212 GENERAL EMBRYOLOGY 



certain conditions, and second, that when cyclic periods of 

 protoplasmic depression do occur the protoplasm may be 

 restored to a condition of normal vigor, either by physical or 

 chemical stimuli, or by fertilization. Supposing, and the sup- 

 position is highly probable though not completely demon- 

 strated, as a fact, that the living processes do tend, in the absence 

 of continued stimulation, to diminish in intensity or otherwise 

 to deviate from the normal, then we find in the process of fer- 

 tilization a natural means of insuring the receipt of stimuli 

 which might otherwise be lacking. The onset of those struc- 

 tural and physiological modifications called senescence, leads 

 to a modification in the behavior of the organisms, i.e., they 

 form gametes and conjugate. 



This becomes clearer when we recall that life itself is re- 

 sponse reaction to the stimuli resulting from changed relations. 

 Such a changed relation may result (a) from changes in both the 

 environment and the cell or organism, or (b) from changes in 

 the environment alone while conditions within the organism 

 remain comparatively uniform, or (c) from changes within the 

 organism while the external conditions remain comparatively 

 uniform. Of these three possibilities the first two are certainly 

 the most frequent in the lives of most free-living Protozoa. 

 But we may interpret fertilization as fundamentally a means 

 of ensuring a changed relation through the realization of the 

 third possibility in the absence of the other two. In a way the 

 Ciliates act so as to ensure automatically a changed relation 

 between organism and environment; when external conditions 

 become too uniform to bring forth the normal vegetative ac- 

 tivities, the form of reaction actually changes and is modified 

 into gamete formation and fertilization, which immediately 

 leads to an internal disturbance and the condition of uniformity 

 is corrected, whenever it may occur. 



Among the Protozoa we find this division of labor between 

 vegetative and conjugative or fertilizing cells occurring when- 

 ever internal-external relations demand it. Among the Meta- 

 zoa, however, such a division of labor must occur at a certain 

 period in the life history, on account of the impossibility of the 



