GENERAL CHARACTERS OF THE VERTEBRATA. 371 



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 



Fig. 153. 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 : <? Embryo ; a Am- 

 nion ; u Umbilical vesicle ; ^Allantoic ; .X Pedicle of the allantois, afterwards the 

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



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 

 nervous centres become prae-oesophageal, whilst others are 

 post-cesophageal. 



Furthermore, the floor of the " primitive groove " in the 

 embryo of all Vertebrates has developed in it at an early 

 period the structure known as the "notochord" or "chorda 

 dorsalis" (fig. 152, B, ch). This structure, doubtfully present 

 in any Invertebrate, is a semi-gelatinous or cartilaginous col- 



