786 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 



time we think that we have adduced weighty evidence in corrobo- 

 ration of Gegenbaur's view. 



As shewn in longitudinal section (Plate 41, fig. 69, iv.r.}, the 

 intervertebral rings are thicker in the middle than at the two 

 ends. In this thickened middle part the division of the cartilage 

 into two parts to form the ends of two contiguous vertebrae is 

 subsequently effected. The curved line which this segmentation 

 will follow is, however, already marked out, and from surface 

 views it might be supposed that this division had actually 

 occurred. 



The histological structure of the intervertebral cartilage is 

 very distinct from that of the cartilage of the bases of the 

 arches, the nuclei being much more closely packed. In parts, 

 indeed, the intervertebral cartilage has almost the character of 

 fibre-cartilage. On each side of the line of division separating 

 two vertebrae it is invested by a superficial osseous deposit. 



The next oldest larva we have had was 1 1 centims. in length. 

 The filamentous dorsal lobe of the caudal fin still projected far 

 beyond the permanent caudal fin (Plate 34, fig. 16). 



The vertebral column was considerably less advanced in deve- 

 lopment than that dissected by Gegenbaur, though it shews a 

 great advance on the previous stage. Its features are illustrated 

 by two transverse sections, one through the median plane of a 

 vertebral region (Plate 42, fig. 78) and the other through that of 

 an intervertebral region (Plate 42, fig. 79), and by a horizontal 

 section (Plate 42, fig. 77). 



In the last stage the notochord was only constricted verte- 

 brally. Now, however, by the great growth of intervertebral 

 cartilage there have appeared (Plate 42, fig. 77) very well- 

 marked intervertebral constrictions, by the completion of which the 

 vertebrae of Lcpidosteus acquire their unique character amongst 

 Fishes. 



These constrictions still, however, coexist with the earlier, 

 though at this stage relatively less conspicuous, vertebral con- 

 strictions. 



The gelatinous body of the notochord retains its earlier 

 condition. The sheath has, however, undergone some changes. 

 In the vertebral regions there is present in any section of the 

 sheath (i) externally, the membrana elastica externa (in.el.) ; 



