266 THE PROTOZOA 



probably justly, been compared with the centrosome and astral rays 

 of Metazoa. This central granule, first observed by Grenacher ('69), 

 was shown to be connected with the pseudopodia by Greeff in 

 the same year. Subsequent observations by F. E. Schultze ('74), 

 Hertwig ('//), and many others, have confirmed these results, and 

 demonstrated the widespread occurrence of the central granule among 

 the Heliozoa. Biitschli ('92) was the first to suggest the similarity to 

 the centrosome with its radiating fibres, a view which O. Hertwig ('93) 

 and Schaudinn ('96), and with them the majority of cytologists, have 

 accepted. The most complete observations have been made upon 

 species of Acanthocystis and Sphcerastrum by Schaudinn ('96). In 

 the latter form (Fig. 144, A) the corpuscle is distinctly granular, and in 

 fixed preparations has a definite alveolar structure. The beginning 

 of division is signalized by the withdrawal of the pseudopodia; the 

 central granule then divides, and with it the entire astral system. 1 

 The daughter-centres are joined together by a connecting strand 

 which Schaudinn regarded as a possible central-spindle (Fig. 144, C). 



The extra-nuclear central granule in these Heliozoa thus acts like 

 the intra-nuclear division-centre of Euglena and other flagellates. The 

 extra-nuclear division-centres in the rhizopod Pammceba and in the 

 cystoflagellate Noctiluca, on the other hand, are much more like the 

 centrosphere in Metazoa ; and in Noctiluca especially, the mitotic 

 figure, while of the protozoan type, is more like that of the Metazoa 

 than of any other known single-celled form. 



On the outside of the nucleus in Noctiluca, in the cytoplasm, and 

 close against the nuclear membrane, is a large, faintly staining spher- 

 ical mass, which acts as a division-centre. During the early stages 

 of nuclear activity, the sphere divides into two similar halves, con- 

 nected by a strand composed of fibres which are formed from the 

 substance of the sphere. These fibres compose the central spindle, 

 and are homologous in every way with the central-spindle fibres of 

 the usual type of mitosis in Metazoa (Fig. 141, C-E). The 

 nucleus then elongates in a direction at right angles to the central 

 spindle, and at the same time it bends in the centre in such a way 

 that the central spindle sinks into a depression in the nucleus, which 

 surrounds it upon three sides. In this way the nuclear plate is finally 

 wrapped about the central spindle in the form of an incomplete ring, 

 a condition which obtains in all higher mitotic figures where the cen- 

 tral spindle is present. The nuclear membrane then disappears 2 

 in that part of the nucleus which is turned toward the central spin- 

 dle, while it is retained unbroken in all other parts of the nucleus 



1 The division of the central granule was first observed by Sassaki ('93) in the marine 

 form, Gymnosphcera. 



2 Cf. Calkins ('98), p. 15 ; Ishikawa ('99), p. 244. 



