NUCLEI AND KINETIC ELEMENTS 69 



]Multiple nuclei are found in Peloniyxa pahisfri<}, Actinosphoerium 

 eichhornii, Calonymphidie and in the majority of Infusoria. 



Dimorphic nuclei are examples of multiple nuclei in which a differ- 

 ent function in the cell is associated with the different nuclei. Such 

 function may be of a sexual nature as in the Myxosporidia where 

 differences in size and structure indicate a differentiation which may 

 be expressed by the terms male and female nuclei since products of 

 two of them, one from each type, unite to form a fertilization 

 nucleus of the young cell (sporozoite) according to the observations 

 of Schroeder and Keysselitz. Or the function may be of a metabolic 

 nature in one t\'pe and reproductive in the other, as in the Infusoria, 

 where the two t^^^es show great differences in form and size. Here 

 the nucleus having to do with metabolism makes up a large part of 

 the volume of a cell and is usually of relatively large size, hence is 

 called the macronudeus , while nuclei having to do with reproduction 

 and fertilization are alwaj'S minute and are called micronuclei 

 (Fig. 30). Usually the micronucleus is closely attached to the 

 macronudeus and, in some cases, may be embedded in its substance 

 (e. g., Blepharisma iinduhnts) emerging only during phases of con- 

 jugation; or it may be partially hidden in a depression or pit in the 

 macronudeus, or it may be entirely independent of the larger nucleus 

 and lie freely in the cytoplasm. A t}7)ical example of dimorphic 

 nuclei is shown by Paramecium caudatum (Fig. 22, p. 57). 



The form assumed by macronuclei and the number in a single cell, 

 varies within wide limits. The most generalized condition is a 

 simple, spherical form; but ellipsoidal, rod-like, horse-shoe-shape, 

 beaded and branched macronuclei are not uncommon. The beaded 

 forms frequently appear like several separated nuclei but the seg- 

 ments are usually enclosed in a common membrane contracted at 

 the nodal points, the entire aggregate forming a single nucleus 

 (Spirostomum, Stentor, Amphileptus, Uronychia, etc.). In other 

 cases, however, multiple macronuclei are formed by repeated nuclear 

 divisions, the eight macronuclei of Uroleptus mobilis, for example, 

 arising by three consecutive divisions of an original single nucleus 

 (Fig. 1). The size of the macronudeus bears no constant relation 

 to the size of the organism (Fig. 30). 



Micronuclei do not differ much in form but vary in structure 

 from t\'pical vesicular to compact massive t\'pes. Their number in 

 the cell likewise varies from 1 to as many as 80 or more (Stentor). 

 They are never connected with one another, but are quite indepen- 

 dent and distributed at intervals along the side of the macronuclei. 



There is little or no evidence of the phylogenetic origin of these 

 dimorphic nuclei which are distinctive of the Infusoria. In onto- 

 genetic origin the macronuclei are invariably derived after conjuga- 

 tion from a division product of the fertilization nucleus, the latter 

 being formed by the union of two micronuclear elements. Hence the 



