Evolution 



Fig. 28.— Volvox. 



plant kingdoms. The body cells, modified in various directions 



for their special pur- 

 poses, could not, and do 

 not^ reproduce complete 

 new individuals. There- 

 fore a generalised type 

 of cell is maintained for 

 the express purpose of 

 the propagation of the 

 race. It is to be ob- 

 served, now, that the 

 process of reproduction 

 in Volvox is not always 

 such as we have de- 

 scribed. Sometimes the 

 reproductive cells are of 

 two kinds. The one type 

 divides into a great num- 

 ber of small ciliated cells, 

 which escape separately 

 and directly to the out- 

 side of the sphere, and 

 swim away. These free- 

 swimming individuals do 

 not form new colonies, 

 but seek out the repro- 

 ductive cells of the other 

 type, which latter still 

 form part of the organ- 

 ism which has produced 

 them. One of the free- 

 swimming cells enters 

 each of those of the 

 other kind, and the 

 nuclei of the two merge into one. The cell so produced, after 

 a longer or shorter rest period, commences to divide and redivide 



38 



Fig. 29. — Volvox. Male, showing packets of 

 sperm cells. 



