STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 779 



wide-meshed reticulum. Surrounding the notochord is the 

 usual cuticular sheath, which is still thin. 



The first indications of the future vertebral column are to be 

 found in the formation of a distinct mesoblastic investment of 

 the notochord. On the dorsal aspect of the notochord, the 

 mesoblast forms two ridges, one on each side, which are pro- 

 longed upwards so as to meet above the neural canal, for which 

 they form a kind of sheath. On the ventral side of the noto- 

 chord there are also two ridges, which are, however, except on 

 the tail, much less prominent than the dorsal ridges. 



The changes which next ensue are practically identical with 

 those which take place in Teleostei. Around the cuticular 

 sheath of the notochord there is formed an elastic membrane 

 the membrana elastica externa. At the same time the basal 

 parts of the dorsal, or as we may perhaps more conveniently call 

 them, the neural ridges of the notochord become enlarged at 

 each intermuscular septum, and the tissue of these enlargements 

 soon becomes converted into cartilage, thus forming a series of 

 independent paired neural processes riding on the membrana 

 elastica externa surrounding the notochord, and extending about 

 two-thirds of the way up the sides of the medullary cord. They 

 are shewn in transverse section in Plate 41, fig. 67 (n.a.), and in 

 a side view in fig. 68 (n.a.}. 



Simultaneously with the neural arches, the haemal arches 

 also become established, and arise by the formation of similar 

 enlargements of the ventral or haemal ridges. In the trunk they 

 are very small, but in the region of the tail their condition is 

 very different. At the front end of the anal fin the paired 

 haemal arches suddenly enlarge and extend ventralwards (Plate 

 41, fig. 67, h.a.}. 



Each succeeding pair of arches becomes larger than the one 

 in front, and the two elements of each arch first nearly meet 

 below the caudal vein (Plate 41, fig. 67) and finally actually do 

 so, forming in this way a completely closed haemal canal. At 

 the point where they first meet the permanent caudal fin com- 

 mences, and here (Plate 41, fig. 68) we find that not only do the. 

 haemal arches meet and coalesce below the caudal vein, but they 

 are actually produced into long spines supporting the fin-rays of 

 the caudal fin, which thus differs from the other fins in being 



502 



