38 Transactions of the Socieiy. 



VIII. — The Thallus of the Diatomacem. 

 By F. KiTTON, Hon. F.E M.S. 



{Iiead 8th January, 1879.) 



The study of the living diatom has lately engaged the attention of 

 many eminent foreign diatomists (M, P. Petit, Paris ; M. J, Deby, 

 Belgium ; Count Castracane, M, Ardres, and others). The latest 

 published observations are those of M. le Dr. Lanzi, of Eome, in 

 his paper* on the " Thallus of the Diatomaceae." By thallus is to 

 be understood the stipes, cushion, tube, frond, or mucous pellicle. 

 The latter is the material by which the film of diatoms is attached 

 to wet walls, buttresses of bridges, &c. He communicates some 

 interesting facts connected with the reproduction of these remarkable 

 organisms. " In a gathering of Epithemia ventricosa made in the 

 Villa Pamphilia, in Eome, I observed that some portions of the 

 pellicle were composed of a great quantity of round granular cor- 

 puscles of a greenish-yellow colour. Most of these corpuscles were, 

 to all appearance, the same as those contained in the interior of the 

 frustules of the Epithemia, and imbedded in a hyaline plasma. 

 Such was the resemblance, that no one could doubt that the 

 granular bodies in the plasmatic thallus and those in the frustules 

 were alike. 



"At another time I made a gathering in a fountain in the 

 interior of the Forum of Trajan, of a Cymbella in a state of 

 reproduction, and I was again able to see the round corpuscles. 

 They were very small, and of the same colour as the endochrome. 

 They were contained in the thallus, and resembled those in the 

 frustules. I followed these germs through their phases of develop- 

 ment ; and by repeated observations I ascertained that, whilst in- 

 creasing in breadth, they preserved their circular form ; that after- 

 wards they commenced to elongate, in order to acquire the lunate 

 and naviculoid outline of the mature frustule. 



" Of these growing forms, some remained attached to the thallus, 

 and some became free. The number of these corpuscles was con- 

 siderable ; and one was easily convinced that they were the result 

 of a new kind of generation. The disparity in size was so consider- 

 able, that it would have been absurd to suppose that they had been 

 produced by fissiparity. 



" I am able to report other similar facts observed in Navicula 

 amhigua, Nitzschia minutissima, Amphora ovalis ; but of these I 

 shall say nothing, in order to avoid useless repetitions, and shall 

 confine myself to describing Gomphonema oUvaceum only, in which 

 I have followed the series of transformations from the time the 

 frustule containing the germs had changed into a sporangial cell, 



* See ' Annales de la Socie'te' Beige de Microscopie," vol. iv. 



