42 



DEGENERA TION. 



Ascidians, have a form totally different from that of 

 their parents. The ^gg of Phallusia giv^es 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. i6). 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 



TAIL 



SPIRACLE 



EYE M0UT8 



ASCIDIAN 



Fig. i6. — Tadpole of Frog and of Ascidian. Surface view. 



researches of the Russian naturalist, Kowalewsky, 

 the resemblance reaches absolute identity when we 

 examine the way in wdiich 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 

 tw^o mouths and in the fact that the Ascidian has 

 one eye while the frog has two. 



