640 



middle line, dorsal to the body cavity. The anterior end of this cord 

 reaches the level of the pectoral fin in 5,5 mm. specimens. When it 

 arrives at the base of the skull, it divides into two branches, each of 

 which grows forward under the ear capsule of its own side, and then 

 up through the still unchondrified floor of that structure to a position 

 inside the anterior end of the ear capsule. The distal ends of these 

 branches begin to expand into air-vesicles in young of about 7,5 mm. 

 in length. The squamosal vesicle is developed only at a much later 

 stage; in specimens about 30 mm. long, it appears as a swelling in 

 the tube leading to the pro-otic vesicle, and rapidly expands to fill a 

 cavity which appears in the squamosal bone. 



The development of the swim -bladder in Stolephorus mitchilli 

 leaves little doubt that, in the Clupeoid fishes, the organ is, morpho- 

 logically, directly comparable to the swim -bladder of the Cyprinidae 

 and consists of a posterior, primitive portion, and an anterior, secon- 

 dary portion. 



2) In Fundulus and Menidia. 



The larval stages of these fishes may be said to be partly passed 

 in the egg, so that very soon after hatching, the young fish begins to 

 swim freely and effectively. The swim -bladder fundament is already 

 well established at this time ; it consists of a posteriorly-extending sac 

 with only a small cavity, and is connected with the right side of the 

 oesophagus by an open pneumatic duct. Very soon the cavity enlarges 

 greatly, probably as the result of the secretion of gas into it; in 

 Fundulus heteroclitus , this process takes place almost immediately 

 after hatching, but in Menidia not until six or seven days later. At 

 this stage of development the duct is still present, opening into the 

 anterior end of the swim-bladder, and, at that end, around the opening 

 of the duct, is formed the red gland. The structure of the swim- 

 bladder at this stage seems to be essentially like that of the swim- 

 bladder of the pickerel. The pneumatic duct soons loses its lumen, 

 and then gradually atrophies. Thus the structure of the adult organ 

 is produced. 



It is important to notice that, in these species, the embryonic 

 pneumatic duct opens into the anterior end of the swim-bladder. This 

 relation appears to be the primitive one, since it is found in the lower 

 types of the swim-bladder. A different condition exists in the higher 

 types, as will be seen from the account of the development of the 

 swim-bladder in the more highly specialized species. 



