COLONIAL PROTOZOA 201 



division of a single large cell to form many small ones, each with 

 two flagella. In V. perglobator these male and female cells are 

 produced in different colonies. In some of the other species, like 

 F. globator, both male and female cells are produced in the same 

 colony, which is therefore said to be hermaphroditic. At the time 

 of fertilization, the microgametes are discharged from the colonies 

 and swim by their flagella until they perish or come in contact with 

 a colony containing ova. The ovum is fertiUzed as it hes in place 

 within the colony. In addition to the somatic cells, and the 

 gametes that take part in sexual reproduction, the Volvox colony 

 contains cells known as parthenogonidia. A parthenogonidium, by 

 repeated divisions, can give rise to a new colony. This is the 

 asexual method of reproduction in Volvox. 



These colonial Mastigophora are of interest because they show 

 that the line between single-celled and many-celled forms cannot 

 be sharply drawn in existing organisms. Since this is the case 

 among the plants and animals now li\ang, it is not unreasonable to 

 suppose that many-celled forms may have arisen, during the early 

 history of organisms upon the earth, by steps somewhat hke those 

 indicated by Fig. 110. This statement does not mean that 

 Volvox and its plant-like relatives, Pleodorina, Pandorina, and 

 Gonium, are the ancestors of animals. They are plants rather 

 than animals, although classified in the IMastigophora (cf. p. 155). 

 What the series shows is a transition, from the single-celled to the 

 many-celled state, which is so gradual that one sees why distino 

 tions cannot be sharply drawm, although the extremes of the series 

 represent distinct conditions. In the protozoan the cell is an 

 independent unit and therefore in a state of physiological bal- 

 ance with respect to its fundamental capacities of metaboUsm, 

 irritability, and reproduction. In the metazoan it is the whole 

 mass of cells that possesses a phj-siological balance comparable with 

 that of the protozoan. In protozoa, the "individual " is the cell; in 

 metazoa, it is the group of cells. These facts have a bearing upon 

 the Organismal Theory as opposed to the Cell Theory {cf. p. 194). 

 The series of colonial Mastigophora that form the basis of the com- 

 parisons here set forth between unicellular and multicellular organ- 

 isms are further considered in the general discussion of reproduc- 

 tion in the next chapter. 



