JAMA. 1960;174(10):1320. doi:10.1001/jama.1960.03030100088019.
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The second stage of one of the most important biological functions of life, nidation or implantation of the fertilized egg in the endometrium, remains an incompletely understood phenomenon.1 Recent studies utilizing modern techniques of microbiochemistry and electron microscopy have revealed new evidence or have confirmed hypotheses that were formulated by means of less precise procedures. Cleavage of the egg begins shortly after fertilization. Mitosis at this phase is a normal process. After the eight-cell stage of the ovum, many of the peripheral cells divide more rapidly than the cells placed centrally. Fluids accumulate in the intercellular spaces between the center and the peripheral structures—the trophoblast. The vesicle, a mass of cells with the eccentrically placed inner cell groups, is identified as the blastocyst. Cell division and differentiation up to this stage occur before the egg has finished the trip down the uterine tube. The time of transit of the


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