ASSORTIVE MATING IN CHROMODORIS ZEBRA 281 



lowed by an unorganized mass of unfertilized ones. Also, if the 

 nudibranch be disturbed during the act of egg laying, the egg 

 string is usually broken off and another (disorganized) one 

 formed subsequently. Very few irregular egg masses are ever 

 found in nature, however, and it is unlikely that the animals are 

 disturbed to any extent while engaged in egg-laying. The num- 

 ber of eggs lost through this agency cannot, consequently, be 

 very great. On the other hand, the abnormal egg masses, in- 

 cluding many unfertilized eggs, which are laid by Chromodoris 

 as the result of insufficient insemination, have an important bear- 

 ing upon the possible significance of assortive conjugation. 



It is well known that most nudibranchs deposit large quanti- 

 ties of eggs (cf. Eliot, '10). For Chromodoris zebra, Smallwood 

 ('10, p. 140) calculated that 8,000-10,000 eggs were contained in 

 each spiral. It is said that in Doris tuberculata 50,000 eggs 

 may be laid in a single mass, and correspondingly large numbers 

 have been observed for some other genera. I made estimations 

 of the nmnber of eggs in the ribbons deposited by C. zebra, 

 and found this number to vary from 2,380 to about 20,000. 

 The eggs when laid are imbedded in a flat, fairly regular, closely 

 coiled spiral ribbon of jelly. The estimates were made by care- 

 fully counting the number of eggs in each of three or five turns 

 at different positions in the spiral, and then multiplying the 

 averages of these counts by the number of turns in the whole 

 ribbon. The data are plotted in figure 21. The egg-masses 

 involved in this tabulation were laid by freshly collected nudi- 

 branchs, segregated in the laboratory, so that the origin of the 

 eggs was definitely known. Egg masses obviously abnormal were 

 excluded from the counts. 



Egg masses collected in natural surroundings were also exam- 

 ined, and found in some instances to contain a little larger 

 number of eggs than the largest one incorporated in figure 21. 

 The range of sizes, and of numbers of eggs, was not, however, 

 significantly different from that for the egg ribbons laid in the 

 aquaria. It is therefore legitimate to conclude that the larger 

 nudibranchs normally deposit many more eggs at a single laying 

 than do the small ones. 



