Journal 



OF THE 



NEW-YORK MICROSCOPICAL SOCIETY. 



Vol. V. OCTOBER, 1889. No. 4. 



NOTE ON THE BINARY SUB-DIVISION OF MICRAS- 

 TERIAS DENTICULATA (BREB.), RALFS. 



BY STEPHEN HELM. 

 {Read October i^th, 18S9.) 



On Saturday the 15th of June at 10.45 P- ^■■> I had the good 

 fortune to perceive a solitary specimen of Micrasterias denti- 

 culata in a state of binary sub-division. 



It had then arrived at what I will call the five-lobe stage, 

 i. e. the lobe on each side of the connecting central one had 

 already divided, making five, which were as nearly as possible 

 equal in size. 



The lobe numbered 2 (Fig. 2) soon gave signs of lateral 

 enlargement, and about 10.55 each top had a small but distinct 

 heart-like depression (Fig. 3). 



As the lobe became broader, this depression deepened until 

 11.30, when the division was completed, and well defined 

 (2, 2a, Fig. 4). About 11.25 lobe number 3 (Figs. 2 and 4) on 

 each side evinced signs of enlargement ; and at 1 1.35 the before- 

 mentioned heart-like depression was apparent. 



I had, by this time become deeply interested in this simulta- 

 neous quadruple growth, when to my intense disgust, and dis- 

 appointment, down came a Cypris and gobbled up my Desmid 

 and brought my observations to a summary conclusion. 



I should not have deemed these observations worthy of 

 record, but for the fact of their leading me to a conclusion 



Explanation of Plate 30. 



Binai-y sub-division of Micrasterias denticulata. 

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

 lower half. Fig. 3.— The " five-lolje stage." Fig. 3.— ■• Heart-like depres- 

 sion" of a lobe. Fig. 4.— The "seven-lobe stage." Fig. 5.- The •'uiue- 

 lobe stage." All figures X l-'>0. 



