DEVELOPMENT OF ATRIAL CHAMBER OF AMPHIOXUS. 455 



actually moves iu course of growth from the mid-line, and 

 rises on to the right side somewhat (Pis. XXX and XXXI, 

 figs. 6, 7, 14, and 14a, I. met.). At the same time the much 

 larger right metapleur is deepened^ and overhangs the slits. 

 Then the little horizontal junction is effected j and we get 

 actually a nearly tubular atrium receiving the openings of suc- 

 cessive gill-slits. With subsequent growth the narrow atrial 

 tube widens and pushes itself right and left, so as to encroach 

 on the space hitherto occupied by the coelom, and finally it 

 extends so far dorsalwards as nearly to surround the alimentary 

 canal (see Figs. 8 and 9). 



The evidence of this history, in the form of careful drawings 

 of various sections, at various stages in the closure of the 

 atrium, together with drawings of whole larvae in two stages 

 of development, is given in the plates (Pis. XXIX, XXX, 

 XXXI, and XXXII) accompanying this paper. It is important 

 to point out that the mode of formation of the atrium as a 

 narrow groove, which closes and sinks (as it were) into the 

 body of the Amphioxus, is really different in important 

 respects from the enclosure of a space by downgrowth of large 

 folds, though ultimately no doubt the two contrasted modes of 

 formation come to the same thing so far as the more obvious 

 morphological relations are concerned. The mode of for- 

 mation which really occurs in Amphioxus is readily har- 

 monised with the existence of the post-atrioporal extension of 

 the atrium which gradually tapers to a fine csecal canal. It 

 also gives us an essentially difi'erent view of the region called 

 " epipleur " by Lankester, and generally so designated, from 

 that which Rolph^s theory necessitated. That portion of the 

 epipleur into which the myotomes of the body-wall extend is 

 seen now to be no downgrowth, no extension or fold. It is 

 the original unchanged body-wall which bounds the sides of 

 the animal's body in front of the atriopore, just as much as it 

 does behind. The only new growth in the atrial region 

 which takes part in the limitation of the surface is the sub- 

 atriai growth formed by the two little horizontal folds which 

 floor in the atrium when it is a mere canal. These in the 



