722 PROFESSOR W. C. M‘IINTOSH AND MR E. E. PRINCE ON 
in a large measure by differences of temperature, light, the condition of the water, and 
other features of the laboratory, though the divergence between a pelagic and a demersal 
ovum in this respect is so marked as not to be fully explained in that way. Thus in 
Gastrosteus a blastoderm, which covered fully one-eighth of the yolk, had embraced in 
twenty-four hours only slightly over a quadrant, while in Plewronectes it had extended 
over nine-tenths of the yolk-surface. Again, in Gadus aeglefinus, when the temperature of 
the tanks was kept lower, epiboly was as slow as in the case of Gastrosteus under a higher 
temperature. When the germ covers barely a quadrant the margin becomes visibly 
thickened, this being the first indication of the embryonic rim (Kuprrer’s Keimsaum, 
OxLLacHER’s Keimwulst), which plays so important a part in the formation of the 
embryo (Pl. IL. fig. 17, br). This appearance of the rim LEREBOULLET connects with 
the thinning out of the germ, and explains it as a process of mechanical transference— 
the central cells passing to the circumference, as indicated by the increased density 
of the latter, which forms “a true pad around the egg” (No. 93, p. 458). The cells 
of the germ undoubtedly become greatly flattened, as we see in Pl. II. fig. 3, as com- 
pared with fig. 17, when extension has proceeded largely ; but such a transmission of 
cells less truly represents the process of peripheral thickening than the inflection of 
old, the reception of new cells described below, and the aggregation of these in a 
marginal band, 
We have referred to epiboly as to all appearance a retrogression,* but it is not really 
so, it is rather a process of invagination such as we find so widespread in the develop- 
ment of animal germs. ‘This process, had the amount of food-yolk present allowed, 
would have resulted in the establishment of an involuted epithelial lining to the gastrula. 
The exaggeration of the trophic mass, which must ancestrally have been even much 
greater, prevents this progress of the ectoderm, and as its extension is not arrested, it 
follows that the yolk-globe is epibolically enveloped. While, as indicated, the germ 
becomes thinner, yet along one radius this decrease is not so great as elsewhere ; 
in other words, the germ, soon after the close of segmentation, shows a thickened 
embryonic radius which never disappears (Pl. Il. figs. 15 and 17). When the 
germinal cavity (gc) is formed, this portion is well marked, as the cavity lies in front 
of it, 7e., eecentrically, and all through development it is thus distinguished by its 
greater thickness, so that LerEBOULLET cannot be correct in saying that the embryonic 
radius only commences when epiboly is nearly complete (No. 93, pp. 495-6). He failed, 
indeed, to notice in his species any trace of the rim until the blastopore is in its final 
stage, then, he says, a very distinct rim is formed around the “trou vitellaire” of Voer, 
In the trout, as OELLACHER shows (see No. 114, Taf. i. figs. 2-5), this radius is well marked ; 
but in other forms it is less apparent at an early stage, though the (embryonic) radius 
in all Teleostean ova is probably distinguishable from the non-radial portion by its 
greater thickness. In sections through the blastoderm before the equator is reached 
(Pl. I. fig. 17, and Pl. IV. fig. 8), the germ consists merely of two layers—ectoderm (ep) 
* Vide E. E. Prince, Annals Nat. Hist., July 1887. 
