THE GILL-SLITS. 



77 



primary slits are present, of which the hinder three or four 

 already open into the atrial cavity, a longitudinal ridge 

 appears in the right wall of the pharynx, above the primary 

 gill-slits. In this ridge six oval thickenings or enlargements 

 appear simultaneously, formed by fusion of the hypoblastic wall 

 of the pharynx with the external epiblast. These fused patches 

 alternate with the primary gill-slits; the first patch (Fig. 37, HP 2) 

 lying above and between the third and fourth primary slits, and the 

 sixth patch, HP 7, above and between the eighth and ninth primary 

 slits. Each patch now becomes perforated by a minute aperture, 

 which by enlargement becomes one of the secondary gill-slits. 



The most anterior of these six slits is usually formed a little 

 later than the remaining five ; and a little later still two more 



CH 



NO 



Gfl ES 



NO 



oc 



HKI3 HP 8 



FIG. 38. The anterior end of an Amphioxus Larva with thirteen primary, and 

 eight secondary gill-slits, seen from the right side. (After Willey.) 



slits are formed in similar fashion, one at each end of the series. 

 In this manner the full number of eight secondary gill-slits is 

 acquired (Fig. 38) ; the first, HP ], lying above and between the 

 second and third primary slits ; and the eighth, HP 8, above and 

 between the ninth and tenth primary slits. A ninth secondary 

 gill-slit is sometimes developed at the hinder end of the series. 



c. Further development of the primary and secondary gill- 

 slits. The secondary gill-slits are at first very small, but they 

 rapidly increase in size, extending down the right side of the 

 pharynx ; as they do so, the primary slits move downwards to 

 the ventral wall of the pharynx, and then extend up its left 



