THE SUN ANIMALCULE 151 



oi 8 mm. in diameter consisting of a nuclear membrane, a 

 network of achromatic (i.e. not easily stainable) substance, 

 containing a more fluid substance in its meshes, and a single, 

 relatively large deeply-staining karyosome consisting of two 

 different substances, namely chromatin and a material which 

 stains differently to chromatin, forms the true nucleoli of tissue- 

 cells, and is called plastin. The karyosome of Actinosphaerium 

 varies very much in shape, being sometimes compact and 

 oval or crescentic, sometimes stellate, variously branched, or 

 broken up into minute chromatin particles which are scattered 

 through the nucleus. When a nucleus is about to divide, 

 conical accumulations of homogeneous cytoplasm make their 

 appearance at two opposite poles, forming the so-called 

 plasmatic cones. The nucleus, previously spherical, becomes 

 flattened between the plasmatic cones, so as to be shaped like 

 a bi-convex lens (fig. 32, D). Meanwhile the chromatin of 

 the karyosome has become distributed along the threads of 

 the achromatic network in the form of branching strings of 

 minute granules. At the same time peculiar structures known 

 as the polar plates make their appearance at the two poles of 

 the nucleus underlying the bases of the plasmatic cones. 

 These polar plates are thin curved plates of a homogeneous, 

 highly-refracting, nuclear substance, and it should be noticed 

 that they arise inside the nuclear membrane, and cannot 

 therefore be homologised with the archoplasm nor yet with 

 the centrosome of tissue-cells. Similar pole-plates have been 

 observed in the dividing nuclei of Amoeba bi-nudeata, in 

 Euglypha, Paramecium and several other Protozoa. The 

 meshes of the achromatic network now become elongated 

 and stretched between the two pole-plates to form the achro- 

 matic spindle (fig. 32, E]. When the spindle is established 

 the chromatin granules, previously distributed along the meshes 

 of the achromatic network, become aggregated at the equator 

 of the spindle in the form of numerous little lumps or rods of 

 irregular shape, each of which may be regarded as a chromo- 

 some. Each chromosome divides into halves which diverge 

 from one another to the opposite ends of the spindle, there 

 to form by fusion with their fellows two continuous plates 

 of chromatin underlying the pole-plates (fig. 32, F\ The 

 achromatic spindle, and with it the whole nucleus, have mean- 

 while become elongated, and finally a constriction divides the 



