596 H. S. 'Jennings 



time. The individual bearing the spine is therefore ^^'^'^'^'^'^'^'^'^ 

 The effect of the ridge forming originally the base of the spine 

 (x-jy, Fig. 4, ^) is shown in the fact that the two individuals did 

 not separate, as usually happens; they remained connected by a 

 sort of bridge passing along the aboral surface (Fig. 4, ^^). Evi- 

 dently the substance formed by the extended base of the spine is, 

 not so easily cut by the processes of fission as are the other parts 

 of the body; it therefore forms the bridge. The two individuals 

 thus connected did not move in unison; there was much pulling, 

 bending and twisting of the slender connecting bridge, so that the 

 latter appeared likely to break. In the course of time this hap- 

 pened; the two individuals separated some time during the next 

 night, before the next fission occurred. 



As will appear in the sequel, this tendency to remain connected 

 even after the adult condition is reached persisted in the progeny 

 of these individuals for many generations. We have therefore 

 something resembling the inheritance of a new characteristic. 

 This matter will be taken up in a separate section. 



The spine still remained near the posterior end of the individual, 

 though not so near it as in the previous generation. The posterior 

 tip has pushed backward from the spine, in the growth that takes 

 place after division. It carries with it some portion of the base 

 of the spine, just as happens in front. 



Eleventh generation. Again the spine went to the posterior 

 individual (night of May 10). As would be expected, the spine 

 is now further forward; it is again nearly straight and at right 

 angles to the body (Fig. 4, "). 



Twelfth and thirteenth generations. During the night of May 

 II there were two fissions, giving three specimens of the normal 

 form, and one with the spine. It appears clear that at the first 

 of these two divisions the plane of fission was just in front of the 

 spine, so that the latter was left almost squarely on the anterior 

 tip of the posterior individual; here it remained till the next divi- 

 sion. This time of course the spine went to the anterior indi- 

 vidual, still remaining almost exactly at the anterior end. In its 

 outgrowth the anterior tip has carried the spine with it, owing to 

 the fact that the latter was almost at the very end. The individual 



