254 The Philippine Journal of Science m$ 



Obviously these devices can be adjusted and used for washing 

 with a flow of water in the opposite direction. For some 

 classes of objects a large cylindrical vessel would serve the 

 purpose as well as a conical one. The special advantage of the 

 conical form is that it provides for the smaller, more slowly set- 

 tling individuals among the specimens. Though prepared espe- 

 cially for washing Volvox, the larger, conical, washing apparatus 

 was successfully used for washing 4 cc of pollen of Cycas 

 circinalis. 



METHODS OF ESTIMATING THE NUMBER OF CELLS IN 

 SPHERICAL SURFACES 



For estimating the number of cells present in Volvox coenobia 

 various methods have been employed. Those used by the earlier 

 workers have been critically discussed by Klein.* 



Leeuwenhoek counted the cells around the periphery of the 

 spheroid and calculated the total number present from the count. 



Ehrenberg based his calculations on the same count, but, as 

 Klein points out, his formula for the calculation was erroneous 

 and gave numbers too large by more than half. Furthermore, 

 his peripheral counts were, naturally enough, too large because 

 of failure to limit the count to cells lying within an optical section 

 no thicker than the average intercellular distance. The proper 

 formula and coefficient for this method have been recently stated 

 by Janet and are given below. 



Cohn 5 counted the number of cells present in a measured area 

 of the spheroidal surface (100 microns square) and calculated 

 therefrom the total number, deriving the area of the spheroid 

 from the radius. A special variety of this method was used by 

 the present writer, and is stated in detail below. 



Drude counted the cells in one-eighth of the spheroidal surface 

 by means of a cross-hair ocular. 



Klein described the method which he found * expeditious for 

 making numerous estimates. Using a camera lucida, he drew 

 the periphery of the coenobium and within it a group of cells not 

 extending so far from the center as to give any considerable error 

 due to foreshortening of the intercellular distances by projection. 

 In this group of cells, drawn as points, he selected 4 to 6 as nearly 

 as possible in a straight line. He then determined how many 

 times the length of this line was contained in the periphery. 



4 Klein, L., Morphologische und biologische Studien ueber die Gattung 

 Volvox. Pringsheim's Jahrb. filr wiss. Bot. 20 (1889) 145-146. 



8 Cohn, F., Die Entwickelungsgeschichte der Gattung Volvox, Festschrift 

 (1875) 15. 



