i9o8.] JENNINGS— HEREDITY IN PROTOZOA. 415 



themselves in the statistical results. The condition just mentioned cannot 

 be absolutely fulfilled, but much effort was directed toward filling it as 

 completely as possible, and where it could not be fulfilled, strict account of 

 that fact was taken. 



The fixing and measurement of the specimens was done by the methods 

 already described (p. 396). 



1. Description of Different Stages of Growth. 



First Stage: the Young Before Separation is Complete. 

 In the earliest stage recognizable, the young Paramecium forms 

 half of a dividing specimen. Before the constriction appears the 

 macronticleus has become band-like, and the mother infusorian is 

 shorter and thicker than the specimens not preparing to divide (see 

 Fig. 2, a). The oral groove and other differentiated parts have 

 become less marked. At the first appearance of the constriction the 

 anterior and posterior halves still retain something of their charac- 

 teristic form, and the body of the mother has extended a little (Fig. 



2, h). The constriction does not pass squarely across the body, but 

 is a little oblique, being farther back on the oral side (Fig. 2, c, d, e). 

 As a result, when the two halves are measured separately, they will 

 seem to differ in length, according to the place where the measure- 

 ment is taken. Thus, if d, Fig. 2, is measured from the ends to the 

 constriction along the oral side, the anterior half measures 96 

 microns, the posterior half 84 microns, while if the measurements 

 are taken along the aboral side these proportions are exactly reversed. 

 Measurements taken from one of the lateral sides give the same 

 length for the two halves. The Paramecia may lie in various posi- 

 tions and this obliqueness of the constricting groove is not always 

 evident. Misled by this fact, I took great pains to measure the 

 precise length of each half in a large number of cases, finding con- 

 siderable differences, though without any marked preponderance of 

 either half. But I am now convinced that in early stages of fission 

 the most accurate measurements of the young are to be obtained by 

 considering each to be one half the length of the two together. 



The breadth of the two halves frequently differs a little, the 

 posterior half being at times slightly broader than the anterior half. 



As the constriction deepens, the two halves lengthen (Fig. 2, 

 b to f ; g to I, etc.). This lengthening progresses with the advancing 



