94 



differences even at places a few miles apart, at the same time, 

 these latter variations being due to cold water ebbing back 

 from the land (which is always colder at this time of year than 

 is the sea). We cannot say, precisely, how long it takes a 

 plaice egg to incubate in the Irish Sea because of all the above 

 variations. The plaice at Port Erin Hatchery always spawn 

 several weeks earlier than they do at Pi el, in the Barrow 

 Channel because of the higher sea (and land) temperature at 

 the former station. There is a rather well-marked mathematical 

 relation between the temperature and the incubation period 

 of a fish egg : the higher the temperature, the shorter the 

 incubation period. So far, however, we have not made 

 experiments stating this relation exactly in the case of Irish 

 Sea plaice. 



Neither has the time required for the later development 

 been made out, though it is known that the baby plaice in the 

 Port Erin spawning ponds have usually become transformed 

 by the end of April. The fish hatches out from the egg as a 

 larva, carrying a large yolk sac, and in the course of about 

 two weeks this organ becomes very small, and then, later on, 

 quite disappears. About the time of its disappearance the 

 transformation (or metamorphosis) occurs ; the body begins 

 to flatten from side to side, and the left eye begins to show 

 on the right-hand side of the head, because of the twisting 

 (from left to right) of the bone between the mouth and the 

 brain-case. In about a month from the date of hatching the 

 metamorphosis has been completed and the fish is become 

 definitely flat. At all stages between the egg and the fully- 

 transformed larva, however, the latter can be identified as a 

 young plaice, though it is very like the flounder and dab. 



The First Shore Stages. 



At about the end of May and the beginning of June, 

 according to the nature of the season, the young plaice first 



