Book - Human Embryology (1945) 1
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Hamilton WJ. Boyd JD. and Mossman HW. Human Embryology. (1945) Cambridge: Heffers.
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Chapter I Introductory Concepts
Reproduction
Members of all multicellular animal species (Metazoa) have a more or less limited life span Consequently if a species is to sur\it.e a mechamsm must exist for the successive production of new generations of that species This process is called reproduction In most metazoan species, including the vertebrates reproduction is efTected b> a complicated process involving the presence of two sexes male and female and the production b> each of these sexes of specialized sex cells called gametes The organs which produce the gametes are known as the gonads or primary sex organs, those of the male are the testes those of the female, the o ones The male gametes are named spermatozoa the female gametes o a The union of a spermatozoon with an ovum is called fertilization and results m the formation of a single cell, a z^gole^ The relationship between the members of one generation of an animal species and those of another is said to be an hereditary or a genetic one It is obvious that a zygote has an hereditary relationship by way of the gametes from the fusion of which it resulted with a male and a female parental organism.
In addiuon to the primary sex organs each sex is usually characterized by the presence oi accessory sex organs which transmit the gametes from the gonads In the male the accessory sex organs may include an intromiitent organ the^mr which enables the sperms to be deposited in the female genital tract In the female the accessory sex organs frequently include a special receptacle for the sperms the vagina and a brood chamber, the uterus for the reception and incubation of the zygote The male and female sexes are usually further distinguished by the presence of yet other differences both physical and mental, such as mammary develop ment hair distribution and patterns of behaviour not directly concerned with reproduction These arc known as secondary sexual characters.
Development and Embryology
The zygote which has been formed by the fusion of a male and female gamete is a single celled organism After a longer or shorter period this unicellular organism wall become pro grcssivcly transformed by the processes of cell division cell migration growth and differentiation into a multicellular mature member of its species The term development is used to describe these progressive changes After a period of matunly, of variable duration depending on the species retrogressive changes leading to senility and eventually death occur.
Consequently in the earlier stages of development structural changes occur and organs appear before the necessity or possibility for their functional activity Thus they have a future or prospectus rather than an actual or immediate value in the life of the developing orgamsm This can be expressed by saying that structure in the embryo is frequently antecedent to function Although It IS obvious that no defimte limit in this total developmental process can be fixed as the end of a stnetiv embryonic period still in general, the processes discussed m this book include almost all of those antecedent to function and most others up to the establishment of the basic functional patterns characteristic of the human foetus dunng the last six months of prenatal life In other words embryology is usually concerned with an organism from the zygote stage up to the anatomical cstabhshment of the definitive organ systems and often into their early functional period An exception to the latter would of course be the reproductive system which matures functionally relatively late.
The term ontogeny is used to describe the complete life period of an individual organism Embryolo^ is the study of the earlier stages of development md in the true mammals including man, is restricted to the developmental processes occurring before birth, i e., in the prenatal period of life It must be stressed, however, that there are no essential differences between prenatal and postnatal development, the former is more rapid and results m more striking changes in the shape and proportions of the organism, but the basic mechanisms are very similar if not identical m both periods Indeed in many lower orgamsms (see Child, 1941, for details) the ability to revert under certain conditions to the embryonic type of development is retained throughout hfe and the phenomena of repair and regeneration, which are present to some degree m the adults of even the highest animal types, present fundamental similarities to embryonic processes. Ageing in most metazoa, however, has a marked retarding effect on developmental reparative changes (du Nouy, 1936).
Developmental changes can be contrasted with the day by day non-progressive and, in generalj much more rapid physiological changes, such as respiratory, circulatory, digestive and nervous activities, which are directly essential for the maintenance of life. When the earlier embryonic stages are considered this contrast is particularly striking, for developmental changes are then at a maximum and the organ systems of the body are not yet established to perform the physiological activities peculiar to them in later life. Such earlier stages may be referred to as the pre-functional period of development, the later stages constituting the functional period It must be emphasized, however, that at all stages of development the embryo is a living organism capable of maintaining itself as such. As it grows and differentiates, new mechanisms are continually coming into activity so that the organism may cope with changing internal and external conditions, for embryos not only grow and differentiate but also live, and the requisite physiological functions must be exercised during these developmental alterations. If embryos are to maintain themselves, their structure must be such that for each developmental period an adequate physiological performance is assured. Equipped with this structural organization an embryo might live indefinitely at any particular stage if no changes in itself or m its environment rendered that level of organization inadequate But changes in the embryo do occur as a function of time and as the requirements for existence are progressively modified. The new needs are met by the development of new devices which one after another are discarded or remodelled when the needs change and pass. “Thus one meets with a series of increasingly complex ephemeral organs and structural arrangements characterizing the periods of development that space off the anabasis of the embryo from the microscopic one-celled egg up to the large, highly specialized fetus of later stages” (Streeter, 1942).
