DEVELOPMENT OF ATRIAL CHAMBER OP AMPHIOXUS. 461 



anterior closure of the atrium completed, and a symmetrically 

 placed right and left series of gill-slits. 



We have taken steps to obtain the critical stages in the 

 living condition during the present summer, and propose to 

 ascertain whether the second row of gill-slits originates by any 

 kind of fission from the first. If not, it is a curious fact that 

 the morphologically median plane of the pharynx of the young 

 larva becomes the left side of the adult, whilst the relations of 

 the mouth to median plane, in adult and larva respectively, are 

 even more curiously divergent. It is probable enough that in 

 these differences the larva does not present the more archaic 

 condition, but an adaptatioual arrangement. We do not at 

 present know what are the conditions of life which render its 

 excessive asymmetry advantageous to the larva. 



The closure of the atrium by the growth of the little hori- 

 zontal sub-atrial ridges from the median face of each metapleur 

 is shown in the sections of various larvse given in Pis. XXX, 

 XXXI, XXXII. 



In the drawings, figs. 4 and 6 of PI. XXIX, we can trace 

 the two metapleura in the still unenclosed region of the 

 pharynx. The right-side metapleur is seen to have its free 

 edge somewhat high on the animal's side, whilst the left 

 metapleur in the perforated pharyngeal region is almost 

 coincident with the median ventral line. (The reference line 

 in fig. 4, PI. XXIX, lettered " edge of left metapleur,^' has 

 been by oversight carried up to the right metapleur. It 

 should stop at the ventral line.) The right metapleur is 

 larger and deeper than the left, which is barely traceable as 

 a thickening of connective tissue, when its fellow of the 

 opposite side is large and provided already with the character- 

 istic lymph-space (see fig. 7, r. met. and /. met., PI. XXX). 



The figures A, B, C, in PI. XXX, represent diagrammatic- 

 ally three stages in the closure of the atrial tube, showing in 

 A the metapleurs or metapleural ridges without any hori- 

 zontal sub-atrial floor ; in B the formation of this floor in the 

 hinder region, where there are no gill-slits; and in C its 

 continued formation so as to enclose the perforations of the 



VOL. XXXI, PART III. NEW SEE. H H 



