Dr. F. Cohn on a new genus of the family of Volvocinese. 345 



tlic young Stephanosphara already becomes free and developes 

 the envelop e-cellj although one or other of the four quadratic 

 segments of the sphere has become constricted but not parted off. 

 Hence originate monstrous forms, since the general envelope-cell 

 then encloses only seven primordial-cells ; but in these cases it 

 is always observed that one of them is distinguished by most 

 curious prolongations or mucous filaments, that it appears twice 

 as large as the rest, that it contains four chlorophyll-utricles in- 

 stead of two as is usual, and that it is also more or less con- 

 stricted in the middle. All this furnishes proof, that here one 

 secondary-cell of the second generation has not been divided the 

 third time like the rest, but occupies by itself the space which 

 is ordinarily filled by two. Very often only six (fig. 6), or even 

 no more than five primordial-cells are found in one envelope- 

 cell; but then two or three of these are twice as large as else- 

 where*. In like manner Alex. Braun figures a Pediastrum com- 

 posed of fifteen instead of sixteen cells, wherein one however is 

 twice as large as the rest (Verjungung, t. ii. 20). 



On the whole, it is obvious that the mode of propagation of 

 Stephanosphcera already examined corresponds completely to that 

 we are already acquainted with as formation of macrogonidia in 

 Chlamydococcus. In both cases it depends upon the envelope- 

 cell remaining unaltered, while the primordial-cells become 

 divided, first into two secondary cells, and then so on in a lower 

 power of two, each of the secondary-cells immediately developing 

 two cilia, and secreting over its whole surface, as do all primor- 

 dial-utricles of vegetable cells, a delicate cellulose membrane, 

 which however becomes gradually removed further from the 

 secreting primordial- cell through absorption of water. The only 

 distinction between Chlamydococcus and Stephanosphcera arises 

 from the formation of a special envelope-cell to each individual 

 secondary-cell in Chlamydococcus, w^hile in Stephanosphara all 

 the generations produced by division form one primordial- cell, 

 become enclosed by a common envelope, and move away as 

 families of cells. On the contrary, the developmental history of 



* Only such imperfect division of a transitional generation gives the 

 possibility of the green cells occurring otherwise than in a power of two, in 

 Stephanosphara, as in all the other VolvocinecB, in which the same law 

 holds good ; at most the normal number might be rendered imperfect by 

 the emission of one or other of the cells which occurs sometimes. On the 

 other hand, definitions like that of Kiitzing's Botryocystis Morum, which is 

 pretended to be composed of six secondary- (primordial-) cells, evidently 

 have their origin merely in imperfect observation and misapprehension of 

 the law of division. In like manner, Ehrenberg's statement that the number 

 of individuals in his Trochogonium varies from six to twenty-one, may de- 

 pend upon a neglect of the proper character. In general, the earlier ob- 

 servers have frequently overlooked the constant numerical relations in the 

 structure of the Volvocine^. 