Subdivisions of Embryology
In its evolution as a science embryology has passed through several stages.* It was at first, and for the greater length of its history, purely descriptive, but as detailed knowledge of the development of related types became established a science of comparative embryology arose. This, in turn, was succeeded by the attempt to introduce an analytical embryology based on experimental methods This experimental embryology, which was first properly established by Roux (1888) as Entwicklungsmechanik (developmental mechanics), has widened the scope of the science, so that now the investigation of the causal mechanisms of development has been added to the descriptive and comparative approaches Observational embryology can merely record the sequence of developmental events, only experiment can acquaint us with the forces involved and their possible modes of action During the past fifty years the experimental approach, using lower forms, has resulted in the elucidation of many fundamental problems of development but, unfortunately, the possibilities of using similar experimental procedures on embryos of the higher, and particularly viviparous, types are still very restricted and, of course, in man are non-existent.
For the history of embryology consult Russell (igiGj 1930)5 Nordenskiold (1929); Cole (1930)5 Needham (1934); Meyer (1936, 1939), and Adelmann (1942).
Human embryology is, strictly speaking still in the descriptive stage However, the application of the concepts of comparative and experimental embryology has added greatly to the rational interpretation of the processes of human development.
Value of Embryology
The value of the study of embryology to the medical student is fivefold —
- From the general biological aspect it gives an understanding of how the different organs and tissues develop from a single cell (the fertilized ovum) into a complex multicellular organism As the study of the development of structure is linked vMth that of function embryology provides a basis for the understanding of the functional activity of the organism during development Further an appreciation of the causal factors underlying development, about which v\e have only the tentative beginnings of knowledge is only possible after normal development has been considered It must be realized that the study of development m a single complex species (e g as in this case, man) will not always be easily understood Indeed in order to obtain a clear understanding and appreciation of developmental processes in man. It is often necessary to refer to sub human ty^ies since in man some sequences of dev elopment have become shortened to such an extent that the basic primitive steps in the process are difficult to discern, or because sufficient stages of human material are not available for study This is particularly the case in the early stages of development Experiments which can be earned out in lower types can not of course be perform^ on the human subject so that the argument from analogy must often be used.
- From the vocational aspect the study of dev elopment in a great many instances gives a rational explanation of the relationships and position of many normal adult structures e g the nervesupply of the diaphragm by cervical nerves the asymmetry of the veins in the abdominal and thoracic cavities the nerve supply to the tongue.
- Embryology includes not onh the development of the embryo but also the development of the membranes which connect the foetus to the mother le placcntation m viviparous vertebrates A knowledge of the development relations and properties of these membranes w essential in order to understand obstetrics and as a basis for advances in this subject Such a knowledge is also obviously necessary for the understanding of the physiological relationship between the foetus and the mother.
- Many pathological conditions can only be understood m the light of normal and abnormal development Until recently the relationship of embryology to medicine was mainly ofa theoretical nature Modern experimental work on lower types has thrown light on such problems as growth and regeneration of tissue formation of certain tumours, transplantation (* e grafting of tissue) and explantation (1 e the growth on a suitable cultural medium, of isolated parts or individual organs) There seems little doubt that these aspects of ebryology are destined to make profound contributions to our conception of growth and r^arative processes and also possibly to our understanding of many pathological phenomena.
- As the student continues his studies through the basic medical sciences and into the c inical subjects embryology will be appreciated more and more as a great correlator of other morphological disciplines such as anatomy pathology, physical diagnosis and surgery and even of many physiological aspects of medicine.
