THE MASTIGOPHORA 173 



the cell body is begun. A new gullet and flagellum are formed 

 on the posterior cell. The old flagellum may persist on the anterior 

 cell, or it may be dropped and a new one produced. Separation of 

 the cells finally occurs. Euglena gracilis thus reproduces itself 

 either by longitudinal or by transverse division of the cell during 

 its active phase. 



Under various external conditions, and perhaps as a result of 

 internal changes, euglenas may undergo encystment by secretion 

 of material that hardens as a thin covering, or cyst, enclosing the 

 cell. In E. gracilis, temporary encystment may occur at the 

 surface of ponds during hot da3^s as shown in Fig. 93, 5. 

 Later, the individuals escape from these cysts without reproduc- 

 tion. Apparently, cysts may also be formed for the purpose of 

 cell division, which in this instance is transverse (Fig. 93, 4). 

 The euglena becomes quiet, loses its flagellum, remains extended, 

 and secretes the cyst. Transverse division now occurs in the same 

 general manner as in the active stage. When all the structures 

 are completely formed for each cell, the individuals escape from 

 the cyst without further division. In the more common form of 

 encystment, however, there may be numerous divisions that are 

 longitudinal. In this instance the euglena casts off its flagellum, 

 becomes spherical, and secretes a delicate cyst. Division then 

 occurs longitudinally. If the culture is kept in sunlight the cells 

 grow as the result of holoph^-tic nutrition, and repeated divisions 

 may ensue, resulting in great distention of the original cyst. 

 Sometimes this is ruptured, the liberated cells form new cysts 

 about themselves, and each repeats the process (Fig. 93 6). 

 When the active phase is to be resumed, flagella are produced, 

 and the euglenas break from the cyst and swim away. 



A process of conjugation, by which cells unite in pairs and 

 become permanently fused into a single cell, having its single 

 nucleus and c>"tosome derived from this double parentage, is 

 well established in many flagellated protozoa. In the genus 

 Euglena it has been described by some investigators but is ques- 

 tioned by others. Unpublished observations by Tannreuther on 

 E. gracilis indicate what is evidently conjugation of free-swimming 

 euglenas (Fig. 93, 7). The cells unite in pairs, and as they fuse a 

 union of the two nuclei occurs. Thus, one cell is formed by the 

 union of two as in fertilization, which is the term appKed to the 

 union of ovum and spermatozoon in many-celled animals. Such 



