204 COMPARATIVE ZOOLOGY. 



The limbs appear as buds on the sides of the body; 

 these lengthen and expand so as to resemble paddles — 

 the wings and legs looking precisely alike ; and, finally, 

 they are divided each into three segments, the last one 

 subdividing into digits. The feathers are developed from 

 the outside cells of the epidermis : first, a horny cone is 

 formed, which elongates and spreads out into a vane, and 

 this splits up into barbs and barbules. 



The muscle-fibres are formed either by the growth in 

 length of a single cell, or by the coalescence of a row of 

 cells: the cell-wall thus produces a long tube — the sarco- 

 lemma of a fibre — and the granular contents arrange them- 

 selves into linear series, to make fibrillse. 



Nervous tissue is derived from the multiplication and 

 union of embryo-cells. The white fibres at first resemble 

 the gray. The brain and spinal marrow are developed 

 from the epiblastic lining of the medullary furrow. Soon 

 the brain, by two constrictions, divides into fore -brain, 

 mid-brain, and hind-brain. The fore-brain throws out 

 two lateral hemispheres (cerebrum), and from these pro- 

 trude forward the two olfactory lobes. From the mid- 

 dle-brain grow the optic lobes; and the hind -brain is 

 separated into cerebellum and medulla oblongata. The 

 essential parts of the eye, retina and crystalline lens, are 

 developed, the former as a cup-like outgrowth from the 

 fore-brain, the latter as an ingrowth of the epidermis. 

 An infolding of the epidermis gives rise to the essential 

 parts of the inner ear, and from the same layer come the 

 olfactory rods of the nose and the taste-buds of the tongue. 

 So that the central nervous system and the essential parts 

 of most of the sense-organs have a common origin. 



Modes of Development. — The structure and embryology 

 of a Hen's egg exhibit many facts which are common 

 to all animals. But every grand division of the Animal 

 Kingdom has its characteristic method of developing. 



