598 MICEOSCOPIC FOEMS OF VEGETABLE LIFE THALLOPHYTES 



sporules or gonids, which, after a period of repose or of incubation 

 inclosed within a cyst, or within a membranous frond, or within n 

 frustule, attain a condition for living an independent life and 

 reproducing in every respect the adult type of the mother-cell ; thus 

 the cyst, the membranous frond, or the frustule, performs the 

 function of a sporaiige. Castracane was of opinion that these gonids 

 or embryonal forms could have no traces of silex in their cell- walls, 

 scarcely yet formed, until a few years ago, 1 among the diatoms 

 of a marine deposit of the Miocene period, he met with a perfect 

 frustule of Coscinodiscus punctatus, which, between the two planes 

 of the valves, and therefore within the cell, exhibited some round 

 marks which admitted of no other interpretation except that of 

 impressions or traces of the embryonal forms surprised by death 

 while still attached to the mother-cell. More recently he met with 

 other cases identical in character, so that he has no longer any 

 doubt as to the presence of silex in the cell -walls of diatoms which 

 have not yet emerged to the light. , 



The formation of * endocysts' within the frustule of diatoms lias 

 also been observed by Comber, Murray, and others. 



No one appears at present to have given attention to a circum- 

 stance described by Castracane 2 in relation to a specimen of Striatella 

 unipunctata, which has passed thousands of times under the eyes of 

 all, without its significance being recognised. The diatoms which 

 we have most frequently under our observation do not ahvjivs 

 exhibit the same arrangement of their endochrome. The attempt 

 has, indeed, been made to found the classification of diatoms on the 

 arrangement of the endochrome, according as it is present in the 

 form of plates or of granules ; thus distinguishing the placochromatic 

 and the coccochromatic forms ; but a difficulty is presented in the 

 way of this classification by certain types which sometimes belong to 

 the one, sometimes to the other class. And this cannot be the 

 result of accident. Such variations might occur in some diatoms 

 as the result of special biological conditions of the individual. 

 There may frequently be seen, for example, a specimen of Melosira 

 varians with its cell-cavity filled with endochrome, not in a condition 

 of unequal amorphous masses, but of uniform rounded corpuscles ; 

 and this demands particular attention, or at least gives good ground 

 for special research. A diligent examination instituted in these 

 cases has demonstrated the existence in them of a special organi- 

 sation ; and the determination of a narrow and well-defined limit of 

 outline seems to prove that these were perfectly distinct and 

 independent of one another. From the perfect resemblance of 

 these to the gonids and embryonal forms seen to escape from the 

 mother -cell by Rabenhorst, O'Meara, and Castracane, he concludes 

 that this special arrangement of the endochrome must be interpreted 

 as a prelude to the process of reproduction. 



These observations may possibly attract the attention of some 



1 See ' Observations on a Fossil Diatom in relation to the Process of Beproduc- 

 tion,' Atti deW Accad. Pontif. del Nuovi Lincei, May 17, 1885. 



2 See 'The Diatoms of the Coasts of Istria and Dalmatia,' Atti dell' Accad. 

 Pontif. dei Nuovi Lincei, April 27 and May 25, 1873. 



