I9& 



(permanent) vertebrae in the middle between the original intervals. 

 A secondary (permanent) vertebra consists consequently ot' the candal 

 arid cranial halves of two adjoining primitive vertebrae t'nsed together. 

 Accoi'ding to Remak there was in the development of the vertebral 

 column one moment, in which tlie blastema, from which tlie vertebrae 

 will originate, is entirelj' unsegmented. For a considerable time 

 Rkmak's theory about the "re-segmentation of the vertebral column" 

 has not been recognised by many anatomists. Recent investigations 

 however have done justice to him. Especially the investigations of 

 V. Ebner have turned the scale here, and in the first place the 

 discovery of the so-called intervertebral-fissure. 



On the frontal section through an embryo (cf. fig. J) one sees 

 on either side of tiie chorda the bodies of the primordial vertebrae. 



Fig. 1. 



Frontal section tliiougli an cmljiyo of Tropidonotus na!i-ix (after v. IlIuner). 



cit = chorda dorsalis ; I.s. = intervertebral fissure ; 



(/.is. = arleria interprotovertebi-alis ; m.c. = myocoel. 



At a certain stage of the de\eioj)ment one sees occur in it the 



differentiation that causes the formation of the [)rodncts that are 



derived from it. 



The primordial vertebra, in which the [)rimordial vcrtebralcavity 

 is situated, shows a medial and a lateral lamella. The lateral lamella 

 is the cutislamella, from which the derm with adnexa takes its 

 origin; the medial one is the muscle-lamella from which the muscu- 

 lature develops itself. Moreover originates from this medial lamella 

 of the primordial segment the blastema (mesenchym) from which 



