44 



THE ORIGIN OF VERTEBRATES 



The interpretation of this stage is that in the invertebrate ancestor 

 the nerve-masses were situated laterally and ventrally to the 

 epithelial tube, and were connected together by commissures on the 

 ventral side of the tube (Fig. 21, A (1)) ; in other words, the chain of 

 ventral ganglia and their transverse commissures lying just ventrally 

 to the intestine, which are so characteristic of the arthropod nervous 

 system, is represented at this stage. 



Subsequently, by the growth dorsalwards of nervous material to 

 form the posterior columns, the original epithelial tube is compressed 

 dorsally and laterally to such an extent that those parts lose all signs 

 of lumen, the one becoming the posterior fissure and the others the 



3 J 



2 



Fig. 21. — A, Method of Formation of the Vertebrate Spinal Cord from the 

 Ventral Chain of Ganglia and the Intestine of an Arthropod, repre- 

 sented in 1 ; B, Method of Formation of the Vertebrate Medulla 

 Oblongata from the Infra-ossophageal Ganglia and the Cephalic 

 Stomach of an Arthropod. 



substantia gelatinosa Rolandi on each side. The original tube is thus 

 reduced to a small canal formed by its ventral portion only (Fig. 21, 

 A (3)). In this way the spinal cord is formed, and the walls of the 

 original epithelial tube are finally visible only as the lining of the 

 central canal (Fig. 21, A (4)). 



When we pass to the brain-region, to the anterior dilated 

 portion of the tube, embryology tells a different story. Here, as in 

 the spinal cord, the nervous masses are grouped at first laterally and 

 ventrally to the epithelial tube, as is seen in Fig. 21, B (2), but owing 

 to the large size of its lumen here, the nervous material is not 

 able to enclose it completely, as in the case of the spinal cord ; 



