642 



side of the vascular (splanchnic) layer. This layer of somatic mes- 

 enchyme cells is the origin of the outer, tough, shining- white, con- 

 nective tissue layer (tunica externa). Into the region lateral to the 

 swim - bladder, grows later on each side the fundameut of the muscle 

 band. This is first visible in a larva about 8 mm. in length as a mass 

 of embryonic cells derived from the muscle plate of the first somite. 

 This mass of cells soon receives a branch from the first spinal nerve, 

 then grows backward around the outside of the group of spinal nerves 

 from the second to the fifth inclusive, and applies itself to the lateral 

 wall of the swim-bladder. This process is completed in specimens of 

 about 12 mm. in length. With the formation of the muscle bands, 

 the fundaments of all the structures of the adult swim -bladder are 

 finally established. 



b 



Fig. 8. Diagrams of the swim-bladder fundament in Opsanus tau, showing the 

 origin of the swim-bladder and the derivation of the posterior chamber from the pneu- 

 matic duct, a origin of the swim-bladder, b forward growth of the fundament, c, d, 

 e transformation of the stump of the pneumatic duct into the posterior chamber. 

 ant.ch anterior chamber. Iv liver duct, oes oesophagus, pch posterior chamber, sbf 

 swim-bladder fundament, sd stump of pneumatic duct. 



Important changes in the development of the organ as a whole 

 have, in the meantime, been taking place. The pneumatic duct, which, 

 on account of the forward growth of the swim-bladder vesicle, comes 

 to open into the posterior end of that structure, begins to atrophy at 

 its oesophageal end (Fig. 8c). It does not disappear entirely, however, 

 but a considerable remnant is left in the form of a stump of cells 

 pointing down toward the mesentery from the posterior end of the vesicle. 

 Very soon the cells of this stump begin to grow backward (Fig. 8d), 

 and to swell out to torm a secondary, posterior cavity (Fig. 8e) which 

 is the fundament of the posterior chamber. 



Coincident with this process occur significant histological changes. 



