444 



last are the general fact, the expression of a constant law according to 

 which all the Vertebrata are developed. The vertebrae of adult fishes, on 

 the other hand, are solid rings, whose presence depends on the type pecu- 

 liar to each species ; consequently their form, and the substance of which 

 they are composed, vary in almost every species. 



" The vertebral divisions * appear very early in the Coregonus almost 

 at the same time as the notochord ; and when the dorsal groove begins to 

 close, they are fine lines, caused, as it would appear, by a greater accu- 

 mulation of embryonic cells, which, like transverse septa, traverse the 

 entire mass as far as the notochord. These divisions extend forwards, as 

 far as the neighbourhood of the auditory vesicles, but there never exists 

 the smallest trace of them in the head itself. At first they are visible 

 only in the middle of the body j by degrees they move forwards, as far as 

 close to the ear, and backwards, towards the tail, as far as it is formed ; but 

 they invade its extremity only when it has attained its full length relatively 

 to the body. At first these lines are all straight and perpendicular to the 

 axis of the chorda j but by degrees, and in proportion as development 

 advances, they become oblique and bend, forming an angle whose apex is 

 directed forwards, and corresponds exactly to the median line of the noto- 

 chord." 



They eventually become the mtermuscular septa. Vogt goes on to 

 say, "The typical structure of the Vertebrata^ then, consists solely in 

 these rings of separation, which are formed around a notochord, and no- 

 wise in the development of a distinct head, or of other solid pieces of 

 the skeleton, such as osseous or cartilaginous vertebrae," illustrating his 

 case by the Amphioxus. 



Each osseous vertebra corresponds to two vertebral segments, namely, 

 to the half of that which precedes, and to the half of that which follows a 

 metasomatome ; for it is where the latter reaches the notochord, that the 

 centra and arches take their origin. The centra arise as a double ring of 

 cartilage, internal and external to the sheath of the notochord. The 

 inter-vertebral spaces always correspond with the middle of the interval 

 between two intermuscular septa, each of which is consequently inserted 

 into the middle of a centrum, while the superior and inferior arches are 

 developed in their plane. They are ossified only long after the centra, 

 which arise as bony rings around the notochord. The intervertebral liga- 

 ments are formed from the sheath of the notochord. 



4. Prof. Owen (Principes d'Oste"ologie Comparee, 1855, p. 184) affirms 

 that " In osseous fishes the centrum is ordinarily ossified from six points, 

 of which four begin in the bases of the two neurapophyses and of the two 

 parapophyses, but the terminal concave plates of the centrum are ossified 



It is not stated on what fish the observations on which this latter asser- 

 tion is based were made. Prof. Williamson has already shown ('' On the 

 Development of the Scales and Bones of Fishes," Phil. Trans. 1851) that 

 it is inconsistent with the structure of the adult vertebra ; it is not sup- 

 ported by any of those writers who have directly observed the develop- 

 * Metasomatomes, their interspaces being the somatomes. 



