. The development of the chick; an introduction to embryology. Birds -- Embryology. 236 THE DEVELOPMENT OF THE CHICK final termination without addition on the part of other cells. The body of the neuroblast forms the nerve-cell, from which, later on, secondary processes arise constituting the dendrites. The view that each nerve-cell with its axone process and dendrites is an original cellular individual, is known as the neurone theory. For the central nervous system this view is generally held, but its extension to the peripheral system is opposed by some on the ground that the axone in periph
. The development of the chick; an introduction to embryology. Birds -- Embryology. 236 THE DEVELOPMENT OF THE CHICK final termination without addition on the part of other cells. The body of the neuroblast forms the nerve-cell, from which, later on, secondary processes arise constituting the dendrites. The view that each nerve-cell with its axone process and dendrites is an original cellular individual, is known as the neurone theory. For the central nervous system this view is generally held, but its extension to the peripheral system is opposed by some on the ground that the axone in peripheral nerves is formed within chains of cells, and is thus strictly speaking not an original product of the neuroblast, though it may be continuous with the axis cylinder process of a neuroblast. This view is discussed under the peripheral nervous system. Each medullary neuroblast is primarily unipolar and the axone is the original outgrowth. Soon, however, secondary proto- plasmic processes arise from the body of the nerve-cell and form the dendrites. These appear first in certain neuroblasts of the ventro- lateral portion of the embryonic cord, whose processes enter into the ventral roots of spinal nerves (Fig. 140). The extent and kind of de- velopment of these dendritic pro- FiG. 140. — Transverse section cesses of the nerve-cells varies through the spinal cord of a extraordinarily in different regions; chick on the fourth day of jrigg. , 140, and 141 give an idea of their rapid development in the motor neuroblasts up to the eighth day. The Ganglionic Neuroblasts. The ganglionic neuroblasts are located, as the name implies, in the series of ganglia derived from the neural crest. It must not be supposed, however, that all of the cells of the ganglia are neuroblasts, for the ganglia contain, in all probability, large numbers of cells of entirely different function. (Sheath-cells, see peripheral nervous system.) It is probable also that the neuroblasts of the spinal gangl
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