Book - Embryology of the Pig 1
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Chapter 1. Foreword to the Student
Foreword to the Student and I are to dart out together to explore some of the regions of embryology. Still vivid memories of the erratic progress of my own first expeditions of that hind have led me to offer my services to you. Perhaps I can help you to avoid some of the difficulties I encountered and lead you to points of interest by routes less devious than you might otherwise find. But it is your own expedition. I am merely a guide.
I can show you the passes to understanding but you must climb them. I can lead you to worth-while things but you yourself must unearth them and carry them away.
Objectives in the Study of Embryology
Most courses in embryology arc planned with one of two ends in view. They aim either to teach undergraduate students the fundamental facts and principles of development, or to show medical students the processes by which the human body reaches its adult form.
The student of zoology rarely finds embryology a â€œduty course.â€ To one at all interested in the how and why of living things, the processes by which animals grow from a single fertilized egg cell to their fully elaborated adult structure seldom fail to be fascinating.
The medical student is likely to be more utilitarian. He sees ahead a definite^ objective. Between him and his objective stand many courses to be surmounted. He expects to work hard and is quite willing to do so, but he quite properly expects each course to contribute definitely to his progress toward his goal. Ion the harassed transitional period of his first yetu" in medical school he rarely takes time to think out just why he is required to study embryology. This is not surprising, for the applications of embryology are, for the most part, indirect. It has no obvious bedside value. Its service lies rather in the rational interpretation of other subjects that it makes possible.
The entering student readily sees the necessity for a thorough knowledge of anatomy. Not until he becomes enmeshed in a maze of anatomical details does he realize his need of some knowledge of how adult conditions became as they are to lead him beyond anatomical memorizing to a comprehension of anatomy. Having no familiarity with the structural and functional abnormalities encountered in clinical practice, he can scarcely be expected to realize how many of these conditions can be interpreted only in the light of embryological phenomena. Knowing pathology, neurology, and obstetrics only by name or by a catalogue prospectus, one cannot expect him to see in advance how much his embryological background will help him in these subjects. It is only fair to the student confronted by a heavy medical curriculum to point out to him that embryology is more than just an interesting field of knowledge â€” that it is in reality a subject which will be of constantly increasing value to him as he goes on with his training and practice.
The Importance of Laboratory Study
Whatever the purpose and scope of a course in embryology may be, it must necessarily be based on laboratory study of actual material. No amount of didactic instruction, however forcefully it may be presented, can take the place of work done by the student himself. In the laboratory, with critical and encouraging guidance, students can be led to become themselves active acquisitors, instead of passive recipients of information.
It is because of its value as laboratory material that we have woven the thread of our story about the development of the pig. If it is desired to illustrate the fundamental processes of mammalian embryology as the basis for a general course, there is no better form than the pig. If a knowledge of human development is the end in view, the pig is equally serviceable. Human embryos in the vitally important stages during which the various organ systems are being established, cannot be procured in sufficient numbers for class use. During these early stages, the development of all the mammals is fundamentally the same. The specific characteristics of any form emerge but slowly, and relatively late. It is, therefore, quite possible, by using young pig embryos, to give students the opportunity of studying at first hand the same processes which go on in the early development of the human body.
There are several other reasons why pig embryos are so frequently utilized as laboratory material. They can readily be obtained in all but the very earliest stages of development in sufficient numbers for class use. The older embryos are large enough to permit gross dissection - a method of study not possible with the embryos of smaller animals and one which is of great value in linking embryology with adult anatomy. Furthermore the pig has been the subject of many important embryological investigations. The information thus accumulated opens an uncommonly wide field to both teacher and student.
