Chap. VI.] GENERAL EMBRYOLOGY. 113 



membranes, a space which contains air. As development pro- 

 ceeds it increases in size, and on the fourteenth day of incuba- 

 tion the chick moves so as to lie lengthways in the egg, with its 

 beak touching the inner wall of the air-space. On the twentieth 

 day the beak pierces this wall, and the chick begins to breathe 

 the contained air. The blood, which has hitherto been oxy- 

 genated in the embryonic respiratory organ (the allantois), is 

 now arterialised in the adult respiratory organ, the lungs. The 

 functionless allantois dries up and shrivels away, as do also the 

 serous membrane and the amnion. The chick breaks the shell 

 with its beak and is hatched. 



In the frog there is neither amnion nor allantois. The 

 urinary bladder, however, arises as an outgrowth of the 

 ventral wall of the posterior end of the cloaca, and has there- 

 fore been regarded as homologous with the similar structure in 

 the rabbit. 



Summary. The frog, the fowl, and the rabbit pass through 

 embryonic development under very different conditions. The 

 primitive embryo, such as that of Amphioxus (p. 114), is 

 free at the gastrula stage. It moves through the water 

 in which it is hatched by means of the cilia with which its 

 surface is freely covered. And from that time forward it has 

 to obtain its nourishment by its own individual exertions. 

 The frog embryo is provided with a supply of food-yolk suffi- 

 cient to carry it over the early stages of development. It 

 is, however, hatched comparatively soon, long before it has 

 assumed anything like its adult form. There is a certain 

 amount of transformation within the egg membrane, but a large 

 proportion of the embryonic development takes place after the 

 organism is hatched, and constitutes metamorphosis. And 

 during metamorphosis the organism is absolutely dependent on 

 its own exertions for the necessary nutriment. The fowl 

 embryo is provided with an abundant supply of food-yolk, 

 sufficient to carry it over, not only the early, but also the later 

 stages of embryonic development. The mother, however, sup- 

 plies under natural conditions the necessary warmth, without 

 which developmental changes cannot proceed. And when the 

 chick is hatched, although the hen affords it protection, and 

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