170 A CENSUS OF THE SEA [PART 11 



cruise III. 59 hauls were made and 13 were negative. The average 

 numbers of cod eggs captured were : — 



per square metre of sea investigated. 



Now consider the relation of eggs in different stages of de- 

 velopment. If we divide all the ova taken into three groups we 

 have the following ratios : — 



The well-known course of a spawning season is as follows : at 

 the beginning of the season few fishes are spawning and the number 

 of eggs per unit area of sea is therefore small ; further, most of 

 these will be in the earlier stages of development. At the middle 

 of the period the maximum amount of spawning occurs ; there is 

 therefore a maximum number of eggs per unit area; and a greater 

 proportion of these are in the later stages of development. At the 

 end of the period the numbers of spawning fishes fall off; the 

 number of eggs per unit area decreases; and there is a greater 

 proportion still of later stages and larvae. Given a spawning 

 period, the duration of which is much longer than the incubation 

 period of the eggs, and this sequence must occur. But just these 

 relations between the numbers of cod eggs taken on the three 

 cruises, and the same relations between the various stages of de- 

 velopment, are exhibited in the above synoptic tables ; and these 

 facts are evidence that the results of the hauls represent closely 

 the natural conditions, which our knowledge of the life-history of 

 the cod would lead us to expect. 



Now from the results of these quantitative plankton hauls 

 Hensen and Apstein calculated that there were produced in the 

 North Sea, during the spawning season of 1895, 354'8 cod eggs per 

 square metre of surface ; but the area of the latter is approximately 

 547,623 millions of square metres, and therefore the total number 

 of eggs produced over the entire area in the spring of 1895 was 

 194,297,000,000,000. 



