No. 3.] GROWTH AND METAMORPHOSIS OF TORNARIA. 423 



morphosis is shown in Fig. 40, PI. XXVI. In hardening, the 

 anterior end of the proboscis has been drawn in somewhat, and 

 therefore is seen to be flattened in the figure. The ectoderm 

 has greatly thickened over the whole surface, which is probably 

 the cause of the opaqueness of the embryo at this period. The 

 longitudinal band has almost disappeared, and the circular band 

 (cb) is by no means so distinct. The method of disappearance 

 of the longitudinal band and the thickening of the ectoderm is 

 shown by Figs. 56 and 57, PI. XXVII. These are from trans- 

 verse sections in the anterior region of the larva, in which ^repre- 

 sents the mid-dorsal line, x the posterior later area bounded by 

 the longitudinal band, y indicates the middle lateral furrow. Fig. 

 56 represents the conditions found in Tornaria, and Fig. 57 a 

 larva during the period of metamophosis. The thickening of 

 the circumoral area at x and y takes place by an increase in 

 number of the few cells forming the ectoderm in that region, so 

 that the ectoderm is gradually built up into the same structure 

 as that over the extra-oral area. At the same time the distinc- 

 tion between ciliated bands and ectoderm is lost, and the whole 

 proboscis becomes ciliated. 



The walls of the digestive tract of Fig. 40 have thickened, 

 but still retain the same divisions seen in the Tornaria. At the 

 anterior end of the oesophagus in the middle line, an evagination 

 of modified cells has taken place at nc. This lies very near to the 

 external opening, but is without doubt formed from endoderm. 

 The oesophageal region of this embryo had not yet been pulled 

 back as Agassiz has described ; later the part nc will lie nearer 

 to the enteroccel. This evagination is the first appearance of 

 the notochord (Bateson). The distinction between stomach and 

 intestine is still seen, although in older stages, after the intes- 

 tine has pulled back, the diaphragm-like partition disappears. 

 Parts of the second and third body cavities are seen cut across 

 in its section. The gill pouches are not shown in this figure, 

 since they lie to the right and left of the middle line, but in 

 Fig. 41 is seen a part of a transverse section through one. This 

 figure shows in detail the structure of the second gill pouch of 

 the series. The pouch gp is applied to the ectoderm wall, and, 

 moreover, the pouch itself is divided by a fold in its wall into 

 two parts. This fold forms the tongue bar which is so conspic- 

 uous a feature of the adult sills. 



