374 THE INFUSORIA 



Among the Acinetaria many curious forms of meganucleus have 

 been described. In full-sized specimens of a species of Ephelota 

 (Fig. 20) the meganucleus has the form of a ring situated in a plane 

 parallel with the crown of tentacles. From this ring several knob- 

 like processes project, some turned towards the crown and others 

 towards the stalk. 



In another species described by Ishikawa the meganucleus of 

 the adult is in the form of a coarse-beaded network (Fig. 21). In 

 Dendrosoma it is a thick smooth band occupying the axis of the 

 branches. 



In some cases remarkable changes in the shape of the mega- 

 nucleus occur which seem to have no connection with the repro- 

 ductive processes. In Dendrocometes, for example, it may take the 

 shape of an elongated spindle and move to almost any position in 

 the cytoplasm. It has even been seen to retreat entirely into one 

 of the arms. It may also be assumed, from its very irregular form 

 in many species of Acineta, Eplielota, and other genera, that the 

 meganucleus normally undergoes amoeboid contortions during the 

 whole period which elapses between successive acts of reproduction. 



Very different accounts have been given of the minute structure 

 of the meganucleus in the group. It is clear, however, that at least 

 two elements enter into its composition a substance that may be 

 called the Chromatin, having a great affinity for ordinary nuclear 

 stains ; and a substance which resists nuclear stains, and may be 

 called the Achromatin. The chromatin is usually in the form of a 

 close-meshed network of fibrils extending through the whole space 

 occupied by the meganucleus, and it gives with low powers of the 

 microscope the appearance of a crowd of granules. Under unfavour- 

 able circumstances this network may be gathered up into a series 

 of bunches, and each bunch may be ultimately torn away from its 

 neighbours, giving the chromatin the appearance of being arranged 

 in a series of irregular and frequently vacuolated granules. If the 

 unfavourable circumstances continue, disintegration of the mega- 

 nucleus may follow, but recovery from the granular condition is 

 quite possible, and the chromatin may again resume the form of 

 an evenly distributed network. Changes in the shape of the mega- 

 nucleus may lead to the deceptive appearance of change in structure. 

 Thus the meganucleus of Dendrocometes when it assumes the spindle 

 form appears longitudinally striated, and in its constriction during 

 bud-formation there is an appearance that might be mistaken for a 

 row of -independent fibres in the narrowest part of the neck (Fig. 31). 

 Careful analysis of these striae and rod appearances proves, however, 

 that in all cases they are due to a rearrangement of the meshes 

 of the primary network. Local thickenings of the fibrils of the 

 chromatin network frequently occur, and in some forms acquire 

 considerable size, but it seems probable that chromatin granules 



