On Platydorina. 425 



the smaller, and slightly overlapped hy those of the contiguous 

 cells. This is due to the intercalation of the cells of the two 

 sides of the plate, and to the fact that the outer ends of the 

 cells are slightly nearer the surface of the plate than are the 

 inner ones. The gelatinous substance within the secondary 

 sheaths does not differ in structure or stainibility from that 

 of the surrounding matrix. As a result of the form of the 

 colony, the amount of the matrix substance is much less in 

 Platydorina than in related forms such as Eudorina. 



The cells of the colony are all of one type, alike in structure, 

 and approximately similar in size. Each is bifiagellate and 

 has a central body of protoplasm with a nucleus, two contrac- 

 tile vacuoles, one stigma, and one chromatophore with a single 

 pyrenoid. 



The number of cells in the colony is either 16 or 32; at 

 least no normal colony with cells of any other number has 

 been detected among the hundreds, if not thousands, of 

 colonies examined. Colonies are frequently seen which, by 

 reason of parasites or from other causes, have lost one or more 

 cells, indeed in some cases all but one or two ; but the form 

 of these colonies is usually preserved, and the empty second- 

 ary sheaths frequently remain as evidence of the original 

 complement of cells. The 16-cell colonies are not mere 

 stages in the development of the 32-cell form, for division of 

 the cells of this type in observed cases leads to the develop- 

 ment of new colonies and not to the formation of the 32-cell 

 stage. As in other nearly related genera of the family — for 

 example, Eudorina, Pandorina, and Pleodorina — the number 

 of cells in the colony varies, within narrow limits, in the ratio 

 of geometrical progression. In Platydorina, however, this 

 pleomorphism is manifested not only by this difference in the 

 number of cells in the colony, but also by a structural dis- 

 tinction — the presence of three tails in the 16-cell, and five 

 tails in the 32-cell, colony. Inasmuch as the two types 

 always occur together, and since this pleomorphism is in some 

 respects similar to that of related genera, it does not seem 

 justifiable to regard the two as distinct species of the genus. 

 They are, I believe, two forms of one species. 



