PRESERVATION OF THE SPECIES 173 



Directions. 



Boil an egg at least ten minutes in water deep enough 

 to cover it. Note which side is uppermost and mark this 

 part of the shell for reference. Remove the egg and pick 

 away the shell from about half the egg, leaving the shell 

 on that portion which was underneath when placed in 

 water. With a sharp knife remove this half of the egg. 

 Note the thickness of the shell. Test its composition (use 

 hydrochloric acid). Find the membrane lining the shell 

 and note that at one end it separates into two parts to in- 

 close an air space. 



1. What is this for.' How does it change after an egg 

 has been incubated for a week or more .'' 



Break an uncooked egg in a saucer. Note the germ spot. Note 

 also the difference in the consistency of the "white" (albumen) and 

 yellow portion (" yolk ")». 



2. Why does the latter retain its shape .' 



3. Why do the white and yolk not mix unless shaken or 

 beaten together } 



4. Look for the chalaza. 



5. What do you think is the use of this structure .'' 



Weigh a fresh egg, place it in a dry atmosphere for a week, and weigh 

 it again. Record result. Why may eggs be kept a long time perfectly 

 fresh if coated with paraffin or if put in " water glass " .' 



Suggested drawings. 



a. The egg as it appears in the saucer. 



b. The egg after part of the shell has been removed. 



2. Early Embryonic Development of an Egg 



Materials. 



Slides or diagrams, showing various stages in the develop- 

 ment of some animal through the gastrula stage. 