Developmental Adaptations
Developmental changes occurring m one group of animals do not necessarily occur in all IS described later (Chapter X\ I) the ova (eggs) of different groups do not contain an equivalent amount of stored nutritive material (yolk or deutoplasm) and this is ^related with adaptive changes in the method of development when the amount of yolk is large, as in birds, the embryonic period of development is long enough to allow adult-like characters to appear before hatching. When the amount of yolk is less, as in Amphibia, the young are hatched as larvae (i.e , tadpoles) which lead an active life and derive nutriment from their external environment before assuming, by a process of metamorphosis, the adult form. Again, eggs of many species are shed into water and undergo their development in an aqueous medium, whereas m other types the eggs are laid on dry land, possibly in desert regions, with no access to water other than that with which they are originally endowed by the maternal organism. The latter variety of eggs show adaptations which tend to preserve the scanty water reserves These include the development of special enclosing shells or membranes and changes in embryonic metabolism such as the excretion of the end products of nitrogenous metabolism as the relatively insoluble uric acid (uricotelic metabolism) and a high resistance to ketosis (Needham, 1942). Yet again, as m the group of true mammals, the egg, with practically no reserves of yolk or of water, is fertilized by the sperm inside the maternal body, and a part of the maternal genital tract, the uterus, becomes modified for the retention and nutrition of the developing individual. This arrangement whereby the egg develops within the uterus long after its meagre yolk supply is exhausted is known as viviparity In such circutnstances a specialized apparatus, the placenta, is elaborated by both embryonic and maternal tissues to serve as a mechanism for the transfer of food and oxygen from the mother to the embryo and of the waste products of metabolism from the embryo to the mother As man is a true mammal this placental mechanism is present during human development and is described m Chapter V. Many other examples of developmental adaptation to special environmental conditions are known and some are referred to m Chapter XVI.
Heredity and Environment in Development
What an organism becomes in the course of development is the resultant of two factors, its heredity and its environment Heredity acts by way of internal factors present in the fertilized egg Itself The modem science of genetics has demonstrated that many, if not all, of the internal factors are present in the nuclei of the gametes They are the so-called genes which are probably complicated protein molecules situated m the chromosomes As the genes of a zygote are derived from both maternal and paternal gametes the characters of the developing organism are derived from both mother and father * It is now well established that the special characteristics of an organism (e.g , hair type, eye and skin colour, etc ) are due to its nuclear genic equipment. The more general characteristics (e g , those enabling us to classify it as a man or a chimpanzee, as a primate or a carnivore, as a mammal or a reptile) are controlled by factors which are not yet understood It is suspected, however, that in addition to the nuclear genes the general cytoplasm of the egg may have some influence in the establishment of the specific, generic and class characteristics of an organism (Needham, 1942, and Harvey, 1942).
Environment acts on the development of the egg as a whole by way of external factors not present in the egg itself Such factors are gravity, temperature, light, chemical agents and nutritive substances There is, however, an interaction within the developing organism of the various genes, chemical substances, and products from the different tissues and organs upon one another. This is often spoken of as the internal environment All of this internal environment is, however, in the last analysis a part of heredity, in contrast to external environment which IS what is ordinarily meant when the problems of heredity and environment are discussed
Heredity and environment are often brought into a false antithesis, since it is assumed that they act in opposition to each other. The characters of an adult organism, however, are produced by the interaction between the genetic factors and the environmental ones An alteration in either of these components may lead to variation or if excessive to abnormal dev elopment or ev en death Genetic \ anaUon results from changes m the genetic constitution due to mutation or more frequentl), to changes in chromosomal or genic pattern (recom bination) Environmental variation results from changes in the environment in v\hich the genes operate The actual organism produced by the normal interaction between heredity and environment is called a phenotype an oi^anism judged by its genetic constitution alone is a senot^pe Phenotypically similar organisms may be genotypically different Thus the genetically hybrid children of a pure dark brown eyed parent and a pure blue eyed one although possessing both blue eye and dark brown eye genes all actually have eyes as dark as their dark eyed parent In other words their phenotype is ‘dark eyed, but their genotype IS a hybrid between dark eyed and blue eyed In genetical terms one parent is homo.^)gous for dark brown eyes the other homozygous for blue eyes and the children are hetero^gous possessing genes for both dark and blue eyes On the other hand alterations in the environment may result m the simulation of characters associated w ith one genoty pe in an organism possessing quite a different genotype Thus genotypically dissimilar organisms can be made to be phenotypically similar (the phenocopies of Goldschmidt 1930)
