5 o8 JOHNSON. [Vol. V 1 1 1 . 



more, and gradually approach the horizontal position {Fig. 

 33, yj). This is accomplished by two distinct processes, — a 

 shoving out of the cytoplasm towards the left, and a depression 

 of the aboral end of the new zone, combined with a strong 

 curvature dorsalward. The backward movement of the zone is 

 brought about by a deep, narrow constriction between its pro- 

 jecting anterior end and the body of the Stentor (Figs. 32, 

 33). This is only a part of the general constriction which 

 now obliquely encircles the animal coincident with the line 

 of fission, but is much the deepest at this point. The contour 

 of the new frontal field varies according as the animal is con- 

 tracted or extended; when in the former condition it is concave 

 (Figs. 31-33, the last being semi-contracted), whereas in the 

 latter it is nearly flat. 



Another factor in the process of fission is the elongation 

 of the posterior portion of the distal zooid. This starts at 

 the beginning of constriction, and rapidly progresses pari 

 pass7i with the constriction. Thus the distal zooid speedily 

 acquires the typical tapered form of Stentor (Figs. 35, 36). 

 The elongation is to be referred to the same cytoplasmic 

 action that produces ordinary extension. There are, then, 

 three distinct factors in the middle and later stages of fission : 

 (i) out-pushing of new zone into a horizontal position, (2) con- 

 striction, and (3) elongation of posterior part of distal zooid. The 

 shifting of the new zone into a horizontal position is accom- 

 panied by a shortening and broadening of the frontal field, 

 and a more complete circumscription of it by the adoral zone 

 (Figs. 35-37). Throughout the middle and later stages of 

 fission the distance from the new pharynx to the fission-line 

 remains nearly constant, but towards the end, when the con- 

 striction has become very great, this space is materially re- 

 duced, although, even in the newly-formed proximal individual, 

 the unenclosed space on the ventral margin of the frontal field 

 is considerably greater than in the ordinary condition. The 

 reduction of the unenclosed space is brought about by the 

 upward movement of the oral spiral, which is the last portion 

 of the adoral zone to take a horizontal position.^ 



1 This is more clearly seen in Y\%. 46 .<% PI. XXV, than in Figs. 35, t^^), because 

 in the latter the zone is viewed obliquely. 



