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 Balfour 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, pi. xv. No. 11, and our 

 PI. IX. fig. 8). The form of the disc varies, changing from the circular outline of the 

 early blastodisc (PI. XXII. fig. l) 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 (PL II. fig. 14), and the subsequent cleavage 

 becomes very complicated. Owing to the increasing pressure of adjacent cells, the 

 rounded form of each cell (PL X. fig. 10) becomes altered, and the polygonal shape is 

 assumed (PL 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 (PL II. fig. 2), as it does permanently 

 in Salmonoids (Lereboullet, No. 93, p. 485 ; Oellacher, Klein) ; 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 II. 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.* Balfour and other in- 

 vestigators have made allusion to this similarity in the size and contour of the cells 

 of the Teleostean blastoderm {vide Balfour, 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. Rieneck t 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 



* Ryder, however, speaks definitely of three layers in the multicelled stage of the Teleostean germ ; hut this does 

 not agree with other descriptions hy the same author. 



t Archivf. Mikr. Anat., vol. v., 1869. : 



