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



the cleavage-plane, occur sparsely over the disc, and especially in its basal portion (PL II. 

 fig. 18). They probably have an important relation to the cleavage-process, as Balfour 

 thought. In sections they occur as clear rounded vacuolations, but are without doubt 

 filled with indifferent fluid, and probably are no other bodies than the clear vesicles 

 scattered over the cortical protoplasm in the ripe unfertilised egg. The vesicles disappear, 

 as we have seen, with the polar segregation of the disc ; but they really persist, and are 

 transferred to the disc, where they accumulate (PI. II. fig. 18), often coalescing and 

 forming larger vesicles, but not to be confounded with the oily extra- embryonic spheres, 

 though Lereboullet does so, saying — " I have seen large transparent spaces like those 

 M. Vogt shows in his figs. 113 and 114 (Embr. of Salmon) produced by oil" (No. 93, 

 p. 486). It is possible that these vesicles, or rather their clear fluid contents, may render 

 mechanical aid during cleavage, filling up with their less consistent matter the furrows 

 formed by the dehiscence of the segmenting blastomeres. After the first furrow, which 

 is perpendicular to the basal plane of the disc, has produced the first pair of blastomeres, 

 the pullulation of the protoplasm is marked, each cell becoming increasingly definite, 

 a feature which Kupffer regards as indicating the appearance of an equatorial furrow 

 (No. 87, p. 196, Taf. ii. fig. 15, &c). Such an equatorial furrow, according to Hoffman, 

 appears before the first perpendicular furrow, and thus the disc would be separated 

 from the marginal protoplasm as well as from the yolk at the first stage in cleavage. 

 A complete discontinuity of yolk and germ produced by cleavage does not accord well 

 with the actual condition in the ovum, and the first furrow would appear to be the 

 primary perpendicular one. When this furrow has penetrated almost to the base, 

 for it does not perfectly bisect the disc, as Lereboullet long ago noticed (No. 93, p. 481 ; 

 see his fig. 18, pi. i.), small furrows directed towards the centre of the disc, appear at 

 right angles to the first cleavage-lines, followed by the appearance, along the course 

 marked by them, of a second cleavage-furrow, which divides the two primary blastomeres 

 into four almost equal segments. Each of the newly-formed blastomeres has a rudely 

 square outline, its two free outer sides being rounded, while the two inner sides are more 

 nearly straight lines, and mark the perpendicular planes which are in apposition to the 

 similar surfaces of the two neighbouring blastomeres. In each blastomere a large nucleus 

 can be made out, though often with difficulty, as Lereboullet noted; but not ill-defined, 

 as the same author further stated (No. 93, p. 483), for the nuclei appear as homogeneous 

 hyaline vesicles with a smooth and distinct contour, the bright contents of which are 

 termed by Auerbach the " ground substance " # (PI. II. fig. 4, a). Nuclear division is 

 not easy to follow in the living ovum, though blastomeric cleavage is readily observed. 

 The ovum of Esox seems well adapted for nuclear observations, as Lereboullet found out 

 when he contrasted this species with Perca, for in the latter the nuclei had greater 

 transparency and were thus less readily seen (No. 93, p. 513). In this species (Esox) 

 Kupffer followed the division of the primary nucleus, and watched the first furrow pass 

 down between the two newly formed nuclei ^No. 87, p. 207). 



* Orcjanolocjische Studien, Breslau, 1873-4. 



