? 
" 
aot 
DEVELOPMENT OF THE SPINAL CORD. 437 
it is at the lateral angles of the lozenge that this subdivision of the lateral wall becomes 
evident. 
This subdivision is a fundamental one, being present in the brain-part as well as the 
cord-part of the neural tube. By it the thick lateral wall is resolved into two longitudinal 
strips (the alar and basal lamine), which extend along the whole length of the tube, and 
which present definite and precise relations with the entering and emerging roots of the 
various cranial and spinal nerves. Confining our attention to the spinal cord, the 
posterior nerve-root is seen to enter the alar lamina, whilst the anterior nerve-root takes 
origin within and emerges from the basal lamina. 
MDL 
Fig. 324.—THREE STAGES IN THE DEVELOPMENT OF THE SPINAL CorD (His). 
AC. Anterior column. BC. Column of Burdach. MVL. Mid-ventral lamina. 
AH. Anterior horn of gray BL. Basal lamina. PC. Early posterior column. 
matter. E. Ependyma. PH. Posterior horn of gray 
AL. Alar lamina. GC. Column of Goll. matter. 
AMF. Antero-median fissure. LC. Lateral column. PMF. Postero-median fissure. 
AR. Anterior nerve-root. MDL. Mid-dorsal lamina. PR. Posterior nerve-root. 
Further Development of the Gray and White Matter of the Cord.—In the 
ventral part of the basal lamina the mantle layer thickens into a mass, which is readily 
recognised as the rudiment of the anterior horn of gray matter, and in this neuroblastic 
cells congregate in much larger numbers than elsewhere. Further, these neuroblastic 
cells begin to arrange themselves into groups, and the axis cylinder processes of a large 
proportion of them converge and form bundles of fibres, which penetrate into the peri- 
pheral layer, and finally pierce the external limiting membrane, to emerge as the fascicles 
of the anterior nerve-roots. Behind the anterior horn the mantle layer still remains very 
thin, and the neuroblasts are few in number. There is, therefore, at this stage no appear- 
ance of the posterior horn of gray matter. Many of the axons of the neuroblasts which 
occupy this region are seen curving forwards, and, after traversing the anterior horn, 
they find their way across the middle line in the mid-ventral lamina. In this manner 
is laid down, at a very early stage, the rudiment of the anterior white commissure of the 
cord. 
The white matter of the anterior and lateral columns is gradually established by axons 
from various neuroblasts in different parts of the mantle layer, entering the peripheral 
reticular layer and taking a longitudinal course within it. The anterior horn is well 
coated with white matter, however, before the lateral column takes definite shape. 
The posterior columns of white matter are formed in a totally different manner, viz. 
by the introduction into the cord of nerve-fibres from without. The fibres of the posterior 
nerve-roots coming from the spinal ganglia strike the outer surface of the alar lamina of 
the lateral wall of the neural tube, and, piercing the external limiting membrane, take a 
longitudinal course in the peripheral reticular layer. On cross section these fibres first 
appear as an oval bundle, which lies in the outer part of the alar lamina (Fig. 324, Pc). 
This bundle is the rudiment of Burdach’s column, and at first it has a somewhat loose 
connexion with the cord; but as the posterior horn of gray matter gradually takes 
shape, the bundle in question increases in volume, and, changing its position, comes: to lie 
on the inner aspect of the posterior horn. The column of Goll gradually assumes form 
