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Henry Gray (1825–1861).  Anatomy of the Human Body.  1918.
 
by their subdivision give origin to four spermatozoa, and the secondary oöcyte and first polar body to four cells, the mature ovum and three polar bodies. In the development of the spermatozoa, as in the maturation of the ovum, there is a reduction of the nuclear chromosomes to one-half of those present in the primary spermatocyte. But here the similarity ends, for it must be noted that the four spermatozoa are of equal size, and each is capable of fertilizing a mature ovum, whereas the three polar bodies are not only very much smaller than the mature ovum but are incapable of further development, and may be regarded as abortive ova.
 
4. Fertilization of the Ovum
 


FIG. 8– The process of fertilization in the ovum of a mouse. (After Sobotta.) (See enlarged image)

  Fertilization consists in the union of the spermatozoön with the mature ovum (Fig. 8). Nothing is known regarding the fertilization of the human ovum, but the various stages of the process have been studied in other mammals, and from the knowledge so obtained it is believed that fertilization of the human ovum takes place in the lateral or ampullary part of the uterine tube, and the ovum is then conveyed along the tube to the cavity of the uterus—a journey probably occupying seven or eight days and during which the ovum loses its corona radiata and zona striata and undergoes segmentation. Sometimes the fertilized ovum is arrested in the uterine tube, and there undergoes development, giving rise to a tubal pregnancy; or it may fall into the abdominal cavity and produce an abdominal pregnancy. Occasionally the ovum is not expelled from the follicle when the latter ruptures, but is fertilized within the follicle and produces what is known as an ovarian pregnancy. Under normal conditions only one spermatozoön enters the yolk and takes part in the process of fertilization. At the point where the spermatozoön

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