August i6, 1906] 



NA TURE 



403 



wliiih Dcciirri'd -.IkhiIv ;iflcr, Ihr liulli which thc-y con- 

 lai ncd. 



l-<)llu«'in}4 up Ih.- iliii- whiih had hpin Ic.i.nd, Municr- 

 Chahiia.^ and hi-, collcaj^iic Sililiimborger examined the 

 shells of a larj^e series of forms, especially of the 

 Miliolidau. It was shown, in a tine series of papers, that 

 the phenomenon of dimorphism was present here too, and 

 mav lind its expression, not only in differences in size of 

 she'll and of central chamber, but also in the plan in which 

 ihe chambers of the two forms are arranged. 



While they differ conspicuously — though, as we shall see, 

 in very varying degrees — in the sizes of the initial chamber, 

 il is by no means the case that in all species, as in those 

 of the genus Nummuliles we have considered, the size 

 attained by the completed test presents so marked a differ- 

 ence. It is, in fact, more usual for the individuals of the 

 two forms of a species to attain approximately the same 

 size on the completion of growth, though standing so con- 

 trasted in the size of the initial chambers. 



The names mcgahisphci'c and microsphere have been 

 given to the large and the small initial chambers, and the 

 iwo forms are generally Unown as the megalospheric and 

 niicrospheric respectively. 



Ihe examination of olher gn>ll|)^ of h'oraminifera has 

 .iliundantly confirmed lhi. view that Ihc phenomenon of 

 dimorphism is widely prevalent among them. 



The Life-history of Polysloiiiella crispa. 

 Turnitig now from the consideration of the shells of 

 Koraminifera to the living animals, let us inquire what 

 light has been gained from them on the problem of the 

 significance of the phenomenon of dimorphism. 



If a large batch of individuals of Polyslometla crispa be 

 killed with a reagent which dissolves the shell, though 

 preserving its protoplasmic contents, il will be found, on 

 examining the casts so obtained, that besides those of the 

 type described and figured by Williamson with a com- 

 paratively large initial chamber (about lio ^), and these are 

 by far the most abundant, there are others in which the 

 initial chamber is much smaller (about lo /j,). In olher 

 words, megalospheric and microspheric individuals occur in 

 Ihe batch, as among the fossil shells of Numniulites, pre- 

 -.erved in the Eocene strata. 



On staining them another point of diflcrence appears. 

 .\ single large nucleus is found in the majority of the 

 megalospheric forms, while in the microspheric a number 

 of small nuclei lie in the chambers most remote from the 

 mouth of the shell. 



The result of observations on the living and preservpil 

 .iiiimals may be briefly stated as follows : — 



The Microspheric Form. 



The microspheric form has many small nuclei, even at an 

 early stage of growth. These nuclei consist of a homo- 

 geneous ground substance with many small nucleoli 

 scattered through it. They lie in the chambers near the 

 centre of the shell, ;md increase in number by simple 

 division. They also exhibit a remarkable phenomenon to 

 which I shall have to recall your attention later. Though 

 several of the nuclei, and especially those that have recently 

 divided, have a rounded contour, nianv of them are highlv 

 irregular in outline, giving off processes which extend in 

 branching irregular strands, staining deeply with nuclear 

 stains, into the protoplasm. Free shreds of such strands 

 lie scattered in the chambers in the neighbourhood of the 

 nuclei, and in large specimens of the microspheric form it 

 is common to find the protoplasm crowded with such deeplv 

 staining strands, and with no trace to be found of the 

 rounded nuclei present in the earlier stages. It is difficult 

 to avoid the conclusion that the nuclei, after increasing in 

 number by amitotic division, give off the strands and are 

 ultimately wholly resolved into them. 



In a culture of I'olystoniclla it is common to find a 

 n\odr of reproduction which on examination will be found 

 lo be that of the microspheric form. It is best followed 

 when occurring in a specimen attached to a glass slide. 

