502 



COMPARATIVE AN ATOM Y. 



tube. This gradually sinks away from the surface of the body, 

 owing to the growth over it of the epiblast, and of parts differen- 

 tiated from the mesoderm. The medullary tube, which is formed 

 in this way, remains as a simple chord in Amphioxus, the most an- 

 terior region of which contains an en- 



O 



largement of its central canal. But in 

 the Craniota, diverticula appear in the 

 most anterior portion before it is com- 

 pletely closed (Fig. 279, a); these form 

 the rudiments of the brain, while the re- 

 mainder of the medullary tube is equally 

 differentiated throughout, and forms the 

 rudiment of the spinal chord. 



In addition to their position, which 

 is always an important point in com- 

 parison, the rudiments of the nervous 

 centre of the Vertebrata have, in common 

 with those of various Invertebrata, certain 

 relations to the higher sensory organs 

 (and especially to the optic organ) ; and 

 in this respect the Tunicata are those 

 which exhibit the closest affinities. In 

 them, as in the Vertebrata, the whole of 

 the medullary tube is not closed in the 

 same way; in the cerebral region, it is 

 in connection with the exterior for a long 

 time. In comparing the nervous system 

 of Vertebrates with that of the Tunicata 

 the chord, which is continued along the 

 dorsal region, and on to the tail in the 

 larvaB of the Ascidias, and in the Appen- 

 dicularia (§ 305), is of importance ; this, 

 which is distinguished by its ganglia, 

 appears to indicate the path by which the hinder portion of tho 

 central nervous system of the Vertebrata was phylogenetically 

 developed, and gradually converted into the spinal chord. As there 

 is such a great difference between the true central organs and this 

 chord, that it is impossible to regard the chord as a true continuation 

 of the central organ, or even as a portion of it, which merely 

 differs in consequence of its position, it must be supposed that the 

 brain, or the most anterior portion of the medullary chord in 

 Amphioxus, represents the more primitive portion of the nerve- 

 centre ; while it is also the first to appear in the embryo. The 

 similarity between the rudimentary spinal chord and brain would 

 then be merely a condition, which had been acquired by the Verte- 

 brata, and which had its starting-point, phylogenetically, from 

 a chord which was continued on from tho primary nerve-centre, 

 such as we now meet with in the Tunicata. According to this 

 view the whole medullary tube is not derived, phylogenetically, 



Fig. 279. Embryo of the Dog 

 seen from behind, with the rudi- 

 ments of the central nervous 

 system, of which the medullary 

 plate (b) forms a groove open 

 superiorly. a The rudiments 

 of the three primitive cerebral 

 vesicles, a' Sinus rhomboidalis 

 in the lumbar region, c Lateral 

 plates, which bound the rudi- 

 mentary body, d Epiblast and 

 mcsoblast. / Hypoblast (after 

 Bischoff). 



