Heredity in Protozoa 595 



the aboral side. Its base had become broad and low, extending 

 between x and y^ Fig. 4, ^ There is reason to think that it actually 

 extended back of x, to the posterior end. It would naturally be 

 carried back in the backward growth of the posterior tip, but owing 

 to the abrupt point naturally found here, there is nothing to mark 

 the end of the base, as there is at y. The anterior point (at y) 

 had dwindled to a mere knob, while the main spine trailed behind, 

 half the length of the body. 



Ninth generation. At the next division (night of May 9), the 

 spine passed, as was to be expected, to the posterior individual 

 ^1.2.1.2.1.2.1.2 Certain interesting changes have taken place in the 

 position and structure of the spine, which throw light on the proc- 

 esses of growth, and which have important consequences for 

 succeeding generations. The free part of the spine is still very 

 near the posterior end, and stands again at right angles to the body 

 (Fig. 4, ^). The broad base of the appendage {x-y, Fig. 4, ^) 

 has been still farther drawn out in the processes of growth, so 

 that it extends forward almost to the anterior end (to the point y. 

 Fig. 4, ^). Posteriorly its end is not evident, but it doubtless 

 reaches to the posterior tip. Thus the base of the spine now 

 extends nearly the entire length of the body, so that it must be 

 cut by the next fission plane. 



It will be observed that up to this time the spine has regularly 

 alternated between the anterior and posterior individuals in the 

 successive generations. This is indicated in the designation 

 employed (^i-2- 1.2. 1-2. 1.2^^ ^.j^g exponent (^) indicating in each case the 

 anterior product of fission, the exponent (^), the posterior product. 

 When situated on the anterior individual the spine lies back of 

 the middle of the body (see Fig. 4, *'6,8,io^ g^.^, -^ When on the 

 posterior individual it has always lain in front of the middle of 

 the body (see Fig. 4, ^'^'''), till in the present generation (Fig. 4, ^). 

 These changes in position are due to the growth occurring after 

 fission; they give us a means of analyzing this growth — a matter 

 to be taken up later. 



Tenth generation. At the next fission (May 10, day) the free 

 portion of the spine went again to the posterior individual, thus 

 breaking the regular alternation which has prevailed up to this 



