176 



NATURE 



\yuly I, 1875 



which are known in other undoubted cell-nuclei, as, for example, 

 in the germinal vesicle of many animals, in the nuclei of many 

 unicelMar plants, the nuclei of many parenchyma cells of the 

 higher plants, and the nuclei of many nerve-cells. The mature 

 Infusorium nucleus is often vesicle-like, and consists of a delicate 

 investing membrane and fine granular contents, precisely as in 

 the differentiated nucleus of many other cells. In many Ciliata, 

 if not in all, there is within the young nucleus a dark, more 

 refringent corpuscle, which has quite the same relations as the 

 nucleolus of a true cell-nucleus. 



Regarded from a physiological, no less than from a morpho- 

 logical point of view, the Infusorium nucleus and true cell 

 nucleus admit of a close comparison with one another. It may 

 be considered as established by the concurrent observations of all 

 investigators, that the nucleus of the Infusoria performs the 

 function of a reproductive organ, though the opinions entertained 

 as to the mode in which it thus acts are extremely divergent. 



It is now admitted that in the reproduction of unicellular 

 organisms both in the animal and vegetable kingdom, the 

 nucleus takes an important part, and by its division as a primary 

 act ushers in the division of the rest of the protoplasm. Even in 

 the cells which form constituents of tissues, the part played by 

 the nucleus is altogether similar, its division always preceding 

 the division of the cell itself. 



In quite a similar way does the nucleus behave in the ciliate 

 Infusoria. The non-sexual reproduction of the Infusoria by 

 division is perhaps universal. In such cases the division always 

 begins by the spontaneous halving of the nucleus, and this is 

 followed by a similar division of the surrounding protoplasm, 

 exactly as in the ordinary simple cell. 



Another phenomenon in which the nucleus plays an imporiant 

 part is named by Haeckel " spore formation." Under this 

 designation he comprehends all those cases in which — the idea 

 of a previous fecundation being rejected — the nucleus breaks 

 into numerous pieces, and each of these, apparently by becoming 

 encysted in a portion of the protoplasm of the mother body, 

 shapes itself into an independent cell — a so-called germ-globule, 

 (Keitnkugel). Now this is a true spore — just as much so as the 

 spores which arise quite in the same way in unicellular plants. 

 The whole process is to be regarded as a case of the so-called 

 endogenous multiplication of cells. 



Most authors, however, take a different view of the nucleus. 

 Following Balbiani, they regard it as an ovary ; and to the frag- 

 ments into which it breaks up they assign the significance of 

 eggs ; while the so-called nucleolus, wtiich lies outside the 

 nucleus, is, as we have seen, believed to be a testis in which 

 spermatozoa are developed for the fecundation of the eggs. 



We must bear in mind, however, that this ' ' nucleolus " has 

 been hitherto found in but a disproportionately small number of 

 species, while the spermatozoal nature of the apparent filaments 

 which have been noticed in it has by no means been proved ; 

 and we have already seen that some observed facts such as those 

 adduced by Biitschli are opposed to the view which woidd assign 

 to them the nature of true spermatozoa. 



As Haeckel remarks, however, even though the so-called 

 nucleolus be really a testis fecundating the eggs or fragments 

 derived from the breaking up of the nucleus, this would afford 

 no valid argument against the unicellularity of the Infusoria, for 

 precisely the same sexual differentiation and reproduction are 

 found in unicellular plants. 



It may now, then, be regarded as proved that the process by 

 which the body of the ciliate Infusorium attains a certain degree 

 of differentiation is repeated not only in other unicellular orga- 

 nisms, but in many parenchyma cells both of plants and animals. 

 The difference, as Haeckel with much force points out, between 

 the differentiation process of these parenchyma cells and that of 

 the Infusorium body consists in the fact that in the parenchyma 

 cells the differentiation is a one-sided one, conditioned by the 

 division of labour in the organism of which they form the con- 

 stituents, while in the Infusorium it is a many-sided one related 

 to all the different directions in which cell-life manifests itself, 

 and resting on a physiological division of labour among the 

 " plastidules " or protoplasm molecules. In other words, the 

 differentiation processes which in multicellular organisms are found 

 distributed among different cells, are united in the single cell of 

 the ciliate Infusorium, thus leading to the formation of an animal 

 very perfect in a physiological point of view, but which morpho- 

 logically does not pass the limit of a simple cell. 



