LIFE CYCLE OF SIMOCEPHALUS VETULUS. 85 



growth of the embryos become retarded, the culture remains 

 nearly at a standstill for some weeks, and if the food supply is 

 exhausted will finally die out, though it has been my experience 

 that they usually linger on indefinitely. By the time the cul- 

 ture has become overstocked cphippial eggs have appeared in 

 considerable numbers, and after having passed the state of 

 equilibrium, when the rate of reproduction has rather abruptly 

 ceased, for some weeks just about but not quite equaling the 

 death rate, the accumulation of ephippial eggs may be taken as 

 proof that the culture has passed through a sexual phase, which 

 in turn is likely to be erroneously considered the cause of the 

 decline of the culture. And now with reproduction at its lowest 

 ebb, and all of the sexual females having passed into the asexual 

 state, as they all do, the observer is naturally likely to believe 

 that the culture has passed into the asexual phase, since all of 

 the sexual members of the culture have yet to live the asexual 

 phase of their lives. Thus there will be an indefinite period 

 when nearly every member in the culture will be asexual. It 

 must be remembered that at this critical period, assuming the 

 ordinary proportion of males, reproduction must be cut down to 

 somewhat less than two during the life time, which is a few weeks 

 or months, of each female. Considering that 25 per cent, of 

 the females are sexual (see table, isolation experiment 3), and 

 that each sexual female is actually carrying an ephippium one- 

 fifth of her life time, even assuming that to be but 14 days it is 

 obvious that only 5 per cent, of the females will be carrying 

 ephippia at any given time. I believe the life time in a large 

 culture is in reality much longer than I have assumed, and 25 

 per cent, far too great, and that it is safe to say that in many 

 instances the ratio of females actually carrying ephippia at a 

 given time is as low as 2 per cent. If it should happen that these 

 few should cast their ephippia at the same time, for a day or so 

 thereafter one could find a sexual individual only by a tedious 

 hunt with the microscope. 



On the other hand, when reproduction proceeds apace, starting 

 with 10 females let us say, each producing only 6 broods, with 

 an average of i and ^ sexual females to a brood, (as was tin- 

 case in isolation experiment 3), we should have 78 ephippial 



