40 MARINE BIOLOGICAL LABORATORY. 



stem, which bears the deep and enduring mark of 

 metamerism. 



So much for the general significance of metamerism. 

 Let us now return to the vertebrate head. If the me- 

 tamerate type of structure precedes and forms the 

 foundation of the vertebrate type, then the question 

 how many primordial segments are represented in the 

 head takes precedence of the question how many ver- 

 tebrae compose the skull. The inquiry takes us back 

 to that interesting stage in which the embryo becomes 

 divided into a chain of segments. But here we find 

 that the transverse lines marking the boundaries of the 

 segments do not extend into the region of the head, or 

 at most only into its hinder portion. But we are not 

 yet satisfied that the head is a thing siii generis, built 

 upon a plan fundamentally different from that of the 

 body. Baffled in the attempt to find direct evidence 

 sufficient to demonstrate the unity of plan which we 

 suspect underlies both the head and the trunk, we 

 next resort to indirect or circumstantial evidence, and 

 begin to question whether the records of ancestral 

 development have been perfectly preserved in the em- 

 bryonic development. It is here that the towering 

 difficulties of the problem come into view, in scaling 

 which investigation rises to its sublimest heights. 



Before the division into segments, there is nothing in 

 the embryo to show even approximately where the head 

 ends and the body begins ; the part which is destined 

 to become the head forms with the rest a continuous 

 whole, as shown in the external form and in the con- 

 tinuity of like structural elements. The cordal axis 

 before alluded to is the precursor of the backbone, and 



