A study of the human placenta, physiological and pathological / by Thomas Watts Eden.

  • Eden, Thomas Watts, 1864-1946.
Date:
[1894?]
Catalogue details

Licence: Public Domain Mark

Credit: A study of the human placenta, physiological and pathological / by Thomas Watts Eden. Source: Wellcome Collection.

Provider: This material has been provided by The Royal College of Surgeons of England. The original may be consulted at The Royal College of Surgeons of England.

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    were present over the greater part of the surface of the ovum. By what precise steps this stage is reached, in man, is not known. We can only assume that it is by a process which does not materially differ from that of mammals lower in the scale than man. We therefore conclude that two structures are chiefly concerned in the formation of the human chorion—(1) the zona pellucida or primitive envelope of the ovum, and (2) the subzonal membrane, or false amnion, which is applied to the inner surface of the zona pellucida, forming a complete sphere within it. The latter envelope is formed, together with the true amnion, from the extra-embryonic portion of the somatopleure. It is composed of an outer epiblastic and an inner mesoblastic layer. Kbllikcr (2l). There has been great conflict of opinion upon the relations of the. allantois to the chorion. The old view was that the allantois grew out from the posterior end of the primitive alimentary canal to the wall of the ovum, in the form of a hollow process, surrounded by mesoblastic tissue; that a direct connection was thus established between the embryo and the envelope, and that along this bridge vessels developed from the terminal bifurcations of the aorta, which vascularised the chorion. This view has recently been opposed by His, and in point of fact their are no direct observations to support it. His (13) points out, among other objections to the theory, that allantoic or umbilical vessels have been found reaching the chorion by a mesoblastic stalk at a period when the allantois itself is only beginning to appear. Without entering into the discussion further, the view of His may be briefly stated, and adopted as most in accord with the facts as they stand at present. The old view assumed that, after the closure of the amnion, a con- nection was established between the embryo and the wall of the ovum by the outgrowth of the allantois. His believes that by the closure of the amnion the feetus is not entirely separated from its envelopes, but remains in connection with them through a bridge of mesoblastic tissue, which is continuous with its posterior extremity. This bridge he terms the ventral stalk. Along it the umbilical vessels grow to reach the chorion. At a later period the allantois grows out towards the chorion, in contact with the ventral stalk ; but it never quite reaches the wall of the ovum. It participates with the ventral stalk and the umbilical vessels in the formation of the umbilical cord. The chorion is thus derived from the embryonic epiblast and mesoblast layers; the epiblast forms the epithelial layer covering the entire membrane and its villi; the mesoblast forms the connective tissue stroma supporting the vessels. The zona pellucida probably disappears early. In the earliest specimens of the human ovum which have been care- fully examined (about the end of the second week), villi were found over nearly the entire surface of the chorion; they, therefore, probably appear at a very early period. They are formed, in the first place,