114 Part III. — Sixteenth Annual Report 





Opaque. 



Mature. 



opecies. 



Diameter in 



Volume in 



Diameter in 



Volume in 



mm. 



c. mm. 



mm. 



c. mm. 



vtiov vxuiuaiu.j . . 



0*9 



0*3817 



1-45 



1*5962 



XvcU. VTU.1 lial Li, • • • 



1*0 



0*5236 



1*55 



1-9498 



Lesser W^eever, 



0*8 



0*268 



1*32 



1 *2042 



AT Q olz" iiT*ol 



l>Xcll/K.cl cij , . , 



0-85 



0*3215 



1*25 



1-0226 



Dragonet, 



07 



0*1795 



1 -0 



0-5236 



V^UUj • « . . 



0*9 



0*3817 



1-38 



1*3706 





I'O 



0*5236 



1*45 



1*5962 



Bib 



0*6 



0*1130 



0*92 



0*4072 



Whiting,' ! \ 



OS 



0*268 



1-2 



0*9047 





0*8 



0*268 



1*16 



0-8172 



Ling, .... 



0* 



0*1795 



1*08 



0*6595 





0*9 



0*3817 



1*3 



1*1503 



Halibut,* 



2-5 



8*1812 



3*8 



28*732 



T inn rr T?rinfTrli T^ili 



\J o 





1 *1 



V 0»7U«7 



0-7 



0-1795 



1*02 



0-5556 



Brill, .... 



0-8 



0*268 



1-3 



1*1503 



Sail-fluke, 



0-75 



0-220 



1-13 



0-7555 



Plaice, .... 



1 ^'^ 



0-6969 



1*93 



3-764 



\ 1-21 



0-9276 



1*88 



3-479 



Lemon Dab, . 



0-8 



0*268 



1-3 



1-1503 



Witch ,, . 



0-75 



0-2209 



1-15 



0*7963 



Common . . . 

 Flounder, 



0-56 



0-0919 



0-85 



0-3215 



0-65 



0*1437 



0-92 



0-4072 



Sole, . . . . 



0-9 



0*3817 



1-4 



1*4367 



Solenette, . . . 



0-8 



0-268 



1-2 



. 0-9047 



It will be observed that the ratio of increase does not always corres- 

 pond in different species, a circumstance no doubt due in part to the 

 true mean diameter not having been ascertained ; partly also to an actual 

 difference in the amount of expansion in different species. 



The fact, however, that a threefold or fourfold increase in volume 

 occurs in pelagic eggs at the end of ovarian growth is, I think it will be 

 admitted, of considerable interest. 



The third important physical change that occurs is in the specific 

 gravity of the ovum, which is much diminished. All opaque eggs sink 

 in sea water, and all mature translucent eggs float in sea water of 

 ordinary density. It is this property of the mature ovum that renders it 

 pelagic, before the change occurs it is a demersal egg. This may be 

 readily shown by cutting out a small portion from an ovary containing 

 both ripe and opaque eggs, and shaking it gently in a beaker of sea 

 water, when the ripe eggs float to the top and the few opaque eggs which 

 have become separated fall at once to the bottom. The specific gravity 

 of ripe pelagic eggs of different species varies considerably, but it may be 

 said to range between about 1023 and 1027. There is also a consider- 

 able variability in the specific gravity of the eggs of the same species and 

 of the same female. I found a batch of fertilised eggs of the plaice in 

 the multicelled stage to possess a specific gravity ranging from 1024 8 

 to 1026. 



I was desirous of determining the density of the large opaque eggs and 

 of the mature eggs for each species, and during last spawning season I 



* The measurements in this case are founded on those given by Brook, Holt, and 

 M'Intosh. It has been overlooked that Brook first described the ripe eggs of the halibut 

 in 1886. See Fomih Annual Report, Fishery Board for Scotlarid, p. 224. 1886. 



