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The amnion, allantois and yolk sac are distinct extraembryonic structures whose relations change as the embryo folds and the umbilical cord forms. The historical umbilical vesicle is the yolk sac; its duct must not be identified with the allantoic channel.

Early in development the embryonic disc lies between the amniotic cavity dorsally and the yolk sac ventrally. The amnion has an epithelial lining facing its fluid and an outer supporting mesodermal component. The yolk sac has an endodermal lining and an external splanchnic mesodermal covering. Outside them is the chorionic cavity, interrupted by the connecting stalk linking the embryonic region to the chorion. The chorion consists of trophoblastic tissues and extraembryonic somatic mesoderm; it is not simply another name for the amnion or an outgrowth of yolk-sac endoderm.

The human allantois appears as a small endodermal diverticulum from the caudal yolk-sac region and extends into the connecting stalk. With incorporation of the caudal region into the embryo it becomes related to the cloaca and developing urinary bladder. Its intraembryonic remnant is associated with the urachus, later represented by the median umbilical ligament. Unlike the large allantoic sac of a bird, the human allantois is not an expanding external urine reservoir or an independent aerial respiratory organ.^[ref]

Embryonic folding converts part of the original endodermal roof of the yolk sac into the gut tube. The remaining sac lies outside the embryo and communicates with the midgut through the narrowing vitelline duct. Vitelline vessels accompany its early vascular apparatus. The sac supports early nutritional and blood-forming functions even though it contains no large yolk store in humans. Its connection normally regresses; persistence of a vitelline duct is a different anomaly from persistence of the urachus. The presence of a duct alone does not make the two structures interchangeable.^[ref]

As the amnion expands, it surrounds the embryo more completely and encloses the roots of the connecting stalk and yolk-sac connection at the umbilical region. Those structures are brought together within an amniotic covering to form the early umbilical cord. The cord initially includes the stalk with allantoic remnants and developing umbilical vessels, together with the vitelline connection and associated structures. The mature cord chiefly contains two umbilical arteries and one umbilical vein embedded in Wharton's jelly, beneath an amniotic surface. The vessels arise within vascular mesodermal tissues; it would be inaccurate to say that the endodermal allantoic tube simply turns into all three blood vessels.

The amnion subsequently comes into apposition and fuses with the chorion, greatly reducing the intervening chorionic cavity. Its surface continues along the cord towards the fetal placental surface. The yolk sac remains outside the amniotic cavity rather than becoming a normal free object floating in amniotic fluid. The mature amnion is avascular, whereas the placental chorion and the cord have crucial fetal vascular connections.^[ref]

These relations can be understood by comparing developmental stages in section: first a disc between two cavities, then a folded embryo with narrowed ventral connections, and finally an expanded amniotic covering around the cord. A section through a later cord cannot by itself reveal every early channel that has regressed. The same membrane names are used across vertebrates, but their relative sizes and functions are not identical in a human fetus and a yolk-rich bird egg.

An answer appropriate to 1867 must distinguish the early nucleated embryonic red blood cells from later populations, and report uncertainty about their origin. Simply stating the modern bone marrow account would miss the historical force of the request for the views then held.

Early embryonic life. In the pre-exam account of William Senhouse Kirkes, a first generation of red corpuscles arose from embryonic cells in the early vascular territory before or around the establishment of blood circulation. The initially colourless cellular material acquired colour, and the early corpuscles were relatively large and nucleated. Division of these early nucleated forms was among the processes considered capable of increasing their number. A second generation subsequently replaced the first. His embryological terminology, including the vegetative layer, should not be silently treated as an exact statement of modern germ layer derivation.^[ref]

Later fetal and adult life. A widely taught hypothesis made pale corpuscles in lymph and chyle precursors of the later red blood cells. In this scheme the pale cells entered the blood, acquired colouring matter and underwent changes associated with loss or alteration of the cell nucleus. Lymph nodes, the spleen and, during development, the liver were consequently central candidates for sites of renewal. Other opinions proposed multiplication of pre-existing red corpuscles by division or segmentation. These were competing interpretations of microscopic appearances, not demonstrations that every adult leukocyte can become a red blood cell. The pre-exam historical record explicitly documents both the pale-corpuscle hypothesis and the alternative multiplication hypotheses.^[ref]

The modern correction is successive developmental waves of haematopoiesis, followed chiefly by bone marrow production after birth, with distinct committed precursors. Primitive erythropoiesis begins in extra-embryonic yolk sac blood islands. Definitive blood-forming populations emerge through additional embryonic processes, including haemogenic endothelium in the aorta-gonad-mesonephros region, and expand in the fetal liver. Bone marrow becomes a major site later in fetal development and is the normal main adult site. The sequence is not a single already mature cell population merely migrating unchanged from organ to organ.^{[ref][ref][ref]}

During definitive erythropoiesis, erythroblasts divide and accumulate haemoglobin; the late precursor extrudes its cell nucleus, producing a reticulocyte, which matures into a red blood cell. The large primitive mammalian erythroblasts also ultimately undergo enucleation; their transient nucleation does not make them adult leukocytes.^[ref] A mature human red blood cell does not normally divide. Adult haematopoietic stem cells maintain progenitor populations giving rise to red blood cells, leukocytes and megakaryocytes; platelets are fragments produced by the last of these, not equivalent nucleated cells. Lymphocytes have their own differentiation and maturation pathways, including development in lymphoid organs. Thus a precursor's pale or lymphoid appearance under a nineteenth-century microscope is insufficient to identify it as an ordinary mature circulating leukocyte.

Two observations explain both the attraction and the weakness of the older theories. Comparing embryonic and adult blood under a microscope reveals genuine differences in corpuscle size, colour and nuclear appearance. But a series of different-looking cells in separate samples is not direct observation of one changing into another. Mixing developmental stages, species or preparations can create a false apparent sequence; reagents that reveal or damage the cell nucleus add another complication. A stronger test samples the proposed blood-forming organs and identifies proliferating precursors there, then connects them experimentally to their descendants. The explicit identification of adult bone marrow as a blood-forming organ in 1868 is therefore a subsequent development, not established background to the December 1867 paper.^[ref]