34 DEGENERATION : I 



The young of some, but by no means of all these 

 Ascidians, have a form totally different from that of 

 their parents. The egg of Phallusia gives rise to 

 a tadpole, a drawing of which placed side by side with 

 the somewhat larger tadpole of the common frog is 

 seen in the adjoining figure (Fig. 16). The young 

 Ascidian has the same general shape as the young frog, 

 but not only this ; the resemblance extends into 

 details, the internal organs agreeing closely in the two 

 cases. Further still, as shown by the beautiful re- 

 searches of the Eussian naturalist, Kowalewsky, the 



ASCIDIAN 



FIG. 16. Tadpole of Frog and of Ascidian. Surface view. 



resemblance reaches absolute identity when we examine 

 the way in which the various organs arise from the 

 primitive egg-cell. Tail, body, spiracle, eye, and 

 mouth agree in the two tadpoles, the only important 

 difference being in the position of the two mouths and 

 in the fact that the Ascidian has one eye while the frog- 

 has two. 



Now let us look at the internal organs (Fig. 17). 

 There are four structures, which are all four possessed 

 at some time of their lives by all those animals which 

 we call the Vertebrata, the great branch of the pedi- 