In some rarer cases the Infusorium body is found to enclose 

 tv/o or more nuclei, and Haeckel admits that such Infusoria 

 must strictly be regarded as multicellular, since the nucleus in itself 



alone determines the individuality of the cell ; but these excep- 

 tional cases have no significance for the main conception of the 

 infusorial organism. The multiplication of the nucleus exerts 

 almost no influence on the rest of the organisation, and such 

 " multicellular ciliata " are to be compared with the colony- 

 building forms of the Acinetse, Gregarinse, Flagellatae, and other 

 undoubtedly unicellular organisms. 



In conclusion, Haeckel considers the systematic position of the 

 Infusoria. That they are genuine Protozoa, having no direct 

 relation to either the Coelenterata or the Worms, must be now 

 admitted. To this result we are led in the most convincing way 

 by all that we know of their development. In all the animal 

 types which stand above the Protozoa, the multicellular organism 

 is developed out of the simple egg cell by the characteristic pro- 

 cess of segmentation, and the cell masses so arising differentiate 

 themselves into two layers — the endoderm and the ectoderm, or 

 the two primary germ lamellae.* Resting on the fundamental 

 homology of these two layers in all the six higher types of the 

 animal kingdom, Haeckel had already f directed attention to 

 the fact that all these types pass in their development through 

 one and the same remarkable form, to which he gives the name 

 of Gastrula, and which he regards as the most important and 

 significant embryonal form of the whole animal kingdom. This 

 gastrula consists of a multicellular, usually oviform uniaxial, 

 body enclosing a simple cavity — the primordial stomach or intes- 

 tine cavity, which opens outward on one pole of the axis by a 

 simple orifice— the primordial mouth, and whose walls are com- 

 posed of two layers, the endoderm or inner germ lamella, and 

 the ectoderm or outer germ lamella. 



This larval form has now been shown by the researches of 

 Haeckel, Kowalevsky, Ray Lankester, and others, to occur in 

 members of all the six higher primary groups of the animal 

 kingdom ; and Haeckel, in conformity with what he has called 

 the biogenetic fundamental law+ — the recapitulation of ances- 

 tral forms in the course of the development of the individual- 

 had already in a former work § concluded in favour of a common 

 descent of all the six higher types from a single unknown ances- 

 tral form which must have been constructed essentially like the 

 Gastrula, and to which he gives the name of Gastnra. 



From this common descent the Protozoa alone are excluded, 

 these not having yet attained to the formation of germ lamellse 

 or of a true intestinal cavity. 



He regards this difference between the development of the 

 Protozoa and that of all the other animal types as so important, 

 that he founds thereon a fundamental division of the whole 

 animal kingdom into two great primary sections- — the Protozoa 

 and the Mdazoa. The former never undergo segmentation, never 

 develop germ lamella:, and never possess a true intestinal cavity ; 

 the latter, which include all the other types of the animal king- 

 dom, present a true segmentation of the egg cell, have all two 

 primary germ lamellse — endoderm and ectoderm — a true intes- 

 tine formed from the endoderm, and a true epidermis from the 

 ectoderm ; they all pass through the form of the gastrula, or an 

 embryonic form capable of being immediately deduced from it, 

 and (hypotheticallyj are all descended from a Gastrrea. 



The only Metazoa which in their existing condition have no 

 intestine are the low worm-groups — Ccestoda and Acanthoce- 

 phala — but these form only an apparent exception, for the loss 

 of their intestinal canal is a secondary occurrence caused by 

 parasitism, and Haeckel regards them as having descended from 

 worms in which the intestine was present. 



Several years ago Haeckel united into a separate kingdom, 

 under the name of Protista, certain low organisms, some of 

 which had been previously placed among the Protozoa, while 

 others had been assigned to the vegetable kingdom. To this 

 neutral group he refers the Monera, the Flagellata-, the Catal- 

 lacta;, the Labyrinthulea;, the MicromycetEc, and the Acytaria: 

 and RadiolariK. After the elimination of these there remain as 

 genuine Protozoa the Amoebina:, the Gregarins;, the Acinstce, 

 and, above all, the the true Infusoria or Ciliata, 



The union of the Protista into a distinct kingdom equivalent 

 in systematic value with the animal or vegetable kingdom, can, 

 however, scarcely be maintained. We already know enough of 

 some of them to justify our assigning these to one or other of 

 the two generally accepted organic kingdoms ; and there can be ^ 

 little doubt that, did we know the whole history of the others, 

 as well as the essential difference between the animal and vege- 



* The comparison of the endoderm and ectoderm of the Coelenterata to 

 the two primary germ lamellae of the Vertebrata was first made by Huxley. 

 t " Die Kalkschwamme," 1872. 

 X "Generelle Morphologic." 

 § ' ' Die Kalkschwamme. 



