ONTOGENY 3 



of the cell body into two bodies, each of which may then form 

 parts corresponding with those carried away by its sister 

 cell, and finally develop into a creature resembling the original 

 organism (Fig. 1). In many unicellular forms, particularly 

 among the Sporozoa, a process of multiple fission or brood 

 formation occurs. This is frequently preceded by growth of 

 the organism to an unusual size; then an uninterrupted series 

 of simple fissions, without intermediate growth or development 

 on the part of the daughter cells, results in the formation of a 



m 



cy 



FIG. 2. Multiple fission in a parasitic Infusorian, Holophrya multifiliis. 

 After Hatschek. A. Normal vegetative individual. B. Cyst containing the 

 products of repeated binary fission; some of the zoo spores are shown leaving the 

 cyst. C. One of the zoospores, enlarged, cr, contractile vacuole; cy, cyst; 

 m, mouth; ma, macronucleus; mi, micronucleus ; t, zoospores. 



large number of small organisms. These usually remain 

 associated, frequently within a cyst formed by the parent cell, 

 until the whole process of fission is completed (Fig. 2). In 

 other cases the nucleus, or nuclei, alone may divide, either 

 successively or simultaneously, into a large number of separate 

 nuclei; the cytoplasm immediately surrounding each nucleus is 

 then cut out as a separate cell, so that the organism appears to 

 fragment simultaneously into a large number of small daughter 

 organisms (Fig. 3). The number of new individuals formed in 

 this way may vary from four, as in some Infusoria, up to as 

 many as several hundred in many different forms, especially 

 among the Sporozoa. When the number is large the process 



