On Platjidorma. 427 



obliqiiel}^ 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, Phitydorina 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 /-i and a polar one 

 of 15-18/1. 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. 



