THE NOTOCHORD AXD ALIMENTARY CANAL 435 



more conspicuous, more and more permanent daring 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 Ammocoetes. 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 



