176 KANSAS UNIVERSITY SCIENCE BULLETIN. 



ment, as it takes about that length of time for incubation to 

 begin. The embryos thus secured are not of exactly uniform 

 age, but a count of the number of somites affords a satisfac- 

 tory check for chicks of from twenty to fifty hours, and the 

 general appearance of the younger stages is characteristic ; 

 ordinarily, for stages older than fifty hours, the incidental 

 variation in immaterial. In this laboratory the "Cyphers" 

 hot-air incubator has proved very satisfactory. During in- 

 cubation the eggs should be turned once or twice a day to 

 prevent adhesions between the fetal and shell membranes. 



After sufficient incubation the eggs are removed and the 

 embryos secured by the following method : The shell is per- 

 forated at one side so as to avoid injury to the chick, which 

 normally floats on the top. Then the shell is carefully re- 

 moved, bit by bit, until a hole about the size of a five-cent 

 piece is made. For this purpose a pair of curved forceps and 

 a needle with a sharply recurved point are very convenient. 

 The chick is now fixed m situ. This is much simpler and 

 more satisfactory than the process frequently employed of 

 floating out the egg in normal salt solution. Incidentally, 

 however, this later process can occasionally be employed to 

 advantage, as a last resort, in case of an embryo that is not 

 normally oriented. The fixative used must be one that does 

 not cause an adhesion between the vitelline membrane of the 

 egg and the embryo. The following mixture has been found 

 most satisfactory : 



FORMO-NITRIC. 



Ten per cent, formalin 3 parts. 



Ten per cent, nitric acid 1 part. 



This fluid requires from five to fifteen minutes for the fixa- 

 tion of embryos up to seventy-two hours old. For older 

 chicks a subsequent treatment in this or some other good fixa- 

 tive is advisable. 



After fixation is completed a disc including the embryo is 

 cut out from the vitelline membrane by means of a pair of 

 curved-pointed scissors. This disc for embryos up to twenty- 

 four hours old should be about the size of a dime ; then up to 

 thirty-six hours of age the incision should follow the mar- 

 gins of the blastodisc, just outside of it; above this age any 



