. Text-book of normal histology: including an account of the development of the tissues and of the organs. ing either in the pro-duction of a peculiar tissue, as that of the voluntary muscle fibre, or 22 NORMAL HISTOLOGY. of external appendages, as the cilia of many unicellular organismsor of the epithelial elements of the higher animals. Since every cell is derived from a pre-existing cell, it follows thatall the cells of the organism are the descendants of the parent ele-ment—the ovum. The ripe mammalian egg, while small in com-parison with many other ova, is among the largest histologicalel
. Text-book of normal histology: including an account of the development of the tissues and of the organs. ing either in the pro-duction of a peculiar tissue, as that of the voluntary muscle fibre, or 22 NORMAL HISTOLOGY. of external appendages, as the cilia of many unicellular organismsor of the epithelial elements of the higher animals. Since every cell is derived from a pre-existing cell, it follows thatall the cells of the organism are the descendants of the parent ele-ment—the ovum. The ripe mammalian egg, while small in com-parison with many other ova, is among the largest histologicalelements, measuring about .2 millimetre in diameter, and, further,possessing all parts of the typical cell. Before the ovum is capable of uniting with the male sexual elementto carry out the changes attendant upon fecundation, it passes througha cycle of preparatory stages collectively known as changes consist in the repeated very unequal division of theovum, resulting in the expulsion of minute portions of its proto-plasm, the polar bodies ; of these latter, usually two are Maturation and fecundation in ova o( ascaris megalocephala : I, k, nucleus of ovum before matura-tion ; s, entering spermatozoon ; //, nucleus («) has passed to periphery of cell preparatory to di-viding ; s, spermatozoon now within the ovum ; ///, nucleus dividing into first polar body (/); >«,male pronucleus resulting from spermatozoon ; IV,p,p, first and second polar bodies, the last stillin process of formation ; m, male pronucleus ; V, p, p, polar bodies ; / and rn, respectively femaleand male pronuclei, in contact but not yet fused ; c. centrosomes, indicating poles of nuclear spindle ;VI, pronuclei now fused ; striation proceeds from centrosomes preparatory to division of ovum. (AfterO. Hert-Mig.) The nucleus which appears within the ovum after the formation ofthe polar bodies is the female pronucleus. Upon the completionof these phenomena, maturation has taken place
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