40 H. B. POLLAKD. 



tecture of the germ plasm is more constant than the quality of the 

 determinants. Exceptions to this rule may be discovered. 



The embryonic development of the tentacles in Ictulurus albidus 

 has been investigated by Ryder. Those present in the adult develope 

 early in situ, and there is no parallelism with the phylogeny. . 



Reversion and Larval Forms. 



From comparison of long lists illustrating the occurrence of the 

 individual tentacles, and from consideration of the fact that they 

 appear sporadically, I have come to the conclusion that it would be 

 extremely rash to maintain that the tentacles have come down in 

 unbroken ancestral line from an early progenitor. In other words, 

 their presence must often be due to reversion. 1 They are not always 

 the most primitive and archaic forms that possess the tentacles most 

 fully developed. 



All parts of an organ may not revert to the ancestral condition, or in 

 other words the reversion may be only partial. Such is the case in the 

 tentacles of Gobitidae where the skeletal axis is not developed. In the 

 language of Weismann the reversion in this case is due to determinants 

 in the skin, the skeletal determinants not being evolved. 



When once a structure has arisen by reversion and been rendered 

 constant by natural selection, it will develope ontogenetically direct to 

 the adult condition, aud therefore it is useless to seek for information 

 as to its ancestral history in its embryological history. 



It will I think be obvious, to anyone fully acquainted with the 

 writings of Darwin and Weismann, that reversion may occur at any 

 free living stage. Laval forms are often supposed to represent 

 ancestral or existing adult forms. The resemblance has no doubt been 

 greatly exaggerated. For instance I am not aware that Ammocoetes 

 shows any approach in positive characters to Myxine or Arnphioxus. 



Nevertheless such characters as the prepalatine piece of tadpoles, 

 and the maxillo-coronoid tentacles of the larva of Dactylethra at its 

 fancied " Siluroid " stage, have to be accounted for. Tentacles do not 



1 Or, in many cases, inasmuch as rudiments of tentacles are almost always 

 present, by "re-development from rudiments" (Darwin). No sharp distinction 

 can be drawn between the phenomena of reversion and re-development from 

 rudiments. 



