140 CONJUGATION, MATURATION, AND FERTILIZATION 



germ nuclei. But iu the simplest form such complication is not appar- 

 ent, for the idiochromidia becomes segregated in masses without 

 nuclear walls, and these masses fuse. This is the case in Ameba 

 Umax, a small free-living ameba common in ponds or decaying matter. 

 It may be easily cultivated on artificial culture media, such as agar, 

 in connection with various types of bacteria serving as food. Under 

 normal conditions of temperature, salt contents, etc., the amebfe 

 reproduce by simple division and by budding (Figs. 56 and 57). 

 Under certain conditions of the cultures, conditions which have not 

 been thoroughly investigated, the organisms encyst and remain so until 

 transplanted to new culture media. Occasionally, and again under 

 conditions unknown, they form sexually mature cells, but this latter 

 condition may also be brought about by suitable temperature changes. 



Fig. 57 



Ameba liniax. Nucleus in upper cell in full mitosis; in lower cell (right) in 

 anaphase of the mitosis. 



Syngamic nuclear union is always preceded by idiochromidia 

 formation within the cyst, but the formation of this material does not 

 necessarily imply sexual maturity. In all cultures, after a time, the 

 nucleus, which consists of a central karyosome and peripheral chro- 

 matin, gives rise to idiochromidia by dissolution of the peripheral 

 portion. The idiochromidia become scattered throughout the cell, 

 and, under ordinary conditions of the culture, are evenly diffused. If 

 the cultures be subjectetl to rapid changes of temperature, the idio- 

 chromidia may be caused to accumulate in masses about the periphery 

 (Fig. 51, p. 122). Sixteen of these masses are usually formed, and 

 then by fusion two by two the number is reduced to eight. This 

 fusion possibly represents a sexual union, or, more strictly speaking, 

 takes the place of sexual union, being the fusion not of secondary 



