PRESIDENTIAL ADDRESS. 591 



stomellas iu a vessel were uearly approaching the formation of flagellated spores — 

 that is, that most of the examples which I tixed and stained presented 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 under a coverslip. In the specimens which had already formed spore 

 nuclei the masses of plasma did not die, but the spores developed quite normally 

 and "swarmed" apart. 1 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 fertilisa- 

 tion 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 witnessing many conjugations. The swarm-cells have, as previously stated, two 

 flagella, and a similar wobbling motion to those of Hyalopus which I have minutely 

 described ; they conjugate in pairs, and cast ofi' their flagella as in Tricko- 

 sphariuni. 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 live-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 manj'-chambered stage described in my earlier publications directly 

 proceeds, so that the life-cycle of Poli/stomella is now complete.' 



We are then, at last, able to give with confidence an answer to the ques- 

 tion — What is the significance of the phenomenon of dimorphism in the 

 Foraminifera P 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 conju- 

 gation of two similar zoospores, produced by individuals of the megalospheric 

 form. 



In the life-histories of Foraminifera belonging to other families — though not, so 

 far as I am aware, in the Nummulitidte, to which Polystmnella belongs — there is 

 clear evidence that the members of the megalospheric generation do not always 

 end their existence by the production 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 asexual 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 megalosphere is much larger 

 than that of the microsphere, and the two forms are thus easily distinguished. 

 There are, however, species (e.g., Penerojjlis, Discorbina 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 i.s 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 reproduction ; and this applies to a 

 wide range of existing species and to fossil forms as far back as the Palseozoic 

 period. The pelagic Foraminifera present a curious and interesting problem in the 



' I have translated the word ' Kopulation ' as ' conjugation,' which in its bio- 

 logical usage describes the nature of the process more accurately than the English 

 equivalent. 



