577 



its muscles belong all to those of the left-side. No nerve and no 

 muscle of the right-side takes part in the provision of the month-cavity. 



Here we are in the presence of a remarkable phenomenon : The 

 month of Amphioxns, as organ of the left-side, cannot l)e homologous 

 with the nnjiaired mouth of vertebrates always developing symme- 

 trically, and we must surmise that on the right-side of the young- 

 larva a similar organ as the mouth, a counterpart of it, is found. 

 This organ is, as I indicated a long time ago already, the so-called 

 club-shaped gland and the mouth together with this glairl form 

 morphologically the first jiair of gill-slits of the Amphioxus-lai-va. 



Instead of gill-,s7//.y, it is more correct to speak of gill-pouches, 

 for in all vertebrates, without any exception, a gill-slit is formed, 

 because a pouch-shaped projecting part of the gut reaches the epidermis, 

 fuses with it on that spot, and afterwards splits to the outside. 

 With Amphioxns this is exactly the same; here also is every gill-slit 

 formed as a pouch-shaped projecting part of the gut, and s|)lits 

 afterwards — before the metamorphosis — to the outside, after the 

 beginning of the metamorphosis, towards the gill-cavity (the atrium). 

 The epithelium of a gill-pouch can partly differentiate to glandular- 

 epithelium, in this way e. g. in all vertebrates the thymus if formed, 

 a glandulous organ, from ihc epithelium of some gill-pouches. 



The club-shaped gland possesses all the essential distinctive pro- 

 perties of a gill-pouch; it is formed on the right-side of the bod^' 

 as a projecting part of the gut, which opens afterwards to the 

 outside and possesses then two openings one inside in Ihc [)harynx, 

 the other to the outside. 



Though the greater part of its epithelium has ditfercntialod in(o 

 glandular epithelium. I found however laterally from it a ring- 

 shaped strip of ciliated epithelium, corresponding entirely (o thai 

 of the other gill-pouches. 



The outside opening of the club-slia[)ed gland is in the begiiniing 

 })laced near to or in the median plane; afterwards it removes in 

 front of the mouth to the left-side of the body. This is again one 

 of the remarkable phenomena of asymmetry in the larva of Am|)hi- 

 oxus, the explanation of which I intend to give in my detailed 

 paper ^), as it would lead me too far hei-e. The clnb-sha|)ed gland 

 disap[)ears in the course of the metamorphosis, and d(^es not leave 

 any vestige behind. 



Has the mouth of the Amphioxus-larva originally also been a 

 gill-slit? In my opinion there is no doubt about i(. II is true that 



^) This paper was oflered last winter to be published in the tiansaetlons ot 

 the Academy. 



