142 



Part III. — Si'ieenth Annual Report 



The ova of Lophius piscatorius may be perhaps regarded as a class 

 distinct from the ordinary demersal ova. When immature they have a 

 volume of about '268 cub. mm., when mature 4:"18 to 5 cub. mm. Those 

 eggs are of especial importance, as they are examples of ova which are 

 dependent for their floating capabilities not on their specific gravity but 

 on that of the gelatinous sheet in which they are imbedded. 



^^ow what is this change in volume due to 1 To answer this question 

 one must find out to what extent the specific gravity is diminished as the 

 volume increases ; the difiTerence in the percentage of water in the unripe 

 and ripe ova ; and lastly, the difference in the nature of the solids in the 

 two conditions. First of all, the alteration in specific gravity must be 

 studied, and I shall take up first of all the pelagic ova. 



CHANGbIS IX THE SPECIFIC GRAVITY OF PeLAGIC Ova DURING 



^Iaturation. 



In order that the eye may rapidly note the differences in specific 

 gravity, I have arranged the results in tabulated form. 



Pelagic Ova. 



Immature. 



Mature. 



Plaice, ..... 



1-084 



1-024-1 025 



Turbot, 



1-070 





Saithe, ..... 



1-046 



1-023 



Whiting, .... 



1-068 



1-024 



Cod, 



1-055 





Haddock, .... 



1-080 



1-020 



Brill, 





1-025 



Flounder, .... 





1024 



Ling, 





1022 



In the plaice ovum, just before the final distension, the specific gravity 

 was found to be 1*027. The immature ova given in above Table were 

 very immature. The Sp. Gr. gradually diminishes as ripening proceeds, 

 but it is in the final stage that the change is most marked. 



Demersal Ova. 



Immature. 



Mature. 



Herring, .... 

 Lophius piscatorius — 



(a) The ova, 



(b) „ in their gelatinous 

 sheet, .... 



Cottus scorpius, 

 Cydopterus lumpus^ 



1-087-1-090 



1-045 



1-009 



1066 



1034 



1-005 

 1-060 

 1-057 



In the different varieties of pelagic ova the specific gravity of the fully 

 ripe eggs is always decidedly below that of the sea water. The results 

 obtained by Hensen are lower than mine, probably because his calcula- 

 tions were based on his measurements of the volume of the ova from the 

 diameter. I think that the displacement method which I employed is a 

 better one. In the fully matured demersal ovum the specific gravity is 

 always decidedly above that of the sea. 



From observations that I have made in different specimens, it seems 

 that all through the process of maturation there is a gradual decrease in 



