222 



THE PROTOZOA 



consider this an indication of sex-difference, but merely the chance 

 fusion of two Polytomas of different age. Nevertheless, the phenom- 

 enon is significant, and indicates that the smaller or less-grown 

 individual has the capacity, whatever that may be, of conjugation, 

 and may be regarded as an intermediate stage, at least, in the devel- 

 opment of sexually differentiated forms (Fig. 120). A somewhat 

 analogous process takes place in Codosiga botrytis, one of the Choano- 

 flagellida, where, as first observed by Stein, an attached form con- 

 jugates with a free-swimming and somewhat smaller animal. 



A similar but more definite sex-difference is seen in the peritri- 

 chous Ciliata, where the individuals are of dissimilar size. In all of 

 these, with the exception of the genus Zoothamnium, a normal-sized 

 individual fuses with a smaller one. Engelmann ('76) made the 



A B C D E 



Fig. 120. — Conjugation of Polytoma uvella Ehr. [DALLINGER and DRYSDALE.] 



interesting discovery that the larger form, or macrogamete, is always 

 one whose sister-buds have given rise by division to smaller forms, or 

 microgametes, which would certainly suggest that a particular condi- 

 tion of the plasm accompanies conjugation. In the genus Zootham- 

 nium, alone, the macrogametes are considerably larger than the 

 normal individuals (Trembley 1747, Ehrenberg, Greeff, Engelmann, 

 and others). 



The microgametes, which may arise by budding, as in Lagenophrys 

 ampulla (Fig. 121), or by repeated divisions, as in Epistylis (Fig. 122), 

 swim about freely until they come in contact with macrogametes, 

 to which they finally adhere. Upon fusing, the microgametes 

 gradually lose their definite structure, until finally they are absorbed. 



Engelmann ('76), watching the process of sexual union in the ciliate, 

 Vorticella, records the following interesting observations : " The buds, 

 at the beginning, swarmed about with constant and considerable 

 rapidity, rotating the while on their axes, but moving more or less in 

 a straight line through the drop. This lasted from five to ten minutes 



