5. The shield of the turtle develops and the reptilian features 



6. The embryo hatches in the form of the adult, there being no 



Development of Birds. So much alike are all the living species 

 of birds that the embryology of a single kind is in all pi 

 a type of that of the others. The development of the domestic 

 fowl has been studied in more detail than any other vertebrate, 

 since it is easy to hatch the eggs artificially, and from their large 

 size they can be examined more readily than the eggs of fishes. 

 Our account of the embryology of birds will be talten from the 

 admirable account by Foster and Balfour in their "Elements of 

 Embryology," and we shall freely use their work, often quoting 

 them, word for word, where it is not possible to farther condense 



The eggs of the hen are fertilized in the upper extremity of the 

 oviduct, whether before or after the 11 white" of the egg is depos- 

 ited is unknown, but at any rate before the shell is deposited 



First day. As the first result of impregnation the germinal 

 vesicle disappears, probably being, judging from the analogy of 

 the bony fishes, bodily ejected from the germinal disk. Then be- 

 gins the process of segmentation of the yolk, which goes on at 

 about the time the shell is formed. Segmentation is partial, being 

 restricted to the germinal disk of the ovarian egg; the result is 

 the formation of the blastodermic disk, which is the beginning of 

 the emliryo, resting on the upper surface of the yolk and appear- 

 ing as a pale round spot seen in the freshly laid egg. This blas- 

 toderm at first eonsi.-,ts of two layers of cells, the upper made up 

 of nucleated cells, and the lower of irregular rounded masses 

 called " formative cells." 



IS T ow begins the marking out of the embryo, which develops in 

 the "area pellucida" a transparent rim (encompassed by the 

 " area opaca ") surrounding the blastoderm. The first step is the 

 origin of an inner germ-layer, the two others having previously 



