PRESIDENTIAL ADDRESS. 457 



pseudo-niuneral stage of evolution, phases in which the nucleus or nuclei have 

 disappeared, having broken up into a number of chromatin-graina or chromidia 

 scattered through the cytoplasm. We do not know as, yet of any Protozoa, 

 however, which remain permanently in the cytodal stage, that is to say, in 

 which the chromatin-grains remain permanently in the scattered chromidial 

 condition, without ever being concentrated and organised into true nuclei ; but 

 it is quite possible that some of the primitive organisms known as Proteomyxa 

 will be found to exhibit this condition and to represent persistent Pseudo- 

 monera or cytodes. 



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. It seems not unreasonable to suppose that a detailed study of the 

 manner in which secondary nuclei are formed in Protozoa will furnish us with 

 a picture, or rather series of pictures, of the method in which the cell-nucleus 

 arose in phylogeny. To judge from the data supplied by actual observation, 

 the evolution of the nucleus, though uniform in principle, was sufficiently 

 diversified in the details of the process. As one extreme we have the formation 

 of a dense clump of .small, separate chromatin-grains, producing a granular 

 nucleus of the type seen in Dinoflagellates, in Hfemogregarines, and in Diatoms. 

 Amongst the chromatin-grains there may be present also one or more grains 

 or masses of plastin forming true nucleoli. At the opposite extreme a clump 

 of chromatin-grains becomes firmly welded together into a single mass in which 

 the individual grains can no longer be distinguished, forming a so-called karyo- 

 some, consisting of a basis of plastin cementing or imbedding the chromatin- 

 grains into a mass of homogeneous appearance. Whatever the type of nucleus 

 formed, the concentration of the chromidia into nuclei does not necessarily 

 involve all the chromidia, many of which may remain free in the cytoplasm. 



In the chromidial condition the chromatin-grains scattered in the cytoplasm 

 are lodged at the nodes of the alveolar framework.-^ Consequently a supporting 

 framework of cytoplasmic origin, the foundation of the linin-framework, was 

 probably a primary constituent of the cell-nucleus from the first. In many 

 nuclei of the karyosomatic type it is very difficult to make out auythiiig of tlie 

 nature of a framework, which, however, in other cases is seen clearly as delicate 

 strands radiating from the karyosome to the wall of the vacuole in which the 

 karyosome is suspended. Probably such a framework is present in all cases, 

 and each supporting strand is to be interpreted as the optical section of the 

 partition between two protoplasmic alveoli. 



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 froloci/tc. 

 It is now the starting-point of an infinite series of further complications and 

 elaborations in many directions. It ie clearly impossible that I should do nnn'o 

 than attempt to indicate in the most summary manner the various modifications 

 of the cell -type of organism, since to deal with them conscientiously would 

 reqaire a treatise rather than an address, and, moreover, many such treatises 

 exist already. The most conspicuous modifications of cell-structure are those 

 affecting the periplasm, or, as we may now tenn it, the cytoplasm. In the first 

 place, the cell as a whole takes various forms ; primitively a little naked mass 

 of protoplasm tending to assume a spherical form under the action of surface- 

 tension when at rest, the cell-body may acquire the most diverse specific forms 

 maintained either by the production of envelopes or various kinds of exo.skeletal 

 formations on the exterior of the protoplasmic body, or of supporting endo- 

 skeletal structures formed in the interior. The simple amoeboid streaming 

 movements become highly modified in various ways or replaced by special loco- 

 motor mechanisms or organs, flagella, cilia, &c., of various kinds. The internal 

 alveolar cytoplasm develops fibrillse and other structures of the most varied 

 nature and function, contractile, skeletal, nervous, and so forth. The vacuole- 



" Cf. Dobell, 'Observations on the Life-History of Cienkowski's Arachnula.' 

 Arch. Protistenkunde, xxxi. (1913), p. 322. The author finds that in Arachnula 

 each nucleus arises from a single chromatin-grain, which grows to form a vesi- 

 cular nucleus. Since the fully-formed nucleus contains numerous grains of 

 ihromatin, the original chromidiosome must multiply in tliis process. 



