REPRODUCTION 147 



pies: Cyclonexis (Fig. 92), Gonium (Fig. 99), Platydorina (Fig. 

 100), Protospongia (Fig. 114), Bicosoeca (Fig. 116), etc. 



Spheroid colony. The individuals are grouped in a spherical form. 

 Usually enveloped by -a distinct gelatinous mass, the component 

 individuals may possess protoplasmic connections among them. 

 Examples: Uroglena (Fig. 92, c), Uroglenopsis (Fig. 92, d), Volvox 

 (Fig. 99), Pandorina (Fig. 100,/), Eudorina (Fig. 100, h), etc. Such 

 forms as Stephanoon (Fig. 100, a) appear to be intermediate between 

 this and the discoid type. The component cells of some spheroid 

 colonies show a distinct differentiation into somatic and reproductive 

 individuals, the latter developing from certain somatic cells during 

 the course of development. 



The gregaloid colony, which is sometimes spoken of, is a loose 

 group of individuals of one species, usually of Sarcodina, which 

 become attached to one another by means of pseudopodia in an ir- 

 regular form. 



Asexual reproduction 



The Protozoa nourish themselves by certain methods, grow and 

 multiply, by the methods described in the preceding pages. This 

 phase of the life-cycle of a protozoan is the vegetative stage or the 

 trophozoite. The trophozoite repeats its asexual reproduction process 

 under favorable circumstances. Generally speaking, the Sporozoa 

 increase to a much greater number by schizogony and the tropho- 

 zoites are called schizonts. 



Under certain conditions, the trophozoite undergoes encystment 

 (Fig. 69). Prior to encystment, the trophozoites cease to ingest, and 

 extrude remains of, food particles, resulting in somewhat smaller 

 forms which are usually rounded and inactive. This phase is some- 

 times called the precystic stage. The whole organism becomes de- 

 differentiated; namely, various cell organs such as cilia, cirri, 

 flagella, axostyle, peristome, etc., become absorbed. Finally the 

 organism secretes substances which become solidified into a resistant 

 wall, and thus the cyst is formed. In this condition, the protozoan 

 is apparently able to maintain its vitality for a certain length of time 

 under unfavorable conditions. The causes of encystment are still 

 the matter which many investigators are attempting to comprehend. 

 It appears certain at least in some cases that the encystment is 

 brought about by changes in temperature, chemical composition, 

 amount of water, food material, and catabolic waste substances, 

 etc., in the medium in which the organisms live. In some cases, the 

 organisms encyst temporarily in order to undergo nuclear reorgani- 



