130 



GENERATIONS 



ists glimpses of other alternations ; but Steenstrup 

 was the first to generalise the results in his work 

 published in 1842, entitled ' On the Alternation of 

 Generations ; or the propagation and development 

 of animals through alternate generations, a peculiar 

 form of fostering the young in the lower classes of 

 animals.' From hydroids and flukes he gave illus- 

 trations of the ' natural phenomena of an animal 

 producing an offspring winch at no time resembles 

 its parent, but which itself brings forth a pvogeny 

 that returns in its form and nature to the parent,' 

 anil distinguished the interpolated generation as 

 the Amme, or 'wet-nurse.' His essay was sternly 

 criticised by Owen in 1849, while "Leuckart at- 

 tempted to treat all the alternations as cases of 

 metamorphosis. Criticism, however, has only ren- 

 dered Steenstrup's generalisation more precise, and 

 the observations of some of the foremost natural- 

 ists have shown that the phenomena are of wider 

 occurrence than was at first supposed, though the 

 form of the alternation varies widely in the different 

 cases. 



(a) The Rhythm between Sexual and Asexual Re- 

 production. The simplest case to start with is that 

 of many hydroids where a sessile, plant-like zoo- 

 phyte a colony of numerous nutritive ' persons ' 

 produces in the summer months modified reproduc- 

 tive individuals which are set adrift as medusoids. 

 These become sexual, and their fertilised ova 

 develop into embryos which settle down and give 

 rise to the sessile /oophyte from which we started. 

 The life-history may be written in the formula : 



M 



M 



M 



( where M and F stand for male and female, and A 

 for asexual generation). 



The life-history of the common jelly-fish ( Aurelia) 

 ( fig. 1 ) illustrates a similar contrast. From the 



Fig. 1. Life-history of the common Jelly-fish : 



1, free-swimming embryo (planula); 2-6, the embryo fixed 

 developing into a ' hydra-luba,' which (7-8) divides trans- 

 versely into a pile of individuals ; these in turn (9) are 

 liberated and grow ( 10-1 1 ) into jelly-fish. ( From Haeckel.) 



large free-swimming sexual jelly-fish embryos are 



Eroduced which develop not into jelly-fish again, 

 ut into sessile tubular organisms or ' hydra-tubse. ' 



A, asexual, produces S, sexual, from fertilised ovum of which 

 A again arises. 



From these, by growth and division in an entirely 

 asexual fashion, the jelly-fish are in turn repro- 



duced. Here the sexual generation is the more 

 stable and conspicuous the reverse of the former 

 case, but the same formula applies, or the preceding 

 graphic notation. In the free-swimming Tunicata 

 (Salpa and Doliolum) the alternation is some- 

 what more complex, but in no essential respect 

 different. 



(b) Alternation between Sexual and Degenerate 

 Sexual Reproduction. The life-history of the com- 

 mon liver-fluke, sketched in the article FLUKE, is 

 in most cases as follows : From the fertilised ovum 

 of the fluke an embryo develops, which produces 

 several asexual generations, the last of which grow 

 up to become sexual flukes. Now the asexual 

 generations are not products of division or budding, 

 but arise from what, though not ova, maybe called 

 precocious reproductive cells ; in fact, they arise by 

 a degenerate process of parthenogenetic reproduc- 

 tion in early life. The facts may be thus ex- 

 pressed : 



where A 2 and A* represent two of the interpolated 

 asexual generations. 



This alternation between sexual reproduction 

 by fertilised ova and reproduction by means of 

 special cells which require no fertilisation prevails 

 in many plants e.g. ferns and mosses. From a 

 fertilised egg-cell arises the ordinary fern-plant 

 with which all are familiar. This, however, pro- 

 duces no male or female elements, but simply 

 ' spores,' which are able of themselves ( when they 

 fall to the ground) to develop a new organism 

 the inconspicuous but sexual ' prothallus. ' This 

 bears male or female organs or both, and from the 

 fertilised egg-cell thus produced the conspicu- 

 ous vegetative, sexless fern-plant once more 

 arises. The facts may be again expressed in 

 notation : 



A, the vegetative sexless fern-plant produces a spore (sp.) from 

 which the sexual ' prothallns,' S, arises, giving origin to 

 fertilised egg-cells, and thereby recommencing the cycle. 



The same formula will apply to the moss. The 

 familiar moss-plant bears male and female repro- 

 ductive organs. From a fertilised egg-cell so pro- 

 duced a sexless spore-producing generation at once 

 develops, and grows like a parasite on the apex of 

 the moss-plant. The spores fall to the ground, and 

 grow out into threads ( 'protonema'), from which 

 there is finally budded the moss-plant with which 

 we started. 



Besides the above alternations there are other 

 rhythms, some more complex, others much less 

 frequent, into which we cannot here enter. In 

 some cases the life-history of the liver-fluke, by the 

 division of the embryo (sporocyst), combines the 

 alternations () and' (b) ; in some midge larva; 

 juvenile parthenogenesis alternates with the adult 

 sexual process ; in not a few cases, as in aphides, 

 the rhythm is between parthenogenesis and normal 

 sexual reproduction ; while finally there is an alter- 

 nation of two different sexual generations in three 

 thread-worms or nematodes. 



Occurrence. Alternation of generations is hinted 



