1 882.] Incubation of the Top- Minnow ( Gambusia). 109 



STRUCTURE AND OVARIAN INCUBATION OF GAM- 

 BUSIA PATRUELIS, A TOP-MINNOW. 1 



BY JOHN A. RYDER. 



OINCE we have taken up our temporary residence at Cherry- 

 ^ stone we have found this interesting genus of cyprinodonts in 

 great abundance in fresh and brackish water streams, also in a fresh 

 water pond in the vicinity, a few miles south of where our station 

 is located. In the latter situation three forms have been collected 

 all of which are in breeding condition — we will not say spawning 

 condition, as they do not, as do most other fishes, commit their 

 ova to the care of the element in which they live, but carry them 

 about in the ovary, where they are impregnated and where they 

 develop in a very remarkable manner. 



Of the manner of impregnation we know little or nothing, ex- 

 cept the evidence furnished by the conformation of the external 

 genitalia of the two sexes. In the adult male, which measures 

 one and one-eighth of an inch in length, the anal fin is strangely 

 modified into an intromittent organ for the conveyance of the 

 milt into the ovary of the female ; a tubular organ appears to be 

 formed by the three foremost anal rays, but one which is greatly 

 prolonged and united by a membrane. At the apex these rays are 

 somewhat curved toward each other, and thus form a blunt point, 

 but the foremost one of the three rays is armed for its whole 

 length with ridges at its base and with sharp recurved hooks at 

 its tip, the other two at their tips similarly with hooks, and be- 

 tween their tips *re two small fenestra or openings which possibly 

 communicate directly with the sperm duct from the testes. The 

 basal elements of the fin are aggregated into a cylindrical col- 

 umnar truncated bony mass, which is prolonged upward into the 

 cavity of the air-bladder for the distance of nearly the eighth of an 

 inch ; from it a series of fibrous bands pass to the dorsal and pos- 

 terior wall of the air-bladder to be inserted in the median line. 

 Whether this bony column serves to steady the fin in the act of 

 copulation, or whether it serves to give passage to the sperm duct, 

 is an unsettled question with the writer. The modified anal fin of 

 the male measures a third of an inch in length. Other peculiari- 

 ties of the male are noticeable— for instance, as the more abbrevi- 

 ated air-bladder or space which also occupies a more oblique 



