286 TADPOLES OF THE SEA 



Ascidian. The third point of agreement in the two tad- 

 poles is that, as in all vertebrates, we find in both a hollow 

 brain and spinal cord running along the back, from head to 

 tail — in fact, a tubular nerve-cord, lying above the skeletal 

 elastic rod or notochord (see Fig. 35). In very young 

 stages of growth of the Ascidian tadpole, the nerve-tube is 

 formed from the outermost layer of the skin by the pro- 

 duction of a trough, or groove, along the back — the sides 

 of which close in. This is the way in which the central 

 nervous mass — the brain and spinal cord — of all verte- 

 brates is formed, and it is so formed in them alone, and in 

 no other animals. The Ascidian tadpole, like the frog's 

 tadpole, has a swollen, rounded brain in its head, from 

 which the pipe-like spinal cord extends right away along 

 the little animal to the end of its tail. The sensitive part 

 of the eye — the retina — is, in all vertebrate animals, in 

 its earliest phase of growth, a bulb-like outgrowth of the 

 hollow brain. That is its nature in the young frog's 

 tadpole. The retina of the eye is not formed as a part of 

 the brain in any other animals besides the vertebrates. 

 But in the Ascidian tadpole the likeness to the vertebrates 

 is kept up in this respect also, giving us a fourth point of 

 agreement between them, shared by no other animals. We 

 find in the transparent young Ascidian tadpole, not two 

 eyes, but a single little "retina" and black-coloured spot of 

 pigment, actually within the hollow brain (Fig. 36). The 

 " retina " is (as in all vertebrates, and in vertebrates alone) 

 part of the brain-wall ; but there is no need for it to grow 

 outwards to the surface, since the light can pass through 

 the transparent tissues to it. It has been wittily 

 remarked of the Ascidian tadpole that " his eye is single 

 and his whole body is full of light." The same is true of 

 the wonderful little fish-like creature called the Lancelet, 

 or Amphioxus. 



Thus, then, we have seen four extremely peculiar and 



