Il8 DEVELOPMENT OF AMPHIOXUS. 



right side is the first gill-sht, opening directly to the 

 exterior. At the time of its actual perforation it lies 

 near the mid-ventral line of the body, but as it increases 

 in size it becomes shifted up to the right side. 



The neurenteric canal is closed up, and the nerve-tube 

 ends blindly behind, being curved round the hinder end of 

 the notochord. Immediately in front of and below the 

 point where the neurenteric canal formerly existed, the 

 amis has now made its appearance, approximately, if not 

 exactly, in the mid-ventral line * (Fig. 64). 



We will now return to consider more closely the exact 

 development of the mesodermic somites, the notochord, 

 and the nerve-cord. 



Development of Central Nervous System. 



As in the craniate Vertebrates, so in Amphioxus the 

 medullary plate arises as a median unpaired longitudinal 

 specialised portion of the dorsal ectoderm. The way in 

 which it becomes separated from the superficial ectoderm 

 has already been indicated above, but it can best be 

 studied in transverse sections. 



In the sections shown in Figs. 65 and 66, the separation 

 of the medullary plate from the ectoderm, and its subse- 

 quent conversion into a closed tube, is so clearly illus- 

 trated, that further description is unnecessary. A unique 

 feature in connexion with the formation of the central 

 nervous system of Amphioxus is, that the medullary plate 

 sinks below and becomes covered over by the superficial 

 ectoderm before it takes on the form of a closed tube, so 

 that for some time it exists as a half-canal open dorsally 



* According to Hatschek, the anus breaks through shghtly to the left of 

 the middle line. 



