DEVELOPMENT AND LIFE-HISTORIES OF TELEOSTEAN FISHES. 681 



dotted appearance would be produced superficially, as in a large number of Teleostean 

 eggs, especially in the comparatively dense capsules of fresh-water forms. These 

 punctures may be comparatively large and distinct, as described by Muller in Perca 

 (No. 1 1 1, p. 1 87-8), and by Leuckart in Esox; or they may be of smaller size, as in Salmo 

 fario (No. 153, p. 101, and fig. 68, c, d), Gastrosteus spinachia; or of extremely minute 

 size, as in certain pelagic forms, e.g., Trigla gurnardus and P. platessa. Frequently 

 the striations are observed to pass only partially through the capsule, and the outer 

 stratum is then imperforate, instances of this condition being the capsule of Clupea 

 (No. 87, p. 177), and Esox (Aubert), Gastrosteus spinachia, and probably Trigla 

 gurnardus. In other ova they traverse the whole thickness of the capsule, as is the case 

 in Salmo fario (No. 4, p. 198), and in Perca fiuviatilis, according to the experiments 

 of J. Muller (No. Ill, p. 188). The distinguished observer just named was convinced 

 that, when he placed the eggs of the perch under pressure, oily matter from the interior 

 of the egg could be squeezed through the canals of the zona radiata, and the canalicular 

 structure of this membrane would appear to be demonstrated in this instance. Other 

 observers, however, strenuously deny this, and, like Andre, pronounce the so-called 

 canals to be nothing more than rectilinear striations directed radially from the inner 

 to the outer surface of the capsule (No. 4, p. 202), precisely like the radial fibrillations 

 in the zona of the fowl's ovum. It is only necessary to observe the effect of desiccation 

 on the egg of the cod, and then the action of water, to prove that a ready interchange 

 occurs through the zona either by pores or by ordinary endosmose. 



Little can be said here as to the origin and growth of the zona radiata, for its 

 development is already complete when the ovum reaches maturity. That it is a true 

 vitelline membrane admits of little doubt ; and Hoffman's opinion, that it is secreted 

 by the vitelline mass as a superficial layer during the intra-ovarian period, and is not 

 separated until it shows an appreciable density and firmness, is probably well founded.* 

 Eansom holds that, after it is defined as an external membrane, it continues to grow 

 interstitially up to a certain stage, when growth ceases, and it performs a jsolely 

 protective function (No. 127, p. 494). Other layers are formed later upon the surface 

 of the yolk after the zona radiata has become detached as an elastic protective capsule, 

 and these may claim to be called vitelline membranes, as indeed they have been styled 

 by various authors. Thus, Oellacher, when speaking of the vitelline membrane in the 

 trout (No. 113), really means the stratum of germinal protoplasm, the polar segregation of 

 which forms the blastodisc ; while Lereboullet uses the same term for the layer 

 of protoplasm which ventrally limits the intestinal tract of the embryonic fish at a 

 comparatively late stage (No. 93, p. 612). Such uses of the term " vitelline membrane" 

 for very different layers (though all of vitelline origin) are not to be approved, and the 

 name zona radiata is at once distinctive and appropriate for that vitelline membrane 



* The development of the vitelline membrane in Triton has been shown in an interesting manner by Mr Iwakawa, 

 and his descriptions and admirable figures (see No. 75, p. 274, and pi. xxiv. figs. 24-26) will apply in the case of the 

 Teleostean capsule. 



VOL. XXXV. PART III. (NO. 19). 5 S 



