458 SUMMARY AND CONCLUSION. 



The special characteristics of apical growth in the craniate stock are the precise 

 limitation to its extent; the sharp differences between the metameres of different 

 groups, or generations; and the differences between the various parts of the same 

 metameres. This is specially notable in the arthropods where many of the sub- 

 phyla, such as the insects, arachnids, etc., are notable fora small, definite number 

 of metameres, and for the sharply graded step-like differences between them. 

 This is in marked contrast with the leaf-like, homogeneous body of the platyhel- 

 menthes; the voluminous and elaborate, but short bodied mollusca; the inde- 

 finitely elongated series of similar metameres characteristic of the annelids; and 

 finally with the small size, feeble definition of organs, but unlimited power of 

 budding so commonly manifested by the acraniates. 



One of the striking features of apical growth that is manifest through the 

 entire range of the arthropod-vertebrate stock is the vigorous and persistent 

 power of producing new metameres that are varied in character, and that have 

 a well marked power of mutual adaptability. It is this adaptability that ultimately 

 leads, especially in the older, more anterior metameres of the higher forms, to the 

 almost complete disappearance of metamerism, and to the substitution for it of 

 a linear arrangement of unlike functions and organs where the location of an 

 organ in the series is determined by its right of historic precedence and by the 

 degree to which the performance of its functions depends on location. 



/ 



On the other hand, the acraniates are universally characterized by the lack 

 of this vital vigor, and the contrast between the method of growth and differentia- 

 tion in these two great subdivisions of the animal kingdom is most instructive. 

 Although the acraniates apparently started at the same time as the craniates, 

 with a very similar structure, and under similar external conditions they gave 

 rise to a multiplicity of feeble, defective, often degenerate subphyla, whose most 

 characteristic features are the degenerate or extremely small size of the neuro- 

 muscular systems, the feeble power of apical growth, and the indistinct metamer- 

 ism. We may attribute this lack of organic definition to some inherent defect in 

 the constituent materials common to them all, and which lies quite beyond our 

 reach. Another great difference between the two groups is the absence of a fixed 

 internal environment in the acraniates, due to the absence of an impervious exo- 

 skeleton. Without it, development is apparently more diffuse, or vegetative, and is 

 marked by an unlimited power of budding. At the same time there is clearly some 

 fundamental defect in their neuromuscular system which checks its development, or 

 leads to its almost complete degeneration. 



B. Asymmetry as a Creative Factor. \Vhile bilateral symmetry, with 

 its accompanying arrangement of unlike parts in linear and transverse series, 

 is apparently an inherent product of apical growth, and is the normal condition 

 in the arthropod-vertebrate stock, it is subject to modifications of unknown origin 

 that produce various degrees of asymmetry. Where there is a measurable quanti- 



