60 A. FRANKLIN SHULL. 



enough to count include one or more groups at both outer (Fig. 5) 

 and inner (Fig. 6) ends of the spindle, and the number is six in 

 either case. 



DISCUSSION. 



From the best evidence I have been able to obtain, the number 

 of chromosomes in Hydatina senta must be placed at 12. The 

 number appears to be the same in male-producers and female- 

 producers. This conclusion is less illuminating in some respects 

 than might have been expected, but simplifies the probable 

 chromosome behavior in certain parts of the cycle. The very 

 sharply denned distinction between male-producers and female- 

 producers, and the irrevocable determination of the nature of the 

 egg at the time of maturation, would be regarded as "explained" 

 if it could be shown that they rested on chromosome differences. 

 If there are such differences, they are not differences of number. 

 Whatever change occurs at maturation, to distinguish eggs that 

 yield male-producers from those that yield female-producers, 

 must, however, be quite as definite an event as the loss of a 

 chromosome to the polar body. 



With regard to the male phase of the life cycle, one may 

 surmise the following consequences of the uniformity of chromo- 

 some number. The male probably develops with the haploid 

 number of chromosomes, and in the maturation of his spermato- 

 zoa the reduction division is either abortive or suppressed. Or 

 the male may at some stage double the number of chromosomes 

 and maturation include a numerical reduction. In either case, 

 the spermatozoa should contain six chromosomes. The sexual 

 egg (identical with the male-producing egg described in this 

 paper) presumably also has six chromosomes. At fertilization 

 the number is restored to 12. 



The status of the females hatching from fertilized eggs, which 

 may be called the stem mothers, is a peculiar one. These stem 

 mothers have been shown by hundreds of determinations to be 

 always female-producers (Shull, '12). Whether, in the absence 

 of differences in chromosome number, the feature which dis- 

 tinguishes a female-producer derived from a parthenogenetic 

 egg from a male-producer is also the feature which makes a stem 

 mother a female-producer can only be conjectured at present. 



