General Morphology of the Protozoa 47 



Nuclear dimorphism 



The Ciliophora are unique in that all species, except the sup- 

 posedly primitive opalinids, have both micronuclei and macronuclei — 

 unless Stephanopogon mesnili (149) is another valid exception. In S. 

 mesnili, all of the nuclei are similar in size and structure, and their 

 division closely resembles that of the micronucleus in typical ciliates. 

 Perhaps this case is analogous to that of Dileptiis (81, 216), discussed 

 below. In the typical ciliates, more than one micronucleus or macronu- 

 cleus may be characteristic of a species and the number of each type some- 

 times varies independently. Stentor coeriileus, for example, may show 

 10-42 micronuclei distributed irregularly along the 7-23 links of the macro- 

 nuclear chain (201). Both types of nuclei have a common origin from 

 the synkaryon formed in conjugation. The division-products of the syn- 

 karyon, presumably identical cytologically and genetically, undergo diver- 

 gent metamorphosis in conjugation. A developing micronucleus under- 

 goes reduction in size and often a decrease in staining capacity. The de- 

 veloping macronucleus increases in size, undergoes changes in internal 

 structure and may show extensive changes in form before reorganization 

 is completed. The nature of the changes involved in the development of 

 macronuclei is still unknown. On the basis of genetic data (203), it has 

 been suggested that the macronucleus is a compound nucleus composed 

 of many units, each with its own diploid set of genes. At each fission, the 

 macronucleus divides amitotically, contributing approximately half of 

 its units to each daughter ciliate. Subsequently, the normal number of 

 units is restored by mitotic processes within each reorganizing macronu- 

 cleus. This theory is interesting, but adequate morphological grounds for 

 such an interpretation are lacking. The ciliate micronucleus, in contrast 

 to the macronucleus, undergoes mitosis during reproduction of the or- 

 ganism. 



Macronuclei vary considerably in form, size and number. The simplest 

 are spherical to ovoid bodies (Fig. 1. 21, C) containing many densely 

 staining granules perhaps embedded in an achromatic framework (159). 

 The macronucleus of Paramecium is stretched in the ultracentrifuge and 

 the contents are stratified in two zones, the chromatin granules appar- 

 ently being denser than the achromatic substance (120). The Feulgen 

 technicjue indicates that different types of granules are stainable with 

 hematoxylin. Uniformly small granules, scattered through the macro- 

 nucleus of certain ciliates, are Feulgen-positive; certain larger granules 

 give a negative reaction (57, 104, 156). The staining capacity of these 

 Feulgen-positive granules in Stentor coeruleiis is not affected by ribonu- 

 clease (223). 



Macronuclei are not always compact. The macronucleus of Euplotes 



