i6 THE PHYSIOLOGY OF REPRODUCTION 



year 1904, the plaice in the open-air ponds at the Port Erin 

 Biological Station started spawning on 3rd March, and those at 

 the Peel (Lancashire) Sea Fish Hatchery (under cover) on 1st March. 



In the Holostean fish, Lepidosteus, which lives in the fresh waters 

 of North America, the breeding season recurs with a wonderful 

 regularity about May. At this time the fish resort in large numbers 

 to shallower water, where the temperature is higher. Here the ova 

 and spermatozoa are emitted during recurrent periods of sexual 

 excitement.! The related fish Amia, of Central and Southern North 

 America, spawns usually in May, the exact season depending some- 

 what upon the temperature of the water. The fish make their way 

 from deep water to the shallow spawning place, which is generally 

 at the end of a swampy lake.^ 



In the Crossopterygian fish, Polypterus bichir, the ova ripen in 

 the summer months from June to September, the breeding season 

 depending upon the period of inundation, as in most of the Nile 

 fishes.^ The other species of Polypterus {P. senegalis and P. lapradii), 

 which inhabit the river-basins of tropical Africa, spawn also in the 

 wet season in July and Aiigust.* 



In the Dipnoan, Geratodus, of Australia the principal time for 

 spawning is September and October, at the end of the dry season.^ 

 In the other two Dipnoans, Zepidosiren of South America and 

 Protopterus of Africa, spawning occurs shortly after' the emergence 

 of the fish from their summer sleep. Kerr, writing of the former, 

 says that the exact time for breeding varies greatly from year to 

 year in correlation with the extreme variability of the climate, the 

 swamps, which the mud-fish inhabit, sometimes -remaining dry for 

 prolonged periods." 



Many fishes migrate, before the commencement of the breeding 

 season, to localities suitable for the deposition of their eggs. Thus, 

 certain marine fishes like the salmon, the shad^ and the sturgeon 

 ascend rivers for long distances before spawning; others meirely 

 migrate to shallower water nearer shore. The eel, on tlie other 



publications of the English and Scottish Fishery Boards, and th,e International 

 Council for Fishery Investigation, should be consulted, rthese reports show 

 that the migratory and reproductive periods of fisheS' are/ affected by the 

 temperature, salinity, etc., of the sea. 



VAgassiz, "The Development of Lepidosteus," Fi-pc':'Amer. Acad. Arts and 

 Science, vol. xiv., 1878. '■ ' ' 



2 Bashford Dean, "The Early Development of Amia,'' Quar. Jour. Micr. 

 Science, vol. xxxviii., 1895. 



3 Harrington, "The Life-Habits of Polypterus," American Naturalist, 

 vol. xxxiii., 1899. 



* Budgett, "On the Breeding Habits of some West African Pishes," 

 Trans. Zool. Soc, vol. xvi., 1901. 



^ Semon, In the Australiom Bush, London, 1899. 



* Kerr, "The External Features in the Development of Lepidqsiren 

 paradoxa," Phil. Trans., B., vol. cxcii., 1900. 



