MITOSIS 97 



secondary cysts. These nuclei are the result of repeated mitotic divisions 

 of the nucleus of the parent. These divisions differ from those described 

 as taking place in the gametes in the secondary cysts, in that definite 

 centrosomes do not occur. Similarly, when the daughter individual in 

 the secondary cyst divides to form the two gametes, its nucleus divides 

 without the formation of centrosomes (Fig. 55). Indications of longi- 

 tudinally arranged fibres can, however, be detected within the nuclear 

 membrane, and also in a cone-shaped portion of cytoplasm which occupies 



huh 







^ 



Fig. 55. — First Nuclear Division in the Secondary Cyst of Aciinospluerium 

 ciclihorni ( x ca. 1,200). (After E. Hertwig, 1898.) 



1. Chromosomes forming in the nucleus. 2. Chromosomes arrtanged as an equatorial plate. 



.3. Daughter chromosomes separating as two j^lates. 



No definite centrosomes appear at any stage. 



the poles of the elongating nucleus. Chromosomes are formed, become 

 arranged as an equatorial plate, and divide into daughter chromosomes 

 in the usual manner. 



As an illustration of another type of mitosis in which a definite spindle 

 and chromosomes are formed associated with disappearance of the nuclear 

 membrane and complete absence of centrosomes, Amoeba glebcB {Hart- 

 mannella glebce), a soil amoeba described by Dobell (1914c/), may be con- 



1. Centrosome with radiations. 



2. Two centrosomes at opposite ]iolcs of nucleus in which chromosomes are commencing to form. 



3. The spindle has formed between the centrosomes. and chi'omosomes have taken u]) a position 



as an equatorial plate. 4. Commencing division of the chromosomes. 



5. The chromosomes have divided and two equatorial ]i!ates are foimcd. 



6. Passage of the daughter chromosomes towards the ctntrdsomes. 



7. Later stage, in whiiji the nuclear membrane is closinu round the chromatin granules. 



8. Two daughter uiulci have formed, though the n'liiiiiiis of the spindle and the radiations from 



the centrosome, which has itself disappeared, are still to be distinguished. 



