278 OUTLINES OF EVOLUTIONARY BIOLOGY 



Such also are the essential characters of the tadpole stage of the 

 frog and of the ascidians, and the vestigial gill-slits of higher 

 vertebrates, which indicate descent from some gill-breathing, fish- 

 like form. 



In the second place there are characters which have been 

 acquired secondarily by the embryo or larva in relation to its 



tn.a-p. 



ex.ap. 



FIG. 129. A Simple Ascidian (dona 

 intestinalis], X . 



a.p., processes for attachment ; in.ap., 

 inhalant aperture ; ex.ap., exhalant 

 aperture. 



FIG. 130. Tadpole Larva of a 

 Compound Ascidian (Diplosoma 

 crystallinum), x 35. (From a 

 photograph.) 



p., papillae for attachment ; t., tail. 



special embryonic or larval requirements. These are termed 

 csenogenetic or modern characters, and it is these which tend 

 more or less to obscure the ontogenetic record. Such are the 

 presence of food-yolk, the foetal membranes of air-breathing 

 vertebrates and the various special larval organs to which we 

 have above referred. It is difficult, however, to draw a hard and 

 fast line between palingenetic and caenogenetic characters, and 

 we must always remember that embryonic stages, as well as the 



