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. 
