74 ORGANIZA TION OF THE INFUSORIA. 



highly characteristic elemental factors, and exhibit among the various 

 members of the class such varied characters and configuration, that their 

 independent treatment is rendered necessary. The morphological corre- 

 spondence of the nucleus or endoplast, as it occurs among the simplest 

 and lowest organized Infusoria, with the nucleus or cytoblast of the ordinary 

 animal or vegetable tissue-cell, is so conspicuously obvious as to render 

 extended comment needless. Among the higher representatives of this 

 group, it. nevertheless, manifests so wide a divergence in various direc- 

 tions from this simple and primitive condition as to render desirable 

 the employment of some equally appropriate but at the same time suffi- 

 ciently distinctive and independent title. This widely felt desideratum 

 has been happily recognized and provided for by Professor Huxley, whose 

 recently introduced term of the " endoplast " in place of that of the nucleus 

 when applied to Infusoria or other Protozoic structures is, as previously 

 related, adopted by the author. The simplest representative type of 

 the infusorial endoplast, and the one which corresponds most closely with 

 the nucleus of the ordinary organic cell, is met with most abundantly 

 among the Flagellate section of the series, but is also represented in not a 

 few of the higher Ciliata. In this initial phase of development it exhibits 

 a simply spheroidal contour, and may or may not enclose a central endo- 

 plastule, the homologue of the histologic nucleolus. The first step towards 

 a departure from this primitive condition is manifested by the tendency of 

 the endoplast, as illustrated by the Flagellate genus Eicglena and its allies, 

 to lose the spheroidal and to assume a more or less ovate outline. A still 

 more attenuate and somewhat sausage-like configuration is exhibited by the 

 endoplast appertaining to the Ciliate genera Balmitidiiun and Nyctothenis, 

 a further element of divergence being here introduced, however, through the 

 fact that the endoplastule or so-called nucleolus is not contained within the 

 substance of the endoplast, but is adherent exteriorly to its lateral peri- 

 phery : this special phenomenon, entirely opposed to what obtains with 

 reference to the nucleolus of the ordinary tissue-cell, occurs repeatedly 

 among the Infusoria. The most pronounced development of the elongate 

 type of endoplast is found associated with the Peritrichous group of the 

 Vorticellidse, and where in many cases it assumes a remarkably attenuate, 

 ribbon-like aspect. Such a ribbon-like or almost filiform configuration of 

 the endoplast is more especially characteristic of the genus Ophrydium, 

 and is also represented, though in a less marked degree, in the Hetero- 

 trichous form Bursar ia truncatella. In intimate connection with the band- 

 or ribbon-like type above described, has to be enumerated that branched 

 variety of this structure common to many of the Suctorial Infusoria, or 

 Acinetidae, and also that modification of the elongate form of the endoplast, 

 similarly band-like in the earlier stages of its development, but which in the 

 mature condition of its existence exhibits a more or less uniform series of 

 constrictions or strangulations. In its most characteristic phase of develop- 

 ment, as prominently illustrated by the genera Steiitor, Condylostonia, and 



