VOLVOX 37 



2. Reproduction 



The Volvox colonies reproduce either asexually or sexually. In 

 both cases the process takes place by means of specialized cells 

 which have become physiologically and structurally differentiated 

 for the purpose of reproduction. Such cells lack flagella and, as 

 will be seen, are otherwise structurally modified. 



In asexual reproduction, certain large cells without flagella 

 (parthenogonidia) begin to divide. The cells thus formed re- 

 main attached to each other and, as a result, very soon a new 

 colony is formed. During the early stages of formation, the new 

 group occupies a place in the wall of the original mother colony. 

 But, as more and more cells are formed, it moves away from the 

 wall into the central cavity of the mother colony, where, together 

 with several other similar groups, it continues to grow by cell 

 division. Finally, the wall of the parent colony is ruptured, 

 thus permitting the new asexually-formed colonies to escape and 

 to take up their independent existence. They have the same 

 structure as the mother colony except that, for a time, they are not 

 composed of so large a number of cells. 



In the sexual reproduction of Volvox a considerable number of 

 large germ cells, without flagella, are formed among the typical 

 somatic cells. These germ cells gradually differentiate into either 

 the male gametes, or sperm, or the female gametes, or eggs. The 

 sperm of Volvox are formed by the repeated division of certain 

 of the germ cells. Thus a plate-like body, composed approximately 

 of 128 sperm cells, is formed from each germ cell of this type. 

 When mature, these male elements separate and escape into the 

 water. As is typically the condition in all organisms, the sperm 

 of Volvox are free-swimming, and thus they are enabled to come 

 into contact with the egg, which is inactive. (W. f. 30, A $ , 9 .) 



In the formation of the eggs, the potential female gametes in- 

 crease in size but do not divide, and so each forms a large egg cell, 

 with which later a sperm cell fuses permanently, thus bringing 

 about a permanent union of the nuclear material of the two gam- 

 etes. This is known as fertilization, and it constitutes the 

 culmination and the fundamental feature of sexual reproduction 

 in all organisms. In Volvox, the sexual reproduction generally 

 occurs in the fall, and the zygotes, which are formed from the 

 fusion of the male and female gametes, secrete a protective covering 



