436 THE NERVOUS SYSTEM. / 
limiting membrane, a series of clefts or open spaces are formed, in which appear large 
numbers of round cells called germinal cells. The precise origin of these germinal cells is 
not at present satisfactorily estab- 
— lished ; but they rapidly increase in 
WS, A P AU SPonciuM number, and in the human embryo 
CELE of four weeks they are seen to form 
an almost continuous layer beneath 
the internal limiting membrane. — It 
is well to note, however, that in the 
thin mid-dorsal and mid - ventral 
laminze no germinal cells are formed. 
Here the wall remains purely spongio- 
blastic. The peripheral portions of 
euaces SN S the spongioblasts likewise undergo 
coun, AY aan Saag a marked transformation. They 
give off branches or processes, and 
by the interlacement of these a 
sponge-like network with irregular 
meshes is formed in the outer por- 
tion of the wall of the neural tube. 
The entire sustentacular framework 
into which the spongioblasts are de- 
veloped is termed the myelosponge. 
The numerous germinal cells which 
are placed in the clefts between the 
inner columnar portion of the myelo- 
sponge are the progenitors of the 
nerve -cells. Many of them show 
karyokinetic stages, and by their 
Fig, 325.—ScHEMA OF A TRANSVERSE SECTION THROUGH THE (division they g give rise to the neuro- 
Earty NEURAL TUBE (Young). blasts or young nerve-cells. A 
The left side of the section shows an earlier stage than neuroblast pr esents a ver Vi character- 
the right side. 
MID-DORSAL LAMINA 
LATERAL WALL 
MID-VENTRAL LAMINA 
istic pear-shaped appearance. From 
the body of the cell a tapering process grows out, and this represents the early axis 
cylinder process or axon of the cell. But the crowds of neuroblasts which are thus formed 
do not remain in their early primitive position beneath the internal limiting membrane. 
They migrate outwards, and in the course of time they come to lie in the part of the 
myelosponge immediately adjoining the reticular meshwork, which is formed by the outer 
parts of the spongioblasts. Here their further outward migration is arrested. The 
reticular meshwork would almost appear to act as a sieve or a filter, which prevents their 
progress towards the periphery of the wall of the tube. It offers no impediment to the 
actively growing axons of the neuroblasts, however, which freely enter it and thread their 
way through it. At this stage the thick lateral wall of the neural tube presents three 
layers, viz. 
1. An inner layer, formed by the columnar part of the myelosponge forsaken by the 
neuroblasts. This is termed the ependymal layer, and it ultimately resolves itself into 
the layer of columnar ciliated epithelial cells which lines the central canal of the cord. 
An intermediate layer, in which the neuroblasts are present, and which is afterwards 
converted into the gray matter of the cord. This is called the mantle layer. 
3. An outer layer, formed of the sponge-like meshwork of the outer parts of the 
original spongioblasts. Into this the axons of many of the neuroblasts are seen threading 
their way. This layer is ultimately transformed into the white matter of the cord, and at 
this stage it may be termed the peripheral reticular layer. 
Alar and Basal Laminz of the Lateral Wall of the Neural Tube.—From 
what has been said, it must be evident that the changes detailed above are confined to 
the thick lateral walls of the neural tube. In these alone do neuroblastic cells arise, 
whilst the thin mid-dorsal and mid-ventral laminze remain spongioblastic throughout. 
But whilst these changes are going on the thick lateral wall begins to bulge outwards in 
an angular fashion, so as to widen the central cavity of the tube and become itself, along 
the line where the cavity is widest, demarcated into two portions—a narrow dorsal strip 
termed the alar lamina of His, and a broader ventral strip called the basal lamina of His. 
The cavity of the tube now appears on transyerse section more or less lozenge-shaped, and 
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