LATBE LARVAL DEVELOPMENT OF AMPHIOXUS. 187 



the right side above the row of primary slits. The secondary 

 thickenings then become perforated and increase in size, and 

 as they do so the primary slits pass under the pharynx, and so 

 to the left side. 



In his figures he places the first secondary slit immediately 

 over the second primary. This is not correct. He did not 

 describe the formation of additional secondary slits, and he did 

 not observe the closure of any of the primary slits. 



He was right as to the number of secondary slits which first 

 appear together, and as to their formation on the right side 

 above the primary slits, with the consequent moving of the 

 latter round to the left side. 



My observations on the gill-slits, therefore, in the first place 

 confirm, in the second place correct, and in the third place add 

 to Kowalewsky's description. Over and above this, the forma- 

 tion of the velum, the origin of the buccal or prseoral cirri, the 

 fate of the club-shaped gland, and the development of the 

 endostyle will be found fully described below. 



In the following account the later larval development will, 

 for convenience, be divided into eight stages. The description 

 of the separate stages will be followed by a brief summary of 

 the stages, and this by a further summary of the facts relating 

 to the various organs ; the paper will be concluded with a few 

 general considerations. 



Stage I.— Figs. 1, 3, 20, and 21. 



This is the stage immediately succeeding the older of the 

 two stages figured in the paper referred to above (8), and the 

 primary gill-slits and metapleurs present very much the cha- 

 racters there described. Thus there are fourteen primary 

 slits (fig. 1), most of which open directly to the exterior ; but 

 the last three or four discharge into the atrial tube, which, 

 while closed behind, is widely open in front, where the right 

 metapleur is seen to overhang the anterior slits. 



The new feature with which we have now to deal is the 

 appearance of a continuous ridge in the right wall of the 

 pharynx above the row of primary slits. 



