MINNOWS GAMBUSIA AFFINIS AND CYPRINODON VARIIEGATUS. 9 



ment during the fall or winter or to rear young for the purpose, in 

 which case the sexes must be segregated at a very early stage. 



As the embryos develop within, the ovary a black spot appears on 

 each side of the abdomen of the parent above and in front of the vent, 

 and these spots gradually become larger and larger. When they 

 become so large that they are about to meet at the ventral surface, the 

 period of parturition is at hand. The process of extrusion of the 

 young was observed repeatedly. There is no uniformity m the man- 

 ner of birth. They may appear singly or by twos and threes at a time. 

 Some come head first, some tail first, and others are delivered in a 

 coiled position. Extrusion may occur quickly and with some 

 apparent force; at other times it is a slow and deliberate process. 

 Some females imder observation delivered nearly the entii'e brood in 

 one position, but others did not. It appears that the yoimg are most 

 frequently born tail first and one at a time. The process invariably 

 takes place during the day. The entire brood may be delivered in 

 the course of an hour or two, or the process may consume an entire 

 day or a portion of two days. During this period the adult swims 

 about as usual and eats food when it is supplied. If hmigry, she 

 devours her own yoimg as rapidly as they are bom. In many 

 instances in the aquarium the mother eats her entire brood on the 

 day they are born. 



The young at the time of birth are from 8 to 10 mm. in length. 

 They are very vigorous, and, as previously indicated, they come into 

 the world with an appetite and well prepared to enter upon an 

 independent career, llie average rate of growth is rapid, but, as is 

 the case with other forms of animal life, each lot has its " runts." The 

 largest female among a lot born in May, 1914, and reared in the 

 aquarium had reached a length of 25 mm. by the middle of September. 

 The recognizable males were somewhat smaller, although the dif- 

 ference in size was not nearly so great at this age as it is among fish 

 that have attained their full growth. The smallest individuals in 

 this lot were only 13 mm. in length. In the field, by about July 30 

 it became difficult to distinguish the first young of the season from 

 the adults. The largest specimens taken at this time and identified 

 as young of the season were 25 mm. in length. From this it would 

 appear that the young in natural habitats grow faster than those iu 

 captivity. Fish born and reared in the aquarium now nearly one and 

 one-half years old have not quite reached the maximiun normal size 

 of their parents. 



The external character distinguishing the sexes is the modified 

 anal fin of the adult male, which is developed into an intromittent 

 organ. In the young, however, the anal fins are similar. The modi- 

 fication of this fin in the male is a gradual process and can not be said 

 to become evident at a stated age or length of the fish. In some 



