MEDULLARY CANAL. 



83 



surface becomes more columnar to 

 form the medullary plate (Fig. 17). 

 As the medullaiy 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 

 tlie 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 tlie 

 medullary canal, which is gradually 

 developed from ])ehind forwards, opens 

 posteriorly into the archenteron by 

 the blastopore and anteriorly to the 

 exterior by the neuropore. As the 

 medullaiy 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 



Via 



15. — Fonnatiou of the 

 <iastriila of Amphioxus (from 

 Claus, after Hatschek). A blas- 

 tosphere ; B commencing in- 

 vagination of tlie 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. 



throughout larval life and only 

 closes on the attamment 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 

 ^'witi^nTr^rot^SlLtolor^Zdtt! Communication with the archen- 

 fr^rnVSchetrn^Serf '"' teron for some time and give rise 



Z — II D 



