46 MARINE BIOLOGICAL LABORATORY. 



in the skull, the primordial cranium, the cranial nerves, 

 the head-cavities, or the gill-clefts, nor, in short, in any 

 one organ or system of organs that could be named in 

 the head. As Professor Dohrn has insisted, both by 

 word and example, nothing less than a complete analysis 

 of the whole head and trunk can furnish a safe founda- 

 tion for speculation on this subject. But the task does 

 not end with the vertebrates. The present vertebrate 

 head represents the cumulative development of unnum- 

 bered aeons, and its ancestral history is only very imper- 

 fectly recorded in its embryonic development. Our 

 analysis must therefore be extended to the worms, the 

 arthropods, the molluscs, and, as it now appears, even 

 to the coelenterates. The history of metamerism must 

 be traced upwards, and the lessons of the simpler types 

 must be our stepping-stones to a knowledge of the 

 higher. 



There is little prospect of ever knowing precisely 

 how many segments the ancestor of the vertebrates 

 possessed. The number varies in the different branches 

 of a common stock ; and we know that this variation is 

 the result of loss in many cases, and suspect that it 

 may be due to addition in others. But we know that 

 this variability in number has very definite limitations 

 in the laws that control the formation of segments. 

 The possibilities in this respect are by no means the 

 same for all regions of the segmented axis. Although 

 the head segments have undergone the greatest modi- 

 fications in form, fixity in munber is he7'e the rule, 

 while variation, if we except degenerate forms, is con- 

 fined to the posterior trunk segments. In the embryo 

 the anterior segments are invariably first in formation, 



