EMBRYOLOGY AND REPRODUCTION. 77 



lies a solid rod of cells (Fig. 23), derived from the 

 hypoblast, which is called the notochord (string down 

 the back) . In the higher vertebrates it only exists in 

 the embryo; when bones begin to be formed round the 

 spinal cord, it is merged in them and lost. It is found 

 in a kind 0/ creature very different from a bird, the 

 Ascidian (Leather-bottle animal), or Sea Squirt, (Fig. 

 84), a marine animal which, is found fixed upon 

 stones and shells. This animal, like a butterfly, has 

 a larval stage which, is different from the grown-up 

 stage. The larval stage has a long tail, which, 

 like the tadpole's tail, sinks in and disappears as 

 the animal grows up. Now, in this tail is found a 

 notochord like that in the chick. It appears not 

 only in the chick, but in all other back-boned 

 animals, at a similar early stage ; and the lower we 

 go in the scale of back-boned animals, the larger 

 we find the notochord, and the longer it persists. In 

 the Amphioxus, which is the lowest of the series, and 

 only by courtesy called a back-boned animal at all, 

 it is large, and persists in the adult ; the same is 

 the case in some fishes. When the coincidence of 

 structure between the vertebrate and the ascidian 

 embryo was first noticed, great excitement was 

 created in the scientific world; and it was thought that 

 we were following up a clue to the answer of the in- 

 teresting question " From what invertebrate type are 

 the vertebrates descended ? " It was popularly sup- 

 posed that scientists traced the pedigree of man 

 directly from the Sea Squirt, an impression that 



