1894.] Evolution of the Vertebral Column of Fishes. 297 



standing of the formation of the vertebral column, because it explains 

 (1) the so-called new segmentation of the axial column, (2) the 

 almost universal occurrence of more than one dorsal and one ventral 

 pair of arcualia (namely, arches and intercalary pieces) in each of the 

 later vertebral segments or skleromeres. 

 The explanation is as follows : 



1. The dorsal half of sklerotome 2 grows downwards and comes to 

 lie behind the ventral half of sklerotome 1. 



2. The ventral half of sklerotome 2 grows upwards and comes to 

 lie in front of and below the dorsal half of sklerotome 3. 



3. The formation of a physiological unit is effected by the combin- 

 ation or fusion of the unequally numbered sklerotomic halves, in such 

 way that the dorsal half lies behind and above the ventral half. 



The new skleromere I (= dorsal sklerotome 2 + ventral sklerotome 

 1) stands now in the following relation to the myoineres; the dorsal 

 end of the skleromere I coincides with myomere I ; the septum 

 between this myomere and the next previous one passes between 

 dorsal sklerotome 2 and ventral sklerotome 3 ; this means to say 

 right across the new skleromere I. This skleromere lies within the 

 influence or range of action of two successive myoraeres. Taken as a 

 whole, the skleromere is " interprotovertebral," more correctly bi- 

 proto vertebral, because it is composed of two successive sklero tomes, 

 namely, the ventral half of one and the dorsal half of a second. 



Consequently, the " resegmentation " or " neugliederung " is 

 brought about in a manner fundamentally different to that hitherto 

 supposed to have taken place. If A and B mean two successive 

 sklerotomes, a and 6 their dorsal, a and ft their respective ventral 



A-!-B 



halves, then the new skleromere is composed of b + & and not of k > 



"Z 



, , . . , , B dorsal . A ventral 



because o-f is the same as + 5 



2 2 



The formation of a skleromere by the combination of alternating 

 dorsal and ventral halves of sklerotomes explains also the presence of 

 eight (four pairs) cartilaginous pieces, namely, basalia (so-called 

 dorsal and ventral arches) and interbasalia (so-called intercalary 

 pieces) for each complete segment. 



The dorsal and ventral halves of the sklerotomes are pyramidal in 

 shape, with their apices pointing respectively downwards and up- 

 wards. Each ventral pyramid extends with its apex above the 

 chorda, and founds there (separated from the ventral mass by the 

 subsequent rapid growth of the chorda and its sheath) a cluster of 

 cells which remains henceforth behind (tailwards from) the basal 

 mass of the dorsal pyramid. The latter founds, with its down- 

 growing apex, a colony of cells below the chorda, and in front of the 

 basal ventral mass. Thus are produced the basalia and interbasalia, 



