DEVELOPMENT AND LIFE-HISTORIES OF TELEOSTEAN FISHES. 701 
or periblast proper, around the margin of the disc. A thin stratum may also be 
distinguished creeping over the segmenting blastoderm as an external pellicle, referred 
to before as probably homologous with Ransom’s inner sac, and this layer sends down 
processes which fill up the interspaces between the large primary blastomeres 
(Pl. HU. fig. 1, p). This appearance, which is distinctly seen in sections of the early 
blastoderm, may, it is true, be really the dilute plasma, or perivitelline fluid, penetrating 
the inter-blastomeric fissures, though more probably it is periblastie protoplasm, forming 
an intermediary substance, such as LEREBOULLET distinctly recognised (No. 93, p. 493), 
and as E. vAN BENEDEN figures (No. 25, pl. iv. fig. 7, &c.). 
To sum up briefly, we may say that the protoplasm interfused with the food-yolk 
continues from a late intra-ovarian stage to collect superficially as a cortical layer, and 
forms— 
(1) The blastodise at the animal pole, and in rare cases a transient pseudo-dise at 
the vegetal pole (Pl. IL. fig. 1, bdm). 
(2) The intermediary, or sub-blastodermic layer (Pl. II. fig. 1, p'). 
(3) The thickened marginal wall or periblast-ring (PI. II. fig. 1, per). 
(4) The superficial envelope and inter-blastomeric substance of the segmented dise 
(PL te nel; 7’). 
(5) The sole intra-capsular envelope of the deutoplasmic globe or yolk, prior to the 
epibolic extension of the blastoderm (PI. II. fig. 1, p’). 
The Subgerminal or Nutritive Disc.—Reference has been made to the layer of proto- 
plasm beneath the blastoderm proper (PI. II. figs. 1 and 15, a, b, c, d, e—ep), and it has 
been distinguished from the periblast proper, 7.e., the thickened peripheral wall, and the 
nuclear zone round the margin of the disc, by various names, such as ‘‘ intermediary layer” 
(BaMBEKe), “ disque huileux” (LEREBOULLET), “ Rindenschicht” (Hrs), “ median lens or 
lentille ” (E. van BrnepEN); while other observers, e.g., HAECKEL and Ransom, have not 
recognised it, the latter indeed saying of the blastodermie surface in contiguity to the 
yolk, that it seems to be merely “ the corpuscles resulting from segmentation in contact 
with the fluid-yolk” (No. 127, p. 467). It appears to arise like the rest of the protoplasmic 
envelope of the yolk by superficial segregation, though BAMBEKE attributes its formation 
to a centripetal extension of the peripheral annulus; but LeREBoULLET’s statement 
probably represents the origin of this sub-blastodermic stratum more truly, when 
he says that in Hsow and Perca it arises simultaneously with the disc, these nutritive 
elements, as he calls them, following the plastic element in their migration to the 
animal pole (No. 93, p. 11), and at the earliest stages may, as Kuprrer supposes, give 
nutriment to the germinal dise (No. 87, p. 194). Ransom did not distinguish a stratum, 
however, but speaks of “a collection of dark oil-granules distinct from the large drops 
which float in the yolk.” He saw granules and globules of oil below the disc, and as 
these are consumed during the development of the germ-mass, it is probable that a 
kind of yolk-digestion goes on, 
