oj the Fishery Board for Scotland. 



149 



The diameter of plaice eggs is about 1*95 mm., in comparison with which 

 the eggs of the flounder are very small, the diameter being 0*92 mm. 



In water of 12° C. the latter all hatched in the course of a few hours. 

 At 10° it took a few hours more, and only at 8° a difference was noticed 

 of half a day. Judging from this it is evidently the larger eggs that 

 show the greatest difference in time of incubation under the same 

 circumstances. 



In Jahresbericht der Commission zar wissenschaftlichen Untersuchung 

 der deutschen Meere in Kiel filr die Jahre 1874-76 (page 32), Dr C. 

 Kupffer has given the following explanation of the cause of the delayed 

 hatching of herring eggs : — 



" The egg-shell, he says, is constantly getting thinner (verdiinnt sich 

 stetig) during development, and the moment of emergence depends upon 

 the relation between the strength of the shell and the muscular action of 

 the embryo." 



This explanation may account for a difference in the time of hatching, 

 under the same circumstances, in general, and so far Kupffer is 

 right; but according to his theory the irregularity in hatching 

 should be the same in different temperatures ; but my experience 

 is that the difference is larger in the lower temperatures than 

 in the high. Another fact also requires to be taken into considera- 

 tion ; namely, the difference in size that has been found to exist amongst 

 eggs even from the same individual. The small eggs of the flounder 

 hatch in four and a half days, while the large eggs of the plaice take 

 twelve days UDder the same temperature (10 2 C), and in the same way I 

 believe a smaller egg will hatch quicker than a large one of the same 

 species. 



Table II. — Showi?ig the Average Rate of Development of the Eggs of Four 

 Species when exposed to Different Temperatures. 



Temperature in 

 Centigrade. 



-2° 



-1° 



0° 



1° 



2° 



3° 



4° 



5° 



6° 



8° 



10° 



12° 



14° 





G. morrhua,* . 

 G. ceglefinus,* . 





H2 

 +42 









23 

 23 



20£ 

 201 



• 



m 



15* 

 17f 



15i 

 1U 



is| 



18| 



6i 



12f 



104 



13 



10* 

 8 

 lOf 

 12 



»! 

 61 

 91 

 101 

 3| 



8* 

 t5f 

 8f 



\ Time of 

 1 incuba- 

 > tion in 

 1 days 

 ' (24 hours). 



Temperature in 

 Fahrenheit, 





302° 



32° 



33-8° 



35-6° 



37-4° 



392 



41° 



42-8' 



46-4° 



50° 



53-6° 



57-2° 





The details in this table, where the average time of evolution is given, 

 are based on observations made with eggs that were hatched in jars 

 without a constant circulation of water. An experiment so arranged 



* The eggs of cod and haddock took about the same time iu incubation ; though 

 I sometimes noticed that eggs of the cod hatched a day or so sooner than the haddock, 

 the average time generally became the same. Cunningham makes the same statement 

 in bis paper • On the Relation of the Yolk to the Gastrula in Teleosteans and in 

 1 other vertebrate Types,' Microscopical Journal, vol. xxvi, new series, page 4. 



t The eggs were kept at the temperature of - 1° G. for 35 days, after which they 

 were transferred to water of +6° C., as it was necessary for me to terminate the 

 experiment. In the latter temperature the eggs hatched with an average time of 2^ 

 days. The whole time of incubation at that temperature being 15^ days ; one-sixth, 

 or 6 days, should be added to the 35 days in water of —1° to give the whole average 

 time for that temperature. 



X The eggs were accidentally lost when the first embryos had been hatched at the 

 beginning of the fifth day. The time stated is consequently to some extent esti- 

 mated, but on a good basis ; and when compared with the statements for the other 

 temperatures, any deviation will be very small. 

 L 



