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). 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 blastopore and anteriorly to the 



exterior by the neuropore. As the 



medullary canal becomes the central 



canal of the nervous system, the 



blastopore is henceforward known as 



the neur enteric canal. It closes soon 



after the commencement of larval life. 



The anterior neuropore persists 



throughout larval life and only 

 closes on the attainment of the 

 adult state. It marks the site of 

 the olfactory pit. 



Meanwhile two pairs of dorso- 

 lateral outgrowths of the archen- 

 teron are formed (Figs. 17, 18). 

 The anterior of these ]-etain their 

 ^"with'„Tr^roTbLto1orf:?d'tt- Communication with the archen- 

 fr^m Sche'lfrnrS^^^^ teron for some time and give rise 



Z— II D 



Fia 



15. — Formation of tlie 

 gastrula of Ampkioxus (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 ; 

 O anterior lip of blastopore. 

 The cilia of the ectoderm are 

 omitted. 



