540 



CEPHALOCHORDA. 



depression leads to the development of the medullary tube. The 

 latter is here formed, not, as in the Ascidia and many Vertebrates, 



through the fusion in the middle line of 

 two lateral medullary folds, the process, 

 although it may be deduced from the 

 above type, being somewhat modified. 

 It might be described as lateral over- 

 growth. The medullary plate (Fig. 282 

 A, hi))) sinks down somewhat and its 

 lateral edges become detached from the 

 rest of the ectoderm. The ectoderm (lib) 

 now grows inward from either side above 

 the medullary plate and unites in the 

 middle line before the plate has become 

 curved into a tube (Fig. 282 B). "The 

 dorsal groove, although completely 

 covered externally, is still open within, 

 under the integument " (Fig. 283). Only 

 later does the medullary plate curve round 

 dorsally, and, through the fusion of its 

 lateral edges, form a closed tube, the 

 medullary tube (Fig. 284). 

 The union of the lateral ectodermal growth above the medullary 

 plate takes place from behind forward, commencing near the remains 



-TOt' 



nip 



Fig. 281. — Embryo of Amphi- 

 oarus with the rudiment of 

 two primitive segments (after 

 Hatschek). mp, pole-cells 

 of the mesoderm ; //*/-, medul- 

 lary groove ; m/' medullary 

 tube ; its', first primitive seg- 

 ment ; «*■", second primitive 

 segment. 



A 



B 



Fig. 282. — A, transverse section through an embryo of Amphioxus with the rudiment 

 oi the first primitive segment (after Hatschek, from 0. Hertwig's Text-book). /I, 

 transverse section through an embryo of Amphioxus, with the rudiments of five 

 primitive segments (after Hatschek, from O. Hertwig's Text-book), ak, ectoderm; 

 ch, chorda-rudiment; dh, archenteric cavity; Jib, layer of the ectoderm that grows 

 over the medullary plate; ik, entoderm; Ih, body-cavity; mk, primitive segment ; 

 m/i, medullary plate. 



