Patten, Bradley M. (Bradley Merrill) P. Blakiston’s Son & Co. 1920
The only method of attaining a comprehensive understanding of embryological processes is through the study and comparison of development in various animals. Many phases of the development of any specific organism can be interpreted only through a knowledge of corresponding processes in other organisms. The beginning student, however, must acquire his knowledge of embryology through intensive study of one form at a time, depending at first on older workers in the field for interpretation of the phenomena encountered. Building on the familiarity with fundamental processes of development thus acquired, he may later broaden his horizon by the comparative study of a variety of forms.
The chick is one of the most satisfactory animals on which student laboratory work in embryology may be based. Chick embryos in a proper state of preservation and of the stages desired can be readily secured arid prepared for study. Used as the only laboratory material in a brief course they afford a basis for understanding the early differentiation of the organ systems and the fundamental processes of body formation common to all groups of vertebrates. In more extended courses where several forms are taken up, the chick serves at once as a type for the development characteristic of the large-yolked eggs of birds and reptiles, and as an intermediate form bridging the gap between the simpler processes of development in fishes and amphibia on the one hand and the more complex processes in mammals on the other. In medical school courses where a knowledge of human embryology is the end in view the chick not only makes a good stepping stone to the understanding of mammalian embryology, but also provides material for the study of early developmental processes not readily demonstrable in mammalian material.
This book on the development of the chick has been written for those who are beginning the study of embryology and has accordingly been kept as brief and as uncomplicated as possible. Nevertheless it is assumed that the beginner in embryology will not be without a certain back-ground of zoological knowledge and training. He may reasonably be expected to he familiar with some of the aspects of evolution and heredity, with the recapitulation theory, the cell theory, the nature of the various types of tissues, and the more general phases of the morphology of vertebrates. Before laboratory work on the chick is begun in any course in embryology the nature of sexual reproduction, and the processes of gametogenesis, maturation, fertilization and cleavage, will have been taken up. It therefore seems unnecessary to include here any preliminary, general discussion of these phenomena. References for collateral reading on this and other phases of the subject are given in the appendix.
Like other sciences embryology demands first of all accurate observation. It differs considerably, however, from such a science as adult anatomy where the objects studied are relatively constant and their component parts are not subject to rapid changes in their inter-relations. During development, structural conditions within the embryo are constantly changing. Each phase of development presents a new complex of conditions and new problems .
Solution of the problems presented in any given stage of development depends upon a knowledge of the stages which precede it. To comprehend the embryology of an organism one must, therefore, start at the beginning of its development and follow in their natural order the changes which occur. At the outset of his work the student must realize that proper sequence of study is essential and may not be disregarded. A knowledge of structural conditions in earlier stages than that at the moment under consideration, and an appreciation of the trend of the developmental processes by which conditions at one stage become transmuted into different conditions in the next, are direct and necessary factors in acquiring a real comprehension of the subject. Without them the story of embryology becomes incoherent, a mere jumble of confused impressions.
A knowledge of the phenomena of development is ordinarily acquired by studying a series of embryos at various stages of advancement. Each stage should be studied not so much for itself, as for the evidence it affords of the progress of development. In the study of embryology it does not suffice to acquire merely a series of « still pictures » of various structures, however accurate these pictures may be. The study demands a constant application of correlative reasoning and an appreciation of the mechanical factors involved in the relations of various structures within the embryo to each other, and in the relation of the embryo as a whole to its environment. In order to really comprehend the embryological significance of a structure one must know not only its relations within the embryo being studied at the time, but also the manner in which it has been derived and the nature of the changes by which it is progressing toward adult conditions. To get absolutely the whole story it is obvious that one would have to study a series of embryos with infinitely small intervals between them. Nevertheless the fundamental steps in the process may be grasped from a much less extensive series. The fewer the stages studied, however, the more careful must one be to keep in mind the continuity of the processes and to think out the changes by which one stage leads to the next.
