EUPLOTES WORCESTEEI: II. 327 



the peristomial cavit}- exteudiDg out from this moiety Ijeoonies reabsorbed 

 and leaves no ridge to mark its original relations, as in E. liar pa. At the 

 stage shown in figure 36,' the inner margin of the peristome which has 

 been formed in the manner just described, has united with the inner 

 wall of the anterior portion of the peristomial invagination which still 

 remains. By the deepening constriction of the bod}' the outer wall of 

 this cavity is carried away, and the medial -wall becomes the margin 

 of this side of the completed peristome. The relation of the inner 

 edge of the aperture of tlie invagination to the medial inargin of the 

 completed peristome can clearly he seen by examining the cirrus marlced f 

 in figures 33 to 36, Plate Till. It lies inside the original aperture, but 

 outside of, i. e., niediad to, the true margin of the peristome. 



I have carefully considered if the new peristome results from an out- 

 growth of the old one, and if the invagination could be formed from a 

 linear depression of the edge of the old adoral zone, or of the region 

 immediately lateral to that zone. There is no evidence that either 

 of these processes occur. Instead, I have been able to follow the process 

 of invagination, as above described, many times in living animals, and 

 also in mounted and sectioned material. The sections leave no trace of 

 doubt regarding the internal position of the new peristome. In the 

 transparent living animals the adoral meml)ranellffi of the peristomial 

 invagination lie at a slight angle to those of the old adoral zone, and 

 since they are all in rapid motion, the relative positions of the two zones 

 can be determined indubitably. 



The formation of the new peristome and pharynx by invagination ma^y 

 be a process developed in order that these parts shall be fully fomied 

 when fission commences, and to insure that the jjosterior daughter animal 

 may be able to take up independent normal existence after fission with- 

 out any delay. It ^dll be noticed that all new structures, of whatever 

 kind, are functionally complete in this animal by the time that fission is 

 accomplished. The peristome extends so far toward the posterior end 

 of an adult animal that a complete peristome can not be formed upon 

 the surface of the body back of this point, while there is also little room 

 between the left edge of the body and the margin of the peristome for 

 such development. It should be noted that fission in this form is not 

 merely a pinching in two of the body, but a process by which material 

 is withdrawn from the anterior half of the body, which results in the 

 formation of two daughter bodies very different in form from a half of the 

 mother body at the time fission began. Therefore, the invagination of 

 the peristome seems to be an anticipatory process related to the develop- 

 ment of the new body and ciiTi. 



There can be no question of overgrowth in the formation of this 

 invagination, as in the more familiar examples of invagination among 

 metazoan embryos. The direction of gro\\dh here is evidently con- 

 trolled by internal tensions either of ectosarc or endosarc. 



