168 Mr. H. J. Carter on the Conjugations of 



but the preliminary steps to the formation of a larger individual, 

 in which the process, still unknown to us, finally takes place. 

 Nor do I see how it can be a modification of the true impreg- 

 native process, as in the former instance large quantities of pro- 

 toplasm, &c., are brought together to produce larger frustules, 

 and in the latter it is smaller quantities to produce them. 



Indeed Prof. Smith observes that '' the ordinary Diatomaceous 

 frustule seems to owe its production to the protoplasmic contents 

 of the sporangial frustule formed by the process of conj-ugation,^^ 

 and that subsequently, under favourable circumstances, its sili- 

 ceous epiderm opens "to permit the escape of the contained 

 endochrome, which is resolved into a myriad of embryonic frus- 

 tules; these either remain free or surround themselves with 

 mucus, forming a pellicle or stratum, and in a definite but un- 

 ascertained period reach the mature form of the ordinary frus- 

 tule.^^ After which he adds that " the size of the mature frus- 

 tule before self- division commences is, however, dependent upon 

 the idiosyncrasy of the embryo, or upon the circumstances in 

 which its embryonic growth takes place ; consequently a very 

 conspicuous diversity in their relative magnitudes may be usu- 

 ally noticed in any large aggregation of individuals, or in the 

 same species collected in difi"erent localities.^' Thus Prof. Smith 

 believed that a diversity in size might commence even in the 

 embryonal development, which, it seems to me, though more or 

 less perpetuated throughout, might still be equalized to a cer- 

 tain degree by the conjugation of a larger with a smaller frus- 

 tule. However the latter may be, Kiitzing surmised, and 

 Rabenhorst affirmed, a similar mode of reproduction in the 

 Diatomese to that described by Prof. Smith (Pritchard, p. 74). 



That the frustule is raised by repeated conjugation to its 

 maximum or required bulk may be inferred from what I have 

 figured and stated, respecting the various sizes of the conjuga- 

 tions and their elements, of Navicula rhomhoides ; but in what 

 the process of impregnative generation consists remains for 

 future observation to determine, and is therefore a question on 

 which we can only speculate now by the aid of analogy. 



For this purpose, then, let us begin by enumerating the full 

 complement of elementary parts which make up a Diatom. 

 They are as follows : — (1) the siliceous frustule; (2) the sarcodal 

 sac within it, containing (3) the nucleus ; (4) the endochrome, 

 (5) the " glair-cell,'^ double, one towards each end ; (6) refractive 

 cells, not spherical; (7) starch-grains; (8) molecular granules. 

 The presence of Nos. 6, 7, and 8 is variable ; and there may, of 

 course, be more organs than have yet been recognized. 



I have not added a layer of sarcode externally, because its 

 presence there may or may not be continuous in duration or 



