H. M. FiTCHS 251 



It should be noticed in the first place that, whereas in all previous 

 experiments in cross-fertilization very dilute sperm-suspensions were 

 used so that the percentages of eggs fertilized should lie between and 

 100, thus allowing of comparison, the fertilizations in the following cases 

 were all made with an excess of sperm. The reason for this was, of 

 course, that the object of the experiments was not to compare per- 

 centages of eggs fertilized under different conditions, but to obtain the 

 maximum number (100°/,_,) of developing eggs for rearing purposes. 



In the first experiment (Table XII) A eggs were self-fertilized (A/a) 

 and B eggs were self-fertilized (B/b). At the same time the reciprocal 

 cross-fertilizations were made between these two individuals (A/b and 

 B/a) and eggs of a third individual G were cross-fertilized with a 

 sperm (G/a). 



TABLE XII. (15.20.1.) 



Percent. Time of Time of Settled Alive, Alive, 



fertiL 4-cells hatching down 8 days old 20 days old 



„ ,, , . f Ala 7 86 19, 37 V. few None — 



Self-fert. ... ' ' ,, 



\ Bib < 1 83 19, 30 None — — 



^ Ajb 100 83 19, 15 Most / Equal number , Fevfest of Ajb, 



Cross-fert. \ Bja 100 83 19,27 Many J of .-1/6 and B/a, | medium num- 



\ Gja 100 86 19, 26 Most i more of C/a \ ber Bja, most 



'^ Cja 



The Table shows that the excess of sperm gave in each cross-fertili- 

 zation 100°/'(, of segmenting eggs. The second column of the Table 

 shows that there was very little difference in the early segmentation 

 rates of the five fertiliza.tions. Both when b sperm was used to self- 

 fertilize B eggs and to cross A eggs, the 4-ceU stage was reached at the 

 same time — 83 minutes. Again, when a sperm was used to self A eggs 

 and to cross G eggs the segmentation rates were identical (in this case 

 86 minutes for 4-cell stage), but when the a sperm fertilized B eggs the 

 latter segmented a little quicker (83 minutes). 



Thus the early segmentation was not slower in the self-fertilized 

 eggs than in the cross-fertilized, but the times of hatching (given in the 

 Table in hours and minutes after fertilization) show that the segmenta- 

 tion rate of the former slowed down a little in the later stages. 



In the column headed "Settled down" is given the proportion of 

 larvae in each case which fixed themselves after their brief free- 

 swimming period. The proportions were roughly estimated by eye — 

 the only method practicable — and are independent of the absolute 

 . numbers of larvae pi-esent in the different cultures, which varies, of 

 course, with the percentages of eggs fertilized. It is striking that none 



