118 PRACTICAL ZOOLOGY. 



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a part of the true egg. la dissecting a bird, the eggs, 

 of various sizes, according to their stages of develop- 

 ment, may be found in the ovary. At this time the 

 egg consists of the yolk, with a thin coat ; the white 

 is deposited around this later during its descent 

 through the oviduct; the shell is last formed, and is 

 absent in the case of most animals. 



In the development of birds all their nourishment, 

 before hatching, must be stored in the egg ; hence its 

 large size. In the higher animals the egg is retained 

 in the body of the mother, and gets its nourishment 

 from her blood, which circulates through the embryo. 

 9. Set a hen on a dozen eggs ; mark the date ; open and 

 examine an egg each day ; if the egg was fertilized, 

 the cells of the germ-spot multiply by division, and 

 soon take definite arrangement ; at the end of twenty- 

 four hours the backbone is outlined; during the 

 second day the brain begins to develop, and the heart 

 appears , on the fourth day the legs and wings make 

 their appearance as flattened buds ; until the sixth day 

 it would be impossible to say whether the embryo was 

 that of a bird, a reptile, or a mammal ; after this, the 

 characters peculiar to birds become evident, the feath- 

 ers begin to develop, and, later, the particular kind of 

 bird may be recognized. 

 The development of the rabbit, guinea-pig, or any mam- 

 mal, including even man, follows nearly the same order as 

 in the chick, the chief differences arising from the fact that 

 the embryo mammal develops in a special portion of the 

 oviduct, the uterus, or womb, and that the growing germ 

 is supplied with maternal blood. 



The eggs of mammals are very minute. These eggs (if 



