4o6 



NATURE 



[August 26, 1897 



and drawn out into a great number of immensely long tentacles. 

 It buds laterally like a Hydra, sending out stolons or runners, 

 which bear new polyps, and separate before long, the polyps 

 becoming independent animals. In the midst of the tentacles 

 of the scyphistoma is a prominence which bears the mouth. This 

 grows upwards into a tall column, the strobila, which is sup- 

 ported below by the scyphistoma. When the strobila is well 

 nourished it divides into transverse slices, which at length detach 

 themselves, and swim away.^ These are the Ephyrse, which 

 had been found in the sea before Sars' time, and were then 

 counted as a particular kind of adult medusre. They are small, 

 flat discs with eight lobes or arms, all notched at the extremity. 

 A pile of ephyrre is produced by the transverse constriction and 

 division of the strobila in a fashion which reminds us of the 

 rapid production of the animals in a Noah's ark by the slicing 

 of a piece of wood of suitable sectional figure. It was thus 

 ascertained that the scyphistoma, strobila, and ephyra are suc- 

 cessive stages of one animal, but for a time no one could say 

 where the scyphistoma came from, nor what the ephyra turned 

 to. At length Sars, aided by the anatomical researches of 

 Ehrenberg and Siebold, was able to clear up the whole story. 

 The ephyra is gradually converted by increase of size and change 

 of form into an Aurelia, a common jelly-fish which swarms 

 during the summer in European seas. The Aurelia is of two 

 sexes, and the eggs of the female give rise to ciliated embryos, 

 which' had been seen before Sars' time, but wrongly interpreted 

 as parasites or diminutive males. These ciliated embryos, 

 called planuloe, swim about for a time, and then settle down as 

 polyps (scyphistomata). There is thus a stage in which Aurelia 

 divides without any true reproductive process, and another stage 

 in which it produces fertile eggs. There is alternation of genera- 

 tions in AureHa as well as in Salpa, and Sars was glad to fortify 

 by a fresh example the observations of Chamisso, on which 

 doubts had been cast. 



It was not long before the alternation of generations was 

 recognised in Hydromedusce also, and then the ordinary Hydro- 

 zoan colony was seen to consist of at least two kinds of polyps, 

 one sexual, the other merely nutrient, both being formed by the 

 budding of a single polyp. The sexual polyp, or medusa, either 

 swims away or remains attached to the colony, producing at 

 length fertilised eggs, which yield plar.uloe, and these in turn 

 the polyps which found new colonies. 



Those of us who are called upon to tell this story in our 

 regular course of teaching should not forget to produce our 

 scyphistoma, strobila and ephyra ; the interest is greatly enhanced 

 if they are shown alive. It is not hard to maintain a flourish- 

 ing marine aquarium even in an inland town, and a scyphistoma 

 may be kept alive in an aquarium for years, budding out its 

 strobila every spring. 



Alternation of generations, when first announced, was taken 

 to be a thing mysterious and unique. Chamisso brought in the 

 name, and explained that he meant by it a metamorphosis 

 accomplished by successive generations, the form of the animal 

 changing not in the course of an individual life, but from 

 generation to generation [foiniaper ge7ie7-ationes, neqnaqiiam in 

 prole scu individtto, vniiata). Sars adopted Chamisso's name 

 and definition. Steenstrup a little later collected and discussed 

 all the examplSs which he could discover, throwing in a number 

 which have had to be removed again, as not fairly comparable 

 with the life-histories of Salpa and Aurelia. He emphasised 

 the alternation of budding with egg-production, and the unhke- 

 ness in form of the asexual and sexual stages. Like Chamisso, 

 he caiefully distinguished between development with meta- 

 morphosis and alternation of generations. All three naturalists, 

 Chamisso, Sars and Steenstrup, laid stress on this point. In 

 an insect, they would have said, there is development with 

 metamorphosis. The same animal passes from larva to pupa, 

 and from pupa to imago. In Aurelia or Salpa, however, the 

 animal which lays eggs is not the animal which buds, but its 

 progeny. The cycle of the life-history includes two generations 

 and many individuals. 



This view has spread very widely, and if we were to judge 

 by what is commonly taught, I think that we should recognise 

 this as the doctrine now prevalent. It is however, in my 



1 Leuckart (Zeits. f. uUs. ZpoL, Bd. iii. p. i8i) remarks that elongate 

 animals tend to divide transversely or to bud axially, while bn ad animals 

 tend to divide longitudinally or to bud laterally. The question has been 

 raised more than once whether the division of the strobila is not really a 

 case of budding. Leuckart shows that budding and fission cannot be separ- 

 ated by any definition ; they pass insensibly into one another (" Wagner's 

 Handb. d. Physiol.," art. 'Zeugung"). 



