648 



the structure known as Bowman's capsule. The fluid excreted from 

 the kidney now has its origin in the glomerulus of Bowman's capsule 

 and in the walls of the tubule, instead of coming through an open 

 duct, as was primitively the case. The analogy of these changes in 

 the phylogeny of the swim-bladder is to be found in the progressive 

 functional substitution of a capillary mass for an open duct, accom- 

 panied by a corresponding modification of the walls of the latter 

 structure. 



IV. Summary. 



1. The Clupeoid swim -bladder is morphologically comparable to 

 the swim-bladder of the carp ; it consists of a posterior portion which 

 represents the primitive swim-bladder, and an anterior portion, derived 

 by a secondary growth from the primitive organ. 



2. The swim- bladder in Fundulus and Menidia is comparable to 

 the swim-bladder of the pickerel (Esox) and differs from it chiefly in 

 the loss of the pneumatic duct. 



3. The epithelial lining of the swim-bladder is derived from the 

 endoderm of the oesophagus ; the inner and middle layers of the walls 

 are developed from the layer of splanchnic mesenchyme which invested 

 the endodermal fundament in the course of its passage up through the 

 mesentery ; the outer, connective tissue layer originates from the som- 

 atic mesenchyme of the neighboring body region; the longitudinal 

 bands of striated muscle in the swim -bladder of Opsanus arise from 

 the myotome of the first somite and are innervated by a branch of 

 the first spinal nerve. 



4. The relation of the pneumatic duct to the swim-bladder in the 

 different types of the organ indicates that two general morphological 

 varieties of the swim-bladder are probably to be recognised : — a pri- 

 mitive type in which the pneumatic duct opens into the anterior end 

 of the swim-bladder, and a derived type in which the pneumatic duct 

 (embryonic) opens into the posterior end of the organ. 



5. The posterior chamber in the swim-bladder of Opsanus, Sipho- 

 stoma, Tautogolabrus, and Tautoga, develops directly from the em- 

 bryonic pneumatic duct. 



6. The pneumatic duct in the eel, the posterior chamber in the 

 swim- bladder of Opsanus, etc., and the oval in the highest type of the 

 swim-bladder are probably to be considered as homologous structures, 

 produced as the result of the progressive reduction of the pneumatic 

 duct and the development of a rete mirabile. 



7. In the primitive swim - bladders the epithelial lining is un- 

 differentiated; in the eel, the epithelium has become columnar and is 



