1892.] MICROSCOPICAL JOUBKAL. 35 



Gemmation. — Reproduction by self-division is not at all peculiar 

 to this sub-class, nor even to the vegetable kingdom. Animals, as well 

 as plants, commonly multiply bv simple fission, and amongst the true 

 infusoria, or animalcules, the operation is so rapid that, in some genera 

 (as, for example, the vorticellas) . one may witness within an hour the 

 complete bisection of a single vigorous creature into two lively counter- 

 parts. The process in the plant-world is generally much slower, but 

 it passes through essentially the same steps and may be traced with 

 equal clearness and certainty. In fact, at their foundation, all modes 

 of reproduction are but forms of fission — the division of one into two 

 or more— and the ultimate nature of the process is not changed by the 

 fact that the lesulting two are often so dissimilar in size and other attri- 

 butes that we feel bound to regard one as the parent and the other as 

 the ortspring. In other words, it is none the less fission because it takes 

 the form we call inidding. 



What is an Individual ? — Continuous gemmation gives rise to 

 very interesting and complicated relations which involve another in- 

 soluable biological puzzle — as to what constitutes an individual. 

 When one regards the cell-units, with theii' circumscribed and appar- 

 ently complete cell-functions, he is disposed to credit them with the 

 real individualitv : but when he looks to the mutual interdependence 

 which usually exists between the cells which retain a physical connec- 

 tion with one another, he inclines to expand the idea so as to include 

 all the members of any communal aggregation, if he does not stretch 

 it still further, as- some authors do, so as to cover all the products of 

 fission or gemmation within a continuous series, whether remaining 

 united or not. In such cases, however, as those of the filamentous 

 and flabellate diatoms, in which there is no absolutely necessary con- 

 nection between one cell and another, and where there is no known 

 reason why anv single frustule would not go on in the performance of 

 all essential functions if it were entirely separated from its sister cells, 

 we seem to be quite justified in regarding the type as physiologically 

 unicellular, even though the anatomical units are not always physically 

 solitary or free. There is at least no evidence that those diatoms which 

 are found within a thallus (as already mentioned) have anytliing more 

 than a mechanical connection with one another, and vital independence 

 is after all the best test we have of their actual individuality. 



Self-Division. — In the diatoms, the first indication we have that 

 self-division is about to take place is the appearance of a sort of uneasi- 

 ness within the nucleus and the formation of a constriction about the 

 middle of the endochrome. This is at the beginning a mere indenta- 

 tion of outline, but it gradually deepens and deepens until it finally re- 

 sults in the cleaving of the endochrome in twain. It is one of the most 

 impressive sights a man can witness. 



Mode of Motion.— Having once seen this manifestation of efiicient 

 energy within the diacom-shell, we shall not much wonder that tlie free 

 frustules move from place to place. We may, however, entertain a 

 livel}^ curiosity as to the direct means by which their locomotion is ac- 

 complished, although, in the present state of knowledge, I am sorry to 

 say, that curiosity cannot be satisfied. 



The motion of the free diatoms is like neither the quick and nervous 

 swimming, effected bv cilia or flagella, nor the slow and measured 



