134 



THE PROTOZOA 



two or more may be formed. These gametes soon divide and form 

 the typical sixteen-cell Pandorina colony. Thus in Pandorina, each 

 of the cells forms both sexual elements, but an advance in differentia- 

 tion is seen in Eudorina elegans, where, according to the rather incom- 

 plete observations of Carter ('58), the thirty-two cells forming the 



colony have a different 

 fate when the conju- 

 gation period comes 

 around. Four of the 

 thirty-two cells situated 

 at the end of the colony 

 form gametes by re- 

 peated divisions in one 

 plane, while the other 

 twenty-eight cells 

 merely develop more 

 amylum granules and 

 turn darker. The ga- 

 metes which are formed 



D 



G 



Conjugation. 



Fig. 76. Pandorina morum Ehr. 



[PRINGSHEIM.] 



A. i6-celled colony. B. Macrogamete. C and . Fusion 

 of macrogamete with microgamete. D and F. Fusion of micro- f rom the Upper f OUr 



gametes. G. Copula. ce ll s are elongate and 



spindle-shaped, with 



two flagella, a red eye-spot, and a long tail. The fate of the different 

 cells was not made out, but there seems reason to believe that if the 

 observations were correct, the gametes represent male elements, the 

 other cells female. 



A still more decided advance is shown by the colonies of Volvox. 



Volvox can scarcely be regarded as a unicellular organism, for differ- 

 entiation has gone so far that the cells if separated, with the excep- 

 tion of the reproductive elements, cannot live. The individuals form- 

 ing the peripheral layer (in Volvox globator about 12,000, Cohn, '75) 

 form the sterile vegetative or somatic cells of the aggregate. A few 

 of these cells, which reproduce asexually, are found upon the inside 

 of the peripheral layer, which protects them like a mantle. Stein 

 finds eight of these asexual cells or parthenogonidia in V. globator, 

 and Cohn, one to nine in V, minor. 



The parthenogonidia, by repeated division, form daughter-colonies 

 from one-quarter to two-fifths the size of the parent colony, which 

 finally make their escape from the latter by rupture of its walls. 

 After a considerable period of such asexual reproduction, sexual 

 elements are formed. These are at first similar to the parthenogo- 

 nidial cells, but are more numerous. Later they can be distinguished 

 as male (androgonidia y Cohn) and female {gynogonidia, Cohn). In 



Volvox globator there may be from twenty to forty gynogonidia and 



