134 COMPARATIVE ANATOMY 



or amitotic division of the nucleus during binary fission is the 

 rule. It is, at all events, certain that amitotic nuclear division 

 does occur. In Amoeba crystalligera the nucleus consists of 

 a central mass or kernel, which stains but feebly with ordinary 

 dyes. This has been described as a nucleolus, but the name 

 is hardly applicable, for it is nothing like the nucleolus of 

 other cells e.g. ovicells. Around this kernel is an envelope 

 of material which stains deeply, and is undoubtedly chromatin ; 

 and outside of this again is a cortical layer which scarcely 

 stains at all. The characteristic alveolar structure of proto- 

 plasm is distinguishable in all three elements of the nucleus. 

 (Fig. 28, E.) When division is about to take place, the nucleus, 

 previously spherical, becomes oval, and is drawn out into an 

 elongated dumb-bell shape (fig. 28, C and F), all three layers 

 sharing in the elongation without obvious change. The connect- 

 ing thread between the two swollen ends of the nucleus becomes 

 attenuated, and eventually snaps, upon which the two ends 

 round themselves off and become new nuclei. At the same 

 time, a deep constriction appears in the cytoplasm, passing 

 towards the place where the nuclear thread divided. This 

 constriction deepens, and eventually divides the body of the 

 Amoeba into two halves, each containing one - half of the 

 original nucleus. In this case there is no trace of centrosomes, 

 spindle, chromosomes, or astral rays, and the division is 

 undoubtedly amitotic. But in Amoeba bi-nucleata a primitive 

 kind of mitosis has been observed. In this species both 

 nuclei act together, and undergo the same changes during 

 division, in such a manner that each daughter form has two 

 nuclei, one derived from each of the two nuclei present in the 

 parent form. The nuclei have very firm nuclear membranes, 

 inside which is a nuclear sap containing numerous chromatin 

 masses. In the prophase of mitosis, the chromatin masses 

 break up, and their minute fragments are scattered through the 

 substance of the nucleus. The nucleus itself becomes some- 

 what flattened, and at each of its two flatter poles a small 

 mass of hyaline structureless protoplasm, apparently the 

 representative of the archoplasm, makes its appearance as a 

 flat cap. Meanwhile the chromatin granules collect in the 

 equatorial plane of the nucleus and form a plate. A true 

 spindle has not been observed, but in the later stages of 

 division fine longitudinal threads were seen stretching between 



