594 ON THE THEORY OF THE VERTEBRATE SKULL 



column. These take the opposite direction to the preceding, tend to enclose the 

 great caudal vessels, and unite in pairs into arches, which lie in a series and 

 correspond with the vertebral segments.'' 



From the segments of that part of the vertebral column which lies between the 

 tail and the head, there grow out, in corresponding places to those in which the 

 crura of the inferior arches take their origin from the vertebral segments of the 

 tail, and in the same manner and at the same time, many cartilaginous processes, 

 which attain, however, only a very slight length, and also take a transverse 

 direction. They might be regarded as lateral pieces of the transverse processes of 

 the higher animals ; but it is more probable that they correspond with the ribs of 

 other Vertebrata. 



"All these processes are connected with the sheath, but not with the core of the 

 notochord." 



As development advances, the ring-like segments increase in breadth, length 

 and I thickness ; at the same time they become somewhat cartilaginous, and 

 then ossify. Each widens somewhat more at its ends than in the middle, and 

 so appears a httle contracted in the centre. It is only after birth that such 

 an internal thickening takes places as to interrupt the cavity of the vertebral 

 centrum. 



The sheath of the notochord is originally of one texture throughout, but the 

 smaller portions, which lie between the vertebral centra, assume a fibrous te.xture, 

 contemporaneously with the appearance of the latter. The included substance of 

 the notochord loses its peculiar dense and elastic character, becomes first gelatinous, 

 then grumous, and finally resembles a thick serum. 



The crura of the upper and lower vertebral arches (in the tail) unite in pairs, 

 and their points of union grow out into spinous processes. 



The ossification of the processes which arise from the vertebree commences 

 at the point of junction of the process with the centrum. " A small bony point 

 arises, which appears to belong to both centrum and process, and from whence 

 ossification e.xtends into both. In each vertebral centrum, therefore, as well 

 of the tail as of the trunk, ossification proceeds from different and distant points." 



2. Cyprinus blicca (Von Bar, ' Untersuchungen iiber die Entw. d. Fische.' 



1835)- 



" At the end of the first day the notochord is covered by something which 

 surrounds it like thin plates ; these are the developing bodies of vertebree. It is 

 clearly observable that these bodies of vertebras are not undivided rings surround- 

 ing the notochord, but that they consist of many pieces united by sutures. This 

 condition also is persistent in the sturgeons. The body of the vertebrse, therefore, 

 is formed of the coalescence of many pieces, and a lateral suture seems to indicate 

 that these processes are elongations of the previously observed upper and under 

 vertebral arches." 



Von Bar imagines that the unconstricted part of the notochord gives rise to the 

 intervertebral ligaments. 



From these observations of Rathke and Von Bar, it would appear as if the 

 annular ossifications which surround the notochord arose by the coalescence of 

 ossific centres, primarily developed at the junction of the apophyses with the 

 centra. My own observations on Gasterosteus, however, show, like those of Vogt 

 on Coregoniis, that the centra ossify from distinct rings deposited immediately 

 round the notochord ; and I am very strongly inclined to believe that the corre- 

 sponding primary annular diaphyses of the vertebras in Cyprinus and Blennius 

 have been overlooked. 



3. Coregonus palea (Vogt, ' Embryologie des Saumones,' 1842, p. 104, et seq.). — 



