210 A HISTORY OF BIRDS 



in open nests. According to some oologists this is a protective 

 device, the structure of the nest being of so loose a character 

 that the light can be seen through when viewed from below, and 

 the white eggs are mistaken for light patches. This is a far- 

 fetched explanation. Such creatures as may prey on these 

 eggs must be able to gain access to the nest from above, whence 

 the eggs would be conspicuous enough. Probably the surround- 

 ing leaves of the overhanging boughs afford protection enough 

 from such enemies, or they are not sufficiently numerous to be 

 seriously harmful, for it must be remembered that even pro- 

 tective coloration is not an absolute protection. 



Finally, it is a matter for comment that there are no black- 

 shelled eggs, inasmuch as black frequently occurs in the form 

 of spots. 



As touching the structure and composition of the egg we 

 need say but little here, yet a few words on this subject are 

 necessary. 



To begin with, what is popularly called the egg is this, and 

 something more. What is this " something more," and what 

 is this egg? If, say, a hen's egg be carefully broken into a 

 cup there will be found a large yellow mass, surrounded by a 

 clear jelly-like elastic coat. On the top of the yellow yolk, in 

 the centre of the field, there will be noticed a small white spot. 

 This represents the germinal protoplasm destined to form the 

 chick ; the cushion on which it rests constitutes an enormous food 

 store which is gradually absorbed as the development of the chick 

 proceeds. The eggs of the lower forms of life, as is pointed 

 out in vol. iv. of this series, though extremely minute in size, 

 consist of a similar mass of germinal protoplasm, including only 

 a very small quantity of food yolk. Hence the developing 

 organism is thrown upon the world in an extremely undeveloped 

 condition, bearing no sort of resemblance to the adult form. 

 Owing to the scanty supply of food material with which it is 

 provided, the microscopic creature has to obtain its food from the 

 surrounding medium, hence it begins life as an aquatic animal 

 or " larva," though it does not always necessarily remain there. 



But to return to our point — the egg in the cup. The cushion 

 of yolk to which we referred is enclosed by an extremely thin 

 and delicate skin, the vitelline membrane, while the yolk within, 

 if examined in a section of a hard-boiled egg, will show that it is not 



