404 



NA TURE 



[August i6, 1906 



set before you on Polystomclla were obtained by both of us 

 independently of one another, though I had not obtained 

 evidence of more than one division of the spore-nuclei or 

 of the number of the flagella of the zoospores. 



The evidence pointed strongly in the direction of the 

 view that the foraminiferal life-history consists of an 

 alternation of generations. While the megalospheric form 

 would, on this hypothesis, arise by a simple vegetative 

 asexual reproduction of the microspheric parent, many 

 considerations seemed to indicate the probability that the 

 microsphere, the initial chamber of the microspheric form, 

 arose by the conjugation of zoospores. In addition to the 

 general probability of the occurrence of a sexual stage 

 somewhere in the life-history, the sizes of zoospore and 

 microsphere fitted in with the view that the latter might 

 be formed by the coalescence of two of the former. Again, 

 the fact of the rarity of the microspheric form in com- 

 parison with the megalospheric was comprehensible, on 

 the supposition that, to be able to conjugate, the zoospores 

 must be of different parentage. The point remained, how- 

 ever, a matter of inference until three years ago, when 

 Schaudinn published an account of the processes that he 

 had observed,' turning inference into certainty. Premising 

 that chroniidia is the name applied to the fragments of 

 staining material distributed in the protoplasm, I will 

 quote the passage : — ■ 



" With the onset of the cold part of the year I observed 

 that many large Polystomellas in a vessel were nearly 

 approaching the formation of flagellated spores — that is, 

 that most of the examples which I fixed and stained pre- 

 sented already the complete filling with chromidia, and 

 others had even formed the spore-nuclei. I now took out 

 at random a large number, and, breaking the shells, 

 squeezed out the plasma imder a coverslip. In the speci- 

 mens which had already formed spore nuclei the masses 

 of plasma did not die, but the spores developed quite 

 normally and " swarmed " apart. I was thus not only 

 able to follow clearly with an immersion lens the twice- 

 repeated division of the vesicular nuclei, which occurs very 

 rapidly, but was able repeatedly to observe directly the 

 conjugation - of the swarm-cells. The reason that I had 

 not succeeded earlier in this latter, though I had often 

 observed the formation of swarm-cells, is that conjugation 

 only occurs between those arising from separate individuals. 

 I proceeded now as in fertilisation experiments with the 

 eggs of sea-urchins ; that is, I crushed a great number of 

 large Polystomellas in sea-water, sucked up the expressed 

 plasma in a capillary tube, stirred it about on the cover- 

 glass of a moist chamber, and then had the joy of witness- 

 ing many conjugations. The swarm-cells have, as pre- 

 viously stated, two flagella, and a similar wobbling motion 

 to those of Hyalopus which I have minutely described ; 

 they conjugate in pairs, and cast off their flagella as in 

 Trichosphjerium. The karyokinesis occurs very slowly 

 (5-6 hours). When it is finished the nucleus of the zygote 

 soon divides by direct division, and the typical growth 

 begins, with formation of a shell. I have cultivated the 

 young microspheric individuals in a moist chamber as far 

 as the five-chambered stage, when they died, probably 

 from want of nourishment. In most cases the nucleus had 

 repeatedly divided. From these small, many-nucleated 

 microspheric individuals the youngest many-chambered 

 stage described in my earlier publications directly proceeds, 

 so that the life-cycle of Polystomella is now complete." 



We are then, at last, able to give with confidence an 

 answer to the question — What is the significance of the 

 phenomenon of dimorphism in the Foraminifera? The 

 .answer is, It results from the occurrence of two modes of 

 reproduction in the life-history, sexual and asexual. The 

 megalospheric form is the product of asexual reproduction, 

 the microspheric form arises from the conjugation of two 

 similar zoospores, produced by individuals of the megalo- 

 spheric form. 



In the life-histories of Foraminifera belonging to other 



families — though not, so far as I am aware, in the 



Nummulitidse, to which Polystomella belongs — there is 



1 " Untersuchuneen iib. d. Fortpflanzung einiger Rhizopoden," Afl>. ti. 



d. Kah. Gestindheitsaiitte, Bd. xix. Heft 3. 1903. 



