raymond f. gasser

ATLAS OF HUMAN EMBRYOS

Preface (Page vii)

The most important and yet most difficult part of gross anatomy is the understanding and remembering of adult anatomic relations. This task is made easier if one is aware of early prenatal conditions when the body is more simply arranged. The most logical and meaningful way to formulate concepts and principles of adult anatomy is to become familiar first with the embryonic form. This knowledge will better enable one to relate and store the voluminous details of adult anatomy.

This atlas was assembled with the hope that it will help students obtain a firm foundation on which to organize and understand adult human anatomy. It may also serve investigators as a reference for the arrangement of normal human embryos during the period when most birth defects occur.

Since the foundations of the definitive body plan are established during the embryonic period, only this portion of development is included. This atlas is divided into eight chapters covering the first eight weeks of prenatal life. The age of each specimen is based on the publications of Iffy et al. (Acta Anat 66: 178–186, 1967) and/or O’Rahilly (Anat Anz 130: 556–559, 1972). The important events in each week are presented in outline form and, when possible, organized according to the body systems. Sincere efforts were made to use Nomina Embryologica (1970) and Nomina Anatomica (1966) terminology. The many micrographs are of representative levels through almost technically perfect specimens. Photographic retouching was used minimally. A bar giving the magnification accompanies most illustrations thus allowing for an appreciation of the actual size of the developing organs. Every effort was made to orient accurately each micrographic section, not only in the embryo itself but also in most of its systems. The level of each section is indicated on drawings taken from wax or graphic reconstructions. A legend points out important features in each section.

After the third week, one embryo illustrates the level of development for each week. Studying the sections in series enables one to follow the morphogenesis of the systems and, at the same time, to become aware of the very important relations one system has to another during the period of organogenesis. An appreciation of the shifts that viscera undergo before they come to rest in their adult position can be attained by comparing the position of each viscus at different stages of development. This information helps explain many definitive anatomical facts (e.g., nerve patterns) and gives meaning to the possible locations of various pathological developments (e.g., thyroglossal cysts).