Archive image from page 51 of The development of the chick. The development of the chick : an introduction to embryology . developmentofchi02lill Year: 1936 34 THE DEVELOPMENT OF THE CHICK Thus at each end of the spindle there are eight dyads. Those at the outer end then enter a httle bud of protoplasm projecting above the surface of the germinal disc, and this bud with the dyads is cut off as the first polar body, which lies in a depression of the germinal disc beneath the vitelline membrane (Fig. 11). Eight dyads, therefore, remain within the germinal disc. A second maturation spindle is th
Archive image from page 51 of The development of the chick. The development of the chick : an introduction to embryology . developmentofchi02lill Year: 1936 34 THE DEVELOPMENT OF THE CHICK Thus at each end of the spindle there are eight dyads. Those at the outer end then enter a httle bud of protoplasm projecting above the surface of the germinal disc, and this bud with the dyads is cut off as the first polar body, which lies in a depression of the germinal disc beneath the vitelline membrane (Fig. 11). Eight dyads, therefore, remain within the germinal disc. A second maturation spindle is then formed almost imme- diately, apparently without the intervention of a resting -stage of the nucleus, and takes a radial position similar to that occupied by the first, with the dyads forming an equatorial plate (Fig. 11). y/?.5p2. Fig. U. — Second maturation spindle and first polar body of the pigeon's egg; a combination of two sections. x 2000. (After Harper.) m. Sp. 2, Second matm-ation spindle, p. b. 1, First polar body. v. M., Vitelline membrane. Each dyad then divides along the preformed plane of division, and the daughter-chromosomes diverge towards opposite poles of the spindle. The outer end of the second maturation spindle then enters a superficial bud of the protoplasm of the germinal disc similar to that of the first maturation spindle; and this bud together with the contained chromosomes becomes cut off as the second polar body. The result of these processes of maturation is the formation of three cells, viz., the two polar bodies and the mature egg. The polar bodies are relatively very minute and soon degenerate completely. After the formation of the second jiolar body there remain in the egg eight chromosomes, each of which represents one quarter of an original tetrad. These form a small resting nucleus known as the egg-nucleus or female pronucleus. It is many times smaller than the original germinal vesicle (Fig. 12), and
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