407 



CHAPTEE XVII. 



embryology of the chick. 

 The Egg op the Fowl. 



It is necessary that we should have a complete knowledge of 

 the structure and formation of the hen's egg before we can 

 consider the stages that take place within it during the devel- 

 opment of the chick. To examine an egg we should be careful 

 first to obtain a fresh one, and then compare it with one that 

 has been kept some time. A fertile and an unfertile ovum 

 should likewise be compared. 



The external envelope is the shell (hg. 197, Sh), which is 

 composed of two layers. The shell is impregnated witli calcic 

 salts ; it is more or less porous, to allow the free interchange of 

 gases, which are necessary for the respiration of the contained 

 embryo during the process of incubation. The shell may 

 become coloured with pigment, both layers of it sometimes con- 

 taining colouring matter. Lining the shell are two thin skins, 

 the shell-membranes (Sh.m), the outermost layer being much 

 the thicker of the two. In a fresh egg these two shell-mem- 

 branes are closely approximated together, but can easily be 

 separated at the broad end of the egg. Every day the egg is 

 kept the farther these two membranes separate at the broad 

 end of the shell, forming in eggs that have been kept some 

 time a cavity called the air-chmnher {Ac). This air-chamber 

 gradually increases in size as the white of the egg becomes 



