782 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 



at first sight appear to have undergone any changes. Viewed 

 from the side, however, in dissected specimens, they are seen to 

 be prolonged upwards so as to unite above with bars of cartilage 

 directed obliquely backwards. An explanation of this appear- 

 ance is easily found in the sections. The cartilaginous neural 

 arches are invested by a delicate layer of homogeneous bone, 

 developed in the perichondrium, and this bone is prolonged 

 beyond the cartilage and joins a similar osseous investment of 

 the dorsal bars above mentioned. The whole of these parts 

 may, it appears to us, be certainly reckoned as parts of the 

 neural arches, so that at this stage each neural arch consists of: 

 (i) a pair of basal portions resting on the notochord consisting 

 of cartilage invested by bone, (2) of a pair of dorsal cartilaginous 

 bars invested in bone (n.a '.), and (3) of osseous bars connecting 

 (i) and (2). 



Though, in the absence of the immediately preceding stages, 

 it is not perfectly certain that the dorsal pieces of cartilage are 

 developed independently of the ventral, there appears to us every 

 probability that this is so ; and thus the cartilage of each neural 

 arch is developed discontinuously, while the permanent bony 

 neural arch, which commences as a deposit of bone partly in the 

 perichondrium and partly in the intervening membrane, forms a 

 continuous structure. 



Analogous occurrences have been described by Gotte in 

 Teleostei. 



The .dorsal portion of each neural arch becomes what we 

 have called the dorsal process of the adult arch. 



Between the dorsal processes of the two sides there is placed 

 a median rod of cartilage (Plate 41, fig. 70, i. s.), which in its 

 development is wholly independent of the true neural arches, 

 and which constitutes the median spinous element of the adult. 

 In tracing these backwards it becomes obvious that they are 

 homologous with the interspinous elements supporting the dorsal 

 fin, in that they are replaced by these interspinous elements in 

 the region of the dorsal fin, and that the interspinous bones 

 occupy the same position as the median spinous processes. 

 This homology was first pointed out by Gotte in the case of the 

 Teleostei. 



Immediately beneath this rod is placed the longitudinal 



