15 



species. Th9 fact that in some forms this precision of pe- 

 riodicity is not dependent upon external influences, while 

 in others there is manifest a marked degree of sensitive- 

 ness to external influences, seems to me to indicate that 

 the tendency has been arrived at by quite different proces- 

 ses, and may be due to quite different requirements in the 

 various creatureSt 



But to return to the dehiscence in Gonionema , not 

 all the eggs, by any moans, which the ovaries contain are 

 liberated at one time. Individuals have been seen to de- 

 posit eggs every night for a week, and while specimens kept 

 in captivity are not very reliable in drawing inferences as 

 to natural processes, this time of sexual activity would be 

 more liable to be shortened than to be prolonged by the un- 

 natural conditions. After the first three or four days on 

 which spawning took place, a small number of ova were left 

 in the gonads, and on the three succeeding evenings these 

 were extruded a few at a time. Late in the summer the spe- 

 cimens taken are usually devoid of sexual products, and the 

 gonads small and shrivelled. 



C. Egg-envelope - In freshly laid eggs the polar bodies 

 are only rarely to be found. They are normally given off 



