DERIVED ORGANIZATION 85 



Multiple nuclei are found in Pelomi/xa palustris, Actinosphaerium 

 eichhornii, Calonymphidae 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, Keysselitz, Naville and others (see p. o2C>). Or the 

 function may be of a metabolic nature in one type and reproductive 

 in the other, as in the Infusoria, where the two types 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 macronucleus, 

 while nuclei having to do with reproduction and fertilization are 

 always minute and are called micronuclei (Fig. 44). Usually the 

 micronucleus is closely attached to the macronucleus and, in some 

 cases, may be partially hidden in a depression or pit in the macro- 

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

 and lie freely in the cytoplasm. A typical example of dimorphic- 

 nuclei is shown by Paramecium caudatum (Fig. 23, p. 50). 



The derived forms assumed by macronuclei and the number in 

 a single cell vary within wide limits. The most generalized condi- 

 tion 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 segments are usually enclosed in a common membrane con- 

 tracted at the nodal points, the entire aggregate forming a single 

 nucleus (Spirostomum, Stentor, Amphileptus, Uronychia, etc.). The 

 size of the macronucleus bears no constant relation to the size of 

 the organism (Fig. 44). 



Micronuclei do not differ much in form but vary in structure 

 from typical vesicular to compact massive types. 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 sides 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 nuclei are invariably derived after conjugation 

 from division products of the fertilization nucleus, the latter being 

 formed by the union of two micronuclear elements. Hence the 

 statement is usually made that macronuclei arise from micronuclei, 

 a statement which is not strictly accurate, since the fertilization 

 nucleus is neither one nor the other, but merely a cell nucleus of a 

 fundamental organization. In some cases macronuclei and micro- 



