igoS.] JENNINGS— HEREDITY IN PROTOZOA. 425 



length and breadth of .3100 (row 10, Table VIII.)- Why there 

 should sometimes be a slight positive correlation, sometimes a nega- 

 tive one, at this stage, will be discussed in the section where we deal 

 with the various factors determining correlation. 



A variation polygon for the youngest stage of lot i of Table 

 VIII. is shown in Diagram 4, p. 440, at a. 



The changes above set forth from statistical data were in a num- 

 ber of cases observed in living individuals. These observations give 

 a number of additional points of importance, so that they will be 

 described. The facts, as illustrated mainly by a typical specimen of 

 the aurelia form, are as follows : 



Some time before fission the body thickens and becomes shorter, 

 taking the form shown at a, Fig. 2, or c, Fig. 3. The form and dimen- 

 sions differ very noticeably from those of the specimens not preparing 

 to divide. How long before the appearance of the constriction these 

 preparatory changes in form begin it is not possible to say, because 

 it is not possible to distinguish with certainty whether a given speci- 

 men is to divide or not until we can see the constriction, and this is 

 at a relatively advanced stage of the process. At the time the con- 

 striction first appears the anterior and posterior halves still differ in 

 form, though they are losing their characteristic features. 



As the constriction deepens the two halves become longer (Fig. 

 2, h to /, Fig. 3, c to d). A specimen of the aurelia form (descend- 

 ant of c) was at about the stage shown at d, Fig. 3, at 12.05; each 

 half measured very nearly 80 microns in length. 



Ten minutes later (at 12.15) ^he connecting portion had become 

 smaller, while the two halves had lengthened, so that each measured 

 about 85 microns in length. The anterior half was more pointed 

 and slightly more slender than the posterior half {f, Fig. 3) ; this is 

 regularly the case. 



Six minutes later (at 12.21) the posterior half measured about 

 90 microns, the anterior half 94. The connecting band was now 

 extremely slender. 



Five minutes later (at 12.26) the two halves separated. The 

 anterior half was still clearly distinguishable from the posterior one 

 by its pointed, somewhat pear-like form. It measured 100 X 44 



PROC. AMER. PHIL. SOC. XI.VH. I90 BB, PRINTED JANUARY 9, 1 9O9. 



