138 T. H. Morgan. 



precaution that I took to isolate the individuals the day before 

 they were to be used. I have rarely seen more than from i to 10 

 per cent, of self-fertilized eggs segment, and in the greater num- 

 ber of cases not a single egg segmented. On the other hand I 

 found, as did Castle, that as a rule 100 per cent, of cross-fertilized 

 eggs develop, to which statement I should add, provided the 

 spermatozoa are in "good" condition. 



What is the meaning of these remarkable facts? Why do 

 not the sperm fertilize the eggs produced by the same individual, 

 and yet fertilize those of any other individual? A number of 

 possibilities readily suggest themselves, and since the following 

 pages record an attempt to test these suggestions they may be 

 briefly mentioned here : 



1. That the spermatozoa are not made sufficiently active by 

 secretions from the eggs of the same individual, but by those from 

 the eggs of any other individual. 



2. That the spermatozoa are not "attracted" to the eggs of the 

 same individual. 



3. That the egg contains, or secretes some substance that les- 

 sens the activity of the spermatozoa of the same individual. 



4. That some mechanical difficulty prevents the spermatozoon 

 from entering the egg of the same individual. 



5. That even if the spermatozoon enters, it can not fertilize 

 the egg of the same individual, in the sense of causing the egg 

 to begin to develop. 



In order to discover if the lack of power to self-fertilize the 

 eggs is due to the absence of some substance around the eggs that 

 excites the spermatozoa, the following experiment was carried 

 out. The eggs of an individual (A) were taken from the ovi- 

 duct. Similarly the eggs of another individual (B) were also 

 taken out. Then the ovary of (A) and that of (B) were 

 crushed separately, and a little sea-water was added. The eggs 

 of (A) were then allowed to soak in the crushed ovary extract 

 of (B) and those of (B) in the extract of (A). After a short 

 time the sperm of (A) with a little water was added to the (A)- 

 eggs, and the sperm of (B) to the (B)-eggs. If the sojourn 

 of the eggs in the extract of the ovary of another individual has 



