Gray's anatomy on blastulation: The segmentation nucleus exhibits the usual mitotic changes, and these are succeeded by a division of the ovum into two cells of nearly equal size.The process is repeated again and again, so that the two cells are succeeded by four, eight, sixteen, thirty-two, and so on, with the result that a mass of cells is found within the zona striata, and to this mass the termmorula is applied. The segmentation of the mammalian ovum may not take place in the regular sequence of two, four, eight, etc., since one of the two first formed cells may subdivide more rapidly than the other, giving rise to a three-or a five-cell stage. The cells of the morula are at first closely aggregated, but soon they become arranged into an outer or peripheral layer, thetrophoblast, which does not contribute to the formation of the embryo proper, and an inner cell-mass, from which the embryo is developed. Fluid collects between the trophoblast and the greater part of the inner cell-mass, and thus the morula is converted into a vesicle, the blastodermic vesicle. The inner cell-mass remains in contact, however, with the trophoblast at one pole of the ovum; this is named the embryonic pole, since it indicates the situation where the future embryo will be developed. The cells of the trophoblast become differentiated into two strata: an outer, termed the syncytium or syncytiotrophoblast, so named because it consists of a layer of protoplasm studded with nuclei, but showing no evidence of subdivision into cells; and an inner layer, the cytotrophoblast or layer of Langhans.
