LATER LARVAL STAGES. 



W.i 



relation of these compartments to the segmentation of the body cannot 

 be made out. 



The anal aperture which originally lay at the posterior end of the 

 body shifts later further forward (Fig. 291), and this leads to the 

 development of the post-anal region of the body characteristic of 

 the Vertebrates. 



We have seen above (p. 545) that the anterior end of the medullary 

 tube, which lies in the region of the first true primitive segment and 

 the so-called cephalic process of the mesoderm (anterior primitive 

 segment of Hatschek) is widened (Figs. 288, 289). This part, 

 according to Hatschek's recent statements (No. 8), becomes dif- 

 ferentiated, in the young Amphioxus, into three consecutive sections 

 which correspond to the three primary cephalic vesicles of the Craniata. 



JT 



izr, 



tic. 297. 



Fig. 2'.'7.--.l. transverse .suctions through the brain of a young Amphioxus (after 

 Hatschek). /. through the (primary) first ventricle; //, through the (primary) 

 second ventricle [aquaeductus Sylvii) ; III, through the (primary) third ventricle 

 i fossa rhomboidalis). B, transverse sectiou through the spinal cord. 



Fig. 298. 



FlG. 298. — Brain with the most anterior nerve-roots of a young Amphioxus (after 

 HATSCHEK). Oh, chorda dorsalis; N, ciliated pit, to the posterior wall of which 

 the olfactory nerve runs ; /, //. ///, the three primary ventricles (diagrammatic). 



The anterior section, the primary fore-brain, shows, in transverse sec- 

 tion, the well-known dilation of the medullary tube which constitutes 

 the first primary ventricle (Figs. 297 A, /; 298 /). The anterior end 

 of this ventricle is continued towards the neuropore (i.e., towards the 

 ciliated olfactory pit) into the infundibulum (the lobus olfadorius of 

 Langerhans) which, in Amphioxus, is curved upward. The second 

 part of the brain (the mid-brain) contains within it the second 

 primary ventricle, winch is represented by a narrowed portion of the 



