44 MANUAL OF ZOOLOGY SECT. 



Multiplication is effected most commonly by the simple 

 process of binary fission (Fig. n, 2b), which may take place 

 either in the active or in an encysted condition. In some 

 cases the fission is multiple, the protoplasm dividing not 

 merely into two, but into a greater number of parts, each 

 destined to develop into the adult form. 



We also meet in the Mastigophora with what may be 

 regarded as the simplest mode of sexual reproduction. In 

 some forms two individuals come together and become 

 completely fused, the process being known as conjugation? 

 and the body formed by the union of the cells being known 

 as a zygote. The protoplasm of the latter divides by mul- 

 tiple fission into very minute spores. These, when first 

 liberated by the rupture of the zogote, are mere granules, 

 but soon the flagella are developed. In some cases the con- 

 jugating cells or gametes are of two sizes, union always taking 

 place between a large cell or megagamete and a small cell or 

 microgamete. In Volvox, which is a free-swimming spheri- 

 cal colony (Fig. 17, , F, G) this difference between the 

 two sets of conjugating cells reaches its extreme, pro- 

 ducing a condition of things closely resembling what we 

 find in the sexual reproduction of higher forms. Certain 

 of the zooids enlarge and form megagametes, others divide 

 repeatedly and give rise to groups of microgametes, each 

 of the latter having the form of a rod-like body with two 

 flagella. The microgametes escaping, swim about freely and 

 conjugate with the motionless megagametes to form a zygote, 

 which, after a time divides to give rise to a new colony. 



Mastigophora occur under the most various conditions, 

 to some of which reference has been already made. Many 

 kinds live in fresh water; others are abundant in the sea. 

 Noctiluca and others among the marine forms are phos- 



1 Conjugation has also been observed to occur in many Rhizopods. 



