234 GARY N. CALKINS. 



gradual tendency for the energy of multiplication to weaken, or, 

 as we may express it, the constructive and destructive processes 

 tend to equalize. With the decline of the division rate comes 

 the advent of the second period in which the most important 

 functions in the life of the protozoon occur. 



The main characteristic of this second general period, or 

 adolescence, is a general decline in the multiplication rate and 

 more or less definite change in form of the cells and in their 

 chemical and physical composition. A single cell, unless it is in 

 the sexual phase, gives little or no clue to its stage in the life his- 

 tory. In the majority of cases it is only by the study of a long 

 series that the student is able to recognize this period in the life 

 cycle. In such a study, which one can easily carry on with cer- 

 tain free forms of ciliates, the decline of the division rate with ad- 

 vancing age of the series is easily followed. In a form like Para- 

 mecium, for example, where there is no sexual dimorphism, one 

 notes at this period a change in the physical constitution of the 

 protoplasm. Such changes, leading in Paramecium to what I 

 have termed the "miscible" state, are certainly the most sinking 

 features in the life history of protozoa and a fortiori of the period 

 of adolescence, for in them we find an explanation of all the form 

 changes that take place in any life history. Such form changes 

 may involve only the cytoplasm as in Paranieciuin or Tctrauritus, 

 only the nucleus as in chromidium formation in rhizopods, or 

 both nucleus and cytoplasm as in gamete formation with or with- 

 out sexual differentiation. 



In Paramecium this condition comes at a period which Maupas 

 designated as sexual maturity. The body size is somewhat less 

 than at earlier periods, although this is by no means a constant 

 feature, size depending chiefly on the rate of division and so only 

 indirectly on age. The plasticity at this period is remarkable, 

 and the cortical plasm is so sticky that two organisms striking 

 each other will fuse at the point of contact. I have had Para- 

 inccia in culture during an epidemic of conjugation with the pro- 

 toplasm so highly miscible that amorphous groups of partially 

 fused Paramccia were formed by the accidental union of from 

 three to nine individuals, dozens of such groups whirling round 

 and round amongst the normally conjugating pairs. 



