120 PARAMCECIUM, STYLONYCIIIA, OXYTRICIIA LESS, x 



forms (see Lesson XIII.), we may consider Stylonychia as 

 the highly-specialized descendant of some uniformly-ciliated 

 progenitor. 



A third genus of ciliated Infusoria must be referred 

 to in concluding the present Lesson. We have seen how 

 the nucleus of a Paramoecium which has just conjugated 

 breaks up and apparently disappears (Fig. 21, A G). 

 In Oxytricha^ a genus closely resembling Stylonychia, the 

 two nuclei have been found to break up into a large number 

 of minute granules (Fig 23), which can be seen only after 



FIG. 23. Oxytricha flava, killed and stained, showing the frag- 

 mentation of the nuclei. (After Gruber.) 



careful staining and by the use of high magnifying powers. 

 This process is called fragmentation of the nucleus ; in 

 other cases it goes even further, and the nucleus is reduced 

 to an almost infinite number of chromatin granules only just 

 visible under the highest powers. From this it seems very 

 probable that organisms which, like Protamceba (p. 9) and 

 Protomyxa (p. 49), appear non-nucleate, are actually pro- 

 vided with a nucleus in this pulverized condition, and that 

 a nucleus in some form or other is an essential constituent 

 of the cell. 



