. Outlines of zoology. k-containing hemisphere is the heavier, and conse-quently is always the lower half of the egg, however this maybe turned about. Round the ovum there is a delicatevitelline membrane, and this is again surrounded by a gela-tinous investment which swells up in water. The formationof polar bodies takes place before the liberation of the eggs. DEVELOPMENT OF THE FROG. 599 The spheres of jelly preserve the eggs and embryos fromfriction, prevent their being eaten by most birds, appear tobe distasteful to Gammarids, and often enclose in theirinterspaces groups of green Algae, wh
. Outlines of zoology. k-containing hemisphere is the heavier, and conse-quently is always the lower half of the egg, however this maybe turned about. Round the ovum there is a delicatevitelline membrane, and this is again surrounded by a gela-tinous investment which swells up in water. The formationof polar bodies takes place before the liberation of the eggs. DEVELOPMENT OF THE FROG. 599 The spheres of jelly preserve the eggs and embryos fromfriction, prevent their being eaten by most birds, appear tobe distasteful to Gammarids, and often enclose in theirinterspaces groups of green Algae, which help in spheres may also be of use in relation to the absorptionand radiation of heat. Fertilisation occurs immediately after the eggs are spermatozoa, which exhibit the usual features of maleelements, work their way through the gelatinous envelopes,and one fertilises each ovum. The first cleavage is vertical, and divides the ovum intoa right and a left half. If one of these two cells be punc-. FiG. 324.—Division of frogs ovum.—After numbers indicate the number of cells or blastomeres. tured, and the ovum be kept still, the other half will,according to Roux, form a one-sided half-embryo. Ata certain stage Rouxs half-embryo regenerated the missinghalf, usually by re-vitalising the remains of the cell whichwas punctured. If the ovum be shaken about after punctur-ing, a readjustment of material is effected, and a half-sizedembryo is formed (Morgan). The second cleavage is alsovertical, and at right angles to the first, dividing an anteriorfrom a posterior half. The third cleavage is equatorial, atright angles to the first two, dividing the dorsal region fromthe ventral. The segmentation is total but unequal, and results in theformation of a ball of cells,, those of the upper hemispherebeing smaller and more numerous than the yolk-laden cells 6oo AMPHIBIA. below. Within there is a small segmentation cavity. Sincethe presence of yolk acts as a
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Keywords: ., bookcentury1900, bookdecade1910, bookidcu3192, booksubjectzoology
