374 University of California Publications in Zoology [Vol. 18 



widens into the ectosarc. As it deepens the bottom of the depression 

 enlarges, making a sack the bottom of which is the original depression. 

 Very early in the enlargement of the sack a heavy staining line is seen 

 on the left side of it, which bends to the left and forms a small almost 

 triangular field surrounded by the dark line. Very early in the 

 development of the new cytostome there appears in the triangular field 

 a series of lines lying almost at right angles to the base of the triangle 

 (pi. 15, fig. 11). These lines are the rows of the basal granules of 

 the new membranelles. As the depression deepens it also elongates 

 in the direction of the long axis of the body. "With it the triangular 

 field also elongates and more membranelles form. This series of mem- 

 branelles increases in length until it reaches about as far forward as 

 the micronucleus (pi. 15, fig. 12). Posteriorly it elongates some dis- 

 tance back past the old peristome. In some cases it seems that the 

 anterior end of the new peristome extends under the membranelles 

 of the old peristome, but it never comes in contact with them. 



As the body of the animal elongates preparatory to constriction, 

 the new peristome is pulled back past the posterior end of the old 

 peristome (pi. 16, fig. 18). Both ends of the new mouth become bent, 

 the anterior end elongating rapidly to follow the constriction which is 

 cutting the organism in two (pi. 16, figs. 20-21). Thus by the time 

 division of the body is complete, the new cytostome with its series of 

 membranelles has nearly assumed its ultimate position and shape. 



The opening to the peristome increases in size and by growth and 

 curvature of the new peristome a triangular adoral field is formed. 

 The right side of the opening to the new peristome becomes the inner 

 edge of the anterior ventral field. The series of membranelles grows 

 over toward the right, keeping almost in contact with the left edge of 

 the constriction. The anterior end of the triangular adoral field grows 

 out in a rather rounded protuberance, forming the anterior lip which 

 comes to lie ventral to the anterior series of membranelles. 



AVhen the two daughter cells separate, the right end of the mem- 

 branelles is at the point of constriction and almost if not quite in con- 

 tact with the fibers from the anal cirri of the posterior daughter cell. 

 That this may be quite significant will be brought out in later discus- 

 sion. The posterior end of the newly formed peristome extends back- 

 ward and dips down into the endoplasm forming the pharynx. 



The above is a rather brief description of the formation of the new 

 peristome, but it will be seen that the process differs little from that 

 described by Griffin (1910) for E. worcesteri. The principal point of 



