26 THE INVERTEBRATA 



In certain cases mitoses repeated several times without dissolution 

 of the nuclear membrane give rise to polyenergid nuclei which possess 

 numerous sets of chromosomes, the sets being finally liberated to 

 form each a daughter nucleus. The polyenergid condition is probably 

 always a provision for spore formation, and may (as in the coccidian 

 Aggregatd) occur only as a transient phase before sporulation, but in 

 other cases (radiolarians) it persists for a long time, the nucleus 

 dividing meanwhile as a whole by "giant mitoses'* in which all the 

 chromosomes take part. A remarkable process in Amoeba proteus is 

 possibly to be interpreted as the multiplication of units in a poly- 

 energid nucleus by cryptomitoses. 



In the Ciliophora (other than the Opalinidae and the Chonotricha) 

 the nucleoplasm, which in other protozoa is contained in one nucleus 

 or in several which are all alike, is divided into two portions, a large 

 amitotic meganucleus which breaks up periodically in *'endomixis" 

 (see p. 35) and also at conjugation, and one or more small micronucleiy 

 by division of one of which the pronuclei of conjugation are provided 

 and the meganucleus replaced when the latter disintegrates. Indi- 

 viduals without micronuclei have been observed and kept through 

 several asexual generations. Thus the meganucleus is capable of 

 conducting by itself the normal vegetative existence of the individual, 

 though the absence of this nucleus at syngamy shows that it does not 

 establish the characters of the race. That function must be performed 

 by the micronucleus, but, since the latter does not exist without the 

 meganucleus, save for a brief period during conjugation, it presumably 

 does not regulate the life of the individual. The chromatin of the 

 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 

 set {microchromosomes) represent the micronucleus. In various cases 

 of gamete and spore formation by members of other classes, especially 

 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 



