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1
But the
medullary
tube
of the Vertebrates originates in the same way.
2
The unsegmented chorda develops between the dorsal
medullary
tube
and the ventral gut-tube.
3
E The
medullary
tube
(mr) is closed; the body-cavity (c) begins to form.
4
Nevertheless, as we know from human embryology, the
medullary
tube
is formed from the cutaneous layer.
5
A epidermis, B
medullary
tube
,
C chorda, C1 inner chorda-sheath, D visceral epithelium, E sub-intestinal vein.
6
In the median line we see the chorda (ch) through the transparent
medullary
tube
(m).
7
Thus is formed the dorsal wall, and the
medullary
tube
takes up a position inside the body.
8
In Figure A the
medullary
tube
(n) and the alimentary canal (a) are still open grooves.
9
The
medullary
tube
is still simple.
10
It originates in the shape of a simple
medullary
tube
from the outer membrane of the embryo-theskin-sense layer.
11
The first rudiment of it is always a simple vesicular enlargement of the fore end of the
medullary
tube
.
12
The brain is very rudimentary at first, a mere bulbous enlargement of the fore end of the
medullary
tube
.
13
The
medullary
tube
of the Chordonia has been formed by the lengthening of the vertical brain on the dorsal side.
14
The two parallel dorsal or medullary swellings that enclose it grow together over it afterwards, and form the
medullary
tube
.
15
The fore-end of the
medullary
tube
expands into a vesicle and forms the brain, which soon divides into five cerebral vesicles.
16
The prorenal duct is visible in this gap even at the time of the severance of the
medullary
tube
from the horn-plate.
medullary
tube
medullary