THE OPALINID CILIATE INFUSORIANS. 17 



is formed. As already noted, the nucleolus is not composed either 

 wholly or in part of chromatin, but is a true plasmosome nucleolus. 



During the whole mitosis the nuclear membrane remains intact. 

 Furthermore there is no considerable shifting of the'' axes of the 

 nucleus with reference to the axes of the body. The longitudinal axis 

 of the nucleus remains constantly parallel, or nearly so, to the longi- 

 tudinal axis of the body. As the chromosomes, both massive and 

 granular, remain attached to the chromatin fibers, and as these have 

 a persistent attachment to the two poles of the nuclear membrane, we 

 see that there is a definiteness and persistence of orientation in these 

 nuclei that is particularly favorable for the study of some problems, 

 which, however, I do not wish to take up in this paper. 



Mitotic division of one nucleus is accompanied by similar division 

 of the other nucleus in the Protoopalinas and the Zelleriellas, but 

 usually one finds that the posterior nucleus lags very slightly behind 

 the anterior nucleus, the difference in mitotic condition being gen- 

 erally just enough to be observable. (See fig. 12, &, p. 35, and also 

 Zelleriella atelopyxena^ fig. 59, h and c, p. 92, in which these relations 

 are reversed.) 



Following the nuclear mitosis there is division of the body. This 

 may sometimes be a longitudinal division, in other cases it is trans- 

 verse.- As these animals have not as yet been reared successfully out- 

 side the host, we have no sufficient data for determining the relative 

 frequency of the two sorts of division. Daughter individuals which 

 have just come from transverse division are a bit more stocky than 

 the usual form, but daughters which have just been formed by lon- 

 gitudinal division are not always observably more slender than usual. 

 It may be that the relative frequency of the two sorts of division is 

 not the same for all strains of a given species and this divergence 

 in division habit, if it exists, may account in part for the divergence 

 in average form found in different infections of the same species of 

 Opalinid. 



One point in connection with these divisions is important to note. 

 When division occurs the division plane passes between the two pairs 

 of daughter nuclei, so that each daughter cell so formed contains 

 both the daughter nuclei coming from the division of one of the 

 parent nuclei. The two daughter nuclei of a single pair are never 

 separated into different cells, but remain together in the same 

 daughter cell, being even connected for a long time by a thread of 

 nuclear membrane. This point is of considerable significance in com- 

 paring the Opalinidse with the higher Ciliata, as we shall later see. 



It is well to give here an outline of the life history, as we have of 

 the structure, of the species chosen, Protoopalina intestinaUs^ for two 

 reasons: first, for reference in our further discussions in this paper, 



