DEVELOPMENT AND LIFE-HISTORIES OF TELEOSTEAN FISHES. 681 
dotted appearance would be produced superficially, as im a large number of Teleostean 
egos, 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. 111, p. 187-8), and by Levekartr in Hsox; or they may be of smaller size, as in Salmo 
fario (No. 153, p. 101, and fig. 68, c, p), Gastrosteus spinachia; or of extremely minute 
size, as in certain pelagic forms, eg., 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 Esow (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 fluviatilis, according to the experiments 
of J. Miter (No. 111, 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 Horrman’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.* 
Ransom 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 solely 
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, OkLLAcHER, 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 blastodise; 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 pl. xxiv. figs. 24-26) will apply in the case of the 
Teleostean capsule, 
VOL. XXXV. PART III, (NO. 19). a8 
