of the Pishery Board for Scotland. 



105 



separate from the outer surface of the zona radiata as a delicate lamella 

 in specimens which have been softened in water. I have also observed 

 it sometimes in carefully rupturing the large mature living eggs, by 

 pressure, as a thin lamina pushed out in lenticular fashion by the yolk 

 oozing from the rupture in the subjacent membrane, and gradually 

 stripped from the latter as the escaped yolk increased in quantity. This 

 superficial pellicle probably represents a membrane upon which the 

 pavement of follicular cells rests, and the minute dots may possibly 

 be due to depressions caused by the follicular cells. They are certainly 

 not, what some observers have apparently thought, the outer openings of 

 radial canals ; although in optical section, with moderate powers, the 

 presence of these minute dots arranged in lines may simulate fine 

 striations passing through the zona radiata. 



The structure of the egg-membrane proper, the so-called zona radiata, 

 has been much discussed. For reasons which will shortly be given, it 

 appears that in the substance of the zona of pelagic eggs numerous creases 

 or plications exist. In the opaque egg, before the mechanical expansion 

 at maturation takes place, the sides of these plications or creases are in 

 contact, but when the egg becomes distended to three or four times its 

 previous size, they are stretched and separated, in much the same fashion 

 as occurs in the expansion of the walls of a concertina when it is drawn 

 out ; in the egg, however, the plications are irregular. The question of 

 whether the zona possesses fine pores or radial canals has been much 

 disputed, but it is unnecessary to enter into the discussion here ; a pretty 

 full discussion of the subject is given by Brock* and by Eigenmann.t A 

 radial appearance may be frequently seen in the membrane of pelagic eggs 

 examined in optical section, with moderate powers, but in many cases at 

 least it is due to the radial lines of dots described above. In mounted 

 sections of the immature eggs of the sole, lemon sole, gurnard, and soleuette, 

 radial striation is obvious (Fig. 7, PI. I.), but in most species it cannot be 

 discerned, although a fibrillated appearance is not uncommon. The 

 existence of radial porous canals in the zona of pelagic eggs appears to me 

 to be unproved, although many facts show that the membrane, like other 

 animal membranes, is porous. 



The question whether there is more than one layer in the egg-membrane 

 of pelagic eggs has also been disputed, although most observers are agreed 

 that there is not. In demersal eggs two or more layers usually exist 

 (Figs. 16, 17, PI. I.; Figs. 10, 11, PI. III.). Sars described several lamellse 

 as existing in the capsule of the cod's egg, j: but he has not been followed 

 in this opinion. The superficial lamella in the egg of the plaice has 

 been referred to, and it probably is present in the eggs of other species 

 in which the dotted pattern exists ; but this condition is of little 

 importance, although it may be the homologue of the outermost of the 

 two layers of demersal eggs. But there is another membrane normally 

 present, I believe, in both pelagic and demersal eggs, which is no doubt 

 of functional importance. I refer to an extremely delicate membrane 

 which encloses the vitellus, and which clothes it after the formation of 

 the perivitelline space, and therefore separates it from the perivitelline 

 fluid. Such a vitelline membrane has been described by several 

 observers, though its existence is usually denied. Ransom mentions it 

 in the egg of the stickleback as " a delicate, colourless, translucent, 

 homogeneous membrane," which he called the " inner yolk-sac," his 

 " outer yolk-sac " being what is usually known as the zona radiata.§ It 



* Moiyh. Jahrb. 'iv., p. 505. 1878. 



t BulL Mus. Conip. Zool. Harv. Col. Vol. xix., No. 2, p. 129. 1890. 



X Op. cit. § Phil. Trans. Roy. Soc, London. Vol. 157, p. 431. 1868. 



