Object and Function of the Egg-stalk. 127 



allied to the Cynipidae have also stalked eggs. In this 

 case we cannot ascribe to the stalk the important 

 functions of a respiratory organ, as the Qgg does not 

 need it ; but nobody can doubt that their great con- 

 formity in outward habit and whole organization proves 

 that these inquilines have been evolved from the 

 Cynipidae, and the peculiarity of the stalked Qg^ has 

 remained with them as a survival. But in their case 

 the egg-stalk is not required to act in the way described, 

 and does not do so ; because the peculiar conditions of 

 embryonic development, which we find existing in 

 Biorhiza aptera, are wanting in them. 



In order to explain the function of the egg-stalk, 

 I have spoken of embryonic development in its ad- 

 vanced stage ; but the egg-stalk plays a part also at 

 the moment when the egg is being laid. It has already 

 been explained that the Qgg cavity communicates with 

 the egg-stalk, consequently part of the contents of the 

 tgg can pass without impediment into the stalk. This 

 happens regularly at the laying of each ^gg. If an egg 

 which has been laid by a fly in a bud, is afterwards 

 removed, the preparation will show the first part of the 

 egg-stalk to be quite full of a finely granular emulsion 

 which forms the egg contents. After a time changes 

 take place in this emulsion, and there are formed in it 

 highly refractive globules of various sizes ; finally the 

 whole contents of the egg-stalk clear, and a fine mem- 

 brane appears, separating the egg-stalk from the egg 

 cavity. These preparatory changes are always a sure 

 sign that egg development is taking its proper course ; 

 a matter as to which we may sometimes be in doubt 



