70- 



COMPARATIVB PHYSIOLOGY. 



these parts together constitute that portion (blastoderm) of the 

 fowl's egg which is alone directly concerned in reproduction, 

 all the rest serving for nutrition and protection. The appear- 

 ance of relative opacity in some of the parts marked oflf as ahove 

 is to be explained by thickening in the cell-layers of which they 

 are composed. 



The Origin of the Fowl's Egg.— The ovary of a young but 

 mature hen consists of a mass of connective tissue (stroma), 



Fig. 68. — Varions stages in the eegmentation of a fowl's egg (KOIIiker). 



abundantly supplied with blood-vessels, from which hang the 

 capsules which contain the ova in all stages of development, so 

 that the whole suggests, but for the color, a bunch of grapes in 



