DEVELOPMENT OF AMPHIOXUS 



171 



formation of the tongue-bars, so that in the adult there is a 

 nephridium to every two gill-slits). Hatschek's nephridium 



A 



B 



Fig. 72. — Amphioxus : transverse sections through larvae showing the 

 development of the atrium. 

 A, early stage in which the coelom (c) is still large, and the atrial cavity 

 (a) is small. B, later stage ; ep, epipleural folds ; fr, fin-ray box ; g, gut ; 

 m, myotome ; ?np, rnetapleural fold ; n, notochord ; nc, nerve-cord. 



arises as a small tube near the preoral pit, but in the adult its 

 opening leads into the pharynx just behind the mouth. 



Primitive features in the development of Amphioxus : 

 Cleavage total ; 



Gastrulation with invagination ; 

 All mesoderm segmented ; 

 Enteroccelic pouches. 



Literature 



Kellicott, W. E. Chordate Development. Henry Holt, New York. 



I9I3- 

 MacBride, E. \V. Text-Book of Embryology, Vol. I. Macmillan, 



London, 1914. 



