l.vi Ki; LARVAL STAGES. 



555 



aperture lias meanwhile shifted from the left side and taken up a 

 more anterior and ventral position. The branchial artery, which 

 lies beneath the rudiment of the endostyle, now occupies the ventral 

 middle line, while the secondary gill-clefts remain on the right side of 

 the body (Fig. 293). A reduction in the number of the primary 

 delta takes place at the same time, the most anterior and some of 

 the most posterior closing and completely disappearing (twelfth in 

 Fig. 293), so that finally the number of clefts on the right equals 

 that on the left. These changes have been defined as the process of 

 symmetrisation of the branchial region or the equalisation of the 

 gill-slits. They lead to a final stage in which an equal number of 

 clefts (seven to nine, usually eight) is found on each side. This 

 stage, which marks a long pause in development daring which only 

 the endostyle-rudiment grows further posteriorly and the clefts 



I €S 



F,G. 293. Ventral side of an Amphioxus larva at a later steg < er Wn lk 

 second, It, vestige of twelfth primary gill-.-lett ; r-VIir first eight 

 clefts; be, buccal cirri; eft, chorda ; es, endostyle; m, mouth ; v, velum. 



increase in height, has been named by Willey the critical stage of 

 larval development. Wilms* points out that the number of clefts 

 at this stage approximately agrees with the typical number oi gill- 

 clefts in Vertebrates. 



The -ill-clefts hitherto present were segmentally arranged, but 

 this relation to the hodv-segments is lost in the tertiary clefts wind, 

 are added later in pairs behind the clefts already formed. The most 

 anterior, originally segmental clefts (primary and secondary) are also 

 then displaced forward. 



The primary clefts become early lengthened in the transverse 

 direction of the body, i.e., vertically ( Fig- 292, 293). The secondary 

 clefts, on the contrary, arc when they first appear, lengthened at 

 right angles to this direction and consequently parallel to the 

 longitudinal axis of the body (Fig. 293). Only in later stages do 

 they also extend vertically. 



