4 E. J. LUND 



as indicated in figures 7, 8 and 9. A clear body of hyaloplasm 

 appears at the cyst hilum (fig. 8, r and h; fig. 9, h). The hilum 

 is a thin part of the cyst which perhaps is readily permeable and 

 through which chemical stimuli may act upon the clear cyto- 

 plasmic body lying beneath it. 2 



Some of these changes during encystment have been partly 

 described and figured by Brauer, '86. 



Cells resulting from division may loose the anlage of the gullet 

 and then encyst without first fully differentiating. The arrows 

 between figures 5, 6, and 7 indicate the possible sequence in the 

 process. When encystment takes place, differentiation of the 

 protoplasm is preceded by increase in size, owing to absorption 

 of water. The upper arrows between figures 9, 8, and 7 refer to 

 the reversal of the morphogenetic processes which take place in 

 the protoplasm and not the cyst. 



3. Spontaneous dedifferentiation and differentiation 



One striking fact about the morphogenetic process in Bursaria 

 is that dedifferentiation of a fully differentiated individual often 

 takes place without any apparent external cause initiating the 

 change. Within an hour or several hours, opening of the peri- 

 stome and gullet takes place with return to the fully differen- 

 tiated condition (figts 1 and 2). The extent of dedifferentiation 

 and closure of gullet is not the same in all these cases. I have 

 histories of individuals in which this process was repeated two 

 and three times during the course of two to ten hours when they 

 were kept in watch glasses containing tap water. Figure 20 is a 

 horizontal longitudinal section of a spontaneously dedifferen- 

 tiated normal individual. Dedifferentiation has proceeded in 

 this case a little farther than in that shown in figure 2, for all 

 trace of membranelles is absent, leaving only the anlage of the 

 gullet (fig. 20, a), . 



2 All cysts are not of the same size, as shown by measurements of cysts from 

 two sets of individuals; one set was fed egg yolk and the other was not fed. 

 Cysts from the well fed individuals were larger than cysts from those of the 

 starved set. Abnormal cysts are often formed. 



