€H. VIIl] A CENSUS OF THE SEA 169 



the duration of the incubation period, Hensen and Apstein were 

 able to estimate the total number of the eggs of various species 

 spawned in the North Sea during the spring of 1895. 



The Nordsee Expedition was made during the period 15 

 February — 1 May 1895 ; thus including practically the whole time 

 of reproduction of the species of fishes investigated. These fishes 

 were cod, haddock, flounder, plaice, dab and long rough dab. Three 

 cruises were made: 15 — 22 February; 27 February — 9 March; and 



23 April — 1 May. On each cruise a difi*erent course was set so 

 that the vessel covered the whole North Sea with a network of 

 lines of observation. The whole time spent in fishing was 

 25h days; and 3397 nautical miles of high sea were traversed, and 

 167 hauls of the quantitative plankton net were made. The fish 

 -eggs contained in each catch were identified and counted and the 

 results were expressed as the average numbers contained in the sea 

 per square metre of surface. Each average was calculated from the 

 results of 167 hauls spread over 3397 nautical miles of sea, and over 



24 months of time. Not only were the individual eggs counted 

 but the stage of development of the embryo was noted. These 

 stages were (1) blastoderm, (2) embryo just visible, (3) embryo 

 marked off, (4) rudiment of eye formed, (5) eye fully formed, (6) 

 larva hatched out. It was of importance to record the eggs found 

 which were in the first stages of cleavage, since an unusual abund- 

 ance of these would be indicative of an aggregation of spawning 

 fishes on the sea bottom, and therefore of a " spawning place." If 

 such an abundance of recently spawned eggs were encountered the 

 instructions given to the naturalists were to cruise round the spot 

 where the haul was made and determine the limits of distribution 

 of the shoal of ova. Then the trawl was to be used and the density 

 of spawning fishes on the bottom investigated. It is impossible 

 to overestimate the skill with which this research was carried 

 out, and the care which was taken to avoid error in the 

 results. 



I have given the details of the catches made in these cruises in 

 the Appendix and the reader will see that fish eggs were taken in 

 158 out of 167 hauls. Now let us consider the case of the cod eggs 

 obtained. In cruise I. 57 hauls were made and 18 of these were 

 negative. In cruise II. 48 hauls out of 51 yielded cod eggs. In 



