THE OPALINJD CILIATE INI USOIUANS. 251 



P. caudata (fig. 19, p. 42), with two nuclei, usually pear-shaped 

 (metaphase condition), but sometimes almost spherical (reticulate 

 condition). When pear-shaped the nuclei are often still connected 

 by a thread. 



P. macrocaudata (fig. 25, p. 50), with two nuclei connected by a 

 thread. Each nucleus, in the characteristic condition, is in a later 

 mitotic phase than is the ordinary resting nucleus of P. caudata. 



P. intestinalis (fig. 27, p. 52), Avith two nuclei frequently not con- 

 nected by a thread and usually in a late metaphase, almost a reticu- 

 late condition. 



P. tenuis (fig. 34, p. 59), with two distinct nuclei, each being 

 usually considerably elongated preparatory to the next mitosis. The 

 internal structure of the characteristic nucleus of this species shows 

 a reticulate or even a prophase condition. 



P. Tegularh (fig. 41, p. 70), with two elongated nuclei, still con- 

 nected by a thread, usually each in an early anaphase of mitosis. 



P. scaphiopodos (fig. 4G, p. 76) , with two distinct dumb-bell-shaped 

 nuclei, each regularly in a very early metaphase. 



P. mexicana (fig. 49, p. 79), with two distinct dumb-bell-shaped 

 nuclei, each usually in a someAvhat later metaphase. 



P. formome (fig. 50, p. 81), with two distinct nuclei, each almost 

 completely divided into tAvo; the four pear-shaped daughter nuclei 

 are still connected in pairs by slender threads. Each is an early 

 telophase. 



P. quadrinucleata (fig. 51. p. 82), with four distinct nuclei, each 

 elongated and in an early stage of the next m.itosis, with distinct 

 chromosomes, not, hoAvever, as yet arranged upon a mitotic spindle. 



P. axonudeata (fig. 52, p. 83), with usually- from G to 10 nuclei 

 in pairs. The nuclei may, in different individuals, be in different 

 stages of mitosis, and in a single indiA'idual there may be some dif- 

 ference in the mitotic phases of the different pairs of nuclei, one pair 

 being, say, in a dumb-bell condition, not yet divided, another showing 

 the daughter nuclei still connected merely by a thread, Avhile a third 

 pair may show the daughter nuclei disconnected. 



It seems that the series of conditions here noted may correspond 

 to the series of evolutionary stages by which originally uninucleate 

 forms acquired the binucleate condition and ultimately passed on 

 to the multinucleate condition. 



Among the Zelleriellas Ave find none that are regidarly uni- 

 nucleate even Avith a dumb-bell nucleus. Of course, after fission 

 one of the daughter cells may be for a time uninucleate. Nor do 

 we find as full a series of characteristic midmitotic resting condi- 

 tions in the several species of this genus. One naturally judges that 

 the flattening of the body, which produced the Zelleriellas, took place 

 after the possession of tAvo nuclei became characteristic of the 



