THE OPALINID CILIATE INFUSOEIANS, 31 



zew's unillustrated descrijDtion of this very interesting species is here 

 given nearly in full. He says, in substance (comments by Metcalf 

 enclosed in square brackets) : 



This species is of peculiar interest since in all individuals the nucleus is 

 found to be in a stage of mitotic division. The infusorian has a slender elon- 

 gated structure ; its anterior end is rounded, its posterior end tapers somewhat. 

 Tlie body is colored with a mass of long cilia arranged in close set parallel fur- 

 rows. The endoplasm is filled with an abundance of almost regular spheroidal 

 inclusions [endosarc spherules of Metcalf]. There is always a single nucleus, 

 which distinguishes this form sharply from Opalina [Frotoopalina] mitotica 

 recently described by Metcalf. In all my material I have found no individual 

 whose nucleus is in the resting condition such as is known in related species. 

 [This so-called resting condition in other species evidently means the reticulate 

 condition, which is not the characteristic condition of any observed Frotoopalina 

 except P. adelnidensis. It is a transiont phase of the mitosis, the nucleus rest- 

 ing in some other mitotic phase.] In my material the nucleus is always in two 

 parts, lying in the longitudinal axis of the body and connected by a constricted 

 portion of different lengths in difTerent cases. In each half of the nucleus one 

 can readily distinguish numerous strongly staining chromosomes of different 

 lengths. The edges of the chromosomes are commonly somewhat irregular, as if 

 toothed, but individuals are found with wholly smooth chromosomes. The 

 chromosome number is evidently not always the same. [Doubtless the number 

 of the definitive chromosomes is constant, but the number of chromatin masses 

 may be different in different nuclei which are not in the middle anaphase con- 

 dition, the stage of mitosis in whicli the definitive chromosomes may best be 

 counted.] Most frequently I have found Opalinas with five chromosomes. 



In cross section it is easily seen that the chromosomes are arranged along 

 the outer edge of the nucleus, while in the interior of the nucleus, in the nodes 

 of the network, lie very fine granules which stain noticeably more faintly 

 than the chromosomes. The chromosomes, as the sections show, are not in 

 actual contact with the nuclear membrane, but are separated from this by a 

 layer of network ["ewe Wabcnschicht "]. In some cases the chromosomes 

 are wholly distinct from one another, while in other instances they are con- 

 nected into a ring by means of delicate weakly staining bridges as described 

 by Metcalf for Opalina [ProtoopaUna] intestinalis.^" [This indicates an early 

 metaphase condition of these nuclei.] 



The poles of both halves of the nucleus are pointed and appear wholly with- 

 out granules, as does the narrow area connecting the two halves of the nucleus. 

 Along the dividing nucleus are seen very numerous, weakly staining spindle 

 threads. 



At the beginning of the division of the infusorian, before the actual division 

 of its nucleus, there follows a division and separation of the chromosomes, 

 which renders clearly visible the spindle composed of achromatic" threads. 

 Thereafter the nuclear bridge breaks, whereupon division of the body itself 

 follows, so that each daughter cell once more enters upon a stage of protracted 

 mitotic division. I have never observed longitudinal division of the Opalina 

 here described. There is scarcely a necessity or possibility of such a division. 

 [This statement is difficult to imderstand. In Frotoopalina intestinalis, for 

 example, nuclear mitosis is very similar to that here described by Awerinzew, 



" The best instance of this lateral union of the chromosomes during the early metaphase 

 to form a ring has been described by Leger and Duboscq (1904, b. III) for ProtoopaUna 

 tsaturnalis (see fig. 37, a, p. 64 ; compare fig. 4, e, p. 12). 



"Metcalf (1909, also section 2 of this paper, p. 15) has described the chief fibers of 

 the spindle as chromatic ; that is, as filose pseudopodia from the chromosomes. 



