THE ANNALS 



AND 



MAGAZINE OF NATURAL HISTORY. 



[FOUKTH SERIES.] 



" per litora spargite museum, 



Naiades, et circiim vitreos considite fontes : 

 Pollioe virgineo teneros hio carpite flores : 

 Floribus et pictum, divae, replete canistrum. 

 At vos, o Nyiuphae Craterides, ite sub undas; 

 Ite, reeurvato variata corallia trunco 

 Vellite muscosis e rupibus, et mihi conchas 

 Ferte, De® pelagi, et pingui conchylia succo." 



If. Parthenii Giannettasii Eel. 1. 



No. 13. JANUARY 1869. 



I. — On the Structure of the Diatomaceous Frustule, and its Ge- 

 netic Cycle. By John Denis Macdonald, M.D., F.R.S., 

 Staff-Surgeon, R.N. 



[Plate III.] 

 Fkom close examination of some of the larger forms of Dia- 

 tomaceas, more especially species of Isthmia and Biddulphia. I 

 have long been under the impression that the commonly 

 received views of the structure of the frustule and its mode of 

 self-division require considerable modification. Though nu- 

 merous inquirers have been engaged in the very inviting study 

 afforded by these little organisms, I am not aware that any one 

 has yet traced out their genetic cycle as satisfactorily as could 

 be wished. 



Having consulted the works of various authorities upon 

 this subject, I find the views expressed in the writings of 

 Dr. Wallich (particularly in his paper on Triceratium, vol. vi. 

 Journal of Microscopical Science, p. 242, and on the Diatom- 

 valve, vol. viii. Trans. Micr. Sci. p. .129) most in accordance 

 with my own independent researches. 



Dr. Wallich appears to have been the first to set forth 

 clearly that the middle piece or " zone " consists, while the 

 frustule is intact, of two distinct plates, the one received 

 within the other, and that the growth of such plates can only 

 take place at the free margins, or those which are not con- 

 nected with the valves. 



Ann. & Mag. N. Hist. Ser. 4. Vol. iii. 1 



