716 PROFESSOR W. C. M'INTOSH AND MR E. E. PRINCE ON 



central lamella can be seen, and that it is " most probably formed by extension under 

 the disc from the bevelled ring outside." Like Agassiz and Whitman, Lereboullet 

 holds that this layer is formed later than the disc, observing that, " at the close of seg- 

 mentation, no trace of the mucous layer is seen, though dispersed vitelline globules are 

 visible out of which this layer is formed" (No. 93, p. 495). 



Three theories of the origin of the periblast are thus held — (l) that it is simply a 

 separation, a superficial segregation of protoplasm interfused in the yolk, and reaching 

 the animal pole too late to enter the disc ; (2) that it does form part of the disc, but 

 afterwards issues from it all round the margin, extending as an extra-germinal layer ; (3) 

 that it is not a mechanical transference, but an actual transformation of yolk-particles. 

 The second and third views, just stated, involve processes less simple than the first, and 

 if a process of simple transference, the segregation of interfused germinal matter, suffice, 

 it is needless to resort to any explanation more complex. The superficial segregation of 

 protoplasm implies that a sub-blastodermic stratum is never wanting, and that, from the 

 first, the blastomeres "do not rest" (in E. van Beneden's words) "immediately on the 

 vitellus ; they are separated from it by a layer of substance which is finely granular " 

 (No. 25, pp. 44, 45). 



For some time the periblast remains homogeneous, devoid of nuclei, and not separable 

 from the yolk-cortex beyond, save by its slightly greater thickness (per., PI. II. fig. 14), 

 and by the occurrence of scattered granules in it, which are distinctly seen at the end of 

 the first day in G. morrhua. Further, the occasional presence of protoplasmic filaments 

 over the area of the periblast seems to indicate its tenacious character (PI. II. fig. 7). 

 It forms in some ova, as Lereboullet and E. van Beneden noted, a considerable 

 thickening below the centre of the germ. This thickened central lamella disappears 

 later, and it is doubtful whether in many species it is ever present. The peripheral 

 thickening is usually well marked * as a prism-shaped ring (per, PI. II. figs. 1-3), which 

 is triangular in cross-section, the disc resting upon one side, the lowest side being in 

 contact with the yolk, while the third is external and free. When segmentation is 

 far advanced and the biconvex form has been assumed, large nuclei begin to appear in 

 close proximity to the margin of the germ (PI. IX. fig. 1 0, n). Though irregularly disposed, 

 two or three rows may be distinguished (PI. IX. figs. 9 and 10, n; and PI. II. fig. 4a), 

 and they rapidly extend outwards over a variable area, which is known as the nuclear 

 zone. The nuclei are large clear vesicles, having a slightly pinkish hue in certain lights 

 (transmitted), well-defined and rounded in form, often slightly elliptical, and sho wing- 

 in some cases granules or nucleoli (PI. II. figs. 6, 8, n ; and PI. IX. fig. 9). At first 

 they are crowded together, but as they extend towards the equator they show a 

 tendency to a regularity of disposition which is very remarkable when they are five 

 or six deep. Kupffer describes these bodies in certain species of Gastrosteus as 

 larger than the nuclei of the germ, separated by regular intervals three times the 

 diameter of each nucleus, and arranged in rows duly alternating, the row nearest to the 

 * Lereboullet descants upon its unusual thickness in the trout (No. 95, p. 14). 



