STUDIES IN THE LIFE CYCLE OF HYDATINA SENTA 
III. INTERNAL FACTORS INFLUENCING THE PROPORTION OF 
MALE-PRODUCERS! 
A. FRANKLIN SHULL 
SIX FIGURES 
PME PO UE LOT AEE res ae sehen tac tape hci c diet tee Sucks d/o: aso) hea Oe Rts ors hg 283 
Obsery stions#an GhexpebianenGses es tard sn. sts 0 ns Ree Ree ee oe io! 284 
Decrease in the proportion of male-producers with long-continued par- 
EHENOSETESIS Ad meee RN Le oe hs 2 A Eee tee eee, 284 
Decrease in size of family with long-continued parthenogenesis........ 288 
Effect of inbreeding on the proportion of male-producers............... 291 
Effect of long duration of the fertilized egg stage on the proportion of 
PC =DLOCUCERS acct Gees ot: ee ae ee PSE 296 
Period at which the nature of a female is determined................... 301 
DIS CUSSLOUS he cscs astray te acy ae ener shee opis. ow cis oo Ree ee 308 
RSS ULIMIT TAY Anes epee ore pei ac tas Ne ah fe aro ES ore eet dn chaos Ca ee ee 315 
[SVICUN@FI ey ol oly ce NRO ee RA CRS AC ee ROR OS. ie 2 th Oe hae eee 316 
INTRODUCTION 
The existence of internal? factors affecting the life cycle of Hy- 
datina senta was demonstrated in my earlier experiments (Shull, 
11 a) in which two or more parthenogenetic lines of rotifers were 
bred under the same external conditions, and yielded different 
proportions of male-producers. Two such lines were crossed, 
and several new lines thus produced. One of these new lines was 
crossed back to one of the parent lines, and another new line 
‘ Contributions from the Zoological Laboratory of the University of Michigan, 
No. 138. 
*In a recent suggestive paper, Woltereck (11) has included me among those 
who hold that the life cycle of Hydatina is determined by external agents, not- 
withstanding that a large section of the paper which Woltereck cites (Shull, ’11 a) 
was devoted exclusively to internal factors. In the hope of correcting this impres- 
sion of Woltereck’s which may be shared by others, the present article will be made 
to include only evidence regarding internal agents, leaving to a future paper the 
further work that has been done or is in progress on external agents. 
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