THE SUN ANIMALCULE 143 



The resting nuclei present some variety of form, but they 

 may in general be described as spherical bodies from -012 to 

 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 body, often called the nucleolus. 

 This so-called nucleolus is described as 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 central body, or "nucleolus," 

 of Actinosphgerium 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. 30, >). Meanwhile the 

 chromatin of the central body has become distributed along 

 the threads of the achromatic network in the form of branch- 

 ing 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 centrosomes 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. 30, ). 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 