 In the early phases these specimens are distinguished by 

 a great increase in the number of pseudopodia issuing from 

 the shell, so that the latter appears when seen by trans- 

 mitted light to be surrounded by a milky halo. The proto- 

 plasm gradually emerges from the shell until, after some 



NO. 1920, VOL. 74] 



hours, the whole of it has come out and lies massed 

 between the shell and the supporting surface and within 

 the area formerly covered by the halo. The internal proto- 

 plasm is darkly-coloured with brown granules, and the 

 whole mass is during this time the scat of involved stream- 

 ing movemenls. Clear spots make their appearance, and 

 gradually the protoplasm collects about these and separates 

 into as many spherical masses, which remain connected' 

 by a felt of hyaline pseudopodia. Some 200 is a commoni 

 number to be found. Not long after they have become 

 distinct it may be noticed that each attains a shining coat 

 — the indication that a shell has been formed, a small 

 aperture being left in each for the passage of the pseudo- 

 podia. After lying in close contact for some hours, the 

 spheres rapidly and simultaneously draw apart from ont- 

 another, and within h:df an hour from the beginning of 

 Ihe movement they are dispersed over a wide area, and 

 each becomes the cenire of a svslem of pseudopodia of 

 its own. 



The whole of the protoplasm of the parent is used up in 

 the formation of the brood of young, the shell being left 

 empty. The process from the first appearance of the halo 

 to the dispersal of the voung is complete in .iljoul twelve 

 hours. 



In a short time the protoplasm which lies oulside the 

 .iperturc of each of the spheres secretes the wall of a second 

 chamber of characteristic shape, and the young individual 

 is ihen clearly recognisable in size and shape as the two- 

 chambered young of the megalospheric form. Each of the 

 spheres was, in fact, a megalosphere. The microspheric 

 parent has given rise lo, indeed it has become, a brood 

 of megalospheric young. 



Even before the formation of the megalospheres small 

 rounded, faintly staining nuclei can be seen in stained 

 preparations of the emerged protoplasm, and the latter 

 takes a deep flush owing to the presence of minute ])ar- 

 licles of chromatin. I am not aware that the origin of 

 these nuclei has been directly observed, but it appears 

 highly prob.ible that they arise by the gathering together 

 about new foci of the staining material distributed through 

 the proioplasm of the niicrospheric parent. 



The Mei:,dospheric l-orm. 



When the megalospheres have become formed their proto- 

 plasm contains abundance of irregular chromatin masses, 

 which arc at first diffused, and obscure the rounded 

 nucleus near the cenire, but I am inclined to think that 

 it is the latter which grows into the large nucleus, the 

 Principal-kern of Schaudinn, which is found throughout 

 the greater part of the life of the megalospheric form. 



.\s growth proceeds and the number of chambers in- 

 creases the nucleus moves on from chamber to chamber, 

 becoming greatly constricted as it passes through the 

 narrow passages of communication. It grows pari passu 

 wilh the growth of ihe protoplasm. Numbers of nucleoli 

 are contained in it, lying in a reticulum, and the nucleoli 

 appear to increase in number and to decrease in size as 

 growth advances. Here, too, as in the microspheric form, 

 the nucleus appears to give off portions of its substance 

 into the protoplasm, the path along which it has travelled, 

 through the earlier chambers, being strew'n with deeply 

 staining particles of irregular size. Towards the later 

 stages the nucleus loses its compact shape and staining 

 power, and ultimately disappears, and multitudes of minute 

 stained bodies may then be detected scattered through the 

 protoplasm. These become aggregated as distinct nuclei, 

 the protoplasm gathers about them, and they divide by 

 karyokincsis. Then follows a second karyoklnetic division, 

 and, the protoplasm having divided correspondingly, the 

 whole contents of the megalospheric shell emerges as a 

 multitude of minute biflagellate zoospores, some 4 il in 

 diameter. 



It so happened that I had been working at the life- 

 history of the Foraniinifera at the same time as Schaudinn. 

 though in ignorance of his work.' The results that I have 



1 F. Scliaudinn, "Die Fortpflanzung der Foraminiferen, und eine neue 

 Art der Kernvermehrunff,'' Bio^ Ceniralblatt,'Bii. xiv. N. 4, February. tPo^. 



' Ueb. d. Dimorphismus der Foraminiferen," Silz. Ber. d. C\-s. 



iialiirf. Fr. zu Berlin, 1895, N. 5. 



I. j. Lister, "Contributions to the Life-history of the Foraminifera," 

 Phil. Trans., vol. c xxxvi. B. (1S95), p. 401. 



