ZOOLOGY AND BOTANY, MICBOSCOPY, ETC. 399 



divides first, the halves separate from one another, and then the rest 

 of the nucleus is cut through. 



Among the Ehizopoda two other kinds of nuclei are also seen ; 

 in one of them we distinguish a nuclear membrane, and substance, 

 within which are scattered, more or less irregularly, particles of 

 chromatic substance. On division these become . arranged into 

 filaments, which, at first coiled, become later on arranged parallel to 

 the long axis of the extending nucleus, and so are equally divided on 

 its constriction. The other form is distinguished by the presence of 

 a cortical zone, generally consisting of granules, which lies just 

 beneath the nuclear membrane. Here there is but little nuclear 

 substance and a large central nucleolus. In division, the nucleolus 

 divides first, and the parts separate from one another ; the cortical 

 zone is then divided equatorially, and finally the whole nucleus is 

 cut through. 



Lastly, the nuclei of some Foraminifera are remarkable for being 

 distinguishable into two halves, one of which is quite filled with 

 chromatic substance, while the other has one or more nucleoli. The 

 mode of their division is as yet unknown. 



The Heliozoa are next taken up, and their nucleus found to 

 consist of a nuclear membrane, clear nuclear substance, and a central 

 nucleolus, or there are several nucleoli ; or, finally, there is a mem- 

 brane, a cortical layer, nuclear substance, and central nucleolus. In 

 this last, division always begins ; when there are several they unite 

 into two plates, which separate from one another. 



In the large nuclei of the intermediate Eadiolaria, we find 

 (a) vesicular forms, exactly like those of many Ehizopoda and 

 Heliozoa, (/3) nuclei with a cortical layer (as in Actinophrys) ; (y) 

 nuclei with a very strong membrane, dark and often granular nuclear 

 substance, in which radiating bands may sometimes be seen ; and 

 (8) nuclei with a plexiform arrangement of the chromatic substance, 

 and nucleoli imbedded in the meshes ; the fissive methods of none of 

 these are satisfactorily known. 



The small nuclei of the multinucleate Eadiolaria are either 

 amoeboid and divide by simple constriction, or are quite round or 

 oval when division commences with the radiate arrangement of the 

 chromatic substance. 



The nuclei of the Gregarinida have a vesicular structure, and one 

 or more nucleoli ; their mode of division is not known. The nuclei 

 of the spores are quite homogeneous and divide by constriction. 



The different groups of the Infusoria are discussed separately ; 

 in the true Flagellata we find vesicular nuclei, which divide by the 

 regular constriction of all the parts, and the formation of parallel 

 longitudinal lines in the nucleolus. In Noctiluca the nucleus forms 

 a granular mass in which nucleoliform corpuscles are distributed ; 

 but in Lejytodisciis the nucleus is formed of a larger, darker, and 

 granular portion together with a smaller and clearer part. Unlike 

 Botalia, the hyaline part here contains the chromatic substance. In 

 Noctiluca, as observed by Eobin, the nucleus elongates, and the 

 central part becomes longitudinally striated. In the Cilio-flagellata 



