THE NOTOCHORD AND ALIMENTARY CANAL 435 
more conspicuous, more and more permanent during the life of the 
animal. Such is, indeed, found to be the case, until at last, in the 
lowest vertebrates, such as the lamprey, and in forms like Amphioxus, 
the notochord persists throughout the life of the animal as a large 
important axial supporting rod. 
This rod has a number of striking characteristics which distinguish 
it from all other structures, and are the only means of guessing its 
probable origin. Its position in the body is always the same in all 
vertebrates and is very significant, for it lies just ventrally to the 
central nervous system, along nearly the whole length of the animal, 
not quite the whole length, for it invariably terminates close to the 
place where the infundibulum comes to the surface of the brain; it 
is, in fact, always confined to the infra-infundibular and spinal cord 
part of the central nervous system. Interpreting this into the 
language of the arthropod, it means that a rod was formed just 
ventrally to the nervous system, which extended the whole length 
of the infracesophageal and ventral chain of ganglia, and terminated 
at the orifice of the mouth. Moreover, this rod was unsegmented, 
for the notochord is devoid of segmentation. 
At the anterior end the rod tapers to a point, as in Fig. 166. 
In its middle part it is very large and conspicuous, cylindrical in 
shape; its interior is filled with a peculiar vacuolated tissue, different 
to any other known vertebrate tissue, which has therefore received 
the name of notochordal tissue. Outside this is a thick sheath 
formed of many layers, of which the external one gives the staining 
reactions of elastin, and is called the external elastic layer. Between 
this sheath and the notochordal tissue a thin layer of lining cells, of 
normal appearance, is conspicuous in Ammoccetes. These cells secrete 
the layers of the sheath, and have originally, by proliferation, given 
rise to the notochordal tissue. In the notochord of Ammoccetes 
there is no sign of either nerves, blood-vessels, or muscles, 
The centre of the notochord presents the appearance of a slight 
slit, as though it had originated from a tube, and that is the opinion 
now generally held, for its mode of formation in the embryo is as that 
of a tube formed from an open groove, as will be explained immediately. 
We may, then, conceive of the notochord as originally a tube lying 
in the mid-line just ventrally to the central nervous system, and ex- 
tending from the original mouth to the end of the body. Translate 
this into the language of the arthropod and it denotes a tube on the 