NO. 1452, VOL. 56] 



opinion, far inferior as an explanation of the facts to thaft 

 adopted by Leuckart, Carpenter and Huxley, who- regard the 

 whole cycle, from egg to egg, as one life-history. Huxley and" 

 Carpenter, differing in this from Leuckart, do not shrink from, 

 calling the whole product of the egg an animal, even though \c 

 consists of a multitude of creatures which move about and seek 

 their food in complete independence of one another. Rather 

 than ignore the unity of the life-history of AureHa or Salpa, 

 they would adopt the most paradoxical language. This attitude: 

 was forced upon them by the comparative method. They re- 

 fused to study Aurelia, for example, as an animal apart ; it hacJ 

 its near and itf remoter relatives. Among these is the fresh- 

 water Hydra, which develops without transformation, buds off- 

 other Hydras when food is plentiful, and at kngth becomes- 

 sexually mature. Budding is here a mere episode, which may- 

 be brought in or left out, according to circumstances. The 

 same individual polyp which buds afterwards procfuces eggs.. 

 The life-history of Salpa cannot be traced with equal facility to» 

 a simple beginning, for it presents points of difficulty, on which 

 the learned differ. In the Polychoet Worms, howerer, we find 

 a beautiful gradation leading up to alternation of generations.. 

 We begin with gradual addition of new segments and increasing: 

 specialisation of the two ends of the body, the fore end be-' 

 coming non-reproductive, and the hinder end reproductive. 

 Then we reach a stage (Syllis) in which the reproductive half 

 breaks off from the fore part, and forms (after separation) a new 

 head, while the fore part adds new .segments behind. In. 

 Autolytus the new head forms before separation, aivd many- 

 worms may cohere for a time, forming a long chain with heads- 

 at intervals. In Myrianida the worms break up first, and after- 

 wards become sexually mature. We should gather from these- 

 cases that alternation of generations may arise by the intro- 

 duction of a budding-stage into a development with trans- 

 formation. The polyp or worm buds while young and lays egg^ 

 at a later time. The separation of the two processes of repro- 

 duction often becomes complete, each being restricted to its- 

 own place in the life-history. As a rule the worm or polyp wilb 

 bud while its structure is uncomplicated by reproductive organs.. 

 It is easy to propagate some plants by cutting one of the leaves- 

 into sections, and making every section root itself, and grow- 

 into a new plant ; but we can seldom do the same thing with a 

 flower. There may therefore be a distinct advantage to par- 

 ticular animals and plants in dividing the life-history into two- 

 stages, an earlier budding, and a later egg-laying stage. 



The advantage to be drawn from budding is easily seen in. 

 those animals which find it hard to gain access to a favourable- 

 site. Thus a Taenia^ is very lucky when it establishes itself in. 

 the intestine. Once there, it goes on budding indefinitely. It! 

 is harder to trace the advantage in the case of many polyps, 

 though some (Cunina, &c.) admit of the same explanation as- 

 Tsenia. There are yet other cases (some Worms, Salpse, &c.)' 

 in which our ignorance of the conditions of life rendeis a satis- 

 factory explanation impossible at present. 



The budded forms often differ in structure from the budding-, 

 forms which produce them, and many writers and teachers make- 

 this difference part of the definition of alternation of generations. 

 I think that Leuckart has suggested a probable explanation irii 

 his es.'-ay of 1851,'-' which is still thoroughly profitable reading. 

 He attributes the peculiarities of the larva mainly to the cir- 

 cum.stance that it is turned out at an early age to shift for itself.. 

 In the budded forms there is no such necessity. The parent has- 

 established itself on a good site which commands a sufficiency 

 of food. Until it has done this, it does not bud at all. The 

 young which it produces asexually need not disperse in infancy, 

 at least until crowding sets in. The tradesman who has founded 

 a business puts his elder boys into the shop ; perhaps the younger 

 ones may be obliged to try their luck in a distant town. The 

 budded forms, reared at the cost of the parent, may therefore 

 omit the early larval stages at least, and go on at once to a later 

 or even to the final stage. Thus the head of Taenia, when it 

 has fixed itself in the intestine, produces sexual segments ; the 

 redia of Distomum produces cercarise or more redite, omitting, 

 the locomotive embryo ; the scyphistoma produces ephyrte. 

 The saving of time must often be great, and the days saved 

 are days of harvest. Think how much a tree would lose if in 

 the height of summer it were unable to bud, and could only 



1 This ca.se is quoted by Leuckart. 



2 " -(Jeber Metamorphose, unge.schlechtliche Vermehrung, Generations- 

 wechsel," Zeits. /. wiss. Zool., Bd. iii. Equally important is the same 

 author's treatise, " Ueber den Polymorphismus der Individuen oder die 

 Erscheinung der Arbeitstheiiung in der Natur," Giessen, rSsi. 



