206 ARTHUK VVILLEY. 



Stage VIII. — Average length of central slits, 21. 



Right Side. — Eight slits; length of endostyle, 45. The 

 two halves of the endostyle are now definitely right and left. 



Left Side. — Eight slits; apparent length of velum^ 0. 



Of the above stages, the fifth is certainly the most eventful. 

 With regard to the last stage, it is well to be reminded that it 

 inaugurates the critical period ; that at the commencement of 

 it there is often a rudimentary or doubtful primary slit behind ; 

 and, finally, that the presence of eight pairs of slits is not a 

 universal rule, as there are sometimes only seven and some- 

 times nine pairs, but never normally more than nine. 



The central feature, perhaps, of the foregoing observations 

 is the development of the endostyle ; next in importance comes 

 the atrophy of the club-shaped gland and the first primary 

 gill-slit during Stage V, and of almost equal importance and 

 novelty is the atrophy of from four to six of the hinder slits of 

 the primary series. 



It goes without saying that there is no hard and fast line 

 between any two consecutive stages. The division into stages 

 is entirely for convenience, all the processes of development 

 and transformation being gradual. 



Summary of the History of the Individual Structures. 

 1. The Gill-slits. 



The primitive or ancestral left row of gill-slits has, by a 

 process which will be discussed later, been made to assume a 

 position on the right side of the larva, where slits form to the 

 number usually of fourteen, but occasionally fifteen. 



The actual perforation in the case of each slit occurs at first 

 at the base of the pharynx, near the mid-ventral line, and then 

 extends upwards on the right side ; the hindermost slits, how- 

 ever, retain their ventral position all through. 



The slits which thus appear at first in a single unpaired 

 series are called the primary slits. 



The right row of gill-slits, which originally corresponded to 

 the primary slits before the latter had forsaken their primitive 



