No. 3-] THE VERTEBRAL COLUMN OF FISHES. 141 



In case the outgrowths described by Gadow and Abbott are 

 really present, we should have around the notochord two more 

 or less complete sheaths of cartilage and perhaps two distinct 

 layers of bone. 



It seems to me that the essential feature of the theory advo- 

 cated by Gadow and Abbott, is to be found in their claim that 

 the basidorsals and the basiventrals which unite to produce the 

 definitive vertebra have had their origin from different proto- 

 vertebrae, and hence correspond to different myomeres. Through 

 the overlap of the myomeres these cartilages are brought into 

 contact and become consolidated. In this way the writers seek 

 to explain the alternation of the myomeres with the sklero- 

 meres. But I do not believe that sufficient evidence has been 

 produced to establish this position. An examination of sagittal 

 sections of Amia at various stages of development fails to show 

 that the ventral arches belong to the anterior myomere, the 

 dorsal arches to the posterior myomere. Nor do I find any 

 proofs of any such slant of the intermuscular septa as the 

 authors affirm and represent in their diagrams. The septa take 

 a course across the vertebral centra which is better represented 

 by this figure ^, the angle on the right hand being directed back- 

 ward and placed near the middle of the height of the centra. 



Certainly the theory proposed by Dr. Gadow and Miss Abbott 

 is not needed in order to escape the acceptance of the hy- 

 pothesis of a resegmentation, or "transverse splitting" of the 

 skeletogenous sheath which surrounds the notochord. At a 

 certain stage this sheath has lost all traces of its original seg- 

 mentation. Why should it regain this segmentation ? The 

 cells proceed to differentiation. Some of those lying opposite 

 the intermuscular septa are transformed into the cartilages 

 of the arches. Of the cells opposite the myomeres, some de- 

 velop inJjO the cartilages which constitute the interbasalia, others 

 into connective tissue which binds the more solid elements 

 together. All this might happen without any transverse split- 

 ting and in an organism in which the myomeres do not overlap. 

 The earliest segmentation around the notochord was due to the 

 independent origin of the various sklerotomes; the later segmen- 

 tation, to differentiation in a mass of similar embryonic cells. 



