118 MICRASTERIAS DENTICULATA. 



Now, the time occupied by my specimen in multiplying from 

 5 to 7 was three-quarters of an hour, but, if I have been clear in 

 my description, the enlargement and subsequent division of No. 3 

 (Figs. 2 and 4) must have continued, and have been completed, 

 before 2 and 2a. (Fig. 4) could have undergone the same process. 



This being the case we should have had i, 2, 2a, 3, 3^, 

 (Fig. 5), or nine processes in all, or in other words, ojie more stage 

 than is described by the authors referred to. 



Both, however, seem to have drawn their inspiration from a 

 paper read by E. G. Lobb, before the Microscopical Society of 

 London — not then Royal — in 1861, and hence their descriptions 

 agree. 



For many years, I had the pleasure of being personally 

 acquainted with Mr. Lobb, and, having spent many hours in his 

 company, I knew him to be a careful observer. 



I am therefore at a loss to account for the discrepancy. 



One other fact makes it still more difficult, and that is, if you 

 allow three-quarters of an hour for the divisions, and assume them 

 to be I, 3, 5, 7, 9, and 13, you have exactly three and a-half hours. 



On the 20th of June I placed ten or twelve specimens in a 

 special tank for the purpose, if possible, of solving my difficulty. 

 But although I examined them almost every evening for two 

 months, and could see their numbers increase to about forty, and 

 could frequently perceive them in the last, or almost completed 

 stage, I have not been fortunate enough to see the beginning, or 

 the middle stage. The denticulation does not appear with the 

 division of the lobes, but is probably coincident with the enlarge- 

 ment of the newly-formed portion of the Desmid, which is not at 

 first so large as the original. 



EXPLANATION OF PLATE X. 



Binary sub-division of Micrasterias denticulatci. 



Fig. 1. — Completed Desmid ; the new, upper half not yet equal in 

 size to the old lower half. 

 2.— The " five-lobe stage." 

 3. — " Heart-like depression " of a lobe. 

 4. — The " seven-lobe stage.'' 

 5. — The " nine-lobe stage." All figures x 150. 



5 J 



5) 



