Developmental Biology 蛙类:卵黄对卵裂的阻碍作用导致卵裂沟延伸缓慢、第 三次纬裂发生不均等分裂。植物极半球分裂的速度始终较 慢,所以囊胚的植物极细胞较大。 Figure 5.7 begins in the animal region of the egg Cleavage of a frog egg Cleavage fur- before the first division has divided the rows,designated by roman numerals, aIs, vegetal cytoplasm. (C) The third div are numbered in order of appearance sion is displaced toward the animal (A, B)The vegetal yolk impedes the ole (D-H)The vegetal hemisphere cleavage such that the second division ultimately contains longer and fewer blastomeres than the animal half (B) lⅡ Ⅱ ⅢDevelopmental Biology 蛙类：卵黄对卵裂的阻碍作用导致卵裂沟延伸缓慢、第 三次纬裂发生不均等分裂。植物极半球分裂的速度始终较 慢，所以囊胚的植物极细胞较大