138 DEVELOPMENT OF AMPHIOXUS. 



the region of the secondary slits. Such of the primary 

 slits as occur behind this region are not affected by the 

 rotatory method of growth, and retain their original position 

 in the mid-ventral line of the pharynx. 



It is to be noted also that there are only twelve primary 

 gill-slits at this stage. Assuming that in the particular 

 larva here figured there were originally fourteen primary 

 slits, the fourteenth has closed up and vanished without 

 leaving a trace, while a vestige of the thirteenth can still 

 be recognised. The actual process involved in the closure 

 and disappearance of a certain number of the primary slits 

 can, as we shall see, be readily observed in the living larva. 



Club-sliapcd Gland and Endostyle. 



The internal aperture of the club-shaped gland into the 

 pharynx is exceptionally plain at this stage, and its refring- 

 ent walls and relatively large size give it a curiously slit- 

 like appearance. We shall find that the gland subsequently 

 atrophies, but the most persistent part of it — that is to say, 

 the last part of it to disappear — is precisely the internal 

 opening with its refringent border. 



The endostyle, whose primary position, as we have seen, 

 was immediately in front of the club-shaped gland, now 

 presents a remarkable condition. It has begun to grow 

 backwards and downwards, being probabl}' pulled down, 

 so to speak, by the general rotatory growth of which we 

 have spoken above ; and so the club-shaped gland no 

 longer lies behind it, but upon it. The gland itself being 

 disconnected with the wall of the pharynx, except at its 

 upper end where it opens into the latter, is not affected 

 by the complicated changes to which the pharyngeal wall, 

 including gill-slits, mouth, and endostyle, is subjected, so 



