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Placentae of 22 one-humped camel concepti with crown-rump lengths (CRL) ranging from 2.5 to 26 cm were studied. The placentae were processed for light, transmission and scanning electron microscopy of exposed surfaces and microvascular corrosion casts. In very early stages of pregnancy (2.5–4.5 cm CRL) three froms of fetomaternal interrelationship are described. (1) Precontact, where the mononuclear trophoblast cells are still separated from the uterine epithelium by a gap containing interareolar histotroph. Both fetal and maternal epithelia develop apical ectoplasmic pads in this location. (2) Apposition, where microvilli of the apical cell membrane of the trophoblast contact the uterine epithelium focally. Multinuclear trophoblast giant cells develop beside the population of already present mononuclear trophoblast cells. Uterine ectoplasmic pads can be observed. (3) Adhesion occurs when apical cell membranes of fetal and maternal epithelia adhere to each other closely, thus forming a ‘normal’ intercellular space of 20 nm width, without any intervening uterine luminal space. Microvillous interdigitation in this location varies from a non-microvillous ‘smooth adhesion’, to a distinctly villiform ‘rough adhesion’, and a ‘semismooth adhesion’ is achieved when trophoblastic microvilli make intimate contact with the non-microvillous uterine apical cell membranes of ectoplasmic pads. This fetomaternal attachment process is sufficient until the conceptus reaches approximately 9 cm CRL. Then, from 10 to 13 cm CRL, additional anchorage of the placenta to the endometrium is accomplished by the growth of temporary grooves and ridges of the allantochorion and the endometrium, which indent each other in a complementary fashion. The height of these groove-ridge structures increases gradually in 14 to 18 cm CRL fetuses, and they also widen at about 25 cm CRL, thus forming globular fetal troughs and irregular, thick maternal ridges. These together create units responsible for improved fetomaternal anchorage and metabolic exchange for the increasing needs of the growing fetus.

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