DEVELOPMENT OF THE BEAIN. 219 



quite internally, the upper edges of the primitive vertebral 

 plates, which penetrate, from right and left, in between the 

 horn-plate and the medullary tube, uniting above the latter, 

 and thus completely embedding it in a closed canal. As 

 Gegenbaur most aptly remarks, " This gradual embedding 

 in the interior of the body must be regarded as an incident 

 acquired in connection with progressive differentiation, and 

 with the consequent higher capacity, by which the most 

 important organ of the system is secured in its interior." 



To every thoughtful and unprejudiced man it must 

 appear an extremely important and pregnant fact, that our 

 mental organ, like that of all other Skulled Animals (Cra- 

 niota), commences in the same way and in exactly the same 

 simple form in which this organ remains for life in the 

 lowest Vertebrate, the Amphioxus (vol. i. p. 420, Fig. 151; 

 Plate XL Fig. 15, m). In the Cyclostomi, that is, in the stage 

 above the Acrania, the anterior extremity of the cylindrical 

 medullary tube begins to extend, at an early period, in the 

 form of a pear-shaped bladder, which is the first distinct 

 rudiment of a brain (Plate XI. Fig. 16, m^. For the central 

 medulla of Vertebrates thus first distinctly differentiates 

 into its two main sections, the brain (m x ) and the spinal 

 marrow (m 2 ). The first faint indication of this important 

 differentiation is discoverable in the Amphioxus, perhaps 

 even in the Ascidian larva (Plate X. Fig. 5). 



The simple bladder-like form of the brain, which is 

 retained for a considerable time in the Cyclostomi, also 

 appears at first in all higher Vertebrates (Fig. 221, hb). In 

 the latter, however, it soon disappears, in consequence of 

 the separation of the simple brain-bladder, by transverse 

 contractions of its circumference, into several consecutive 



