VIII] OF DISCOID CELLS 579 



is so far simplified that we see at once that the partition cannot be 

 parallel to the extended plane, but most cut the cell, somehow, at 

 right angles to that plane. In short, the problem of dividing a 

 much flattened sohd becomes identical with that of dividing a 

 simple surface of the same form. 



There are a number of small algae growing in the form of small 

 flattened discs, and consisting (for a time at any rate) of but a 

 single layer of cells, which, as Berthold shewed, exempUfy this 

 comparatively simple problem;* and we shall find presently that 

 it is admirably illustrated in the cell-divisions which occur in the 

 egg of a frog or a sea-urchin, when it is flattened out under artificial 

 pressure. These same httle algae which serve to exempHfy the 

 partitioning of a disc also illustrate, now 

 and then, a curious feature of its contour. 

 Such a small^ green alga as Castagna 

 (Fig. 229) shews, and many Desmids 

 shew just as well, a sinuous border 

 running out into rounded crenations or 

 lobes. This is a surface-tension phe- 

 nomenon. A little milk poured over an 



apple-pie gives a homely illustration of Fig. .229. Castagna pdycarpa. 



the same sinuous outlines; a drop on a ^^^^^^^^ZlT^ ^''''"^ ^^^"*'' 

 greasy plate spreads in the same uneven 



way, and does so indeed unless the utmost care be taken to ensure 

 absolute cleanhness and surface equihbrium*. 



Fig. 230 1 represents younger and older discs of the little alga 

 Erythrotrichia discigera; and it will be seen that in all stages save 

 the first we have an arrangement of cell-partitions which looks 

 somewhat complex, but into which we must attempt to throw some 

 light and order. Starting with the original single, and flattened, 

 cell, we have no difficulty with the first two cell-divisions; for we 

 know that no bisecting partitions can possibly be shorter than the 

 two diameters, which divide the cell into halves and into quarters. 

 We have only to remember that, for the sum total of partitions to 



* Cf. Quincke's " Ausbreitungserscheinungen," in Poggendorff^s Annalen, cxxxix, 

 p. 37, 1870; also Tomlinson's papers in Phil. Mag. vm-xxxix; and Van der 

 Mensbrugghe, Mem. Cour. de VAcad. R. Belgique, xxxiv, 1870; xxxvii, 1873. 



t From Berthold's Monograph of the Naples Bangiaceae, 1882. 



