. The cell in development and inheritance. Cells. i8o FERTILIZATION OF THE OVUM crossing produces both greater vigour and greater variability. In view of all the facts, however, we are constrained to the admission that the essential nature of sexual reproduction must remain undetermined until the subject shall have been far more thoroughly investigated, espe- cially in the unicellular forms, where the key to the ultimate problem is undoubtedly to be soughjt. A. Preliminary General Sketch Among the unicellular plants and animals, fertilization is effected by means of conjugation, a process in w
. The cell in development and inheritance. Cells. i8o FERTILIZATION OF THE OVUM crossing produces both greater vigour and greater variability. In view of all the facts, however, we are constrained to the admission that the essential nature of sexual reproduction must remain undetermined until the subject shall have been far more thoroughly investigated, espe- cially in the unicellular forms, where the key to the ultimate problem is undoubtedly to be soughjt. A. Preliminary General Sketch Among the unicellular plants and animals, fertilization is effected by means of conjugation, a process in which two individuals either fuse together permanently or unite temporarily and effect an exchange. Fig. 89. — Fertilization of the egg of the snail, Pkysa. [Kostanecki and WIERZEJSKI.] A. The entire spermatozoon lies in the egg, its nucleus at the right, fiagellum at the left, while the minute sperm-amphiaster occupies the position of the middle-piece. The first polar body has been formed, the second is forming. B. The enlarged sperm-nucleus and sperm-amphiaster lie near the centre; second polar body forming and the first dividing. The egg-centrosomes and asters afterward disappear, their place being taken by those of the spermatozoon. of nuclear matter, after which they separate. In all the higher forms fertilisation consists in the permanent fusion of two germ-cells, one of paternal and one of maternal origin. We may first consider the fer- tihzation of the animal egg, which appears to take place in essentially the same manner throughout the animal kingdom, and to be closely paralleled by the corresponding process in Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Wilson, Edmund B. (Edmund Beecher), 1856-1939. New York, Macmillan
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectcells, bookyear1911
