1. Introduction.- 2. Historical Survey.- 3. Development of the Guinea-Pig Placenta.- 3.1. Breeding and Age Determination.- 3.2. Early Development and Formal Genesis.- 3.3. Morphogenesis.- 4. Rough Structure of the Mature Placenta.- 5. Vascular Supply of the Mature Placenta.- 5.1. Maternal Arterial Blood Supply.- 5.2. Maternal Venous Drainage.- 5.3. Fetal Arterial Blood Supply.- 5.4. Fetal Venous Drainage.- 5.5. Intralobar Circulatory Conditions.- 6. Ultrastructure and Functional Morphology.- 6.1. Physiological, Technical and Morphometrical Data.- 6.1.1. Physiological Data and Perfusion Technique.- 6.1.2. Influence of Fixation on the Structure of the Placenta.- 6.1.3. Morphometry of the Main Placenta.- 6.2. Ultrastructural and Histochemical Development of the Main Placenta.- 6.2.1. 6th to 14th Day.- 6.2.2. 15th to 20th Day.- 6.2.3. 21st to 35th Day.- 6.2.4. 36th to 50th Day.- 6.3. Functional Morphology of the Different Regions in the Mature Main Placenta.- 6.3.1. Interlobium.- 6.3.2. Transitional Zone.- 6.3.3. Periphery of the Lobe.- 6.3.4. Middle of the Lobe.- 6.3.5. Centre of the Lobe.- 6.4. Subplacenta and Junctional Zone.- 6.4.1. Development and Regression of the Subplacenta.- 6.4.2. Functional Significance.- 6.5. Yolk Sac and Margin of the Placenta.- 6.5.1. Section 1 of the Yolk Sac.- 6.5.2. Section 2 of the Yolk Sac.- 6.5.3. Section 3 of the Yolk Sac.- 6.5.4. Section 4 of the Yolk Sac.- 6.5.5. Section 5 of the Yolk Sac.- 7. Possibilities of Comparison with the Human Placenta.- Acknowledgements.- 8. References.- 9. Subject Index.