REGULATION IN VORTICELLA. 345 



means of a sharp dissecting needle. A typical case is shown in 

 Figs. 2-8. Soon after the cutting, the zooid (Fig. 2) settled 

 down on its oral surface. In an hour, cilia began to push out in 

 a circlet near the aboral pole. They elongated rapidly, and 

 began to beat around the oral-aboral axis (Fig. 3). The oral 

 disk turned in upon itself in the manner characteristic of the 

 normally free zooid (Fig. 4). One hundred and five minutes 

 after the operation, the zooid swam away (Fig. 5), indistinguish- 

 able in every respect from the normally free form. After five 

 minutes of active locomotion, it came to rest on its aboral end, 

 became attached, and unfolded its oral disk (Fig. 6). At once 

 the stalk began to grow and the aboral cilia to disappear. In 

 ten minutes no aboral cilia were to be seen (Fig. 7). Two hours 

 and a half later, the organism appeared as in Fig. 8. 



The development of the stalk appears to be dependent on 

 contact at the aboral end; while the development of aboral 

 cilia is conditioned by physiological isolation from the stalk 

 whether achieved experimentally or by a narrowing of proto- 

 plasmic connection in the ordinary course of fission. 



