66 DISCOVERY REPORTS 



Cyclothone braueri metamorphosis occurs at a length of from n to 14 mm. (Jespersen and Taning, 

 1926), but two individuals of lengths 17 and 26-5 mm. (St. 3094, i50o(-o) m., 21. v. 54) still had no 

 deposit of fat around the swimbladder. However, in another fish of length 26 mm. there was a small 

 cushion of adipose tissue at the anterior end of the sac. In a 31 -mm. fish the larval swimbladder had 

 lost its ellipsoidal shape and appeared as a glistening white sphere (with a diameter of 0-6 mm.) in 

 the middle of a blimp-shaped mass of fat, having a length of about 3 mm. 



rsb ft 



C D 



Text-fig. 33. Diagrammatic reconstructions of four stages (a-d) in the fatty investment of a regressing larval swimbladder 

 in Cyclothone livida. The gas-gland is shown black, ft, fatty investment of swimbladder; lu, lumen of swimbladder;/)/), pig- 

 mented peritoneum ; rsb, regressed swimbladder ; sbw, swimbladder wall. 



Above and below the vertical diameter of the sphere, there was only a thin covering of fat. Finally, 

 in a 56-mm. individual the remnants of the larval swimbladder measured no more than 0-15 mm. in 

 diameter and needed to be carefully looked for in its fatty investment. It would thus appear that the 

 swimbladder is fully adipose in adults of about 35 mm. and upwards. 



In Cyclothone livida (also from St. 3094) the sequence of changes is much the same and is sum- 

 marized in Text-fig. 33. As in C. braueri, the larval swimbladder was completely regressed and 

 surrounded by a sausage-shaped mass of fat when the fish had reached a length of about 37 mm. 



This most advanced stage in the development was examined microscopically. In Text-fig. 34 is 

 a drawing of a transverse section at the level of the regressed swimbladder (see also PI. I, fig. 2). The 

 left-hand figure shows the reticular system of fat-storing cells surrounding the central regressed part 

 of the larval swimbladder. This part is shown enlarged on the right. An outer fibrous layer encloses 

 a mass of regressed gas-gland cells, mostly without nuclei, and the remnants of the retial capillaries. 

 The remaining space is filled up with fine loosely-woven reticular tissue, which is undoubtedly the 

 remains of the submucosa. The tunica externa must be represented by the outer fibrous layer, which 

 has much the appearance of the outer circular layer fibres described by Nusbaum-Hilarowicz (1920). 

 The mass of fat-charged cells is almost entirely bounded by the darkly pigmented peritoneum with 

 its fine mosaic of melanophores. Thus the fat is deposited between the peritoneum and the tunica 

 externa and in this way comes to invest the larval swimbladder as it regresses. 



