i CEREBROSPINAL NERVOUS SYSTEM. 417 
anastomosis between the dendrites of neighbouring cells, or between the dendrites 
of the same cell. It would appear, therefore, that nothing in the shape of a net- 
work is formed by these processes. 
In the chapter upon Embryology it has been shown that in the early condition 
of the cerebrospinal axis the brain and cord consist simply of a thin-walled tube 
(p. 20). The wall of this tube is formed of a single layer of tall columnar neuro- 
epithelial cells, and in its deepest or most internal part large round cells make their 
appearance in the intervals between the epithelial columns. These new cells are 
termed the germinal cells, and from them the nerve-cells are derived. They are 
MID-DORSAL Cen’ 
= —,| NEURO- at 
—ApLAST fS VAI 
LATERAL WALL 
<< t\ Ve \ \ q Vy VA ai ms i, \ 
/. = \ AN WO \) ——s tp A » We Wy 4 : 
q pe : = SS ~ “ss A N ASG En Bite ee 
te SSS CENTRAL [2S Oy ys 
Cm SS cana EY 
= - => \| GERMINAL SU gees 
— CELL {G—= < 
a SPONGIO- ES Yg' 
———— 3 : BLAST KL SE eS 
Se CED ae SN NS =z U 
=, é z CILIATED fC . Sal 
= =| = = 53 : ; re y z 
Se] FESS 
—————— Z atl es ] eh 
2 e 
b a 
Fic. 310.—THE DEVELOPMENTAL 
STAGES EXHIBITED BY A PYRA- 
es Ze ie Yi RS Ss : Ssh J ; MIDAL CELL OF THE BRAIN (after 
LP 2 ey Danni NY SS : Ramon y Cajal). 
q S a, Neuroblast with rudimentary 
Ne, axon, but no dendrites ; 6 and ¢, 
The dendrites beginning to sprout 
out ; ¢@ and e, Further develop- 
Le 
iM 
MID-VENTRAL LAMINA 
Fic. 309.—TRANSVERSE SECTION THROUGH THE EARLY NEURAL ment of the dendrites and ap- 
TUBE, diagrammatically represented (Alfred H. Young). pearance of collateral branches 
The left side of the section exhibits an earlier stage of on the axon, 
development than the right side. 
present in considerable numbers, and towards the fourth week of embryonic life 
they form an almost continuous layer. Although these cells ultimately become 
nerve-cells they are absolutely without processes in their early state, and therefore 
at this period, although there is a nervous system, there are, as His remarks, no 
nerves. In course of time, and as the wall of the neural tube thickens, the 
germinal cells begin to migrate in an outward direction. They leave the deep part 
of the wall of the neural tube and proceed to take up the positions they occupy 
in the gray matter of the cord and brain of the adult. These migrating cells 
assume a pyriform shape, and are termed neuroblasts. The drawn-out portion or 
stall of the pear-shaped neuroblast represents the early axon, and this continues 
to grow and increase in length until it ultimately attaims the terminal relations 
characteristic of the adult. The study of embryology presents few more remark- 
able phenomena than the manner in which this axon grows out, and, in the 
efferent nerve-fibres, emerges from the central axis, and yet pursues its allotted 
path with the most unerring exactitude and precision until it ultimately reaches 
the nerve-cell or the peripheral tissue element with which it becomes associated. 
The growing point of both it and its collaterals is shghtly bulbous, and it is out of 
this that the terminal arborisation is formed. 
The dendritic processes of the nerve-cell appear at a later period than the axon. 
The surface of the neuroblast becomes rough and then somewhat spiny. By the 
growth and subdivision of these spiny projections the dendrites are formed. As 
27 
