643 



The epithelial cells lining the original (anterior) vesicle increase in 

 height with the growth of the organ, while the cells lining the secondary 

 (posterior) vesicle become thinner and thinner as it grows backward. 

 The extreme of this differentiation is reached in a larva of about 

 16 mm. in length,- which is approximately the size of a young toadfish 

 about to break away from its attachment. A sagittal section of the 

 swim-bladder of such a larva is shown in Fig. 9. At this stage, the 

 posterior chamber is lined with flat epithelium; the anterior chamber 

 contains only a slight cavity and is lined on all sides with a layer of 



Fig. 9. Sagittal section of swim-bladder in 16 mm larva of Opsanus tau. ant.ch 

 anterior chamber, cp capillary bundles, cnt outer connective tissue layer of swim- 

 bladder, epi epithelium lining the anterior chamber of the swim-bladder, mb lateral 

 band of striated muscle, post.ch posterior chamber. X 125. 



very tall columnar cells; ventral to the tube connecting the two chambers 

 is a conspicuous whorl of capillaries from which is to be formed the 

 blood supply of the red gland. 



Soon after this stage, the larva absorbs the yolk-sac, breaks away 

 from its attachment, and begins its free-swimming existence. Appar- 

 ently coincident with this process, the swim-bladder becomes functional, 

 or at any rate it assumes the structural relations of the adult organ. 

 The cavity enlarges greatly, either as a result of, or as an ad- 

 justment to, the secretion of gas; the tall columnar cells lining that 

 portion of the anterior chamber just in front of the passage to the 

 posterior chamber remain as a single layer of columnar epithelium 

 and form the fundament of the red gland; all the remainder of the 

 lining cells become transformed into flat epithelium. I have not yet 

 studied this process in detail, but its careful investigation might add 

 much to our knowledge of the swim-bladder, since this transformation 

 of these very tall cells into flat epithelium during a relatively short 

 time, may be intimately related to the little understood process of gas 

 production by the swim-bladder. 



From a general point of view, the most important fact in the 

 development of the swim-bladder of the toadfish is that its posterior 

 chamber develops directly from the embryonic pneumatic duct. In 



41* 



