56 



ZOOLOGY 



SECT. 



convex ; the blastopore marks the posterior end and is distinctly 

 dorsal in position. Cilia are developed from the ectoderm cells, 

 and by their vibration cause the embryo to rotate within its 

 membrane. 



The ectoderm cells forming the median portion of the flattened 

 dorsal surface now become differentiated and sink below the rest, 

 giving rise to the medullary plate (Fig. 760, A, mp). The ordinary 

 ectoderm cells on each side of this plate rise up as a pair of longi- 

 tudinal medullary folds (hb), extend towards the middle line and 



B 



hb 



r~ mk 



FIG. 760. Amphioxus lanceolatus. Four stages in the development of the notochord, 

 nervous system, and mesoderm. ok. ectoderm ; eft. notochord ; dh. cavity of arc.henteron ; 

 hb. ridge of ectoderm growing over medullary plate ; ik. endoderm ; Ih. coelome ; mk. 

 cffllomic pouch ; mfc 1 . parietal layer of mesoderm ; mk*. visceral layer ; mp. medullary plate ; 

 n. neuron ; MS. protovertebra. (From Korschelt and Heider, after Hatschek.) 



unite (B, Tib], covering over the medullary plate. The latter bends 

 upwards at the sides so as to become trough-like instead of flat (0), 

 and, its two sides coming in contact with one another above, the 

 plate is converted into a tube, the neuron (D, n), enclosing a central 

 canal, the neuroccele, continued dorsally into a narrow cleft. The 

 medullary folds extend behind the blastopore so that, when they 

 unite, the latter aperture opens into the neuroccele by a neurenteric 

 canal (Fig. 761, A, en). Anteriorly the folds remain apart up to a 



