460 HISTORY OF THE HUMAN BODY 



The question rests upon the assumption that the precursors 

 of the vertebrates were, like Amphioxus, headless but com- 

 pletely segmented, and that the head was first formed by the 

 union of a certain definite, though probably rather small, num- 

 ber of somites, each with its similar set of organs, into a single 

 body unit or complex. A similar process has been postulated 

 in the case of the head of insects, and the two cases have much 

 that is analogous, especially the gradual addition of primarily 

 trunk somites in proceeding from lower to higher forms. In 

 a myriapod, for example, or still better, in an annelid like the 

 earth-worm, the somites are practically alike, each containing 

 one pair of nerve ganglia, one pair of segmental organs (in the 

 latter case), one set of metameric muscles, and one pair of ex- 

 ternal appendages, and it is by comparing the number of gan- 

 glia, of appendages, or other metameric parts, that morpholo- 

 gists attempt to resolve the head complex into its primary 

 somites. In much the same way the body somites of Amphi- 

 o.rus, or, to a lesser degree, of a fish, are also similar, and the 

 pairs of nerves, the gills, the nephridia, and especially the myo- 

 tomes, are metameric, at least in the embryo. By thus ascer- 

 taining the original number of each of these metameric elements 

 that exist in the head, as shown in the embryological record, 

 morphologists have sought here also to reconstruct the early 

 conditions and translate the head of modern vertebrates into a 

 definite series of somites, each with its metameric parts. Thus 

 far the views of investigators are widely apart, and the sug- 

 gested number of primary somites varies from three or four to 

 eighteen, or even more ; the most usual results agree, however, 

 in placing the number between the limits of nine and eleven. 



The posterior portion of the head in fishes, and especially 

 in certain primitive selachians, shows a definite metamerism, 

 marked in the cartilaginous gill-arches, the arterial arches r 

 and the cranial nerves (Vagus group), but anterior to the 

 otic region this becomes effaced, and it is extremely difficult 

 to see here any suggestions of segmentation, even in the 

 embryo. Some, have, indeed, asserted that the prcechordal and 

 parachordal portions of the head, divided at about the hypo- 



