708 PROFESSOR W. C. M‘'INTOSH AND MR E. E. PRINCE ON 
regularity of geometrical progression. The size of the blastomeres is likewise far from 
uniform after the 8-cell stage, and in the 14- to 16-cell stage especially, they vary 
very much in size and shape, the outer being large and somewhat rectangular, while those 
more central are smaller and ellipsoidal. This distinction between the more external and 
the inner cells Batrour noted in Elasmobranchs (No. 11, p. 392), and compared 
it to the horizontal furrow which separates the smaller pigmented spheres from the 
larger spheres of the vegetal pole in Rana (cf. figs. 3, 4, and 5, pl. xv. No. 11, and our 
Pl. IX. fig. 8). The form of the dise varies, changing from the circular outline of the 
early blastodise (Pl. XXII. fig. 1) to a more or less regular quadrate figure (PI. X. fig. 9), 
and reassuming the circular form when the multicelled stage (morula) is reached (PI. II. 
fig. 13, a). The first furrow parallel to the base of the disc passes across the median 
horizontal plane at about the 50-cell stage (PI. II. fig. 14), and the subsequent cleavage 
becomes very complicated. Owing to the increasing pressure of adjacent cells, the 
rounded form of each cell (PI. X. fig. 10) becomes altered, and the polygonal shape is 
assumed (PI. II. fig. 19). The size of the blastomeres shows much variability, though 
the variation is now within narrower limits. In profile the disc up to this stage has 
maintained the plano-convex outline, which is often retained until the 180-cell 
stage or later (Pl. X. fig. 10); but when the cells are so subdivided as to appear 
almost of one size, a marked bi-convexity is assumed, and upon the yolk a depres- 
sion is formed in which the blastoderm rests (PI. II. fig. 2), as it does permanently 
in Salmonoids (Leresouier, No. 93, p. 485; OELLACHER, KiErN); but later it spreads 
out in Gadoids and other forms, and appears as a flattened plaque in which several 
layers of similar cells can be distinguished (Pl. Il. figs. 3 and 15, e). There is 
no marked difference in the cells of the various strata, and the blastodermic layers 
are not readily distinguished, as they are in Elasmobranchs.* Batrour and other in- 
vestigators have made allusion to this similarity in the size and contour of the cells 
of the Teleostean blastoderm (vide Batrour, No. 11, p. 551; and LeReBouLLET, 
op. cit.). 
It is true, as already pointed out, that in very early cleavage the marginal cells are 
distinguished from the inner cells by a marked difference in size (Pl. IX. fig. 8); 
nor is the distinction lost with the appearance of the horizontal furrows, though it 
cannot be due, as is undoubtedly the case in Elasmobranchs and Amphibia, to the 
greater proportion of yolk-matter present in the outer germinal protoplasm, for there 
does not appear to be any conspicuous difference in their physical character. 
In the Elasmobranch blastoderm of about one hundred cells, the ectoderm is readily 
distinguished from the endoderm or “ lower layer ” cells by their smaller size, and marked 
columnar character. Rreneck + observes that the upper cells of the germ give rise to 
a two-layered sensory lamina (or leaf), and that some of the lower cells fall to the bottom 
* Ryper, however, speaks definitely of three layers in the multicelled stage of the Teleostean germ ; but this does 
not agree with other descriptions by the same author. 
+ Archiv f. Mikr, Anat., vol. v., 1869. 
