HABITS, MORPHOLOGY, AND EMBRYOLOGY OF GAMBUSIA AFFINIS. 1 89 



Pigmentation begins comparatively early. Scattered pigment spots first appear on 

 the dorsal surface, being more closely aggregated on the posterior region of the head and 

 along the dorsal mid-line of the trunk. These pigment spots become more numerous 

 and more closely aggregated until at birth pigmentation is almost complete. 



Embryos which still retain a yolk sac of considerable size when removed from the 

 ovary show nearly all the characteristic markings of the adult. At birth the yolk sac 

 is completely absorbed. The newborn fish answers fairly well, except with respect to 

 dimensions, to the diagnostic description of the species. Its color is light olive, darker 

 dorsally than ventrally. The number of scales in the lateral and transverse series, respec- 

 tively, correspond to the number of scales in these series, respectively, in the adult. The 

 number of rays in the dorsal, anal, and caudal fins also correspond to the number of rays 

 in these fins, respectively, in the adult. The fine dark line along the side is already present. 

 The two or three transverse rows of dark spots on the dorsal, the dark margin on the anal, 

 and the three or four irregular rows of dark spots on the caudal fin, characteristic of 

 adult females, are already becoming differentiated. The dark purplish blotch on the 

 side above the vent (absent in males) is not yet apparent. The modified anal fin of the 

 male was not observed in newborn fishes. 



The newborn fishes are 9 to 10 millimeters in length and are very vigorous. Having 

 been protected from many of the dangers which beset the larvae of oviparous fishes, they 

 are now well prepared to enter upon an independent existence. 



Embryos still carrying a yolk sac of considerable size, being removed from the 

 parent, were able to swim freely in water, where they continued to live, the yolk sac 

 being gradually absorbed. Such embryos were kept in small aquaria with occasional 

 changes of fresh water for a period of 10 days. At the end of this time the yolk sac was 

 completely absorbed and the young fishes were apparently in a healthy condition. 



SUMMARY. 



1. Gambusia affinis is known primarily as a fresh-water species, but occurs also in 

 brackish water. Under experimental conditions, fishes transferred from brackish water 

 to sea water were kept alive and apparently in a normal condition for a period of 10 days. 



2. The ovary of Gambusia is a paired tubular organ without a distinct median wall, 

 which opens directly into the urogenital sinus. Each ovum is contained in a separate 

 cellular follicle in which fertilization takes place and the embryo is developed. At the 

 completion of development the ovarian follicles which are attached to the central rachis 

 by a slender stalk are ruptured and the young fishes are extruded directly through the 

 urogenital aperture. 



3. The modified anal fin of the male which functions as an intromittent organ is con- 

 trolled by a powerful muscle which is inserted on the proximal end of the anal fin rays 

 and has its origin on a bony process projecting ventrally from the fourth to the last 

 abdominal vertebra and the modified haemal spines of the first three caudal vertebrae. 

 The third, fourth, and fifth rays of the anal fin are enlarged, greatly elongated, and 

 variously curved, bearing short spines on the distal portions. The interhaemal which 

 articulates with the third ray is enlarged and articulates with the two anterior 

 processes on which the muscle controlling the anal fin has its origin. 



19371°— vol 33—15 13 



