LATER LARVAL DEVELOPMENT OF AMPHIOXUS. 195 



stage is that the patch of modified epithelium which we have 

 already spoken of as the endostyle has definitely commenced 

 to push its way past the club-shaped gland, so that the latter 

 now lies upon instead of behind it. This can also be expressed 

 by saying that the endostyle has begun to fall away from its 

 previously oblique position, so as ultimately to assume a longi- 

 tudinal horizontal and ventral position. The incipient change 

 of position of the endostyle is not accompanied in this stage 

 by any growth in its length. The movement in bulk of a patch 

 of modified epithelium from one position to another is suffi- 

 ciently remarkable. It is probably, however, effected by the 

 same transverse growth which affects the primary gill-slits. 

 A consideration of the figures will show that it would be quite 

 possible for a growth of this kind to have the effect of 

 dragging down the at first very oblique endostyle. 



It is not so much the primary obliquity of the endostyle, as 

 its primary anterior position in the region of the first myo- 

 tome, which should be especially noted. 



A view from the left side of a larva of this stage is given in 

 fig. 6, the object being chiefly to show to what extent the 

 primary slits have come round to this side. The first primary 

 slit is hardly recognisable from this side, but can just be seen 

 with a median focus. In this larva there were fourteen 

 primary slits ; but the more usual number for this stage is 

 perhaps thirteen, and, moreover, the thirteenth is usually iu 

 course of closure. 



The oral hood, both as to its dorsal and ventral halves, is 

 now well marked, and the cartilaginoid elements of the buccal 

 cirri, of which there are six, are growing out into distinct ten- 

 tacles, which give a crenate margin to the lower half of the 

 oral hood. The external orifice of the club-sliaped gland is 

 undoubtedly present, but is covered over by the buccal cirri. 



The apparent length of the mouth is rather greater in fig. 6 

 than is usual at this stage, and in this respect the condition 

 of the mouth represented in fig. 26 is much more typical ; a 

 glance at this drawing will make it possible to understand 

 that, while the contour of the mouth undergoes a complete 



