240 B. H. GRAVE. 



presence of the spermatic fluid from the fact that none of the eggs 

 initiated development. After some hours had elapsed these eggs 

 were artificially fertilized and a considerable percentage of them 

 went through cleavage stages, showing that they were normal, 

 mature eggs, and that they had not previously been subjected to 

 spermatozoa. 



This experiment, first performed in 1920, was verified during 

 the past summer. 



Both sexes are apparently affected by the change from running 

 to quiet water (tide changes), presumably also by the change in 

 pressure between high and low tides (mechanical shock), and most 

 certainly by moonlight. 



It is evident that these factors have not been analyzed satisfac- 

 torily, but Mayer ('07 and '08), by actual controlled experiments, 

 has contributed at least a first step toward a solution. The writer 

 is of the impression that Scott's ('09) explanation, in the case of 

 Ainphitntc, that the periodic maturing of the sexual products is 

 due to the warming effect of the sun at periods of especially low 

 tides, can not be generally applied and is not applicable to Chccto- 

 plcura. 



The non-committal statement made by Lillie and Just ('13), that 

 the maturing of the sexual elements of these animals is dependent 

 upon phases of the moon, involving, through lunar tidal variations, 

 rhythmical alterations of conditions of nutrition, is less open to 

 criticism and is as exact as the known facts warrant at the present 

 time. 



Many interesting suggestions concerning the cause of periodicity 

 in spawning have been made by various authors, but most of them 

 are undoubtedly wide- of the mark. 



Friedlander ('9S-'oi), after reviewing various possibilities, 

 finally admits that he is wholly in the dark and arrives at the con- 

 clusion that no theories yet proposed are adequate to explain perio- 

 dicity in spawning. 



Mayer ('07 and '08) states, upon the authority of experimental 

 data, that moonlight is the effective cause, and that the tides are 

 unnecessary but contributing causes. 



Hemplemann ('n) believes that the moonlight, acting through 

 a period of several days, is the stimulus to the maturing of the 