The outstanding idea to be kept in mind by the student beginning the study of embryology is that the development of an individual is a process and that this process is continuous. The conditions he sees in embryos of various stages are of importance chiefly because they serve as evidence of events in the process of development at various intervals in its continuity, as historical events are evidences of the progress of a nation. Just as historical events are led up to by preparatory occurrences and followed by results which in turn affect later events, so in embryology events in development are presaged by preliminary changes and when consummated affect in turn later steps in the process.
In certain respects the laboratory study of embryological material involves methods of work for which courses in general zoology do not entirely prepare the student. Some general. suggestions as to methods of procedure are, therefore, not out of place.
In dissecting gross material it is not unduly difficult to appreciate the complete relationships of a structure. The nature of embryological material, however, introduces new problems. Embryos of the age when the establishment of the various organ systems and processes of body formation are being initiated are too small to admit of successful dissection, but not sufficiently small to permit of the satisfactory microscopical study of an entire embryo, except for its more general organization. To study embryos of this stage with any degree of thoroughness they must be cut into sections which are sufficiently thin to allow effective use of the microscope to ascertain cellular organization and detailed structural relationships. In preparing such material the entire embryo is cut into sections which are mounted on slides in the order in which they were cut. A sectional view of any region of the embryo is then available for study.
While sections readily yield accurate information about local regions it is extremely difficult to construct a mental picture of any whole organism from a study of serial sections alone. For this reason it is necessary to work first on entire embryos which have been prepared by staining and clearing so they may be studied as transparent objects. From such preparations it is possible to map out the configuration of the body, and the location and extent of the more conspicuous internal organs. In this work the fact that embryos have three dimensions must be kept, constantly in m’ind and the depth at which a structure lies must be determined as well as its apparent position in surface. view. While conventionally entire chick embryos are represented in dorsal view, much additional information may be gained by following a study of the dorsal, with a study of the ventral aspect. Unless the preliminary study of entire embryos is carefully and thoroughly carried out the study of sections will yield only confusion.
In studying a section of an embryo it is necessary first of all to determine its location. The plane of the section under consideration, and the region of the embryo through which it passes should be ascertained by comparing it with an entire embryo of the same age as that from which the section was cut. Only when the exact location of a section is known can the structures appearing in it be correlated with the organization of the embryo as a whole. Probably nothing in the study of embryology causes students more difficulties than neglect to locate sections accurately with the consequent failure to appreciate the relationships of the structures seen in them. Too great emphasis cannot be laid on the vital importance of fitting the structures shown by sections properly into the general scheme of organization as it appears in whole-mounts. It must by no means be inferred that the possibilities of the whole-mounts have been exhausted by the preliminary study accorded them before taking up the work on sections. ~ Further and more careful study of entire embryos should constantly accompany the study of serial sections. Many details which in the initial observation of the whole-mount were inconspicuous or abstruse will become significant in the light of the more exact information yielded by the sections.
In the discussion of structures and processes in embryology, it is necessary to use .terms designating location and direction which are referable to the body of the embryo regardless of the position it occupies. The ordinary terms of location, which are primarily referred to the direction of the action 1 of gravity, such as above, over, under etc. are not sufficiently accurate. In gross human anatomy, there still persist many terms that are referred to gravity, and are .therefore, because of the erect posture of man, not applicable to comparative anatomy or to embryology. The most confusing of these are anterior and posterior as used in gross human anatomy to mean, respectively, pertaining to the belly and to the back. In comparative anatomy and in embryology, anterior has reference to the head region and posterior to the tail region. The use of these terms in embryology in the sense usual in gross human anatomy is likely to lead to confusion and is entirely avoided in this book. The terms anterior and posterior have been replaced to a large extent by their less confusing synonyms, cephalic and caudal.
In addition to the adjectives of position, such as dorsal, ventral, – cephalic, caudal, mesial, lateral, proximal. distal, corresponding adverbs of motion or direction are commonly used in embryology. These adverbs are formed by adding the suffix -ad to the root of the adjective, as dorsad meaning toward the back, cephalad meaning toward the head, etc. These must not be used as adjectives of position but should be applied only to the progress of processes, or to the extension of structures toward the part indicated by the root of the adverb. Cultivation of the use of correct and definite terms of position and direction in dealing with embryological processes will greatly aid accurate thinking and clear understanding.