LARVAL DEVELOPMENT. 1 43 



number of primary slits is reduced to the same number ; 

 namely, eight. 



Since it is usual for the primary slits to break through 

 in the first instance to the number of fourteen, no less 

 than six of them must close up and disappear before the 

 stage with only eight gill-slits on each side of the body is 

 arrived at. The six slits which are to close include the 

 first and the five posterior primary slits. In the larva 

 shown in Fig. 80, the tenth and eleventh primary slits 

 would have to close at a later stage ; the twelfth is on the 

 point of closure, and its walls present the characteristic 

 coarsely granular appearance spoken of above, while the 

 thirteenth and fourteenth slits have entirely vanished. 



In addition to the fact of the closure of these primary 

 slits, it is important also to emphasise the fact that they 

 disappear without leaving a trace behind. In the higher 

 Vertebrates there are a number of structures not only di- 

 rectly connected at some stage of development with the 

 pharyngeal wall, but also at some distance removed from 

 it, which various morphologists have interpreted as the 

 remnants of ancestral gill-clefts, without sufficiently con- 

 sidering the question whether gill-clefts were in the habit 

 of leaving their mark behind them.^ In Amphioxus, at 

 all events, they do not. 



77;^? Adjustment of the Month, etc. 



While the gill-slits have been adjusting themselves to 

 their definitive positions, the mouth has also been sub- 

 jected to a peculiar kind of growth, which results in its 

 bending round the front end of the pharyngeal wall, and 

 ultimately assuming an anterior and median position, as 

 we find it in the adult. 



