CHROMOSOMES AND THE LIFE CYCLE OF 

 H YD ATI N A SENTA. 1 



A. FRANKLIN SHULL, 

 UNIVERSITY OF MICHIGAN. 



A reexamination of the chromosomes of this rotifer seemed 

 desirable owing to the results of some of my early experiments 

 (Shull, '12), in which it was shown that whether one of these 

 rotifers is to be a male-producer or female-producer is irrevocably 

 decided within a period several hours in duration occurring in 

 the course of maturation of the parthenogenetic egg from which 

 the individual develops. Influences acting upon the egg before 

 and after that period were found to be without effect. This result 

 indicated that some very brief event occurring in maturation 

 decided the fate of the female arising from the egg, and it seemed 

 possible that the event might be a change in the number of 

 chromosomes, perhaps comparable to the chromosome change 

 in the aphids and phylloxerans, in which the diminished number 

 in the male is still greater than the haploid. Sharp distinction 

 between male-producers and female-producers is known in the 

 phylloxerans without any difference in the chromosome number 

 (Morgan, '09) ; but in these insects the time at which the 

 differentiating event occurs is unknown. 



A difference in the number of chromosomes between male- 

 producers and female-producers was not excluded by the results 

 of previous studies of the cytology of Hydatina. Lenssen ('98) 

 states the number of chromosomes to be 10 or 12 in the female- 

 producing egg. In one passage it is plain that these numbers 

 imply uncertainty as to the correct number. Perhaps uncer- 

 tainty was intended to be implied by the other passages in which 

 the expression " 10 or 12 " is used, but there is room in one case 

 to suppose that Lenssen observed sometimes 10, sometimes 12, 



1 Contribution from the Zoological Laboratory of the University of Michigan. 

 For aid in the laborious task of preparing the specimens from which this study was 

 made the author is indebted to the trustees of the Bache fund. 



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