88 AMPHIOXUS. 



on the contrary, increase greatly in number, new ones being 

 added on at the hinder end of the series, apparently throughout 

 the life of the animal. Each new gill-sl it (Fig. 35, lQ becom es 

 ^ivided into two , at an early stage in its d evelop ment, by th e 

 growth down wa rds of a tongue-bar from its dorsal borde r ^_just 

 as in the earlier formed slits.. The slits further become divided 



transversely by the horizontal bars characteristic of the adult 

 (Fig. 35, l). Owing to this increase in number of the gill-slits, 

 without any increase in the number of the myotomes, the corre- 

 spondence between the two sets of structures is speedily lost ; 

 the alimentary canal, and the body generally, each acquiring a 

 metamerism of its own. 



The Reproductive Organs. 



The reproductive or gans ar e formed by prolife ration of the 

 epithelial walls of the septa wh ich divide the successive som ites 

 from one another . Each of these septa (cf. Fig. 31) is formed by 

 the coalescence of the posterior and anterior walls of adjacent 

 somites, and consists in the young Amphioxus of a thin connec- 

 tive tissue lamella, clothed on each surface by a single layer of 

 flattened epithelial cells, these latter being really j)arts of the 

 walls of the protovertebrse. 



In young specimens of Amphioxus, of about 5 mm. length, 

 the epithelial layer becomes modified over a very small patch at 

 th e outer and lower corner of the septu m^in the angle between 

 t he parietal wall or cutis laye r, andt he visceral wall ot the 

 protovertebra (cf. Fig. 41, or) : at this spot the cells become 

 cubical or columnar in shape, while over the rest of the septum 

 they remain flattened. 



Th is modificat i"^ ^1""° "" ot occur along the whole length of 

 tb e_ body, bui_ia_from the first confined to the somi tes in 

 Wh ^h the reprodu ctive organs lie in the adul t anima l ; i.e. t he 

 pa tches ofmoditLeH"epithelium are found on the se xita_ fcrming th e 

 walls of the somites from the tenth to the thirty-sixth inclusive. 



The modification affects the cells of botJi surfaces of each 

 septum, but the cells of the posterior surface are almost from 

 the first of larger size than those of the anterior surface, and, 

 growing much more rapidly than these latter, push the septum 

 forwards, and project into the segment in front of that to which 

 they really belong, as a small stalked knob, to which the cells of 



