REPRODUCTION IN DROSOPHILA. 223 



arise, is applied against the dorsal wall of the uterus, while the 

 opposite surface lies against the ventral wall. The anterior end 

 of the egg, bearing the micropyle is lodged in the anterior pouch 

 of the uterus ; the micropyle cone is directed toward the opening 

 of the ventral receptacle, and in some cases seems to be almost 

 in contact with it. 



Owing to the size of the egg no space is left between its chorion 

 and the wall of the uterus; both of these come in close contact 

 thus preventing the spermatozoa from being diverted from the 

 anterior pole of the egg, where the penetration of a spermatozoon 

 takes place. 



6. THE EXTERNAL PHENOMENA OF FERTILIZATION. 



Under this heading I will consider all the phenomenal leading 

 to the penetration of the egg by a spermatozoon, the discharge 

 of the sperm from the ventral receptacle and spermathecae, and 

 the way in which the last process is accomplished. The actual 

 penetration of the egg by the spermatozoon was never observed 

 owing to the lack of transparency in the walls of the anterior por- 

 tion of the uterus, which prevents the detection of isolated sper- 

 matozoa, and also to the fact that it is very difficult to ascertain 

 the moment in which an egg enters into the cavity of the uterus. 



A series of experiments showed that in all probability the first 

 spermatozoa used in fertilization are those stored in the ventral 

 receptacle, those kept in the spermathecse being used later, when 

 the contents of the former are exhausted. When planning these 

 experiments it was thought that the best way to ascertain the 

 facts would be to allow several females to be fertilized and ex- 

 amine them at definite periods after which the condition of the 

 ventral receptacle and spermathecse could be observed by dissec- 

 tion. The time chosen for the first lot of flies was seven days. 

 The food was changed daily, and the eggs laid counted every day. 

 There is, however, an important obstacle, namely, that the rate 

 of laying is variable in different females. But this proved to be 

 very helpful, for, instead of the ventral receptacle being found 

 empty in all the flies, a rough relation between the number of 

 eggs laid and the amount of sperm stored in this receptacle was 

 found. Ten flies were used in this experiment ; two of them died 



