THE OPALINID CTLIATE INFUSORIANS. 29 



The Protoopalinas have in general two nuclei, though one known 

 species is said to have but one, and one species has often as many as 6, 

 8, or even 10 nuclei. The nuclei are large, having in different species 

 a major diameter of from 10 to 45^ micromillimeters. They have few 

 chromosomes, from 4 to 10 massive chromosomes having been 

 counted in different species. The granular chromosomes seem to 

 show corresponding numbers. Nuclear conditions in Zelleriella re- 

 semble those in Protoopalina. In the mitosis of Protoopalina and 

 ZeJJeriella the massive chromosomes are carefully halved and the 

 daughter chromosomes of each pair are distributed to opposite 

 daughter nuclei, whereas in Cepedea and Opalina some of the chro- 

 matin masses may remain undivided and pass bodily to one daughter 

 nucleus, or it seems ma}- even be extruded from the nucleus after 

 being caught bj^ the constricting nuclear membrane at the equator of 

 the dividing nucleus. In Cepedea and Opalina the nuclei are more 

 numerous, from four to several hundred. These nuclei are small, 

 having a major diameter of from 2.6 to 7.75 micromillimeters, or in 

 a very few species over 9 micromillimeters. The chromosomes are in 

 general more numerous and this fact, together with the small size 

 of the nuclei, makes it difficult to count them. Opalina ranm'^Tn and 

 Cepedea dimidiata are thought by Neresheimer to have each twelve 

 macrochromosomes. 



Subfamily Protopalininae. 



The species of Protoopalina are circular, or nearly so, in cross 

 section; the species of Zelleriella are flat, some being very thin, while 

 others are intermediate in shape, being oval in cross section, with 

 different proportional thiclaiess in different species. There are cor- 

 responding shapes among the multinucleated forms, the cjdindrical 

 species belonging to the genus Cepedea and the much flattened 

 species to the genus Opalina. 



Perhaps the most remarkable feature of the Protoopalininae is 

 that in many species the nuclei are regularly found to be resting in a 

 midmitotic condition instead of in the reticulate condition character- 

 istic of " resting " nuclei in general. The mitotic phase in which the 

 nucleus comes to rest is different in different species of Protoopali- 

 ninae. Only a few species have the nuclei characteristically in a 

 reticulate condition. Most numerous are species whose nuclei are 

 characteristically in an early or a late anaphase, or in an early, or a 

 medium, or a late telophase. Equatorial plate nuclei are not charac- 

 teristic of any species thus far studied, nor have nuclei in the skein 

 stage been observed as the characteristic resting nuclei of any species, 



