of the Pishery Board for Scotland. 



made a large number of observations by using sea water mixed with a 

 saturated solution of common salt or with distilled water, and a delicate 

 Challenger hydrometer was used for this purpose. I found, however, 

 that these observations were vitiated, inasmuch as the penetration of the 

 salt solution into the eggs was too rapid to allow an accurate determina- 

 tion to be made, and the method had to be abandoned. Dr. Milroy, 

 however, has determined the specific gravity in a number of cases (see 

 p. 135) quite sufficient to bring this change in the physical properties of 

 the eggs into line with the others. Thus the opaque eggs of the plaice, 

 with a mean diameter of l'13mm., had a specific gravity of 1*070, 

 while the mean specific gravity of the mature translucent eggs is 

 about 1-025. 



Another physical change, not so obvious, but still easily demonstrated, 

 takes place when the enlargement of the ovum occurs at maturation ; 

 and it is a change of considerable interest as showing the nature and 

 comparative rapidity of the expansion. The egg-capsule or zona radiata 

 becomes attenuated and the surface markings separated by the stretching 

 of the membrane. The change is precisely of the nature one would 

 expect to be presented by the patterns on the surface of an elastic 

 spherical shell which had undergone mechanical distension ; but the 

 degree of attenuation does not correspond to what calculation shows 

 ought to occur in a perfectly homogeneous elastic membrane stretched to 

 the same extent. For example, the thickness of the capsule of the large 

 opaque eggs of the plaice is about 0*054mm., and that of the capsule of 

 the mature translucent egg about 0"025mm., while calculation shows 

 that the latter ought to be 0*01 63mm., if it were uniformly extensile. 

 The explanation of this circumstance is to be sought in the structure of 

 the capsule, previously referred to (p. 105) ; the creases or plications are 

 opened out by the forcible expansion of the ovum. The basket-work 

 pattern is therefore much more coarse and obvious in the capsule of the 

 distended mature ovum than in that of the opaque ovum, and, moreover, 

 the dots of the superficial layer are drawn apart. In the eggs of the 

 torsk {Brosmius brosme) the surface of the capsule is characterised by 

 the presence of relatively large distinct points or pits ; in the opaque egg 

 seven or eight of these were found to lie between two lines 0*05mm. 

 apart, while there were only two or three, and never more than four, in 

 the same distance in the fully mature egg (Fig. 25^^, PI. I.). The appear- 

 ance of the basket-work pattern in the opaque and mature eggs of the 

 sole, under the same power, is shown in Fig. 25"^, PI. I. 



The thickness of the egg-membrane in a few species is appended. 



Species. 



Opaque, in mm. 



Mature, in mm. 



Grey Gurnard, 



0*042 



0-026 



Red Gurnard, 



0-044 



0-020 



Cod, 



0-04 



0-022 



Haddock, 



0-043 



0-024 



Bib, 



0-016 



0-01 





0-027 



0-016 



Four- bearded Rockling, .... 



0-02 



0-009 



Brill, 



0-026 



0017 



Witch 



0 026 



0-016 





0-054 



0-025 





0-036 



0-02 



Sole, 



0-03 



0-019 





0-018 



0-01 



