208 BULLETIN: MUSEUM OF COMPARATIVE ZOÖLOGY. 
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 IL), 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? 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 fertilization. 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 carefnlly 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. 
