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 II. 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 periblastic protoplasm, forming 

 an intermediary substance, such as Lereboullet distinctly recognised (No. 93, p. 493), 

 and as E. van Beneden figures (No. 25, pi. 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 blastodisc at the animal pole, and in rare cases a transient pseudo-disc at 

 the vegetal pole (PI. II. fig. 1, bdm). 



(2) The intermediary, or sub-blastodermic layer (PI. II. fig. 1, p 1 ). 



(3) The thickened marginal wall or periblast-ring (PI. II. fig. l,per). 



(4) The superficial envelope and inter-blastomeric substance of the segmented disc 

 (PI. II. fig. l,p 2 ). 



(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 z ). 



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 — cp), and it has 

 been distinguished from the periblast proper, i.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 " (His), " median lens or 

 lentille " (E. van Beneden); while other observers, e.g., Haeckel and Ransom, have not 

 recognised it, the latter indeed saying of the blastodermic 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 Esox 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 Kupffer supposes, give 

 nutriment to the germinal disc (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. 



