ORGANS IN THE EMBRYO OF THE FOWL. 95 



ciple rests very largely on inference, analogy and a few doubtful 

 pathological conditions. 



Discussion of this subject belongs, however, to the conclusion 

 rather than to the introduction, and the foregoing remarks are 

 intended only to define the problem. 



II. METHODS OF OPERATION. 



In making the operations one must work as far as possible 

 under antiseptic conditions. Instruments, etc., must be steri- 

 lized ; this is most readily done by passing the needles, knives, 

 scissors, etc., through a flame immediately before each is used. 

 In spite of all precaution a great many eggs are infected. In 

 my experiments only about 20 per cent, of the eggs remained 

 alive until the time of examination for the results of the experi- 

 ment, two to five days after the operation. The causes of the 

 mortality in the remaining 80 per cent, are two: (i) Fatal in- 

 jury of the operation (about 40 per cent. ?) ; (2) infection with 

 mould or bacteria (about 40 percent. ?). There is a very notice- 

 able difference between different lots of eggs ; some bear opera- 

 tions much more readily than others and are less prone to infec- 

 tion. These differences in the relative powers of resistance of 

 different lots of eggs are due to the relative freshness of the eggs 

 when incubation is begun, and also to the time of year. It is 

 noticeable that in a lot of eggs in which a relatively large pro- 

 portion, over 50 per cent., fail to develop in the incubator, the per- 

 centage of failures in the actual experiments is usually very high. 



The method of procedure in my experiments was as follows : 



1. The eggs are not turned in the incubator, so that one may 

 be sure of locating the position of the embryo in the unopened 

 egg exactly. The upper side of each egg is marked with a pencil. 



2. A small opening is made through the shell and membrane 

 over the embryo. 



3. The operation is then made. For cauterization I employ 

 either a needle heated red hot in the flame, or an electric cauter- 

 izing needle. The heated needle cools very rapidly, so that the 

 operation must be hastily performed, and it is difficult precisely to 

 delimit the injury. The electric cautery, on the other hand, is 

 apt to give too intense heat. Each method possesses certain 

 advantages. 



