90 PRINCIPLES OF ANIMAL BIOLOGY 



The series finds its culmination in the genus Volvox, the species 

 of which show considerable range in size and complexity. The num- 

 ber of cells in the mass ranges from 200 to 2000 in the smallest 

 species, Volvox aureus, and to more than 50,000 in V. rousseletii. In all 

 the species the cells are arranged near the surface of a hollow sphere of a 

 gelatinous or mucous material. In old specimens of V. perglohator 

 the cells are widely scattered while in V. rousseletii they are compactly 

 arranged. The cells of Volvox are of two sizes, the small cells being in 

 great abundance. 



New colonies of the species of Gonium, Pandorina and Eudorina 

 are formed by the repeated division of each of the old cells of the parent 

 colony. In this manner as many new colonies are formed as there were 

 cells in the parent. In Pleodorina only the large cells are capable of 

 producing new groups of cells by fission, the small cells having lost this 

 power. In Volvox the function of reproducing ascxually is possessed 

 only by very large cells, the parthenogonidia. Since all the small cells 

 have lost this function they resemble cells of the bodies of higher animals 

 and hence are called somatic cells (from soma, body). The partheno- 

 gonidia are irregularl}^ distributed among the somatic cells. Their 

 number varies in different species. They are provided with chloroplasts 

 and in yoang stages with two flagella. Numerous protoplasmic strands 

 connect the parthenogonidia with the neighboring somatic cells, in the 

 same manner as the somatic cells are connected with one another. When 

 a parthenogonidium divides it produces a plate of cells which by sub- 

 sequent movement of the cells becomes molded into a hollow sphere 

 which for some time has an opening to the exterior. As the sphere in- 

 creases in size it sinks into the cavity of the parent and finally comes to 

 lie entirely within the parental cavity. After having attained consider- 

 able size, these small spheres, the young individuals, escape from the 

 parent by a rupture in the wall of the latter. Sexual reproduction occurs 

 in all the genera of the Volvox series by means of specialized cells, the 

 gametes or mature germ cells, two of which fuse to form the oosperm. 

 The oosperm then divides to form a new individual. The variations in 

 the reproductive processes of this series cannot be described here, but when 

 they are discussed in a later chapter it will be found that in its method of 

 sexual reproduction as well as in its structure Volvox is more complex 

 than the other members of the series. 



This series shows increasing complexity in aggregation, a complexity 

 which involves not only increase in the size of the aggregation as a 

 whole, but also a modification in the size, form, and function of some 

 of the component cells. These modifications ma}^ be traced as a series 

 of stepB, beginning with colonies of unmodified cells and extending to 

 aggregations possessing three types of cells. 



