This work attempts to sketch a coherent picture of the amniotic fluid com­ partment, its borders, and the interactions occurring between fetus and amniotic fluid, for it is this compartment which for 10 months provides the habitat in which the fetus and its functions develop. As a matter of course, our mor­ phological studies had to be limited to the situation prevailing at the full term of pregnancy. This portrayal purposely neglects the placenta, which is already the subject of an extraordinary profusion of literature. The other aspects of the embryonic sac have been overshadowed to date by the preeminent role 2 of the placenta, although they represent around 1200cm of contact surface in the maternal compartment. From our point of view, the fetal membranes not only act as a seal or simply a diffusion barrier, but actively participate in the exchange processes between compartments. The author also took pains to compile (albeit incompletely) the widely scattered findings reported in clinical journals in order to present them as a well-rounded summary of the mor­ phological picture and the functional happenings involved in fetal develop­ ment. The author's own studies of a specifically histomorphological nature provided the groundwork for this synopsis. A special branch of embryology, embryonal physiology, is treated only incidentally in today's embryology textbooks. This work offers several encounters with this field, which should help make it the focus of greater interest. 2 Definition of Terms Fig. 1.