328 GENERAL HISTORY OF THE INFUSORIA. 



happens in Vaginicola. When loose, this membrane not imfrequently falls 

 into plaits or folds. It is represented in Cohn's figure of the nucleus of 

 Nassula elegans as a very distinct and stout tunic. 



The rule is, that the nucleus is single, and it has been assumed as a fact 

 that the appearance of a double nucleus or of two nuclei is a general indica- 

 tion of the approaching or progressive act of fission. However, Stein in a 

 recent figui'e of Stylonychia mijtilus (XVIII. 10), delineated in Carus's Icones 

 Zootomicce, represents two ovoid nuclei as present without the accompanying 

 process of self- division. In CMlodon Cucullulus, he also represents the 

 nucleus (XXX. 48 e) to be composed of a moderately thick external or cor- 

 tical portion siuTounding a clear cavity, in the centre of which the opaque 

 solid nucleolus is placed. The cortical lamina, he affirms, consists of the usual 

 homogeneous granular substance which makes up the mass of most nuclei, 

 but rather firmer; and its internal free surface towards the cavity is, he 

 says, undulated or dentated. The interspace between the nuclear lamina 

 and the nucleolus is not always clear, but occasionally occupied by a cloudy, 

 finely-granular matter, — w^hence the nucleus acquires rather the characters of 

 a homogeneous tissue, having a central, well-defined nucleolus. Although 

 the last-named structure is probably never absent, it has nevertheless escaped 

 Stein's notice in very young specimens. The nucleus of Sjjirochona in young 

 specimens is either solid and homogeneous, or transversely divided into tw^o 

 by a crescentic space (XXX. 28/) ; the nucleolus occupies the middle of the 

 nuclear cavity, and has around it a finely-dotted areola (XXX. 17). 



In the case of Paramecium both Cohn and Stein describe the nucleolus to 

 be included in a depression or hilum on one side the nucleus. Like the 

 nucleus it is formed of a membranous coat and homogeneous contents (XXIX. 

 28 d) ; the connexion between the two appears to be only by the adhesion of 

 their membranes, an adhesion readily broken through by pressure or by the 

 action of acetic acid. Further, in the long band-hke nuclei, the nucleoli 

 seem to be multiplied in number. 



On the subject of its chemical nature. Stein concludes from the reaction of 

 tincture of iodine, and of acetic acid ^^ith a solution of sugar, that the nucleus 

 is a proteine compound, like the other contents, except the fat-corpuscles. 



Although its office in secreting a spermatic fluid may be justly called in 

 question (direct obsei'vation being contrary to it), yet this so-called testis, or, 

 perhaps more correctly, this nucleus, certainly plays a most important part 

 in the well-observed mode of propagation by spontaneous fission ; for whenever 

 fission, whether longitudinal or transverse, is about to occur in an animalcule, 

 the fii^st change observed is a progressive constriction of the nucleus, suc- 

 ceeded by that of the body generally. This constriction goes on till division 

 is complete, each segment of the body being consequently provided Avith a 

 nucleus. The division of the nucleus, as an essential element in the process 

 of spontaneous fission, may be well observed in the transverse division of 

 Paramecimn, Bursaria, or Chilodon. 



Professor Owen, in his learned and able Essay on Parthenogenesis, refers 

 to the initiative, assumed by the nucleus of Infusoria, in their rej)roduction 

 by spontaneous fission, between which and the essential contact of the sper- 

 matozoon with the germ -cell, as a prehminary to the primary process of 

 self-division of the latter, in the course of the development of more perfect 

 animals, he indicates an analogy ; and, after having completed the comparison 

 of the results in the two cases, goes on to say, — " This is certain, that the 

 analogy between these phenomena in the midtiplication of the parts of the 

 germ-mass, and those of the nucleus in the multiplication of monads, is so 

 close, that one cannot reasonably suppose that the nature and properties 



