LATER LARVAL DEVELOPMENT OP AMPHIOXUS. 189 



On the left side (figs. 20, 21) the large lateral mouth is seen 

 with its margin ciliated, A ciliated groove leads from the 

 praeoral pit to the antero-dorsal margin of the mouth. Ante- 

 riorly the mouth ends in a sharp point ; but in fig. 21, on close 

 inspection, a very slight protuberance can be seen in the edge 

 of the mouth projecting into the ciliated groove. This minute 

 protuberance is the first indication of the combined forward 

 and transverse growth by which the mouth moves from a 

 lateral to an anterior median position. 



Just below the mouth in front is the external orifice of the 

 club-shaped gland, and some way behind this (fig. 20) is seen 

 a small circular piece of homogeneous tissue, which is really 

 a differentiation in the mesoblast of the lower lip of the mouth, 

 showing through the outer integument, and is, in fact, the 

 first element of those peculiar cartilaginoid structures which 

 form the skeleton of the prseoral cirri. In fig, 21 there are 

 two such elements, and they go on increasing in number by 

 additions at both ends. 



Running below the mouth, and then bending up to assume 

 a dorsal position, is a ciliated band composed of columnar 

 hypoblastic cells, forming a slight ridge in the pharyngeal 

 wall. It is continuous in front with the base of the lower arm 

 of the endostyle. On the other side there is a similar band 

 running from the upper arm of the endostyle, but it cannot be 

 seen in surface views at this stage. 



In these, and all drawings of the left side, the anterior 

 aperture of the nerve-tube described by Hatschek (3 and 4) is 

 shown very clearly. 



Stage II.— Figs. 3, 22, 23, and 24. 



The distinction of this stage lies in the perforation of the 

 secondary thickenings. The second to the fifth inclusive of 

 the thickenings become perforated, as a rule, before the first 

 and sixth ; and, again, the first is usually perforated rather 

 sooner than the sixth. This order, however, is subject to 

 frequent exceptions. In fig. 3, for instance, the sixth secondary 

 slit is open, but the first is not. 



