56 MARINE BIOLOGICAL LABORATORY. 



the fact that growth takes place mainly at one end (the 

 posterior) of the embryo, differentiation becoming more 

 marked as we proceed forwards along the antero-poste- 

 rior axis of the body. For the sake of brevity, this may 

 be called apical or tuiipolar growth. To these funda- 

 mental morphological peculiarities we may add a third 

 — that in all vertebrates the body appears to be built 

 up, in a greater or less degree, by the union along the 

 median line of parts that are laid down in the embryo 

 as separate bilateral foundations. This I shall term 

 coiiC7'escence} Vertebrate morphology therefore presents 

 three fundamental problems for solution, viz., the origin 

 of (i) metamerism, (2) apical growth, and (3) concres- 

 cence ; and these three lie at the root of all others. 



Let us now turn to the invertebrate types. Do any of 

 these possess the three characteristics in question ? 

 There are two such types, namely, the artJiropods (in- 

 sects, arachnids, Crustacea, etc.) and the annelids (earth- 

 worms, leeches, and a great number of marine worms). 

 The arthropods may, however, be left aside, since the 

 annelids are in every respect simpler and less specialized, 

 and there is strong reason to believe that the leading 

 features of arthropods are inherited from annelid-like 

 ancestors. The annelids have a typical metamerism 

 and they are the lowest of segmented animals ; apical 

 growth appears among them in its clearest and simplest 

 form ; the phenomena of concrescence are nowhere so 



^ The term is here used in a wider sense than is ordinarily employed. 

 The occurrence of concrescence in the more restricted sense among the 

 vertebrates is not generally admitted to be a fact, and very eminent 

 authority can be cited both for and against it. However, this very 

 division of opinion in regard to so deep-lying a question only serves to 

 render its investigation more interesting and important. 



