144 



COMPARATIVE MORPHOLOGY OF VERTEBRATES 



with a neural groove between them. The folding process continues 

 until the folds meet above and the plate is rolled into a tube, the 

 edges of the folds fusing, and the tube being then cut off from the rest 

 of the ectoderm (fig. 153, ^4). The thinner portion of the primitive 

 groove becomes the floor plate of the tube, while a similar roof plate 

 is formed dorsally, where the neural folds met. The sides of the tube 



Fig. 152. — Section of embryo sea-bass (Serranus) after H. V. Wilson, c, notochord; 

 ec, ectoderm, developing in the middle line a distinct keel, n, forming a solid cord in 

 which a lumen appears later; en, entoderm; m, mesothelium. 



are several cells thick, and the lumen of the tube is called the central 

 canal. From the method of its formation it will be understood that 

 the inner surface of the tube is homologous with the outer surface of 

 the general epidermis of the body, a point of importance in connexion 

 with the development of the eye. 



Fig. 153. — A, diagram of early spinal cord; B, later, showing increase in size and 

 consequent ventral fissure, c, central canal; e, ectoderm; /, floor plate; g, anlage of 

 spinal ganglion; nc, neural crest; r, roof plate; s, sulcus limitans (sulcus of Monro); 

 V, ventral fissure. 



This general account of the formation of the neural tube will not exactly 

 apply to cyclostomes, teleosts and some ganoids. In these forms the neural 

 plate thickens into a deep keel (fig. 152) dipping deep into the embrj'O. Later 

 a central canal is formed in the keel, so that the final result is the same as that 

 described above. 



Spinal Cord 



From this simple tube, the brain and the spinal cord are differen- 

 tiated. After the separation of the tube from the rest of the ecto- 



