STRUCTURE OF SEA-SQUIRT'S TADPOLE 287 



important structures in which the Ascidian tadpole and 

 the frog's tadpole agree structures which are possessed 

 by all vertebrates and by no other animals. And not 

 only do these structures, as shown in the Ascidian tad- 

 pole, agree in their shape and relations to other parts 

 with the similar structures of vertebrates, but actually in 

 their mode of building up, cell by cell, from the primitive 

 egg-cell, the structures compared are demonstrated by 

 careful microscopic study to be identical. What, then, 

 must we conclude ? This is a case of cumulative evidence. 

 If one met say, near the North Pole a race of men with 



FIG. 36. A drawing of the head of an Ascidian tadpole seen as a 

 transparent object under the microscope. C marks the tail, only 

 partly shown ; K is the gullet ; N is the nervous system tapering 

 to form the spinal cord n behind and expanding in front to form 

 the hollow brain N'. On the inner wall of this we see O the eye 

 and a an auditory organ; o is the aperture of the mouth; ch the 

 notochord (unshaded) ; d the intestine ; s is one of a pair of ad- 

 hesive outgrowths by which the tadpole fixes itself permanently to 

 a piece of rock. 



the colour and appearance of New Zealanders, one would 

 say, " It is very curious ! They are like New Zealanders." 

 But if one found that they could speak the Maori language 

 and no other, it would be rendered nearly certain that 

 they were New Zealanders. If after this you found that 

 they decorated themselves with the tattoo-markings of 

 New Zealanders, and then that they possessed weapons and 

 carvings of the Maori pattern, your conclusion would be 



