Dr. C. A. Kofoid on Platydorina. 553 



expressed by the arrangement of the cells and by structural 

 features of the envelope. In regard to polarity, also, the 

 new genus is thus the most highly specialized member of the 

 Volvocina?. 



The reproduction of Platydorina has been observed by me 

 repeatedly in the past five years, but only the asexual phase 

 has thus far been discovered. All of the cells of the organism 

 are gonidial, each dividing to form a daughter colony. The 

 sequence of the divisions and the position of the successive 

 planes are of the type found in Eudorina and Pleodorina, 

 the resemblance being so close that the figures illustrating 

 the asexual development of Pleodorina illinoisensis (1898, 

 pl.xxxvii.) might almost be used for cleavage in Platydorina. 

 There is one difference, however, for in Platydorina the 

 curved plate of cells, which becomes first cup-shaped and 

 then ellipsoidal, subsequently flattens, the cells of the two 

 faces intercalating during the pi*ocess. The daughter colony 

 acquires the adult form, including the tails and the torsion of 

 the plate, before it escapes from the maternal matrix, the 

 young colonies moving about for some time in the disinte- 

 grating matrix before making their escape through the 

 ruptured outer sheath. The secondary sheath surrounding 

 the gonidial cell becomes the outer or primary sheath of the 

 new colony. No stages of sexual reproduction have been 

 seen, though the collections examined represent a considerable 

 range of season and locality. It may be that these are to be 

 sought upon the bottom rather than in the superjacent strata 

 of water where plankton collections are usually made. 

 Aquaria about to dry up were also searched in vain for sexual 

 stages of Platydorina. 



The mode of development of Platydorina is significant of 

 its systematic position and its relationships. The number 

 and the original arrangement of the cells, the type of deve- 

 lopment, and the character of the envelope, all indicate that 

 Platydorina is a more highly specialized form descended from 

 some Eudoi ina-\ike ancestor, and that it is more closely 

 allied to Eudorina than to any other existing genus. 



Throughout this paper the customary term " colony " has 

 been used to designate the organism herein described and 

 others related to it. The wide use of the term in the literature 

 of the subject is doubtless due to the fact that, as a rule, the 

 organisms are composed of similar cells arranged in symme- 

 trical form with no pronounced axial differentiation, without 

 contact or protoplasmic connexion, separated from each other 

 by a non-living gelatinous matrix, and each capable of per- 

 forming all the functions necessary for its own life and the 



