Abstract
Sexual differentiation in the human embryo has taken place by the end of the seventh week of embryonic life. From this time until the third month simple growth and cellular multiplication occurs. With the third month the early maturation phases of the ova are found. These phases consist of the usual maturation phases present in the adult male germ cells, the formation of the leptonema, synizesis, pachynema, and diplonema, stages which do not occur in the adult female. Furthermore, in the development of the leptonema, prochromosomes like those of insects are formed and then resolved into the leptotene threads. Prochromosomes have not hitherto been seen in the mammalia. By five and one-half months the prochromosomes have nearly all disappeared and the type of maturation is the same as that in the adult male.
The appearance of prochromosomes seems to be a recapitulation stage such as is found in some other organs of the developing embryo. The maturation of embryonic ova may plausibly be due to the action of a maternal hormone incapable of affecting male embryonic germ cells.
The embryonic germ cells disappear before adult life is reached and the ova developed during adult life do not pass through the above maturation phases preliminary to the maturation divisions.
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