THALLOPHYTA: ALGAE 



31 



reproductive cells. Such a "division of labor" is not seen in lower mem- 

 bers of the order. 



Volvox reproduces asexually by the formation of new colonies inside 

 the old one. A few of the vegetative cells, seldom over 10 or 12, retract 



B 



Fig. 20. Volvox. A, mature colony with young colonies inside; B, j'oung colony in rim 

 of mature colony; at the right, a vegetative cell has lost its cilia and is starting to form a new 

 colony; C, a group of sperms derived from a single vegetative cell, one of which, to the left, 

 has lost its cilia and is enlarging; D, an egg shortly before fertilization and, to the left, an 

 egg beginning to develop from a vegetative cell; E, a mature zygote; A, X170; B to E, 

 X780. {After Chamberlain.) 



their cilia and increase slightly in size. Each divides to form a small 

 group of cells that enter the colony and give rise to a new colony, remain- 

 ing inside until the old colony dies (Fig. 20B). 



In sexual reproduction, Volvox is heterogamous. Any cell may retract 

 its cilia, enlarge by the accumulation of food, and become an egg (Fig. 

 20D). Another cell may enlarge and, at the same time, divide to form 

 many small biciliate sperms (Fig. 20C). These arise as a hollow sphere 

 or plate of cells that later separate. The sperms and egg? escape into 



