22 



THE INVERTEBRATA 



meganucleus is known as trophochromatin^ that of the micronucleus 

 as idiochromatin. 



A similar distinction between trophochromatin and idiochromatin 

 is discernible in various other protozoa. In the Opalinidae (Fig. 21 C) 

 and Chonotricha there are two sets of chromosomes, an outer and an 

 inner, which divide successively at mitosis. The members of the outer 

 set (megachromosomes), larger and less regular than those of the inner, 

 are held to represent the meganucleus of other ciliophora : the material 

 of which they are composed is known to be cast out of the nuclei of 

 the Opalinidae before gamete formation. The members of the inner 



. * - 



yi'y 



Fig. 21. Nuclear phenomena in protozoa. A, Extrusion of the germ nucleus 

 from the somatic nucleus in Gregarina cuneata before gamogony. The somatic 

 nucleus will break up and disappear. B, Extrusion of the substance of the 

 endosome from the gamont of Eimeria schuhergi, the germinal part of the 

 nucleus remaining in the centre of the body. C, Mitosis in Opalina ranarum, 

 megachromosomes in anaphase in outer part of nucleus, microchromosomes 

 in metaphase internal to them. A, after Milojevic; B, after Schaudinn; 

 C, after Tonniges. 



set {microchromosomes) represent the micronucleus. In various cases 

 of gamete and spore formation by members of other classes, especi- 

 ally of the Sporozoa, there is a destruction (Fig. 21 A, see legend), 

 or a casting out from the body (Fig. 21 B), of a portion of nuclear 

 substance which is probably trophochromatin. It has been suggested 

 also that the obscurity of cryptomitoses is due to a veil of tropho- 

 chromatin dividing amitotically around the idiochromosomes. It may 

 be that all protozoa contain chromatin in both these conditions ; and 

 it is perhaps in this respect, as well as in restriction of function, 

 that the cells of metazoa differ from protozoa. 



