I 



July 2, 1891] 



NATURE 



203 



shaped gland." It opens to the exterior on the left side, 

 just in front of the big laterally- placed mouth, whence it 

 can be traced, bending down across the median line and 

 passing up at right angles to the long axis of the body 

 along the deep surface of the right wall of the buccal 

 cavity. It opens at its apex, as Mr. Willey has shown, 

 into the buccal cavity. Its earliest appearance (as de- 

 scribed by Hatschek) resembles that of a gill-slit, though 

 it precedes both the mouth and the first gill-slit in date. 

 Mr. Willey suggests that it is a modified gill-slit. By the 

 side of this club shaped gland and in front of it, imme- 

 diately associated with it. is a band-like tract of modified 



but the <-shaped epithelial tract does not ; it grows 

 rapidly at its angle along the line or interspace between 

 the two series of slits, forming a double tract of modified 

 epithelium consisting of parallel extensions of the two 

 limbs of the <. It is now the epithelium of the hypo- 

 pharyngeal ridge or endostyle. 



Mr. Willey regards the club-shaped gland so intimately 

 associated with the first stages of the endostyle as a 

 modified gill-slit belonging to the secondary (the per- 

 manent right-side series). Its early development in 

 front of the mouth indicates this ; since, when the mouth 

 acquires a median position (passing from the left towards 



Mouth 



■ Fig. I. 



Jl 10 9 8 



I, 2, 3. — Diagrams showing three stages in the development of the gill-slits and endostyle of Amphio.xii?. Figs, i and 2 are seen 

 from the rigtit sids ; Fig. 3 from the ventral aspect. In Fig. i the position and shape of the mouth, which lies on the left side of the 

 animal, are indicated by a dotted oval. The primary series of gill-slits are numbered in all the figures. The secondary series are not 

 numbered. Fig. 2 shows the rotation downwards of the primary series of gill-slits and their nearly complete disappearance from view 

 on the right side ; at the same time the secondary series have developed to the number of eight, and the endostyle has become 

 <-shaped,and is pushing its angle between the two rows of gill-slits. Fig. 3 shows the atrophy of the most anterior primary gill- 

 slit, whilst some of the hindermost have disapp?ared and numbers 10 and 11 are in course of closure and disappearance, a, pra:oral 

 ciliated pit, opening on the animal's left side, but seen through the transp.-irent integument ; />, the endostyle (<;-shaped tract o( 

 modified epithelium); c, the club-shaped gland ; d, the edge of the right meta-pleur (the atrial cavity is not yet formed in the anterior 

 phai'ngeal region) ; e, the six thickenings which develop the six anterior gill-slits of the secondary (permanent right-side) series ; /, the 

 praeoral tentacles. 



intra-buccal epithelium. When there are about eight 

 gill-slits of the primary series present, it is noticeable that 

 the apex of the club-shaped gland is bent over, so that 

 the gland teads to become ^-shaped, with the angle 

 directed backwards ; the adjoining epithelial tract faith- 

 fully follows the bend. At first the upper limb of the < 

 is a good detl smaller than the lower, but as the primary 

 series of gill-slits move from the right side of the 

 pharynx to the left, the two limbs of the < become 

 nearly equal in length, and the angle takes up a position 

 between the primary and the new secondary series of 

 slits. The club-shaped gland-tube now atrophies entirely, 



NO. 1 131, VOL. 44] 



the right by a relative growth, the reverse of that which 

 brings the primary gill slits from the right to the left !), 

 structures just in front of it would be thrown round to the 

 right side, the side of the secondary series of slits. He 

 suggests that it is the early-developed anterior member 

 of the secondary series of gill-slits ; and points out that 

 just as this modified gill-slit atrophies, so does its pair in 

 the primary series, viz. the first. 



Mr. Willey points out the possible importance of these 

 facts in reference to the views of Dohrn and of Van 

 Beneden, and makes an interesting comparison between 

 the Ascidian tadpole and the Amphioxus larva, with a 



