788 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 



have called in our description of the adult the osseous roof of 

 the spinal canal. 



A comparison of the neural arch at this stage with the arch 

 in the adult, and in the stage last described, shews that the 

 greater part of the neural arch of the adult is formed of mem- 

 brane-bone, there being preformed in cartilage only a small basal 

 part, a dorsal process, and paired key-stones below the ligamen- 

 tum longitudinale superius. 



The haemal arches (Plate 42, fig. 78) are still largely carti- 

 laginous, and rest upon the sheath of the notochord. They are 

 invested by a thick layer of bone. The bony layer investing 

 the neural and hsemal arches is prolonged to form a continuous 

 investment round the vertebral portions of the notochord (Plate 

 42, fig. 78). This investment is at the sides prolonged outwards 

 into irregular processes (Plate 42, fig. 78), which form the com- 

 mencement of the outer part of the thick but cellular osseous 

 cylinder forming the middle part of the vertebral body. 



The intervertebral cartilages are much larger than in the 

 earlier stage (Plate 42, figs. 77 and 79), and it is by their growth 

 that the intervertebral constrictions of the notochord are pro- 

 duced. They have ceased to be continuous with the cartilage 

 of the arches, the intervening portion of the vertebral body 

 between the two being only formed of bone. They are not yet 

 divided into two masses to form the contiguous ends of adjacent 

 vertebrae. 



Externally, the part of each cartilage which will form the 

 hinder end of a vertebral body is covered by a tube of bone, 

 having the form of a truncated funnel, shewn in longitudinal 

 section in Plate 42, fig. 77, and in transverse section in Plate 42, 



fig- 79- 



At each end, the intervertebral cartilages are becoming 



penetrated and replaced by beautiful branched processes from 

 the homogeneous bone which was first of all formed in the peri- 

 chondrium (Plate 42, fig. 77). 



This constitutes the latest stage which we have had. 



Gegenbaur (No. 6) has described the vertebral column in 

 a somewhat older larva of 18 centims. 



The chief points in which the vertebral column of this larva 

 differed from ours are : (i) the disappearance of all trace of the 



