522 JOHNSON. [Vol. VIII. 



fission may follow closely the supposed abortive fission without 

 change of the conditions of life, and the inevitable occurrence 

 of the process when the adoral zone has become defective 

 through atrophy or injury, have led me to adopt Balbiani's 

 explanation as the true one. 



I. Cytoplasmic Phases. 



At the outset, the neoformation leading to regeneration is 

 indistinguishable from that leading to fission. In both cases 

 a new adoral zone is initiated in precisely the same manner 

 and place. The first mark of distinction is the appearance 

 of a new contractile vesicle if it is to be fission, and none 

 if regeneration. But my observations of the process agree 

 so well with Balbiani's description that there is no occasion 

 to give them at length. Different stages of the regeneration 

 are shown in Figs. 38, 39 A, and 39 B. It is interesting to 

 see how perfectly the new membranellae [a.s."^) become assim- 

 ilated to those of the old zone. The modus operandi by which 

 the new mouth (<?.!) is gradually brought into the position of 

 the old, and the slow advancement of the new zone to the 

 anterior end of the animal, I have not been able to compre- 

 hend, nor does Balbiani offer any explanation. There is evi- 

 dently a shortening of both the old and new zones. 



During their formation, and until the new mouth has ad- 

 vanced nearly to the position of the old, the new membranellae 

 exhibit the same slow, undulating vibration as in fission. This 

 is gradually quickened into the more vigorous vibration of the 

 old membranellae. 



I have once seen two successive regenerations coming at 

 a very short interval — a sort of hypertrophy of the process. 

 (Figs. 39 A, 39 B.) When first observed the animal was at 

 a slightly earlier stage than that represented in Fig. 39 A. 

 The process, so far as the first-formed mouth {o.^) and zone 

 {a.z.^) were concerned, was taking place normally. The old 

 mouth [o) had nearly atrophied. To my surprise a second 

 regeneration was also under way, the adoral zone being already 

 visible. It developed rapidly, while its predecessor as rapidly 

 disappeared (Fig. 39 B). The condition represented in Fig, 



