92 



ANATOMY OF AMPHIOXUS. 



the central nervous system of the higher Vertebrates, a 

 stage is passed through which may be compared broadly 

 with the permanent condition of things in Amphioxus. 

 But in the former the anterior portion of the medullary 

 tube quickly becomes greatly enlarged in contrast to the 

 spinal cord proper, and becomes divided by constrictions 

 into fore-, mid-, and hind-brain, which constitute the three 

 primary divisions of the Vertebrate brain. Then the 

 brain undergoes a flexure round the anterior end of the 

 notochord. This curvature of the primitively horizontal 

 brain-region in the craniate Vertebrates is known as the 

 cranial flexure. (Cf. Figs. 23 and 24.) 



Among the numerous longitudinal nerve-fibres which 

 compose the bulk of the spinal cord of Amphioxus, there 



are some which stand out in 

 marked contrast to the great 

 -G majority on account of their 

 large size. These are the so- 

 called ^/(rw/'-yfi'^/rj, and they form 

 one of the greatest peculiarities 

 in the spinal cord of Amphioxus. 

 According to Rohde there are 

 no fewer than twenty-six of these 

 giant-fibres present, and each of 



Fig. 47. — Transverse section 



tiirough spinal cord in region them arises from a correspond- 



of giant ganglion-cell G. (After :„ - , ;- 7, ^^^^ 



ROHDE.) "^8 S"-™^ ganglion-ccll. These 



a. Process of giant-cell A. g.f. so-called giant-cells have many 



Giant-fibres. 



processes, i.e. they are multi- 

 polar, but they each send out one main stem, which is a 

 giant-fibre. The giant-cells lie across the middle of the 

 central canal, and the giant-fibres pass outwards alter- 

 nately to the right or left of the central canal, and then 

 bend downwards and pass below the central canal and up 



