. A manual of zoology. Zoology. Fig. 100.—Unequal cleavage of the egg of Petroimjzon. (After Shipley, from Hat- schek.) A, stage of eight cleavage spheres; B, blastula in meridional section. The dissimilarity of the cleavage spheres begins with the equatorial furrow. --^^ J Fig. 101.—Unequal cleavage of a snail's egg, Nassa nmtahiUa. (After Bobretzty.) I, tlie first meridional furrow lias divided the egg into unequal parts; II, the second meridional furrow has formed three smaller and one larger cleavage sphere (seen from the side); 111, the equatorial furrow has formed four smaller animal and
. A manual of zoology. Zoology. Fig. 100.—Unequal cleavage of the egg of Petroimjzon. (After Shipley, from Hat- schek.) A, stage of eight cleavage spheres; B, blastula in meridional section. The dissimilarity of the cleavage spheres begins with the equatorial furrow. --^^ J Fig. 101.—Unequal cleavage of a snail's egg, Nassa nmtahiUa. (After Bobretzty.) I, tlie first meridional furrow lias divided the egg into unequal parts; II, the second meridional furrow has formed three smaller and one larger cleavage sphere (seen from the side); 111, the equatorial furrow has formed four smaller animal and four larger but unequal vegetative cells (seen from the animal pole). cells are in straight lines (fig. 100, J), but in otliers the cells alternate (fig. 102); this is called spiral cleavage and is common in several groups. fi. Meroblastic Eggs with Partial Cleavage. 3. Discoidal Cleavage.—The yolk is so collected in the vegeta- tive iiortion of the egg that it jircveutscleavage;cleavage, there- fore, is limited to the region around the animal pole and here forms a disc of small cells, the anlage of the embryo, or hla^fo- (leriii (telolecithal, meroblastic eggs) (figs. 90, 103). 4. Siqjerficial Cleavage.—The yolk is collected in the centre of the egg and prevents cleavage; in conse(iuence of this only the outer layer of the egg divides into cells, which, in the form of a continuous superficial layer, enclose the unsegmented central mass (centrolecithal eggs) (fig. 104).. Fig. 103.—Spiral cleavage in Crevidiila (After Conlilin.). Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Hertwig, Richard, 1850-1937; Kingsley, J. S. (John Sterling), 1854-1929. New York, H. Holt and Company
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