176 



PRINCIPLES OF ANIMAL BIOLOdY 



colony into 8 small cells of c(iiuil size. Two gamcites fuse to form the 

 zygote. In this instance both gametes are much smaller than the vege- 

 tative cells. In Heteromita lens (Fig. 134) and in some other species the 

 isogametes are of the same size as the vegetative cells. In Pandorina 

 morum (Fig. 135) there are gametes of two sizes, and two combinations 

 are possible: two small gametes, or one small and one large, may fuse 

 but not two large ones. 



Eudorina elegans (Fig. 136) shows well marked differentiation into 

 macro- and microgametes and union always takes place between a large 

 and a small gamete. Union of unlike gametes is known as anisog- 

 amy in contrast to isogamy (union of like gametes). The macrogamete 



Fig. 134. Fig. 135 



Fig. 134. — Isogamy in Heteromita lens. (After Kent.) 



Fig. 135. — Reproduction in Pancloriria morutn Borg. A, normal colony; B, daughter 

 colonies arising by division of mother cells within old colony, x475; C—H, gametes (g), 

 formation of zygospore (z) and its development. (From West after Pringsheim.) 



of Eudorina is not only large but is well supplied with nutritive material. 

 It is unlike the ovum of the metazoa in that it is motile. The micro- 

 gamete is small, has little stored nutritive material and has motor organs. 

 Pleodorina and Volvox have similar reproductive methods. The macro- 

 gamete of Volvox may be properly called an ovum, since the cell has no 

 motor organs (at least in its later stages), and something comparable to 

 maturation appears to occur. The microgametes of Eudorina, Pleodo- 

 rina, and Volvox are similar in their general features. 



In the Sporozoa differentiation of gametes into micro- and macro- 

 gametes is well marked. In Coccidium schuhergi (Fig. 132) the micro- 

 gametes are formed by repeated divisions of a large cell (Xllb-XIId.) 

 The microgametes are minute, possessing little cytoplasm. With their 



