54 



CELL-DIVISION 



which leads the Avay in cell-division by dividing into two similar 

 halves to form the centres of division. "Thus we are justified," said 

 Van Beneden, "in regarding the attraction-sphere with its central 



•7^^ 



■^ 



Sf-*: 



\ 



B 



A 



' 'l \ 



Z> 



Fig. 21. — The prophases in cells (spermatogonia and spermatocytes) of the salamander. 

 [DrUner.] 



A. Spermatogonium in the spireme-stage ; the chromatin-thread lies in the linin-network, still 

 surrounded by the membrane ; above, the two centrosomes, the central spmdle not yet formed. 

 B. Later stage (spermatocyte) ; the nuclear membrane has disappeared, leaving the naked chro- 

 mosomes; above, the amphiaster, with centrosomes and central spindle; astral rays extending 

 towards the chromosomes. D. Following stage ; splitting of the chromosomes, growth of the 

 aster; mantle-fibres and central spindle clearly distinguished. C. The fully formed mitotic figure 

 (metaphase) ; the chromosomes, fully divided, grouped in the equatorial plate. 



corpuscle as forming a permanent organ, not only of the early blas- 

 tomeres, but of all cells ; that it constitutes a cell-organ equal in rank 

 to the nucleus itself ; that every central corpuscle is derived from 

 a pre-existing corpuscle, every attraction-sphere from the pre-existing 



