90 ELEMENTS OF BIOLOGY 



divisions develops into a daughter colony. It is not unusual to ob- 

 serve several daughter colonies o£ this type within the central cav- 

 ity of a flourishing volvox. 



The sexual process of reproduction is more complex, involving 

 the setting aside in the colony of two different sorts of reproductive 

 cells. One of these becomes larger and is passive, its cytoplasm be- 

 coming dense with stored-up materials. This is the female element, 

 the OVUM. The other type is an actively swimming cell, derived by 

 division from a single cell intruded into the cavity. These active 

 cells are the male elements, the sperms or spermatozoa. Collectively, 

 reproductive cells are called gametes. In the cavity a single sperm 

 cell, on contact, fuses with an ovum, a process known as fertiliza- 

 tion. The now fertilized tgg, called a zygote, no longer a single 

 simple element, embarks on a series of cell divisions resulting in the 

 formation of a new colony. Usually the egg is fertilized by a sperm 

 from another colony, but this is not invariably true. 



Metazoon Characters of Volvox. A third manner in which 

 Volvox anticipates multicellular organization is in its geometrical 

 form. If one studies the history of any multicellular animal from 

 the tgg to the adult, without exception it is found that early in 

 the process of development the new individual consists of a single- 

 layered, hollow sphere, or some modification of such a structure. 

 This fact is frequently regarded as indicating the common origin 

 of all multicellular animals. 



Volvox, then, consists of something more than a chance associa- 

 tion of like cells; it is an individual, the cells being subordinate to 

 the colony. Its locomotion results from the coordinated motor 

 activity of the cells; reproduction is ejected by the assignment of 

 this function to certain types of cells; the gelatinous matrix that 

 holds the colony together is the result of the collective metabolism 

 of the cells. Among Protozoa and Protophyta there are many other 

 examples of colonial aggregations, exhibiting collective efforts and 

 various degrees of specialization among members of the colony. 



