REVERSIBILITY OF MORPHOGENESIS 6 



midway between the anterior and posterior ends of the individual. 

 Figure 18 is a cross section of a fully differentiated normal indi- 

 vidual as shown in figure 1 or figure 6. Figure 16 is a cross sec- 

 tion through the middle of one of the daughter cells at the stage 

 in division shown in figure 4. The membranelles, septum and 

 gullet have disappeared or dedifferentiated leaving only the anlage 

 (fig. 16, a). Figure 17 is a cross section from the same level in 

 the cell as the sections shown in figures 16 and 18, but at a stage 

 in differentiation after division shown in figure 5. Differentia- 

 tion proceeds by inward movement and expansion of the anlage 

 to form the gullet. As the gullet deepens the membranelles 

 appear on its dorsal wall ; the septum with its long slender cilia 

 appears and the peristome opens. The real mouth lies at the 

 inner end of the curved gullet (fig. 1, o). The action of this 

 feeding mechanism has been described in connection with the 

 reactions of Bursaria to food (Lund, '14). 



Differentiation takes from thirty minutes to an hour for its 

 completion, or about the same length of time as the process of 

 dedifferentiation, depending upon conditions such as tempera- 

 ture, mechanical stimulation, medium, etc. Dedifferentiation 

 during normal division is never complete, a trace of the gullet 

 and peristome always remaining. This I have called the anlage 

 of the gullet. Figure 19 is longitudinal sagittal section through a 

 normal fully differentiated individual such as shown in figure 1. 



2. Before and after encystment 



The anlage of the gullet and peristome completely disappears 

 before encystment takes place. The cell becomes spherical or 

 very nearly so, showing that internal tensions are more nearly 

 equalized than at division, The process of dedifferentiation pro- 

 ceeds farther than at cell division. Figure 7 shows the cell as a 

 dedifferentiated ciliated sphere. In this condition as well as at 

 division (figs. 3 and 4), the cell is strongly thigmotactic, stick- 

 ing to the bottom of the watch glass or surface film. During 

 encystment (fig. 8), the cilia melt away. The protoplasm ap- 

 parently loses water, reduces in volume and becomes more dense, 



