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II. Embryology of the Swim-bladder. 



1) In Stolephorus mitchilli. 



The swim-bladder in the majority of the Clupeoid fishes consists 

 of a much elongated, tubular or fusiform chamber which extends along 

 the whole length of the body cavity. The pneumatic duct passes from 

 a point near the middle of the swim-bladder to the blind end of the 

 stomach sac. The swim -bladder of Stolephorus mitchilli, however, 

 differs from that of the typical Clupeoid swim -bladder in that it is 

 divided into an anterior and a posterior part by a constriction. The 

 posterior part consists of a large, expanded, thin-walled chamber which 

 occupies the greater portion of the hinder region of the visceral cavity; 

 the pneumatic duct opens into this part of the swim - bladder, just 

 behind the constriction. The anterior part of the swim-bladder is in 

 the form of a tube which runs up forward to a point just posterior 

 to the skull; its walls are thick and made opaque by a considerable 

 deposit of pigment. Just behind the anterior end of the swim-bladder, 

 there comes off from its dorsal side a very small tube which soon 

 divides bilaterally. Each branch extends up into the skull and ex- 

 pands into squamosal and pro-otic bullae which have the same ana- 

 tomical relations as in other Clupeoid fishes. Species of Pellona have 

 the swim -bladder divided into posterior and anterior divisions as in 

 Stolephorus mitchilli (de la Beaufort, '09). 



The external anatomy of the swim -bladder in these species sug- 

 gest at once a close resemblance to the swim-bladder of the carp. 

 That this resemblance is not merely superficial, is shown by the de- 

 velopment of the swim -bladder in Stolephorus mitchilli. The eggs 

 and larvae of this species are to be found in great abundance in 

 Narragansett Bay during July and August. The eggs are pelagic; they 

 are about 7,5 mm. in diameter, and have the segmented yolk like other 

 Clupeoid fishes. The swim -bladder first appears in larvae of about 

 3,5 mm. in length. It grows out of the left side of the dorsal wall 

 of the gut just in front of the liver duct; it passes upward through 

 the mesentery, and then grows backward and expands into a chamber 

 of considerable capacity. This stage seems to correspond to the early 

 stage of the carp swim -bladder, as described by Moser ('04), and 

 probably represents the primitive organ morphologically equivalent to 

 the swim-bladder of the adult salmon. The next step also takes place 

 in a similar way as in the carp. In specimens of about 4,5 mm. in 

 length, there arises from the anterior end of the swim-bladder funda- 

 ment a bud, from which a solid cord grows forward in about the 



