LATER LARVAL STAGES. 555 



si pertu re has meanwhile shifted from the left side and taken up a 

 more anterior und 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 

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

 the most posterior closing and completely disappearing (twelfth in 

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

 that on the left. These changes have been denned 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 during which only 

 the endostyle-rudiment <rrows further posteriorly and the clefts 



Fi<;. i><:3. Vt-ntrtil side of an Amjihwxu* larva at a later stage (after WILLEY). 

 second, 12, vestige of twelfth primary gill-cleft ; /- VIII, first eight secondary 

 Irt'N ; f>r, bneeal cirri; <-h, chorda; >'*, endostyle; ///, mouth ; v, velum. 



increase in height, has been named by WILLEY the critical at aye of 

 larval development. WILLEY points out that the number of clefts 

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

 clefts in Vertebrates. 



The gill-clefts hitherto present were segmentally arranged, but 

 this relation to the body-segments is lost in the tertiary clefts which 

 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 (Figs. 292, 293). The secondary 

 clefts, on the contrary, are, 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. 



