2o6 



THE EVOLUTION OF SEX. 



further stepping-stones. Observers such as Van Beneden and Boveri, 

 in their masterly accounts of the morphological facts, have not left the 



Fig. 70. — Illustrating the Mechanism of Cell-Division, — (a) the chromatin 

 or essential elements of the nucleus forming an "equttorial 

 plate" in the one figure, drawn toward the poles to form two 

 daughter-nuclei in the other; (/') the almost "muscular" 

 threads; (c) the protoplasmic center from which these radiate. 

 — From Boveri. 



problem of the actual dynamics unessayed ; while the title of Berthold" 

 book on " Protoplasmic Mechanics," shows how the biologist persis- 

 tently seeks the aid of the student of physics in his endeavor to explain 

 the architecture of the living organism. 



IV. Protoplasmic Restatement. — In the above helpful sugges- 

 tion, Spencer has emphasized the reasonableness and general necessity 

 of cell-division at the limit of growth, refraining from the deeper 

 question of the actual mechanism involved. In truth such cautious 

 reserve must still be maintained, but Spencer's analysis admits of 

 being expressed in lower and more definite terms. The early growth 

 of the cell, the increasing bulk of contained protoplasm, the accumu- 



