lyO 



NATURE 



[October 14. 1915 



datory type of organism, the data at our disposal 

 appear to me to indicate very clearly the nature of 

 the changes that took place, as well as the final result 

 of these changes, but leave us in the dark with regard 

 to some of the actual details of the process. The 

 chief event was the formation, round the biococci, 

 of an enveloping matrix of protoplasm for which the 

 term periplasm (Lankester) is most suitable. The 

 periplasm was an extension of the living substance 

 which was distinct in its constitution and properties 

 from the original chromatinic substance of the bio- 

 coccus. The newly formed matrix was probably from 

 the first a semi-fluid substance of alveolar structure 

 and possessed two important capabilities as the result 

 of its physical structure ; it could perform streaming 

 movements of various kinds, more especially amoeboid 

 movement ; and it was able to form vacuoles in- 

 ternally. The final result of these changes was a 

 new type of organism which, compared with the 

 original biococci, was of considerable size, and con- 

 sisted of a droplet of alveolar, amoeboid periplasm in 

 which were imbedded a number of biococci. Whether 

 this periplasm made its first appearance around single 

 individual biococci, or whether it was from the first 

 associated with the formation of zoogloea-like colonies 

 of biococci, must be left an open question. 



Thus arose in the beginning the brand of Cain, the 

 prototype of the animal — that is to say, a class of 

 organism which was no longer able to build up its 

 substance from inorganic materials in the former 

 peaceful manner, but which nourished itself by cap- 

 turing, devouring, and digesting other living 

 organisms. The streaming movements of the peri- 

 plasm enabled it to flow round and engulph other 

 creatures ; the vacuole-formation in the periplasm 

 enabled it to digest and absorb the substance of its 

 prey by the help of ferments secreted by the biococci. 

 By means of these ferments the ingested organisms 

 were killed and utilised as food, their substance being 

 first broken down into simpler chemical constituents 

 and then built up again into the protein-substances 

 composing the body of the captor. 



A stage of evolution is now reached which I pro- 

 pose to call the pseudo-moneral or cytodal stage, since 

 the place of these organisms in the general evolution 

 of life corresponds very nearly to Haeckel's conception 

 of the Monera as a stage in the evolution of 

 organisms, though not at all to his notions with 

 regard to their composition and structure. The bodies 

 of these organisms did not consist of a homogeneous 

 albuminous "plasson," but of a periplasm correspond- 

 ing to the cytoplasm of the cell, containing a number 

 of biococci or chromatin-grains. In the life-cycles of 

 Protozoa, especially of Rhizopods, it is not at all 

 infrequent to find developmental phases which repro- 

 duce exactly the picture of the pseudo-moneral stage 

 of evolution, phases in which the nucleus or nuclei 

 have disappeared, having broken up into a number uf 

 chromatin-grains or chromidia scattered through the 

 cytoplasm. 



The next stage In evolution was the organisation 

 of the chromatin-grains (biococci) into a definite cell- 

 nucleus. This is a process which can be observed 

 actually taking place In many Protozoa in which 

 "secondary" nuclei arise from chromidia. With the 

 formation of the nucleus the cytode or pseudo-moneral 

 stage has become a true cell of the simplest type, 

 for which I propose the term protocyte. It Is now 

 the starting-point of an infinite series of further com- 

 plications and elaborations In manv directions. With 

 all the diverse modifications of the cell the nucleus 

 remains comparatively uniform. It may, indeed, vary 

 Infinitely in details of structure, but In principle it 

 remains a concentration or aggregation of numerous 

 grains of chromatin supported on some sort of frame- 

 NO. 2398, VOL. 96] 



work over which the grains are scattered or clumped 

 in various ways, supplemented usually by plastin or 

 nucleolar substance either as a cementing ground- 

 substance or as discrete grains, and the whofe marked 

 off sharply from the surrounding cytoplasm, with cr 

 without a definite limiting membrane. There is, 

 however, one point In which the nucleus exhibits a 

 progressive evolution of the most important kind. 

 I refer to the gradual elaboration and perfection of 

 the reproductive mechanism, the process whereby, 

 when the cell reproduces itself by fission, the chro- 

 matin-elements are distributed between the two 

 daughter-cells. 



The chromatln-constltuents of the cell are regarded, ' 

 on the view maintained here, as a number of minute 

 granules, each representing a primitive independent 

 living individual or biococcus. To each such granule 

 must be attributed the fundamental properties of 

 living organisms in general ; in the first place meta- 

 bolism, expressed in continual molecular change, in 

 assimilation and in growth, with consequent repro- 

 duction ; in the second place specific individuality. 

 As the result of the first of these properties the chro- 

 matin-granules, often perhaps ultra-microscopic, may 

 be larger or smaller at different times, and they 

 multiply by dividing each into two daughter-granules. 

 .\s a result of the second property, chromatin-granules 

 in one and the same cell may exhibit qualitative dif- 

 ferences and may diverge widely from one another 

 In their reactions and effects on the vital activities of 

 the cell. The chromatin-granules may be either in 

 the form of scattered chromidia or lodged in a definite 

 nucleus. When in the former condition, I have pro- 

 posed the term chromidlosome for the ultimate chro- 

 matinic individual unit; on the other hand, the term 

 chromlole Is commonly in use for the minute 

 chromatin-grains of the nucleus. 



In the phase of evolution that I have termed the 

 pseudomoneral or cytodal phase, in which the 

 organism was a droplet of periplasm containing 

 scattered biococci or chromldiosomes, metabolism 

 would result in an increase In the size of the cytode- 

 body as a whole, accompanied by multiplication of 

 the chromldiosomes. Indlvidualisation of the cytodes 

 would tend to the acquisition of a specific size — that 

 Is to say, to a limitation of the growth— with the 

 result that when certain maximum dimensions were 

 attained the whole cytode would divide into two or 

 more smaller masses amongst which the chromldio- 

 somes would be partitioned. 



In the next stage of evolution, the protocyte with 

 a definite nucleus. It Is highly probable that at each 

 division of the cell-body, whether Into two or more 

 parts, the primitive method of division of the nucleus 

 was that which I have termed elsewhere "chromldlal 

 fragmentation " ; that is to say, the nucleus broke 

 up and became resolved into a clump of chromldio- 

 somes, which separated into daughter-clumps from 

 which the daughter-nuclei were reconstituted. In- 

 stances of nuclear divisions by chromldlal fragmenta- 

 tion are of common occurrence among the Protozoa, 

 and represent probably the most primitive and direct 

 mode of nuclear division. 



It is clear, however, that if the chromatin-grains 

 are to be credited with specific individuality and 

 qualitative differences amongst themselves, this 

 method of nuclear division presents grave imperfec- 

 tions and disadvantages, since even the quantitative 

 partition of the chromatin is Inexact, while the quali- 

 tative partition is entirely fortuitous. 



It Is not surprising, therefore, to find that the 

 process of nuclear division undergoes a progressive 

 elaboration of mechanism which has the result of 

 ensuring that the twin sister-granules of chromatin 

 produced by division of a single granule shall be 



