APPEARANCE OF THE SOMITES IN THE CHICK. 1 23 



After closing the opening I place over it a piece of sterilized 

 cotton about 5 cm. square and holding down the edges of the 

 cotton on the sides of the shell, slowly revolve the egg until the 

 closed window is on the lower side. It is then placed in a watch- 

 glass and incubated the desired period of time. In addition to 

 holding the piece of shell in place, the cotton prevents the egg 

 from rolling and facilitates handling. 



Inverting the egg serves a double purpose. In the first place, 

 no matter how careful one may be a certain number of eggs are 

 sure to become infected by germs falling on the albumen from 

 the air. Now, when the egg is revolved, the yolk turns until 

 the blastoderm is uppermost and hence removed as far as pos- 

 sible from the region of possible infection, which spreads too 

 slowly to reach the blastoderm and interfere with the develop- 

 ment of the embryo, especially if the egg is incubated but a few 

 hours. In the second place, by revolving the egg the blasto- 

 derm is brought into an environment almost, if not entirely, 

 normal. Mitrophanow ('97) has shown that varnishing the shell 

 above the blastoderm retards development by limiting the supply 

 of oxygen, and produces abnormalities. A similar effect is un- 

 doubtedly produced by placing an extra piece of shell above the 

 embryo and sealing down its edges with strips of membrane — 

 a method used by some workers. Any abnormalities thus pro- 

 duced are to be avoided, because they complicate the correct in- 

 terpretation of one's experimenal results. 



In order to test whether inverting the egg brings the blasto- 

 derm into a normal environment, some eggs were thus turned, 

 while others were allowed to remain with the covered window 

 uppermost, or turned but slightly to one side. In general, the 

 latter were delayed from two to four hours, while the former 

 developed equally with the controls. 



It may seem that the above precautions are a bit tedious and 

 unnecessary, but one is fully repaid for the trouble thus taken, 

 as shown by the following statistics. During the period in 

 which this series of experiments was carried on over 400 opera- 

 tions were made and but five eggs were infected, or less than 2 

 per cent., and during the present year about 100 operations have 

 been performed with not a single case of infection noted. Of 



