598 EMBRYOLOGY. 



that a complete fusion takes place, and that cartilaginous neural 

 spines are formed. The part of the membrane which lies between 

 the carEilaginous arches furnishes the ligamentous apparatus. 



In the process of chondrification the nascent bodies of the vertebrae 

 have a fixed position relative to the primitive or muscle-segments ; 

 it is such that on either side of the body they are adjacent to two 

 of the latter, one half to a preceding segment, the other half to a 

 following one ; or, in other words, the bodies of the vertebrce and the 

 muscle-segments do not coincide, hut in their positions alternate with 

 each other. 



The necessity of such an arrangement follows from the very 

 function which vertebral column and musculature together have to 

 fulfil. The axial skeleton must possess two opposite properties 

 united : it must be firm, but also flexible, — firm, in order to serve as 

 a support for the trunk ; flexible, so as not to impede the motions of 

 the latter. Since a continuous cartilaginous rod would not have 

 possessed sufficient flexibOity, the process of chondrification could not 

 take place throughout the whole extent of the skeletogenous layer, 

 but there must be left more elastic tracts, which allow a movement 

 of the cartilaginous pieces on one another. But a movement of the 

 cartilaginous pieces would obviously be impossible if they should lie 

 so that the muscle fibres had their origin and insertion on one and 

 the same vertebral element. In order that the fibres of a muscle- 

 segment may operate upon two vertebrae, the muscular and vertebral 

 segments must alternate in position. 



This process, which is easily intelligible in the way in which it has 

 been outlined, has given occasion for the assumption of a " reseg- 

 mentation of the vertebral column." This conception originated with 

 Remak, and since then has been for a long time tenaciously held to 

 in the literature. 



Remak, like other embryologists before him (Baer), perceived in 

 the primitive segments of the Chick the material for the establishment 

 of the vertebral column, and therefore gave them the name " proto- 

 vertebrsB." But inasmuch as he found that the cartilaginous vertebrae 

 did not afterwards correspond in position with the protovertebrae, he 

 announced the proposition that a new " segmentation of the vertebral 

 column takes place, from which arise the secondary, permanent bodies 

 of the vertebrae." 



Both the name " protovertebra " and the assumption of a reseg- 

 mentation of the vertebral column should be dropped, and for the 

 following reasons : — 



