502 JOHNSON. [Vol. VIII. 



but gradually, as the young membranellae develop from the 

 exposed endoplasmic ridge, the motion is restricted to undula- 

 tions sweeping lengthwise along the new adoral zone. (Figs. 

 26, 27, a.z.^) The wave-like motion becomes more and more 

 pronounced as the membranellae increase in length and vigor 

 of action (Figs. 30, 33, 35-37). It continues throughout all 

 subsequent stages of fission, and even for a short time after 

 division is accomplished. It is but rarely seen in old mem- 

 branellae, the action of which is more rapid and uniform. 



The posterior extremity of the new zone, reaching about 

 two-fifths of the way down the body when the Stentor is 

 extended, becomes more sharply curved than the anterior end. 

 Just within the curve the pharyngeal invagination appears 

 (Figs. 27, 28, o.^). At this and the preceding stages, the 

 aspect of the new zone is different according as the animal is 

 in the extended or contracted state. When contracted the 

 zone forms nearly a semicircle, and has strongly incurved 

 extremities (Fig. 28). The new membranellae are quiescent, 

 and the whole zone is apt to stand out in strong relief upon 

 the surface of the animal. 



A structure of much morphological interest which makes its 

 appearance very early, is a narrow light band running parallel 

 with the zone along its right side (Figs. 27, 30, 32, p^), and 

 having much the appearance of a clear stripe of unusual width. 

 It persists throughout the phases of division, and is seen in 

 the fully-formed frontal field, which it encircles just within 

 the adoral zone (Figs. 6, 30, /), in the manner described by 

 Schuberg. His interpretation of this band as a rudiment of 

 the peristome of the lower Heterotricha, seems the most 

 reasonable explanation. It is certain, as Schuberg (p. 234) 

 maintains, that the frontal field (" Stirnfeld ") of Stentor and 

 its near allies, Climacostomum and Folliculina, cannot be 

 regarded as homologous with the true peristome of the lower 

 Heterotricha. Neither the structure of the frontal field nor 

 its mode of formation are the same as those of the peristome. 

 As Schuberg has pointed out, the frontal field of Stentor is a 

 piece of the ventral body-wall, cut out, as it were, by the 

 adoral zone during fission, finally shifted to a horizontal posi- 



