LATER LARVAL DEVELOPMENT OF AMPHIOXUS. 219 



as the prffioral pit, while the right one becomes the expanded 

 head-cavity of the proboscis-like end of the body. 



The anterior chamber with its two lateral horns, together 



TnQS.J). 



Fig. 1. 



Fig. 2. 



Explanation of the Woodcuts. 

 Fig. 1.— Diagrammatic dorsal view of an embryo of Clavelina Rissoana 

 (modified after van Beneden aud Julin). The mesoblast is represented 

 in an extremely diagrammatic way. It consists really of large polyhedral 



cells. 



a. p. Anterior neural pore. n. t. Nerve-tube, lying on the hypoblast 

 in front and on the notcehord behind, p. v. Prsechordal vesicle. /. d. 

 Lateral diverticulum (paired) of proechordal vesicle, mes. p. Mesoblastie 

 or archenteric pouch : only one pair, equivalent to the first pair of pouches 

 in Amphioxus. mes. Mesoblast. nch. Notochord. 

 Fig, 2.— Dorsal view of an embryo of Amphioxus of a corresponding 

 age (after Hatschek). 



Letters as above. The nerve-tube lies entirely on the notochord. 

 There are numerous pairs of mesoblastie pouches. 



