554 Dr. C. A. Kofoid on Platydoriua. 



continuance of the species. Furthermore, the destruction of 

 individual cells does not impair the life of the other cells of 

 the organism, for so long as a single cell remains it continues 

 its customary activity. The use of the term colony is, however, 

 objectionable. A number of facts militate against this con- 

 ception of the organism, and the discovery of the new genus 

 here described adds to the array. (1) The cells are not 

 always similar, for in all forms with poles physiologically 

 or otherwise differentiated the anterior stigmata are brighter 

 than the posterior, and in Pleodoiina there are two kinds of 

 cells, the vegetative and the gonidial, the former distinctly 

 smaller than the latter. (2) There is in all of the higher 

 genera a well-defined physiological polarity accompanied by 

 the difference in the anterior and posterior stigmata, and 

 also, in Platydoriua, by a differentiation of the poles by the 

 arrangement of the cells and the structure of the envelope, 

 and by the further differentiation of a transverse axis. 



(3) In Pandorina the cells are almost in contact, and in 

 Volvox they are actually connected by protoplasmic processes. 



(4) The beginnings of histological differentiation are also 

 evident in the cells composing the so-called colony. In 

 Eudorina, according to Carter (1858), the cells are differen- 

 tiated into male and female in definite regions, the male cells 

 developing from the anterior quartet and the remainder 

 becoming female; in Volvox sexual and asexual reproduction 

 alike are limited to a few of the cells; and in Pleodorina the 

 asexual process is confined to the posterior hemisphere. The 

 cells of the organism are thus histologically and functionally 

 differentiated in this particular in these higher genera. 

 Although the degree of differentiation is slight, it is never- 

 theless appreciable. (5) In the matter of locomotion the 

 activities of the individual cells of the organism are not 

 independent of each other, but are correlated, the flagella 

 acting together to produce rotation, its reversal, or its cessa- 

 tion. The piedominance of the direction in the higher genera 

 plainly exhibits the phenomenon of correlated locomotor 

 activities of the constituent cells. 



The facts above cited emphasize the desirability of regarding 

 each of these so-called colonial fhigellates of the subfamily 

 Volvocinas as a unit, with an organization of its own, and not 

 as a colony, that is, an aggregation of independent and similar 

 cells associated merely as a result of descent from a common 

 parental cell, the form being a matter of chance or circum- 

 stance. The group of cells as a whole, and not each of the 

 constituent cells, is the unit of descent, of form, and of func- 



