CHAPTER XIX. THE DEVELOPMENT OF THE CHICK. 



1. The Fowl's Egg. Although we have not taken a 

 bird as one of our types in comparative anatomy, we do so 

 in embryology for two reasons first, because circumstances 

 make it the easiest of all the higher Vertebrate types to 

 study minutely ; and secondly, because without the study of 

 the development of either a reptile or a bird, that of the 

 rabbit would be too puzzling to be profitable. 



The fowl's ovum is about as many times larger than the 

 frog's, as the frog's is larger than that of Amphioxus. This 

 enormous size is due to an excessive gorging with yolk. 

 Thanks to this abundant food-supply, the young bird has 

 no need to go through the dangers of a free larval stage, 

 but can remain within the egg until it differs but little 

 from its parent. At the same time the presence of so 

 much yolk necessitates a much greater deviation from the 

 simple course of early development than was the case in 

 the frog. 



The real ovum is what we commonly call the " yolk " or 

 " yellow " of the egg. This is, at first, a single cell, with 

 its nucleus (germinal vesicle) close to the animal pole. 

 Around the nucleus is a small disc-shaped area of proto- 

 plasm, free from yolk the germinal area. All the rest of 

 the ovum consists of a meshwork of protoplasm filled up 

 with yolk spheres. The whole is surrounded by the vitel- 

 line membrane, for (as with the frog) the ovum escapes 

 from its follicle when ripe. The difficulties of observation 

 have, as yet, prevented any one from actually seeing the 

 extrusion of polar bodies, but there are indications that 

 this process does take place. Fertilization occurs about 

 the time that the egg enters the oviduct, up which the 

 spermatozoa must make their way from the cloaca, where 

 they are placed by the male bird. As the ovum travels 

 down the oviduct the walls of this tube secrete coverings 

 for it first the albumen or " white," and, lower down, the 



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