98 DEVELOPMENT OF ORGANS. 



nus paloea) was found by Vogt* to pursue an almost similar course to 

 that observed in the toad and triton. Very large cells are early observed 

 in the position occupied by the chorda; these shortly disappear, leaving a 

 transparent, faintly granular substance, from which the subsequent cells 

 of the chorda appear to be produced. 



Vertebral column. The nature of the changes undergone by the fibrous 

 sheath of the chorda dorsalis of Alytes obstetricans, by which the develop- 

 ment of the bodies of the vertebrae is effected, has also been minutely inves- 

 tigated by Vogt. In the first " laying down " of the individual segments 

 of the vertebral column, the sheath of the chorda dorsalis can exercise no 

 share, for, at the period at which this occurs, the sheath is not yet formed 

 from that part of the substance of the chorda to which it owes its origin. 

 From the time of its first appearance, however, the sheath becomes so 

 intimately connected with the mass of cell-substance surrounding it, that it 

 can at no time be completely separated therefrom. The first trace of soli- 

 dification of the divisions of the vertebral column is observed immediately 

 external to the sheath of the chorda : and presents itself in the form of car- 

 tilaginous rings adherent internally to the sheath, and externally to the sur- 

 rounding cell- substance, which gradually assumes the characters of muscular 

 tissue. At first, the lines of separation between the muscular, cartilaginous 

 and fibrous layers of which the vertebral system is at this time composed, 

 are very obscure; but, shortly, the distinction between the two former be- 

 comes manifest, while, between the cartilaginous layer and the fibrous sheath 

 of the chorda, no line of demarcation can ever be perceived even with the 

 aid of the microscope. Indeed these two latter tissues merge insensibly 

 one into the other, so that in fragments of the sheath examined beneath 

 the microscope, cartilage-cells may be observed scattered throughout its 

 fibrous structure. Eventually this fibrous structure entirely disappears, 

 and the whole sheath becomes cartilaginous ; being thus converted, by 

 change of tissue, into the bodies of the vertebrae. This account is in 

 accordance with the observation made by Professor Miiller,f that in some 

 of the frog-like Amphibia and the Salamandrinse, the ossification of the 

 vertebrae takes place in the sheath of the cord. Coincident with the con- 

 version of the sheath into cartilage, and the encroachment of this on the 

 substance of the chorda, the cells of this substance gradually disappear and 

 eventually are found only in the spaces between the bodies of the ver- 

 tebrae. They are never directly converted into cartilage-cells. 



Towards the end of the embryonic period of life cartilaginous rings 

 begin to be developed also around the central parts of the nervous system 

 lying in the axis of the embryo. These rings, which become the vertebral 

 arches, result from the transformation of the internal portions of the two 

 oval masses situated one on each side of the groove or canal containing 



* Histoire Naturelle des Poissons d'eau douce, par M. Agassiz, t. i. p. .08 ; and Entwic- 

 kelungs-geschichtc der Geburtshelferkrotc, p. 47. t Physiology, p. 1613. 



