DEVELOPMENT OF NERVOUS TISSUES. 



77 



cells, increase by the continued division of the germinal cells. The indifferent cells 

 later differentiate into the spongioblasts, from which the characteristic constituents 

 of the neuroglia are derived, and the neuroblasts, which are directly converted into 

 the neurones. 



The neuroglia is evolved by the gradual transformation of the spongioblasts and 

 their descendants into fibrillae and the production of a more definite framework that 

 replaces the primary myelospongium and eventually, in conjunction with the proc- 



elm 



FIG. 105. Segment of wall of neural tube of pig- embryo of 10 mm.; radial strands (r) of syncytium and 

 differentiation of ependymal (a), nuclear (6) and marginal (m) layers; Urn, elm, internal and external 

 limiting membrane ; g, dividing cell ; p, pia mater. X 690. (Hardesty.) 



esses derived from the ependymal cells, gives rise to the completed supporting tissue 

 (page 70). The neurones are directly derived from the neuroblasts. The latter are 

 distinguishable from the spongioblasts as soon as they are provided with nerve- 

 processes. These appear as outgrowths from the peripherally directed and pointed 

 ends of the developing nerve-cells. The first, and for a considerable time the only, 

 processes which the motor neurones possess correspond to axones that become the 



FIG. 106. Transverse section of part of developing spinal cord from pie embryo of 30 mm.; c. central canal ; 

 ep, ependymal layer; n, nuclear layer; m, marginal layer; r, radial fibres. X 240. (Hardesty.) 



axis-cylinders of efferent (motor) nerves. Subsequently the dendrites grow out in 

 various directions from the cell-bodies of the young neurones. 



The peripheral nerves, according to the teachings of His and the views of most 

 anatomists, are essentially outgrowths from the nerve-cells, since the axis-cylinder of 

 the entire nerve-fibre is formed by the peripherally directed growth of the original 

 nerve-process of the neuroblast. The opposed opinion, that the nerve-fibre arises by 

 the fusion of a number of segments, is not in accord with the most recent embryo- 

 logical data. The motor neuroblasts within the spinal cord and the sensory cells 

 within the spinal ganglia send out processes of considerable thickness, which give rise 

 at their extremities to groups of fibrillse. These increase in thickness and length 

 and, in turn, at their extremities give rise to new groups of fibrils. The latter proceed 

 at first as naked bundles, but soon become surrounded by the sheath-celts, which are 