- I have translated the word " Kopulalion" as "conjugation,'" which in 

 its biolosical usage describes the nature of the process more accurale'y 

 than the English equivalent. 



NO. 1920, VOL. 74] 



clear evidence that the members of the megalospheric 

 generation do not always end their existence by the pro- 

 duction of zoospores. The protoplasm may emerge from 

 the shells and break up into a brood of megalospheres, as 

 in the reproduction of the microspheric form. In such 

 Foraminifera, therefore, we have to conclude that the 

 megalospheric phase may be repeated in the life-history, 

 and that there may be a succession of megalospheric forms 

 before the sexual stage recurs in the life-cycle. Such a 

 repetition of the ase.xual mode of reproduction is a common 

 phenomenon in the life-histories of other groups of 

 Protozoa. 



In the great majority of cases the size of the megalo- 

 sphere is much larger than that of the microsphere, and 

 the two forms are thus easily distinguished. There are, 

 however, species (e.g. Peneroplis, Discorhina globularis) in 

 which the range of variation of the small megalospheres 

 overlaps that of the microspheres, and we have to rely on 

 other characters for discrimination of the two forms. 



We must not, however, too hastily apply these results 

 to all the organisms included among the Foraminifera. 

 Wherever there is dimorphism, as expressed in the sizes of 

 the initial chambers, it is clear evidence of the occurrence 

 in the life-history of the sexual and asexual modes of re- 

 production : and this applies to a wide range of existing 

 species and to fossil forms as far back as the Palaeozoic 

 period. The pelagic Foraminifera present a curious and 

 interesting problem in the fact that their initial chambers 

 are, at least in the great majority of cases, of uniformly 

 small size, a condition which I suspect to depend on their 

 peculiar mode of life. Again, in the simpler groups 

 (Gromiida2 and Astrorhizidse) the covering appears, in many 

 cases at least, to expand with the growing protoplasm, so 

 that the evidence of their initial condition is not preserved 

 in the shells. In these cases also we have to seek for 

 evidence of the course of the life-history in nuclear and 

 other characters. 



Review of Nuclear Characters. ■ 

 Turning now to the nuclear changes which are found 

 in Polystomella, there are many features which are worthy 

 of attention. In their feeding, locomotion, and the mode 

 of forming the shell, in fact in all that concerns their 

 vegetative existence, the megalospheric and microspheric 

 forms are, so far as I am aware, exactly alike ; yet in 

 one the economy is dominated by a single nucleus, and in 

 the other by many. Richard Hertwig has compared a 

 uninucleate organism, whether a whole protozoon or a 

 metazoan cell, to an absolute monarchy, and the multi- 

 nucleate organism to an oligarchy, in which the rulers, 

 though many, perform identical functions. In the life- 

 history of Polystomella the apparently revolutionary change 

 in government occurs at each reproductive phase, yet the 

 internal and external relations of the State, as far at least 

 as its vegetative life goes, appear to remain unaltered. 

 Why the nucleus of the microspheric form should divide 

 up into a number of daughter nuclei, while that of the 

 megalospheric form remains single, is, to me at least, 

 entirely obscure. 



The separation of portions of the chromatic substance 

 of the nuclei, in both forms of the species, and the ulti- 

 mate resolution of the whole of it into such shreds, dis- 

 persed through the protoplasm, appeared at first a puzzling 

 and obscure phenomenon. In metazoan cells, which are 

 advancing to the formation of the reproductive elements, 

 the nuclear divisions occur in regular succession, and the 

 nucleus of a germ-cell may be regarded as the daughter 

 nucleus, granddaughter, great-granddaughter, and so forth, 

 of some other nucleus which went before It. The aphorism 

 omni'i iiiicleiis e vuclco appears to hold good for the 

 metazoa, but how does It find Its application in the case 

 we are considering? Is there any recognition of the 

 hereditary principle when the change of government of our 

 State occurs? Light has recently come on this obscure 

 phenomenon, and, as usual, by the results obtained In 

 other groups of Protozoa. In the introductory essay, " Die 

 Protozoon und die Zelltheorie." which he contributed to 

 the first number of Schaudinn's " Archlv " Richard Hert- 

 wig drew attention to morphological elements of the 

 protozoan body, distinct from the protoplasm on the one 



