ASSORTIVE MATING IN CHROMODORIS ZEBRA 289 



Between forty and fifty random individuals of various sizes 

 were placed in a large jar of running sea water. Two days later 

 nineteen mating pairs were simultaneously secured in this popu- 

 lation, and measured. The method of measurement was as 

 given in the early part of this paper. The accompanying table 

 (table 6) and figure (fig. 23) present a summary of the evidence 

 thus obtained. The number of pairs (19) is small, yet affords 

 adequate ground for the contention that at this season (De- 

 cember) the intensity of homogamy with respect to size is as 

 great as that found in April. 



It seemed that toward the end of the first week in June, 1917, 

 the abundance of Chromodoris in shallow situations began no- 

 tably to decrease; this was not due to my ''overfishing" in the 

 area most intensively studied, since it was equally true in other, 

 unmolested, areas normally frequented by this nudibranch. In 

 December they began again to increase in number in shallow 

 regions near the shore. If it were to be held that these months 

 mark approximately the termination of one 'breeding season' 

 and the initiation of the next succeeding 'reproductive period,' 

 we should be faced by the difficulty that individuals of such 

 diverse sizes are about equally engaged in propagation at the 

 beginning and at the conclusion of a 'reproductive season.' 

 This condition is, however, consistent with, and adds something 

 to, a conclusion elsewhere (Crozier, '17b, p. 381) favored, namely 

 that the more or less periodic shoreward flocking of C. zebra 

 does not signify the incidence of a special propagative period. 



In December, under laboratory conditions, two individuals of 

 average size (10 cms.) will conjugate, or attempt to, at intervals 

 of four or five days. One pair laid four egg masses (two each) 

 in twelve days, and other couples deposited eggs in six-day to 

 seven-day periods; however, after the third egg laying, which com- 

 prised a relatively small number of eggs, no further masses were 

 obtained.^ Egg laying, at this season, occurs two and a half to 

 three days after conjugation; in June, within one to two days. 

 The number of eggs contained in a single mass is, for animals of 



* In most cases after a variable number of weeks, even without food, eggs 

 are again deposited. 



