8 BRANCHIAL CAVITY. 



growth push the original clefts over to the same side as the mouth. Each of 

 the fresh clefts becomes divided into two, which form the permanent clefts of 

 their side. 



The gill slits at first open freely to the exterior, but during 

 their formation two lateral folds of the body wall, containing a 

 prolongation of the body cavity, make their appearance (fig. 4 

 A), and grow downwards over the gill clefts, and finally meet 

 and coalesce along the ventral line, leaving a widish cavity 

 between themselves and the body wall. Into this cavity, which 

 is lined by epiblast, the gill clefts open (fig. 4 B, br.c). This 

 cavity which forms a true peribranchial cavity is completely 

 closed in front, but owing to the folds not uniting completely 

 behind it remains in communication with the exterior by an 

 opening known as the atrial or abdominal pore. 



The vascular system of Amphioxus appears at about the 

 same time as the first visceral clefts. 



BIBLIOGRAPHY. 



(1) A. Kowalevsky. " Entwicklungsgescliichte des Amphioxus lanceolatus." 

 Mem. Acad. Imper. des Sciences de St Pltersbourg t Series vn. Tom. xi. 1867. 



(2) A. Kowalevsky. " Weitere Studien iiber die Entwicklungsgeschichte des 

 Amphioxus lanceolatus." ArcJiiv f. inikr. Anat., Vol. xm. 1877. 



(3) Leuckart u. Pagenstecher. " Untersuchungen iiber niedere Seethiere." 

 Miillcr's Archiv, 1858. 



(4) Max Schultze. " Beobachtung junger Exemplare von Amphioxus." Zeit. 

 f. iviss. Zool., Bd. III. 1851. 



(5) A. M. Marshall. "On the mode of Oviposition of Amphioxus/' your, 

 of Anat. and Phys., Vol. x. 1876. 



