140 DEVELOPMENT OF AMPHIOXUS. 



tions of them are visible from this side. In some of the 

 secondary slits the dorsal margin, which had previously 

 begun to curve downwards, has now reached the ventral 

 margin and fused with it (Fig. 78, third secondary slit). 

 In this way is the tongue-bar formed, and the primitively 

 simple gill-opening is divided into two distinct halves. 

 The formation of the tongue-bars occurs in the secondary 

 slits considerably in advance of the primary, both actually 

 and relatively, since the latter have existed all through the 

 earlier period of the larval development without a trace of 

 tongue-bars. 



Peripharyngeal Bands. 



The endostyle has now grown a long distance behind 

 the club-shaped gland, and extends backwards between 

 the two rows of slits as far as the middle of the second 

 secondary slit. From the anterior part of the upper half 

 of the endostyle, which is now nearly equal in length to 

 the lower half, arises an epithelial tract in the wall of the 

 pharynx, which appears in the form of a band of ciliated 

 cells, and proceeds backwards below the notochord to the 

 end of the pharynx. A corresponding ciliated band occurs 

 in the left wall of the pharynx, proceeding from a similar 

 point in the lower limb of the endostyle. In their course 

 below the notochord the two bands take part in forming 

 the hyperpharyngeal (dorsal) groove of the pharynx. 



Atrophy of First Primary Gill-slit and Club-shaped 



Gland, etc. 



We have already seen indications of a reduction in the 

 size of the first primary slit. This reduction has advanced 

 considerably in the stage we are now describing (Fig. 78), 

 where the slit in question is only recognisable in side 

 view as a small groove. 



