6 GENERAL INTRODUCTION. 



a dauo-hter-nucleus (Text-fig. V. C, D). Tlie cell, meanwhile, divides in a plane passing through the equator of the 

 spindle, so that each daughter-cell contains a daughter-nucleus and one of the asters. 



The achromatic figure is undoubtedly to be regarded as a mechanism or apparatus by means of which cell-division is 

 effected, but its precise mode of action is still in doubt. According to the view now generally prevailing (which was 

 orio-inated by Van Beneden'), the fibres of the spindle and of the asters are contractile elements, analogous in their mode 

 of action to muscle-fibres. By the contraction of these fibres the daughter-chromosomes are believed to be drawn apart, 



B 



C D 



Fig. V. 



Fig. V. — Diagrams of the cell at rest and during division. 



A. Resting-cell, showing the cyto-reticulum (R), the linin network (L) within the nucleus (N); C, the chromatin; A, the centrosome containing two centrioles (the latter often 



regarded as true centrosumes). 

 E. Cell in process of division. The centrosome has divided to form two asters (A), each of which contains one of the centrosomes. Between the asters extend the spindle- 

 fibres, which have been formed from the linin network, while the astral rays have been differentiated out of the cyto-reticulum. The nuclear membrane has disappeared. C, the 

 chromosomes (derived from the chromatic network of the nucleus) already split lengthwise. 



C. Later stage of karyokinesis. The daughter-chromosomes (DC) are moving apart towards the respective asters. (IF), the interzonal fibres (Verbindungsfasern) stretching between 



the diverging chromosome-halves. 



D. The cell has divided into two. Each group of daughter-chromosomes has given rise to a daughter-nucleus (DN), beside which lies a centrosome (A) derived from the 



preceding aster. 



and the cleavage of the entire cell finally effected. Other observers, however (e.g., Strasburger), reject this interpretation 

 and leave the precise nature of the mechanism in doubt. There can, however, be no doubt that the amphiaster is in some 

 manner an expression of the forces by which cell-division is effected. 



Origin of the Amphiaster. — By the early observers the amphiaster was supposed to disappear entirely at the close of 

 cell-division. In the year 1SS7, however, the discovery was independently announced by Boveri,- Van Beneden,'' and 



iRecherches, etc. Arch. Biol., III., pp. 552, 564, etc. See also Van Pjenedeu and Neyt, Nouvelles recherches sur la fecondation, etc.; Bull. Acad. Roy. de 

 Belgique, XIV., 1887, pp. 279, 280. 



•■^Boveri. Zellstudien, II. 18SS. p. 60. 



^ Van Beneden et Neyt. Nouvelles recherches, etc., 1. c. 1887. 



