DXTBLIlf NATURAL HI8T0RT 80CTETT. 211 



of a new cell- wall being interposed between them, eventually fonning 

 two new segments. At first the new growth is simple in outline, and 

 pale in colour, but afterwards assumes the characteristic, more or less 

 complex form and degree of tint of the species, and becomes filled with 

 endochrome, exactly similar to the older segments ; in the free species 

 separation taking place, each older segment bearing with it a new one, 

 to replace that from which it has been separated by the above-mentioned 

 process of growth. According to Hofineister {loc. ctL), '* the new 

 halves are at first lined only by the protruded portions of the pellicle of 

 the contents belonging to the older half cells," and '4t is the margin of 

 the half shells which constitute the rings evident in many species, e. g., 

 Closterium, Docidium," &c. I believe that the portion of each new 

 segment first formed to be the end-lobe, beneath which, at each side, are 

 then gradually evolved the lateral lobes. Nor can it be that certain spe- 

 cimens in Micrasterias and Euastrum, occasionally met with, are more 

 than an apparent exception to this, in which the end-lobe is not only 

 seemingly absent, but in which, in its place, a more or less deep sinus 

 exists. For I should think the phenomenon referred to is due to the 

 arrest of growth of the end-lobe, and which, not keeping pace with the 

 expansion of the lateral lobes, is left behind, thus producing the sinus. 



I would here like to remark, parenthetically, that I do not find 

 that Hofineister, in his paper alluded to, makes any reference to 

 the circumstance of there being, occasionally at least, cast off, imme- 

 diately after division, from each of the new segments a loose trans- 

 parent coat, sometimes looking almost like two empty segments, with 

 their ends towards each other, or back to back. What I refer to is well 

 shown by Mrs. Herbert Thomas {loc. cit., Plate V., Figs. 13 and 21) in 

 the object of her study, Cosmartum margaritiferum and C. Thwaitesii. 

 I have seen similar in other species, especially smooth ones, such as Cos- 

 marium Ralfsiiy Cosmarium undulatum, &c. It does not appear to me 

 evident what this pellicle-like production may be. It seems to me to 

 be possibly only, as it were, the matrix of gelatine formed during the 

 act of division and fresh growth, which may have become denser and 

 firmer, and from which the fronds then emerging and leaving it behind, 

 give rise to a membranous appearance with the doubly cup-shaped 

 outline ; or it may possibly be a secretion deposited superficially during 

 growth, even more comparable than the usual gelatinous investment to 

 what in the higher plants is called ** cuticle," and which in them is oc- 

 casionally separable by peculiar treatment. In the growing Desmidians 

 referred to, this investing pellicle-like production, however, does not 

 extend beyond the new segments, ceasing at the sutures. It is some- 

 times particularly remarkable in Docidium (in D, clavatum I have no- 

 ticed it), because it forms two lengthened tubes (each, indeed, as long as 

 one of the new segments, and closely applied thereto), in apposition at the 

 closed ends and open at the opposite, from out of which the fix)nd8 are 

 to emerge, and indistinguishable till the process has commenced. This 

 may be witnessed under the microscope during its accomplishment, 

 when it is seen that near the open ends the cast-off tube is of a slightly 



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