REPRODUCTION IN EUDOEINA 13 



tion into male and female cells. The process of reproduction 

 by division occurs in Eudorina as in Pandorina — ^that is to say, 

 each cell undergoes successively four or five divisions, and thus 

 forms four or five new daughter colonies. When, however, the 

 moment comes for a renewal of this process of division, the 

 various colonies behave differently. Some of the colonies are 

 composed exclusively of male cells, others of female cells. The 

 latter remain as they were — that is to say, their division follows 

 a normal course. The male cells, on the contrary, undergo a 

 peculiar process of division, the result of which is the formation 

 of a mass of " zoosperms," whicji are distinguished from the 

 female ceUs (egg-cells) by their different structure, small dimen- 

 sions, and greater facility of movement. Here, therefore, we 

 have in simple expression the familiar antithesis between male 

 and female cells. 



In the course of the evolution of the various types we find 

 this difEerentiation becoming ever more prominent. It would, 

 however, be erroneous to conclude that the difierentiation of the 

 reproductive elements goes hand in hand with increasing com- 

 plexity in the structure of the organism. We find, on the con- 

 trary, compHcated difierentiations between the reproductive 

 elements of certain Algse, which disappear again in most of the 

 higher plants. It is not on the degree of complexity in the 

 structure of the organism that the degree of sexual differentia- 

 tion depends, but rather on the conditions in which, in each 

 individual case, the conjugation or coming together of the two 

 kinds of reproductive elements is effected ; as also on the con- 

 ditions necessary for the subsequent development of the product 

 of that conjugation. 



B. — Reproduction in the Higher Metazoa. 



Oscar Hertwig, in 1875, by his experiments on the egg of the 

 sea-urchin, marked a turning-point in the science of embryology. 

 His beautiful researches revealed the fact that only one out of 



