MEDULLARY CANAL. 



33 



surface becomes more columnar to 

 form the medullary plate (Fig. 17). 

 As the medullary plate extends to the 

 hind end of the dorsal surface, the 

 blastopore is included in it. The 

 lateral part of the ectoderm now be- 

 comes detached from the medullary 

 plate, and grows over it (Fig. 17). 

 This overgrowth begins at the hind 

 end of the medullary plate, so that 

 the blastopore is covered over and 

 comes to open into the space between 

 the overgrown ectoderm and the 

 medullary plate (Fig. 18). I Later the 

 medullary plate curves over dorsally 

 (Fig. 17), and by the junction of its 

 two lateral edges forms the walls of the 

 medullary canal (Fig. 26). From what 

 has been said it is clear that the 

 medullary canal, which is gradually 

 developed from behind forwards, opens 

 posteriorly into the archenteron by 



the 

 the 



medullary canal becomes the central 

 canal of the nervous system, the 

 blastopore is henceforward known as 

 Hhe neur enteric canal. It closes soon 

 after the commencement of larval life. 

 The anterior neuropore persists 



throughout 



the blastopore and anteriorly to 

 exterior by the neuropore. As 







FIG. 15. Formation of the 

 gastrula of Amphioxus (from 

 Claus, after Hatschek). A blas- 

 tosphere ; B commencing in- 

 vagination of the lower surface 

 of the blastosphere to form 

 the endoderm ; C later gas- 

 trula ; all in optical section ; 

 anterior lip of blastopore. 

 The cilia of the ectoderm are 

 omitted. 



larval life and only 

 closes on the attainment of the 

 adult state. It marks the site of 

 the olfactory pit. 



Meanwhile two pairs of dorsb- 

 lateral outgrowths of the archen- 

 teron are formed (Figs. 17, 18). 

 The anterior of these retain their 

 communication with the archen- 

 hdt rf anTHeid i e a r) SChek teron for some 'time and give rise 



D 



