Evolution 



plant kingdoms. 



FlG. 28. Volvox. A female, showing egg cells. 



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 

 The cell so produced, after 



FlG. 29. Volvox. Male, showing packets of 

 sperm cells. 



nuclei of the two merge into one. 

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



38 



