. A manual of zoology. Zoology. GENERAL EMBliYOLOGT. 151 3. Cleavage Process. Arrangement of the Cleavage Planes.—The fertilized egg-cell divides in rapid succession into 2, 4, 8, 16, etc., cells, "wlnoli become continually smaller, since the mass of the egg does not increase. The cells are called cleavage spheres, or blastomeres, the whole process the cleavage process, or segmentation, because, at each division, furrows arise on the surface which continue to penetrate more deeply (fig. Ho). As a rule each. Fir;. 96.—The equal cleavace of Aiiiiihinr-iis hnirrnlfifii^. (After Hatschek.) I,
. A manual of zoology. Zoology. GENERAL EMBliYOLOGT. 151 3. Cleavage Process. Arrangement of the Cleavage Planes.—The fertilized egg-cell divides in rapid succession into 2, 4, 8, 16, etc., cells, "wlnoli become continually smaller, since the mass of the egg does not increase. The cells are called cleavage spheres, or blastomeres, the whole process the cleavage process, or segmentation, because, at each division, furrows arise on the surface which continue to penetrate more deeply (fig. Ho). As a rule each. Fir;. 96.—The equal cleavace of Aiiiiihinr-iis hnirrnlfifii^. (After Hatschek.) I, division" into two (fofmation of tlie firjit meridional furrow): II, df\nsion into four (second meridional furrow) forming; four cleavage splieres (fourth ishidden); III, division into eight (equatorial furrow; the seventh and eiglitli cleavage spheres hidden); IV, blastula in optical section. A single layer of cells surrounds tlie cleavage cavity. In I, II, III, a polar body is sliown. new plane of cleavage is as nearly as jwssible perpendicular to the preceding. Hence the first three cleavage planes, which cause the division into 2, 4, and 8 parts, are similarly arranged in almost all animals. Using the glolje as a basis tor comparison, one speaks of a first and a second meridional furrow (I, II), and calls the third the equatorial furrow (III). Tlie intersections of the two meridi- onal furrows form the poles of the egg, the animal and the vegetative, so called because the material of the one is used chiefly for animal organs (nervous system), the material of the other for vegetative organs (digestive tract). Influence of the Yolk upon Segmentation.—Different kinds of cleavage processes are distinguished, the peculiarities of \vhich depend upon two factors: (1) npon the quantity of material, food- yolk, serving for nourishment of the egg; (2) npon the arrange- ment of this. The food-yolk hinders the division, since it is a material which is incapable of active mo
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