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Part III. — Thirteenth Annual Report 



might give results a little different from mine, especially at the lower 

 temperatures, but I have no reason to think any such difference will 

 be considerable. The emergence of the embryo at the hatching 

 period seems to be quite irregular, and this will influence the calculation 

 of the average time. But a confined quantity of water ought to have 

 a little influence on the rate of development and delay it, if the portion 

 of water is small or not frequently changed. 



On one occasion I had an opportunity to see the result in this respect. 

 The water in a jar with eggs of the cod, a few days before the hatching 

 period, had not been changed, and when examining it I found a number of 

 the eggs had succumbed and had sunk to the bottom ; others, again, were 

 tending to sink, being scattered at various levels throughout the water. 

 Some of these I examined and found all alive ; but the beats of the 

 heart had been considerably diminished. The average of four observations 

 was 29 beats of the heart per minute in the embryos in those sinking 

 eggs, the specific gravity of which had increased to about 1027*0 ; other 

 eggs had the usual specific gravity, and floated at the surface of the 

 water. The number of heart-beats in the latter averaged 45 per minute. 

 The hatching, however, of these surviving eggs took place about two 

 days later than what is usually found in the same temperature. The 

 eggs of cod, haddock and plaice seem to thrive as well at a temperature 

 near zero as in warmer water ; the eggs at the low temperatures, were 

 not, however, brought to the hatching stage ; with one exception (in - 1°) 

 they all succumbed owing to difficulties in attending to them, but as they 

 all were iu a heaithy condition for from twenty to twenty-five days when 

 the formation of the embryos had begun, there is no reason to think they 

 should not be able to resist the low temperature for the rest of their de- 

 velopment. A slight movement of the heart was noticed in the embryos in 

 some of the eggs that had been kept at - 1° for thirty-five days, and when 

 transferred to water of 6° C, the embryos emerged after an average time 

 of two and a half days. The freezing point of sea-water, with a specific 

 gravity of about 1027*0, is near - 2° C, so it occasionally happened that 

 the eggs kept in water of that low temperature became inclosed in lumps 

 of ice that floated on the surface ; when those again were melted while 

 the bottle was left quiet, a layer of fresh water was found floating on the 

 top of the salt water. All the eggs were then seen floating exactly at 

 the same level — at the limit between the fresh and salt water. Eggs of 

 plaice and haddock appeared to be most capable of resisting these changes, 

 while the whiting eggs gradually succumbed. As a whole, the latter did 

 not seem to do so well in a temperature at zero or below it, while the 

 eggs of the haddock especially appeared very hardy. The eggs of the 

 cod, as a rule, I found to be the least hardy amongst them all. 



Plate I. gives a graphic representation of the time of incubation of the 

 various species referred to in table II. 



The vertical lines represent the number of days from the time of 

 fertilisation of the eggs till the embryo emerged, while the horizontal 

 lines give the temperatures to which the eggs were exposed. 



The figures given in the table are represented by a cross at the 

 corresponding places in the plate. A line drawn between the marks 

 representing the same species (cod and haddock are taken together), will 

 not be quite regular ; but, no doubt, if observations were made with the 

 greatest care and with a constant supply of water to the eggs, as in a 

 hatching apparatus, the line that represents the time of incubation at 

 various temperatures would be quite regular, and perhaps mathematically 

 so. Such curves I have constructed as much in correspondence with the 

 data as possible. 



The study of these curves of incubation is rather interesting : as a rule 



