30 Mr. H. G. Seeley on the Origin 



In the median lateral line between the great lateral muscles 

 slight transverse processes are sometimes developed; and these 

 may be upon the centrum, or upon the neural arch, or upon 

 the heemal arch, according to the arrangement of the muscles. 

 But the point is one of detail, and not a fundamental part of 

 the vertebrate common plan. As the caudal vertebrge progress 

 forward towards the head, they encounter the viscera on the 

 haemal side ; and then the hsemal arch widens and embraces 

 the viscera, so that the parts called luemapophyses, which in 

 the tail are directed downward, come in the thorax to be lifted 

 up the side of the centrum and directed outward, sometimes 

 attached to the median lateral osseous process, and often con- 

 nate with it. When the viscera extend to a great length 

 down the body, the lateral transverse processes are not deve- 

 loped as distinct processes ; when the viscera have a short 

 extension and the tail is long, they are considerably developed, 

 and then pass forward as epipliyses upon the visceral region, 

 being developed at the point of junction of the heemapophysis 

 .with the part of the centrum which supports it. In this form 

 the hgemal arch is called a rib. And as the arch widens, new 

 elements come to be introduced into the circle which it consti- 

 tutes — formed toward the ventral surface by the increased 

 expansion given to the venti-al strips of muscles, which often 

 become blended with the lower lateral strips. 



In this way I conceive the vertebrate common plan to have 

 been elaborated, so far as its osteological structures are con- 

 cerned, by the mechanical machinery with which it is inevi- 

 tably accompanied. And if so, it will be evident that all 

 subsequent variations it may assume in form will be due to a 

 different distribution of the muscular machinery resulting 

 from kinetic growth, while the different proportions of the 

 different regions of the column will be due to potential growth. 



In first conceiving of a vertebrate I introduced two ideas — 

 the tail and its product, the head. In obtaining a similar 

 generalized idea of the head to that just given of the body, it 

 may be as well to remark : — that the extension forward of the 

 vertebree will have maintained the spinal cord of approxi- 

 mately uniform size up to the point where, like the constricted 

 neck of a bottle, it abuts against the enlarged terminal part ; 

 and that the transition in the dorsal region from neural 

 matter covered by a vertebra to the brain covered by the 

 skuU is not dissimilar in kind to the transition seen on the 

 haemal surface, where the tail suddenly expands and covers 

 the viscera, only with this difference — that while the brain ex- 

 periences but very slight fluctuations in size, the viscera are 



