OF THE PHYTOZOA. 121 



The cell-contents divide into two or more segments, each of which can 

 further develope around itself a gelatinous investment, and enter on an inde- 

 pendent existence. In Euglena, self-division occurs longitudinally into two 

 portions ; and the newly- developing half is of smaller size than the other, but 

 becomes complete in all its parts before its severance is effected. 



The motile cells of Cldamydococcus undergo fission into two or four seg- 

 ments (XIX. 23-26) : this takes place in the protoplasmic or primordial cell 

 contained within the hyaline spherical enveJope-ce]l ; when division is com- 

 plete, the latter is ruptured, the sections escape as independent beings, each 

 throws out around itself its envelope- cell, and in aU points goes through the 

 same cycle of development as the parent-cell. Many Monads also divide 

 into two beings, whilst others separate into foiu\ In the above-cited ex- 

 amples the fission is complete, and each segment, on detaching itself from the 

 other, becomes an independent, free being. 



But this same act of fission may proceed under different circumstances ; 

 and instead of a single organism, a colony may be formed, consisting of 

 several individual cells united together, either permanently or only for a 

 time, within a common envelope. These aggregate Phytozoa are especially 

 represented in the family Volvocinece. 



In this second mode of fission the process is repeated a greater number of 

 times — for instance, some 3, 4, or 5 times — the result being a higher midtiple 

 of 2, the product of the first act of scission. 



Each repetition of the process of fission, from the commencement until 

 its completion, constitutes, in Xiigeli's language, a transitional generation, 

 whilst the final repetition produces the permanent generation. For example, 

 in Stephanos]jha;ra two segments are produced by the act of fission, which re- 

 present the first generation (XIX. 45) ; then each of these subdivides, and so 

 developes four portions (XIX. 40, 46) — the second generation ; and, lastly, 

 each of the foiu' separates into two, and in that way produces eight segments 

 — the third, and in this organism the final or permanent generation (XIX. 

 41, 42, bQ). 



Unlike the segments resulting from a single act of division, or, as may 

 happen, from this act once repeated, each newly-formed i^rimordial cell 

 does not commonly siUTound itself with an envelope and enter on an isolated 

 existence, but the whole eight or more continue to live within a common 

 tunic, which presently expands by endosmotic action and acquires a more or 

 less spherical figure (XIX. bQ, 57, 58). Simultaneously with this expansion, 

 the previously contiguous particles are drawn away more and more from 

 each other, and chsposed within the common envelope, after a more or less 

 regular fashion, characteristic of the species to which they belong (XIX. 42, 

 58). In general, the separation of the primordial cells is not complete; 

 bonds of union between them in their early state, when closely approximated, 

 become cbawn out, and ultimately present themselves as interciuTcnt thj-eads. 

 When this series of changes is terminated, we have before us a reproduction 

 of the aggregate organism of which the dividing primordial cell was but an 

 individual member. Braiin has styled this variety of reproduction by fission, 

 development by ' Diacrogonidia.^ It is well illustrated in Steplianosphcera, 

 above cited, in Volvox (XX.), in Goniiim and Pandorina (XIX. 35, 36, 37, 

 and 62-66), and also in undoubted Algae, the Hydrodictyon or Water-net 

 for example. 



But the segmentation of the cells of Phytozoa occurs in yet another form ; 

 i.e. the fission, instead of stopping at the third or foiuih generation, pro- 

 ceeds still further, until 32 or 64, a hundi'ed, a thousand and upwards of 

 minute cell- structures are produced, technically called ' micror/onidia,' in- 



