448 BULLETIN 120, UNITED STATES NATIONAL MUSEUM. 



camera drawings] within the limit of 8.9 sq. mm., is the stimulus 

 that initiates nuclear division." Division of one of the nuclei de- 

 creases the mass per nucleus by an amount represented by an area 

 of 14.7 sq. mm., and this in spite of the fact that the body as a whole 

 has increased in size. " There is an increase in area per nucleus up 

 to the point where [when] one nucleus divides, then a conspicuous 

 decrease following nuclear division, and subsequently an increase 

 during the period when the nuclei resulting from division regain 

 their full size, ending in a size at which the area per nucleus is ap- 

 proximately that present at the beginning. * * * The size of 

 the entire specimen increases during nuclear multiplication and 

 growth * * * but the area per nucleus remains almost con- 

 stant." The data " favor the conclusion that the stimulus that ini- 

 tiates division acts as a rule on only one nucleus at a time, and 

 that the division of this nucleus restores the nucleo-cytoplasmic 

 ratio," explaining " why nuclear divisions in Opalina are not syn- 

 chronous." "As the number of nuclei increases, their average volume 

 and surface decreases " [an observation confirmed by my own prep- 

 arations of Opalina ranarum]. 



See Appendix 1 for review of two other papers. 



10. A LIST OF INSTITUTIONS IN WHICH ARE DEPOSITED SETS OF 

 SPECIMENS OF THE SPECIES OF OPALINIDAE. 



In dealing with species whose distinctive character is not very well 

 marked, as is the case with numerous Opalinids, it is, of course, far 

 better to have an actual specimen of the animal than to have merely 

 a description. It is almost certain, in the case of some of the species 

 described in this paper, that other students, with more, or better, or 

 dijfferent material, will find it necessary to revise the taxonomy here 

 adopted. In order to minimize so far as possible the confusion likely 

 to result, the autlior has placed a set of type specimens in the United 

 States National Museum and has asked also 16 museums and other 

 institutions, where interesting Opalinid material is likely to be 

 available or where protozoologists are likely to be working, to accept 

 sets of paratypes of the new species, subspecies, and formae de- 

 scribed in this paper and to allow competent students, who may so 

 desire, to have access to them. The author retains one set. "With 

 these paratype specimens are included slides of most of the species 

 hitherto known. The author has, however, no specimens of the 

 following species to include in these sets: Pi'otoopalina acuta 

 (Rajff), P. dorsalis (Raff), P. hylarum (Raff), P. primordialis 

 (Awerinzew), P. tenuis (Raff), Zelleriella hinucleata (Raff), Z. 

 macronucleata (Bezzenberger), Cepedea{\') fiava (Stokes), C. lan- 

 ceolata (Bezzenberger), Opalina cincta (Collin), 0. coracoidea (Bez- 

 zenberger), O. lata (Bezzenberger). He has but one slide each 

 of Cepedea virgula (Dobell) and Opalin^a {larvarum^ and his mate- 



