208 bulletin: museum of comparative zoologv. 



the dishes were by chance interchanged, a certain amount of cross fertili- 

 zation may of course have been possible. 



On the three succeeding days (fourth, fifth, and sixth) greater precau- 

 tions were taken, and the jars were dried as well as washed before the 

 experiment was repeated. It will be observed that the proportion of 

 eggs fertilized on those days was distinctly less than on the first three 

 days. 



Taking an average of the thirty cases in which eggs were laid in 

 aquaria containing each two individuals (Table II.), we find that 94% 

 of the eggs laid were fertilized. 



The single instance in which none of the eggs laid in one aquarium 

 were fertilized may be explained by a failure on the part of one of the 

 two animals confined together to emit the sexual products on that par- 

 ticular occasion. Table I. indicates that such cases sometimes occur; 

 for in two instances out of sixty no eggs at all were laid. 



Comparing the results of the two tables, we see that under conditions 

 allowing of only close (self-) fertilization (Table I.), less than 5% of the 

 eggs developed ; whereas under conditions permitting of cross fertiliza- 

 tion (Table II.) at least 90% of the eggs developed. 



The question now arose, Do eggs laid by isolated individuals fail to 

 develop because the parent does not discharge sperm at the proper time 

 (perhaps for want of stimulation by another individual), or do the eggs 

 fail to develop because they are incapable of fertilization by sperm from 

 the same parent 1 To settle this point if possible, resort was had to arti- 

 ficial fertilization. 



Third Experiment. 



The same animals employed in the second experiment were also used 

 in this one. Half of the individuals of each lot were taken for an attempt 

 at close fertilization, the other half being reserved for an attempt at 

 cross fei'tilization. Each animal was dipped in 90% alcohol to kill any 

 spermatozoa which might be adhering to it ; the fingers and instruments 

 used were treated in the same way. Eggs and sperm were removed 

 from the sexual ducts of the animal, and thoroughly mixed in a dish of 

 clean fresh sea-water, the dish having been previously carefully washed 

 and then dried. 



The second ten were treated in exactly the same way, except that the 

 sexual products — both male and female — of two individuals were mixed 

 together in a single dish. 



The proportion of fertilized eggs in each dish was subsequently care- 

 fully observed. The results are given in Tables III. and IV. 



