86 CELL-DIVISION 



muscle-fibres, drag the daughter-chromosomes asunder; while other 

 spindle-fibres act as supporting and guiding elements, and probably 

 by their elongation push the spindle-poles apart. The contractility 

 hypothesis is, however, difficult to apply in certain cases, and is prob- 

 ably an incomplete explanation which awaits further investigation. 

 The functions of the astral rays are involved in even greater doubt, 

 being regarded by some investigators as contractile elements like 

 those of the spindle, by others as rigid supporting fibres like those 

 of the central spindle. In either case one of their functions is prob- 

 ably to hold the kinetic centre in a fixed position while the chromo- 

 somes are pulled apart. Whether they 'play any part in division of 

 the cell-body is unknown ; but it must be remembered that the size 

 of the aster is directly related to that of the resulting cell (p. 51) 

 a fact which indicates a very intimate relation between the aster and 

 the dividing cell-body. On the other hand, in amitosis the cell-body 

 may divide in the absence of asters. 



These facts show that mitosis is due to the co-ordinate play of an 

 extremely complex system of forces which are as yet scarcely com- 

 prehended. Its purpose is, however, as obvious as its physiological 

 explanation is difficult. It is the end of mitosis to divide every part of 

 the cJiromatin of the mother-cell equally between the daughter-nuclei. 

 All the other operations are tributary to this. We may therefore 

 regard the mitotic figure as essentially an apparatus for the distri- 

 bution of the hereditary substance, and in this sense as the especial 

 instrument of inheritance. 



LITERATURE. II 



Auerbach, L. Organologische Studien. Breslau, 1874. 



Van Beneden, E. Recherches sur la maturation de Toeuf, la fecondation et la 



division cellulaire : Arch, de Biol., IV. 1883. 

 Van Beneden & Neyt. Nouvelles recherches sur la fecondation et la division 



mitosique chez 1'Ascaride megalocephale : Bull. Acad. roy. de Belgique, 1887. 



III. 14, No. 8. 

 Boveri, Th. Zellenstudien : I.Jena. Zeitschr.,XXl. 1887; \\.Ibid. XXII. 1888; 



III. Ibid. XXIV. 1890. 

 Brauer, A. Ober die Encystirung von Actinosphaerium Eichhorni : Zeitschr. 



Wiss. Zool, LVIII. 2. 1894. 

 Driiner, L. Studien liber den Mechanismus der Zelltheilung. Jena. ZeitscJir., 



XXIX., II. 1894. 

 Erlanger, R. von. Die neuesten Ansichten liber die Zelltheilung and ihre Mechanik : 



Zool. Centralb., III. 2. 1896. 

 Flemming, W., '92. Entwicklung und Stand der Kenntnisse liber Amitose : Merkel 



und Bonne? s Ergebnisse, II. 1 892 . 

 Id. Zelle. (See introductory list. Also general list.) 



