AMCEBA 139 



the elongation without obvious ^change. The connecting 

 thread between the two swollen ends of the nucleus becomes 

 attenuated and finally 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, chromo- 

 somes or astral rays, and the division is undoubtedly amitotic. 

 But in Amoeba M-nucleata^ a primitive kind of mitosis has been 

 observed. In this species there are two nuclei which act 

 together and undergo the same changes during division, in 

 such a manner that each daughter Amoeba has two nuclei, one 

 derived from each of the two nuclei present in the parent 

 form. The nuclei have very firm nuclear membranes, and 

 inside them 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 

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

 mass of hyaline structureless protoplasm, apparently the 

 representative of the centrosphere, 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 

 the chromatin particles, now separated into two groups, of 

 which one travels towards each pole of the nucleus. After 

 the separation of the chromatin groups, the two nuclei divide 

 to form four, and division of the cell-body follows. Indica- 

 tions of mitosis have been described in the binary division of 

 Ama'ba proteus and A. verrucosa. 



Encystment, followed by the sub-division of the body into 

 a number of minute reproductive bodies or spores, has been 

 observed only once in A. proteus. In this case the encysting 

 Amoebae were kept under observation in a laboratory during 

 the winter, and the whole process occupied from two and a 

 half to three months. Before encystment the animals with- 

 drew their pseudopodia and assumed a spherical shape. The 

 streaming movements of the endoplasm became slower and 



