106 Unircrsity of California Publications in Zoology [Vol. 20 



Near the distal limits of the ectoplasmic differentiation the longi- 

 tudinal myonemes fade out in the endoplasm, without showing distinct 

 attachment areas. Their function here, as in Trichonympha, seems 

 to be concerned with the mobility of the anterior tip of the bodj'. 

 The globular form of Leidyopsis as compared with the elongate one 

 of Trichonympha allows for much less activity, yet the same tj-pe of 

 sidewise movements, though greatly restricted, may be observed in 

 the living, active flagellate. 



Nucleus : The nucleus is found in the anterior part of the body, 

 a short distance posterior to the centroblepharoplast (fig. A, «.). It 

 is a rotund ellipsoid, with its longer axis, which lies in the transverse 

 plane of the body, exceeding its shorter axis by nearly a third of its 

 own length. No rhizoplast could be detected between it and the centro- 

 blepharoplast. 



The structure of the nucleus is like that of Tri-chonympha cam- 

 panula. The membi'ane is thin and overlies a narrow granular zone, 

 beneath which is an alveolar zone. These alveoli are rounded out- 

 wardly where they abut upon the granular zone, with the romaining 

 facets closely pressed together. The alveoli are fairly uniform in 

 size, yet in stages which show distinct chromosome changes prior to 

 the prophase of division, great modifications may often be noticed. 

 The alveoli may be fewer in number and larger in size. This may 

 be the result of a breaking down of the walls between several adjacent 

 alveoli, since in the earh^ prophase their disappearance is usually 

 complete. 



The remainder of the nucleus inside the alveolar zone is occupied 

 by an irregular linin network in which loose ends may often be 

 detected. The network is usually encrusted with chromatin, with 

 large granules of the same substance at tlie nodes. The disposition 

 of the chromatin, however, varies considerably^ in different individuals 

 aiid is probably conditioned by the chromosome cycle, which here 

 apparently follows a course similar to that previously outlined for 

 Trichmiynipha camrpanula (Kofoid and Swezy, 1919c). The nuclei 

 shown in figures 5 to 8 on plate 13, give different phases of nuclear 

 structures in individuals which do not yet give other indications of 

 the approach of division. In these nuclei the network progressively 

 breaks up, the chromatin moving out from the granules along the 

 threads which become thicker and split lengthwise (fig. 6). From 

 these threads the definitive chromosomes are produced. 



