A STUDY OF METAMERISM. 467 



It is difficult to see how by simple elongation of a larva to 

 meet a supposed larval advantage, in the first place the 

 organs in the ectoderm should get separated into exactly the 

 same number as the number of gut-pouches, and in the second 

 place that all of these should correspond so as to give organs 

 repeated symmetrically in both ectoderm and mesoderm. I 

 cannot, I admit, resist the conviction that metameric repeti- 

 tion is far too difficult and fundamental a problem to be ex- 

 plained as the result of mechanically pulling out the supposed 

 Anlagen of all the organs of the body. 



The phenomena of metameric repetition and apical growth 

 are closely associated together. Whether the latter stands in 

 any causal relation to the first cannot be definitely asserted, or 

 if it could we should have no means of determining whether 

 the relation is an ontogenetic or a phylogenetic one; whether 

 the apical elongation was established in the embryo or in the 

 adult (if, indeed, we can draw any line of any value between 

 the embryonic and adult appearance of any organs). 



Nevertheless it is important to emphasise the fact of the 

 connection, for we find both in Vertebrates and Annelids the 

 two phenomena closely bound up together. Further, we have 

 the interesting case of the star-fishes and brittle-stars, where 

 at five radial points there is apical growth, and the five arms 

 are segmented. In the higher plants also there is a repetition 

 of similar parts (phytomeres) and apical growth. 



Whether the repetition of the calcareous skeleton and tube- 

 feet of the arm of a star-fish is a repetition comparable to the 

 repetition in the Vertebrate and Annelid will depend largely 

 upon definition of terms. As to the fact of a symmetrical 

 repetition and the presence of apical growth there can be no 

 question, and that is the main point. There are no grounds 

 for assuming that the repetition of the parts of the star-fish 

 arm was ever connected with any attempt of the animal, in the 

 past, to reproduce itself at five equidistant points, nor would 

 such a suggestion be believed by anybody for a moment. The 

 method of growth in these arms, where there is a terminal 

 piece carrying the eyes and a subterminal growing region, 



