208 BIOLOGICAL LECTURES. 



second polar globule, and a large cell the mature egg. With 

 this division the life-cycle is completed, the mature egg repre- 

 senting the last generation of cells. 



Beginning with the formation of the primary oocyte, the phe- 

 nomena of cell-division recall those in the cleavage stages of 

 Amphitrite. In the oocyte, as in the cleavage-blastomeres, we 

 meet with cells which cease to divide temporarily and with 

 others of a definite generation which, left to themselves, never 

 divide. The peculiarities in the rate of division of the oocytes 

 are rendered more significant by the fact that in almost all 

 known cases, in both animals and plants, the same phenomena 

 obtain, viz., that not more than two successive divisions of the 

 primary oocyte take place (unless fertilization occurs to initiate 

 a new cycle of division) before there is a permanent cessation 

 of mitosis. 



It is now pertinent to ask, What are the factors which deter- 

 mine the rate or time of division of the oocytes, and why does 

 the maturated egg cease dividing until fertilization takes place ? 

 That the behavior of the oocytes is not a " function of their 

 position" is evident from the enormous differences in the 

 physical surroundings of various eggs at this period. Is there 

 any known structure within the cell by whose activity the divi- 

 sion may be incited and in whose absence the cell cannot 

 divide ? Boveri's theory of the centrosome, which has been 

 endorsed by a large number of eminent workers, gives an 

 affirmative answer to this question. According to this theory, 

 the centrosome is necessarily present in mitosis; the attraction 

 sphere, astral rays, and spindle fibers of the mitotic figure arise 

 under its influence. The centrosome is, in fact, the special 

 organ of cell-division, " the active centre of cell-division in 

 the cell-body." 



It is interesting that a large part of the evidence in support 

 of this theory has been drawn from the behavior of the centro- 

 some in the maturated ovum, the cell now under discussion. 

 According to Boveri, this cell is unable to divide because it has 

 lost its centrosome through degeneration. It may resume 

 karyokinetic activity only when a new centrosome is brought 

 in by the entering sperm. " The ripe egg possesses all of the 



