DEVELOPMENT. 199 
many bones in a skeleton as centres of ossification. But 
the actual number in the adult animal is much smaller, as 
many of the centres coalesce.”* The development of the 
backbone (as, in fact, the growth of the whole chick) is 
not from the head or from the tail, but from a central 
point midway between: there the first vertebrze appear, 
and from thence they multiply forward and backward. 
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 ont into a vane, and 
this splits up into barbs and barbules. 
The muscles 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 sarcolemma 
of a fibre, and the granular contents arrange themselves 
into linear series, to make fibrille.**’ 
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 primitive stripe—that pale-white line on the cic- 
atricula, which almost from the beginning is conical, fore- 
shadowing head and tail. Soon the brain, by two con- 
strictions, divides into fore- brain, mid - brain, and hind- 
brain. The fore-brain throws out two lateral hemispheres 
(cerebrum), and from these protrude forward the two ol- 
factory lobes. From the middle-brain grow the optic 
lobes; and the hind-brain, afterward separated into cere- 
bellum and medulla oblongata, is the origin of the ear- 
sacs. 
Modes of Development.—The structure and embry- 
ology of a Hen’s egg exhibit many facts which are com: 
