On Platydorina. 427 



obliquely outward, the direction alternating, however, in 

 conformity with the arrangement of cells in the central area, 

 as may be seen in a view of the edge of the colony (Fig. 3). 

 The alternation of the cells in the colony as a whole is the 

 same whichever face is presented, the right-hand cell of the 

 posterior row of four cells always presenting the basal end 

 uppermost. From this as a starting point the regular alter- 

 nation of stigma and pyrenoid can be traced from cell to cell 

 throughout the whole colony. An examination of twenty-five 

 colonies showed that all conformed to the same plan of alter- 

 nation, there being no case of reversal. In the arrangement 

 of the cells in the colony, Platydorina is thus more like 

 Eudorina than like Gonium, being, not a simple plate like the 

 latter genus but, in reality, a flattened ellipsoid so much 

 compressed that the cells of the two faces intercalate 

 regularly, and thus give to the colony its superficial resem- 

 blance to Gonium. 



The individual cells are all substantially alike in size and 

 structure. They have the form of an oblate spheroid, slightly 

 larger in the outer hemisphere.* Some cells, especially the 

 marginal ones, often exhibit a slight flattening or even a 

 depression at the outer pole. In the full-grown colony the 

 cells have an equatorial diameter of 15-20 yu and a polar one 

 of 15-18yu. The cells of young colonies still within the 

 maternal matrix do not exceed 4-6 M in diameter. I do not 

 find that the cells of the 16-cell colonies are appreciably 

 larger than those of the 32-cell. 



The protoplasm is small in amount, consisting of a very 

 thin pellicle (Fig. 4, p.) on the surface of the cell on the out- 

 side of the chromatophore, and an axially-placed knob- 

 shaped mass (pr.) located somewhat nearer the outer pole 

 than the inner one. Near the center of this mass lies the 

 spherical nucleus (Fig. 4, n.), containing a single spherical 

 nucleolus (ncl,). Within this protoplasmic mass lie the two 

 contractile vacuoles (c. v.) and the stigma (st.), while from 

 the outer end of the cell arise the two flagella (/.). 



* As in the case of Eudorina and Pleodorina, the terms "outer" and -inner" are 

 used to designate respectively the ends which bear the stigma and the pyrenoid. 



