VERTEBRATA: GENERAL CHARACTERS. 



445 



development is going on the "germinal area" becomes 

 elevated into two parallel ridges, one on each side of the 

 middle line, enclosing between them a long groove, which is 

 known as the " primitive groove " (fig. 239, A, B). The ridges 

 which bound the primitive groove are known as the " laminae 

 dorsales ; " and they become more and more raised up, till they 

 ultimately meet in the middle line, and unite to form a tube, 

 within which the cerebro-spinal nervous centres are developed. 

 It follows from its mode of formation that the inner wall of the 

 tube formed by the primitive groove, which remains as the 

 septum between the cerebro-spinal canal and the body-cavity, 

 is nothing more than a portion of the primitive wall of the 

 body of the embryo. And there appears to be little doubt, as 

 believed by Remak and Huxley, that the cerebro-spinal nervous 

 centres are "the result of a modification of that serous layer 

 of the germ, which is continuous elsewhere with the epidermis " 

 (Huxley). 



Another remarkable peculiarity as regards the nervous sys- 

 tem is found in the fact that in no Vertebrate animal does the 



I) 



Fig. 239. Embryology of Vertebrata. A, Portion of the germinal area of the ovum 

 of a Bitch, showing the primitive groove (after Bischoff). B, Profile view of the 

 same. C, Diagram representing the amnion and allantois : e Embryo ; a Am- 

 nion ; u Umbilical vesicle ; b Allantois ; f Pedicle of the allantois, afterwards the 

 urinary bladder. D, Head of an embryo, showing the visceral arches (v v). 



alimentary canal pierce the main masses of the nervous system, 

 but turns away to open on the opposite side of the body. In 

 most Invertebrates, on the other hand, in which there is a 

 well-developed nervous system, this is perforated by the gullet, 

 so that an cesophageal nerve-collar is formed, and some of the 



