I/O DEVELOPMENT OF AMPHIOXUS. 



account of its close proximity to the thyroid cartilage of 

 the latter, the older anatomists assuming a functional 

 relation between the two structures. 



We know perhaps more about the morphological than 

 about the physiological significance of the thyroid gland, 

 since it is the vestige of the very actively functional endo- 

 style or hypobranchial groove of the Ascidians, Amphioxus, 

 and Ammocoetes. 



Morphology of Club-shaped Gland of Amphioxus. 



In describing above the formation of the second row of 

 gill-slits in Amphioxus, we found that the first secondary 

 slit paired with the second primary slit. It now remains 

 to consider what has become of the antimere of the first 

 primary slit. 



The probability is that, unlike the antimeres of the suc- 

 ceeding primary slits, that of the first has not suffered a 

 retardation of development, but is present from the very 

 beginning of the larval development, although in a some- 

 what modified form. I refer to the club-shaped gland. 



The club-shaped gland fulfils the requirements of a gill- 

 slit in so far as it opens at one end into the pharynx, and 

 at the other to the exterior. Since, as we have shown, the 

 morphological mid-ventral line lies high up on the right 

 side, immediately above the primary gill-slits, it is evident 

 that its anterior continuation would pass through the en- 

 dostyle precisely at the point where the latter is redoubled 

 upon itself. But the internal opening of the club-shaped 

 gland lies above the upper limb of the endostyle, and 

 therefore it is placed not only on the actual right side of 

 the larva, but in opposition to the first primary slit, on the 

 morphological right side as well. 



