20 GENERAL EMBRYOLOGY. 
which are continuous in front of the groove but separate behind, where they 
embrace the anterior end of the primitive streak. The neural groove increases 
in length both in front and behind. The backward increase takes place at the 
expense of the primitive streak, whilst the anterior increase is due to the rapid 
growth of the anterior part of the embryonic area; at the same time, not because 
of, though comeident with, an increase of the mesoderm which has grown beneath 
them, the medullary folds are gradually elevated, and their apices bending inwards 
unite together over the neural groove, which is thus converted into a tube or canal 
—the neural tube. 
Along the line of union the neural tube is connected, for a time, with the 
surface ectoderm by a ridge of cells, the neural crest. The crest soon separates 
PS 9. SG (GCi) @SBONG | CCl esc 
; 5 \ | / Yi ee EC \ 
; 2 = : : eer 
a. sy 
= SpP 
J 
SoP 
EN 
IMC 
Fig. 14.—TRANSVERSE SECTION OF FERRET EMBRYO. 
Showing the closure of the neural groove, the formation of the neural crest, the outgrowth of the spinal 
ganglia, the commencement of the separation of the paraxial mesoderm from the lateral plates, and the 
differentiation of the intermediate cell mass. 
C. Celom. GC. Germinal cell. PA. Primitive aorta. SG. Spinal ganglion. 
CC. Central canal. IMC. Intermediate cell mass. PS. Protovertebral somite. SoM. Somatic mesoderm. 
EC. Ectoderm, N. Notochord. SB. Spongioblast. SoP. Somatopleure. 
EN. Entoderm. NC. Neural crest. SC. Spinal cord. SpM. Splanchnie mesoderm. 
SpP. Splanchnopleure. 
from the surface, but it remains connected with the neural tube and is utilised in 
the formation of the cranial and spinal nerve ganglia, whilst the walls of the neural 
tube are converted into the nervous and sustentacular tissue elements of the whole 
of the central nervous system (brain and spinal cord). 
Before the tube is closed the neural groove is dilated at each end (see Fig. 18). 
The posterior dilatation is single; 1t constitutes the rhomboidal sinus, which under 
ordinary circumstances soon disappears. Anteriorly, numerous dilatations are 
distinguishable at first. The exact number of these dilatations (neuromeres) is 
said to be eleven. As the tube closes they resolve themselves into three distinct 
vesicles termed the primary cerebral vesicles. These constitute the rudiments of the 
fore, mid, and hind brains, and their respective ventricular cavities. The remainder 
of the cavity of the tube becomes the central canal of the spinal cord. 
After the separation of the neural crest from the surface, the mesoderm com- 
pletely surrounds the whole of the neural tube, and from it are formed the 
membranes of the brain and spinal cord and their skeletal environments. 
The ectodermal cells which form the wall of the primitive neural tube are ill- 
defined, but they soon differentiate into two sets, spongioblasts and germinal cells. 
The spongioblasts are the more numerous, they are columnar in form, and all 
extend from an internal limiting membrane which is developed round the peri- 
phery of the central canal, to an external limiting membrane which forms the outer 
limit of the neural tube. There is frequently considerable difficulty in recognising 
their columnar character, even in the early stages, partly because their nuclei do 
not all lie at the same level, and partly because they are so closely apposed. The 
spongioblasts are converted into the sustentacular tissue, or myelospongium, of the 
brain and spinal cord, but all do not undergo precisely the same transformations. 
The inner portions of those spongioblasts whose nuclei lie near the central canal retain 
a columnar form, and cilia grow from their free surfaces into the lumen of the canal ; 
in other words, they are converted into the ciliated epithelium of the central canal, 
