Physiological Changes with Age in Fish 189 



group measured 52-3 mm. on August 14 and contained 42-7 

 embryos. Under laboratory conditions four fish gave birth to 

 a decreasing number of young. They averaged 30 • 8 young in 

 the first two broods, 10-0 in the third, and 7-0 in the fourth. 



There may be a period of reproductive sterihty in Gamhusia 

 late in life. In one collection the largest female was not gravid 

 while other sexually mature individuals were. The ovaries of 

 this female were examined microscopically, but no ova were 

 found. 



The only other pertinent observation in this connexion was 

 made by Fraser and Renton (1940) on Heterandria formosa, 

 the dwarf top-minnow. Successive broods were followed in a 

 single female from April 1934 until death in May 1936. She 

 grew to a remarkably large size (40 mm.), and produced a 

 total of 170 young, 150 of which were born during the first 

 eight months. She showed diminishing fertility during the 

 last few months of life. At the time of death the ovary was 

 examined microscopically and found to be a "mass of de- 

 generating tissue". 



Egg-laying fishes 



Bidder (1925a, 19256, and 1932) raised an extremely provo- 

 cative question when he proposed that fish and certain other 

 aquatic animals are potentially immortal. He based his 

 argument on reproduction and growth in the plaice (Pleuro- 

 nectes platessa). He noted that these fish continue to grow 

 throughout their lives and that the oldest individuals retain 

 their reproductive capacity. His idea was immediately 

 challenged by Wallace (1925) who pointed out that male 

 plaice had a higher mortality rate than females and that "this 

 apparently implies natural death". Bidder answered by 

 making a distinction between "senile death" and "parental 

 death". Parental death refers to that which occurs as a result 

 of the reproductive act. He supported this definition by draw- 

 ing upon the same plaice data which Wallace had used to 

 show that six-sevenths of the males die after the first spawning 



