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 Iaterally to such an extent that those parts lose all signs 
of lumen, the one becoming the posterior fissure and the others the 
Fic. 21.—A, MretHop or FoRMATION OF THE VERTEBRATE SPINAL CORD FROM THE 
VENTRAL CHAIN OF GANGLIA AND THE INTESTINE OF AN ARTHROPOD, REPRE- 
SENTED IN 1; B, MrtHop or ForMATION OF THE VERTEBRATE MEDULLA 
OBLONGATA FROM THE INFRA-GSSOPHAGEAL 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; 
