LATER LARVAL DEVELOPMENT OF AMPHIOXUS. 209 



has commenced to grow in length (fig. 18), and now extends 

 between two or three of the anterior secondary and primary 

 slits. 



In Stage VII (fig. 14) it has greatly increased in length, and 

 is quite horizontal and ventral in position ; but the upper and 

 lower (i. e. right and left) halves are still perfectly distin- 

 guishable. 



In Stage VIII the two halves of the endostyle, which have 

 hitherto been in the relation to one another of upper and lower 

 — though the lower has been in the later stages at a deeper 

 focus than the upper — come to lie alongside of one another, 

 and so become definitely right and left respectively. 



In fig. 16 the two halves can just be recognised in front 

 for the last time. 



From the anterior extremity of each half of the endostyle a 

 ciliated ridge composed of the pharyngeal epithelium passes 

 dorsalwards, and becomes on each side the hyperpharyngeal 

 band. 



The anterior portion of the ciliated bands, where they curve 

 dorsalwards from the endostyle, is undoubtedly to be compared 

 with the pericoronal ridges of Ascidiaus which pass from the 

 front end of the endostyle round the mouth to the dorsal lamina. 



The following table shows the proportionate average increase 

 in the length of the endostyle, together with the concomitant 

 growth in length or vertical height of the gill-slits during the 

 critical period. The unit of measurement, as before is ywu 

 millimetre. 



Length of central slits 



1 ... 27 units 



2 . . . 32 „ 



3 . . . 35 „ 



4 . . . 41 „ 



Length of endostyle, 



52 units. 

 56 „ 

 66 „ 

 76 .. 



The endostyle does not reach the posterior end of the pharynx 

 before the close of the critical period. 



3. The Club-shaped Gland. 

 The development of this gland was made out by Hatschek (3). 

 To his description of it has been added the discovery of the 



