The blastoderm is the first step in the development of the ovum after segmentation of the yolk-substance (vitellua), and it gives rise to two germinal layers of cells, the epiblast and hypoblast, between which there soon develops a third layer of cells, the mesoblast, and from these three germinal layers of cells all the structures of the mature adult are formed.
There appears at one part of the epiblast the earliest trace which is at all characteristic of form; this is called the primitive streak or groove, in close relationship with which the central nervous system, or cerebrospinal axis, is developed. While this is going on, blood vessels are formed within the mesoblast which become distributed over the blastoderm.
Eight to ten days are supposed to have elapsed since the ovum was fecundated; it is now about the size of a pea; the first characteristic human shape, the neural canal, has appeared; it is still in the Fallopian tube, where it was fertilized, but presently passes into the uterus, where it finds lodgement for the remainder of the gestation period. While these early changes have been taking place in the ovum, and while it is yet in the Fallopian tube, certain preparatory changes also occur in the uterus, the mucous membrane of which, through a sympathetic reflex trophic influence, becomes swollen and thrown into folds, so that when the eight or ten-day-old ovum reaches the uterine cavity it is stopped in its descent by becoming lodged in one of these folds.
The ovum, now resting in a cup like cavity on the surface of the uterine mucous membrane, and being endowed with a wonderfully energetic vitality, seems to exert a peculiar irritative influence upon the area immediately surrounding it, so that the edges of this cup like cavity grow up around it and finally meet so as to include and retain it. Thus is formed what is called the decidua reflexa (turned-back decidua). In addition to this there is a layer of membrane formed, closely applied to the uterine cavity, which is called the decidua vera (true decidua), and of this that portion which lies immediately adjacent to the fecundated ovum becomes specially modified to form the decidua serotina, at which site the future placenta is developed. Thus there is a threefold division of the deciduous membranes called deciduous from the fact that they are discharged at the time of birth.
These deciduous membranes, developed from the uterine mucous membrane, and so of maternal origin, form an external investment for the ovum, while within these are formed yet other membranes of embryonic origin, constituting the foetal parts of the embryonic sac.
While these changes are going on in the formation of the membranes the embryo itself continues to grow.
The embryo at this very early period, i.e., after the formation of the neural canal, possesses a thickened anterior extremity, the head or cephalic end, and a caudal extremity or tail. From these extremities hollow pouches develop, which finally meet and coalesce to form a single shut sac inclosing the embryo. Thus is formed what is called the amnion. This amniotic sac contains a bland, serous fluid, the liquor amnii, in which the embryo floats; eventually this sac fills the entire uterine cavity, being closely applied to the inner surface of the deciduous membranes, and constitutes one of the membranes composing the "bag of waters", which ruptures when labor comes on.
Sometimes during birth a portion of the amnion adheres to the child's head as a skull cap, which event is regarded as an auspicious omen by superstitious midwives, who then say that the child was "born with a caul". The allantois is a membranous pouch which springs from the embryo and ultimately envelops it, so that the embryo is completely invested by allantois as well as by amnion and deciduous membranes.