EXTENSION OF THE MESODERM. 21 
but the outer portions of the same cells are transformed into fibrillar processes 
which terminate externally by fusing with the external limiting membrane. The 
remaining spongioblasts entirely lose their columnar form, they become much 
branched, and their branches interlace with the fibrillar processes of the ciliated 
epithelial cells, and with similar branches of neighbouring cells, forming the 
reticular sustentacular tissue or myelospongium ; the external hmiting membrane 
is produced by the close interweaving of the peripheral myelospongial fibrils. 
The germinal cells are spherical in outline, and contain clear protoplasm and 
darkly-staining nuclei. They lie between the inner ends of the spongioblasts close 
to the central canal where, at the fourth or fifth week, they form an irregular layer, 
and very soon give rise to a new generation of cells, the neuroblasts, or young nerve- 
cells. Each neuroblast rapidly becomes pyriform by the outgrowth of an axial 
process or axon, which projects from its outer end towards the periphery of the 
tube. 
Shortly after their formation the neuroblasts migrate outwards, and ultimately 
those of the cord are arranged in longer or shorter columns in the myelospongium, 
whilst those of the brain are grouped together in definite areas to form the 
cerebral nuclei. 
Each neuroblast as it develops gives off many processes, which vary in length 
and thickness. The first formed of these is the axial process or axon already referred 
to. It carries impulses from the cell, gives off lateral branches, and terminates 
either in association with a special end-organ or by ramifying amidst other nerve- 
processes or round a nerve-cell of the central or peripheral nervous system. The 
remaining processes of the neuroblast are called dendrites or protoplasmic processes. 
They are ‘usually shorter and more branched than the axon, and they carry impulses 
to the cell. The whole system of cell body, axon, and dendrites into which a neuro- 
blast develops is termed a neuron. 
Every neuron is a separate and distinct entity. Its processes neither anastomose 
together nor with the processes of other neurons. They le, however, in close 
contiguity with either the body or processes of other neurons or with special end- 
organs, and it is possible for impulses to pass from one neuron to another although 
there is no structural continuity between them. 
Extension of the Mesoderm, and Division of the Blastodermic Membrane 
into Areas.—It has already been pointed out that when the primitive streak first 
appears it consists of a thickened ridge of ectoderm situated in the posterior part 
of the embryonic area and resting upon the entoderm. The anterior end of the 
ridge soon fuses with the entoderm beneath it, and from its sides and posterior 
extremity a lamelliform outgrowth, the mesoderm, is projected between the 
ectoderm and entoderm. At its commencement the mesoderm is an outgrowth 
from the primitive streak, but during its subsequent extension it is probably added 
to by cells proliferated from the entoderm. 
As it extends the mesoderm forms a semilunar sheet of cells, the concavity of 
the semilune being turned forwards, whilst the convexity is oradually projected 
beyond the margins of the embryonic area. The cornua of the semilunar sheet 
grow forwards on either side of, and at some little distance from, the middle line, 
immediately beneath the medullary folds. Each cornu on reaching the anterior 
end of the embryonic area bifurcates, and the resulting processes unite with their 
fellows of the opposite side. 
At the same time the mesoderm grows from its convex margin, and extends 
over the rest of the ovum as a continuous sheet. But even when the extension is 
completed, in the majority of mammals, three areas on the upper aspect of the ovum 
remain devoid of mesoderm, and consist only of ectoderm and entoderm. 
The largest of these areas hes directly in front of the embryonic region. In 
many mamunals it is folded upwards and backwards over the head of the embryo, 
when this becomes distinguishable, and it takes part in the formation of one of the 
protecting foetal membranes, viz. the amnion; it is therefore called the proamnion, 
and the area from which it is developed constitutes the proamniotic area. Probably 
it is not present in the human blastoderm, or if it exists it is very transitory. 
The second of the areas into which the mesoderm does not extend les in the 
