LATEE LARVAL DEVELOPMENT OF AMPHIOXDS. 197 



position, and has grown a long way past the club-shaped 

 gland. The latter is still present with its large intra-buccal 

 orifice, but when seen in the living animal it seems to present 

 signs of disintegration, the constituent cells assuming a 

 loosely aggregated and turgid appearance preparatory to dis- 

 solution. The intra-buccal orifice is apparently the most 

 persistent part of the gland. In fact, before the end of this 

 stage the club-shaped gland atrophies altogether. From 

 observations which I made on numerous larvse belonging to 

 this stage, I am inclined to believe that the cells of the gland 

 break away from each other and pass into the alimentary 

 canal, where they are possibly absorbed. 



The ciliated hyperpharyngeal baud of the right side, which 

 has been referred to above, can now be seen proceeding from 

 the upper arm of the endostyle; while the corresponding band 

 of the left side proceeds from the lower arm of the endostyle. 

 These upper and lower portions become, at a later stage, 

 respectively the right and left halves of the endostyle ; and, 

 indeed, the latter — namely, the lower half — when looked at 

 from the right side, is found to lie at a deeper focus than the 

 right or upper half. 



One of the most curious events of this stage is the closure 

 of the first primary slit, which occurs very shortly after, if not 

 at the same time as, the atrophy of the club-shaped gland. 

 In fig. 7, which is a drawing of a younger larva than those 

 represented in figs. 8, 9, and 10, the first primary slit can just 

 be seen in side view ; on the other hand, in the larva of which 

 a three-quarter ventral view is given in fig. 9 it could not 

 have been distinguished at all in side view ; but a ventral view 

 showed it in the condition of a minute aperture with cilia 

 working in it, surrounded by cells in such a state of aggrega- 

 tion as to present a coarsely granular and dark appearance, 

 which, combined with the loss of a sharp outline to its wall, is 

 the distinctive feature of a closing slit, and enables it to be 

 placed in marked contrast to a newly formed slit, of which the 

 wall is clear and refringent. 



In fig. 10 there is the merest trace of the first primary slit 



