600 MARCUS M. HARTOG. 



cautiously supplying very dilute boiled paste to the colony. 

 The latter require to be supplied with smaller Infusoria, for 

 which purpose Cryptochilus nigrescens is raised in hay- 

 decoction ; the drop of the liquid with this forna is always 

 examined under the microscope to ascertain its freedom from 

 other forms, and only after this control drawn oflP with the 

 pipette to be given as food. In the research on fission observa- 

 tions were made daily, and entered with the temperature in 

 the journal. Twenty species were examined, and their rate 

 of bipartition carefully studied. For any given species this 

 rate varies with (1) the temperature, (2) with the quantity, 

 and (3) the quality of the food ; while light or darkness is 

 absolutely without influence. 



It is found with each species that for temperature there is a 

 minimum below which no bipartition takes place, an optimum 

 at which it proceeds most rapidly, and a maximum beyond 

 which, again, there is no occurrence of reproduction. Ex- 

 haustion of the food-supply determines encystment if the 

 culture is of close relations; conjugation when the indi- 

 viduals are of mixed origin, and their age is suitable. 

 This is a discovery on which the second research was largely 

 based, as Maupas could now induce conjugation at will. 



The greatest frequency of bipartition at temperatures of 

 15° — 18° was observed in Glaucoma sci nt ill ans, namely, 

 five times in the twenty-four hours, and the least in 

 Spirostomum teres, viz. once in forty-eight hours, or one 

 tenth the rate of the former species. 



Maupas started the cultures with an individual that had just 

 separated from its partner in conjugation, and which he terms 

 an " exconjugate.^' In this way it has been possible to ascer- 

 tain whether the rate of bipartition increases or diminishes 

 as we recede from the ancestor; and he finds that neither 

 change takes place at first, but that the rate is fairly constant 

 for all the individuals of a single cycle for a long period, after 

 which it diminishes : there are, however, fairly marked dif- 

 ferences between the descendants of different ancestors of 

 the same species. 



