemmiparous, 

 or other, assumes the character peculiar to 

 the class to which the intermediate form is 

 analogous (e. g, the polypiform reproduction 

 of the Acalephae, the confervoid growth 

 and multiplication of the proembryo of the 

 Mosses). The product of the vegetative 

 reproduction is either like or unlike the 

 body which produces it : in the former case 

 the vegetative reproduction will be re- 

 peated ; but in the latter case the product is 

 usually provided with sexual organs, and 

 the cycle of development is completed by 

 the reproduction of a fertilized ovum. In 

 the latter case we have what is called an 

 alternation of generations. 



It will be evident that we here exclude 

 from consideration the metamorphoses 

 within the sphere of the individual shoot on 

 plants that is, the metamorphosis of the 

 leaf, the morphological element of the higher 

 plant. It appears to us that these are not to 

 be taken as parallels to the metamorphoses 

 of animals comprehended by Steenstrup 

 under the name of alternation of genera- 

 tions, which would rather be found in the 

 cases where bulbs, bulbils, tubers, &c. ap- 

 pear in the place of shoots, as the product 

 of branch-buds. The analogy would hold 

 also with the gemmae of the Mosses, &c., and 

 with the gonidia of the Thallophytes. Our 

 space does not admit of a more minute exa- 

 mination of the subject. Illustrations of 

 the phenomena in vegetables will be found 

 under FERNS, MOSSES, CONFEBVOIDE^E, 

 LICHENS, certain Fungi, e. g. ERYSIPHE, 

 PENICILLIUM, &c. 



