106 



Part III. — Sixteenth Annual Report 



has also been described by Vogt in the eggs of Coregonus* by Oellacher 

 in the eggs of the trout,t and by others. Among pelagic eggs, Scharff 

 found it in sections of the gurnard J, but only in the young eggs, and 

 Calderwood has described it in a section of the egg of the sole.§ 

 Eigenmann I| thinks its existence to be unproved. 



I am able to affirm its existence in the eggs of Gadus ceglefinus, G. 

 vireTiSj and Fleuronectes flesus. In dealing with fresh (opaque) eggs 

 under pressure it was sometimes revealed as a delicate pellicle projecting 

 from a rupture in the capsule, bounding and retaining the protruding 

 yolk (Fig. 8, PI. I.), and then bursting and emitting the yolk with a 

 rush on the slightest increase of pressure. At other times, on diminish- 

 ing the pressure, the yolk would be partially sucked back within the egg, 

 and the walls of the protruding membrane were then well observed (Fig, 9, 

 PI. I.). In certain other cases it could be seen within the zona when the 

 yolk had separated from the latter (Fig. 10). Usually it was ruptured 

 with the zona radiata and not distinguished. I have not been able to 

 satisfy myself of its presence in mounted sections. It is not a mere 

 condensation of the surface of the vitellus, at all events morphologically, 

 whatever it may be in origin. Under a high power this membrane 

 appeared to be quite homogeneous, and showed a faint double contour. 

 Its function is no doubt of importance in the mature egg after it has 

 been shed, and the perivitelline space formed between the zona and the 

 vitellus by the imbibition of sea water. It will then intervene between 

 the sea water and the periblast. Probably, although I have no observa- 

 tions on the point, this vitelline membrane is continuous with the inner 

 margin of the micropyle before the formation of the perivitelline space. 

 In the living fertilised egg of the flounder, at an early stage, I was able 

 to trace it, quite distinct from the periblast, up over the edge of the 

 blastodisc, where it was lost on the surface of the latter. In some eggs 

 of the same species after death, when the cortical protoplasm had begun 

 to retract, as it always does in dead eggs, from the zona radiata, the 

 delicate membrane was carried with it and was thrown into wrinkles in 

 the neighbourhood of the blastodisc, the lines converging towards the 

 latter in a radial manner, and sometimes passing over the edge of the 

 blastomeres. I cannot but think that further investigation will show 

 the existence of this membrane in other pelagic eggs. It may best be 

 shown by the patient examination of fresh specimens under pressure. 



With regard to the yolk in the large opaque eggs, there is nothing to 

 add to what has been already said. The yolk-spherules form the great 

 bulk of the egg. In fresh specimens under pressure they may be watched 

 escaping with the fluid matrix in which they are imbedded ; the applica- 

 tion of heat or of hardening agents coagulates this fluid, in which case 

 the spherules are observed to be attached to and imbedded in the granular 

 or flocculent colloid substance. After treatment with strong solution of 

 common salt, the contents come out in a clear, syrupy, coherent stream. 

 As has been said, the interstitial matter in demersal eggs is larger in 

 amount and less fluid that in pelagic eggs. 



The germinal vesicle is large and central ; I have never observed it 

 markedly excentric in the pelagic ovum. In the fresh condition it is 

 globular, or it may be somewhat ovate, and smooth, but in sections it 

 usually appears irregular in outline, or even stellate, apparently from 



* Embryologie des Salmones, in Agassiz's Hist. Nat. des poissons d'eau douce de 

 I'Europe centrale. 1842. 



t Zeitschr.f. Wissensch. Zool. Bd. xxii. 1872. 

 X Op. cit., p. 68. 



§ Journ. Marine Biolog. Assoc., New Series, Vol. ii,, No. 4, p. 307. 1892. 



II Op. cit. 