The Plan of Presentation Used in This Book
In planning the subject matter of this book, one of the first problems to present itself involved the question of the preliminary training the student of embryology might be expected to possess. It is of the utmost importance that new work should start on firm ground. On the other hand nothing but wasted effort results from unnecessary repetition of work already covered and it seems reasonable to assume that one taking up the study of mammalian embryology will not be without a considerable background of biological information. The exact extent of this background necessarily will be variable and is exceedingly difficult to evaluate. I have assumed that it was unnecessary to give any preliminary review of such essentials as protoplasmic activity, cell structure, cell division, or the aggregation and specialization of cells to form the fundamental tissues. Certainly most students will be well grounded in these things, and be familiar as well with some aspects of evolution and heredity, sexual reproduction, the recapitulation theory, and in a general way at least with the adult structure of vertebrates.
Many students will have had some preliminary work in embryology before taking up the development of mammals. Even in the case of such readily available forms as the pig, very young embryos are difficult to obtain in large numbers. For that reason, some other embryos such as frogs or chicks are commonly utilized to illustrate early embryological processes. In fact, the mammalian type rarely is studied by students until it is relatively advanced in development. Under these circumstances it is important that the stage of development at which work on the preliminary form is discontinued, be equivalent to that of the mammalian embryos to which attention is transferred. A common procedure is to make the transition from chicks of about three or four daysâ€™ incubation to pig embryos of approximately 5 to 10 mm. in length. This works well because the degree of development in the various organ systems is readily comparable in both embryos, and so the story of development can be carried forward with little break in continuity.
Because preliminary work is usually done on the early stages of some other form, and the study of the pig begun on fairly welladvanced embryos, it has not seemed necessary to go into great detail as to its early stages. Nevertheless it would be unwise to start the consideration of any developmental process elsewhere than at its beginning. Embryology is a progressively constructive series of events. A knowledge of preceding stages 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. Just as historical events arc led up to by preparatory occurrences and are 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.
The stage of mammalian development which commonly receives most attention in the laboratory is that in which the body form is well defined and the various organ systems are just appearing. I'his, for the beginner, is the most critical part of the subject. If the student successfully follows the emergence of the various organ systems from undifferentiated primordial tissues to a point where he can recognize the beginnings of familiar adult structures, his troubles are largely passed. The chief difficulty in embryology lies in getting a start among strange names and unfamiliar structural conditions. It is in recognition of this fact that so large a proportion of the laboratory work in most courses is spent on embryos of this stage. Nothing but actual contact with these unfamiliar structures can fix them in the studentâ€™s mind and give him a clear understanding of their origin and their relation to one another. In this text, for the same reason, the critical stages in the early differentiation of the organ systems have been especially emphasized.
In dealing with the later phases of development it has seemed best to pass lightly over, or omit altogether, certain things. In doing this I have been guided by two considerations, first, the degree of general interest of the subject, and second, the ease with which it can be presented in the laboratory with actual material as a basis for study. Thus a discussion of the later development of the organs of special sense has been omitted because the preparation of adequate laboratory material of these organs involves a greater exp>enditure of time than is practical in most laboratories. The later changes in the muscular system have been omitted because it was believed that the space necessary for their consideration could advantageously be devoted to things of more general interest. Similarly the detailed configuration of the intestinal tract and mesenteries has been dismissed with but a word of comment.
Selection of the histogenesis of bone and the organogenesis of teeth for special consideration was made because these processes as they occur in the pig are essentially typical for all mammals. Moreover laboratory material covering the more important phases of these phenomena can be prepared without undue expenditure of time. I offer no apology for presenting these examples of histogenesis in some detail nor for failure to present additional examples of this type. Either of these processes carefully studied from actual material will give the alert student a first-hand acquaintance with fundamental cytological activities, and with the development of specialized cell products. If such acquaintance be an appreciative one, even though it be limited to a single process, it will be worth far more than superficial contact with many facts categorically presented.
The figures in this book have been planned primarily to tell a graphic story running parallel to that of the text. In many cases, however, it has been found possible to include in the figures details not discussed in the text. My purpose in so doing was to provide a readily available basis for supplementary instruction, and to encourage further study by the student on his own initiative.
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Cite this page: Hill, M.A. (2019, October 20) Embryology Book - Embryology of the Pig 1. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Book_-_Embryology_of_the_Pig_1
- © Dr Mark Hill 2019, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G