Owing to the nature of the hereditary mechanism, however, blending of particular characters of mother and father does not usually oecur On this point, and on genetics generally, the student should consult a textbook of genetics, e g , Gruneberg (1947), Ford (1942), Castle (1940), Waddington (1939)1 Goldschmidt (1930)
Owing to intrauterine gestation the environment of the developing mammalian embryo IS tolerably constant and more or less optimal Environmental variation in this vertebrate class therefore is difficult to study m the prenatal penod It is well established, however that environmental differences of quite subtle kinds (eg number of embryos in uterus age of mother number of previous pregnancies and certain virus diseases such as rubella m the mother) do influence the course of development Maternal hormones too, may have some effect (Chapter V)
The experiments of Walton and Hammond (1938) have demonstrated very clearly the effect of environment on mammalian development These workers by artificial msemina tion produced reciprocal crosses of the Shetland pony and the Shire horse At birth the cross bred foal from the large (Shire) mother and the small (Shetland) stallion is three times as large as that from the small (Shetland) mother and the large (Shire) stallion As the cells of both foals have similar chromosomal contents and presumably similar genes the size differences must be due to the environment provided by the mother How the size is controlled to suit the size of the maternal organism has not been determined It may be limited by the amount of nutrition provided by the maternal circulation by the maximum size of the uterus or by some unknown (maternal placental or foetal) hormonal influence.
The occurrence in man of like {identical) and unlike {fraternal) twins provides excellent material for assessing the importance of changes in environment on the subsequent history of individuals of identical or different genetic structure The results of such assessment are considered in Chapter VIII.
The question whether environmental influences affect only the individual organism con cerned or whether such influences have a transindmdual action (1 e the effects are earned over to the next generation) has been much debated Modem biologists are almost unanimous in their opposition to the so called transmission of acquired characters in the Lamarckian or neo Lamarckian senses of the term (Huxley 19412) All animal characters are acquired in the course of development by the interaction of the genetic equipment with the environment and as has been stated earlier variations in cither of these scu of factors may result in alterations in the course of development There is however no acceptable evidence that a character of the body of an organism arising in response to an environmental stimulus is able so to impress Itself upon the genes that in subsequent gcneration the character will appear in the absence of the stimulus The internal factors (genes) can be permanently changed as the result of direct action upon them of irradiation which is of course an environmental stimulus However environmental stimuli such as those of bght, temperature and food which act directly on the body tissues, but hav e no direct action on the genes within the germ cells cannot produce hereditary modifications.
Epigenesis and Preformation
The major problem m embryology is the appearance, during development, of complexity of form and function where previously no such complexity existed. Historically two contrasting points of view have been held on this problem. One of these, the so-called theory of epigenesis, considered that during development there is actually the creation of new structures; whereas the other, the theory of preformation, maintained that a pre-existing diversity is already present in the fertilized egg (or in the sperm) and that future development consists merely in the unfolding and rendeiing visible of this innate diversity The embryological investigations of the past hundred years have demonstrated most conelusively that the actual processes of development are of an epigenetic nature but the doctrine of preformation has been reintroduced, in a much modified form, in the explanation of the facts established by modern genetics. “The modern view IS rigorously preformatiomst as regards the hereditary constitution of an organism, but rigorously epigenetic as regards its embryological development” (Huxley and de Beer, 1934).
Fundamental Processes in Development
Needham (1933 and 1942) has classified the fundamental morphogenetic mechanisms under the headings of giowth, diffeientiation and metabolism. Growth is increase m spatial dimensions and in weight, it may be multiplicative (increase in number of nuclei and (or) cells), auxetic or intussusceptive (increase in the size of cells) or accretionary (increase in the amount of non-living structural matter). Differentiation is increase in complexity and organization This increase may be in the number of varieties of cells and may not at first be apparent (“invisible” differentiation, e g , determination of fates, segregation of potencies, loss of competence, etc , see Chapter VIII), but, when apparent (“visible” or “manifest” differentiation), constitutes histogenesis. Differentiation may be manifested as an increase in morphological heterogeneity resulting in the assumption of form and pattern and in the appearance of recognizable organs or organ pnmordia {organogenesis). Metabolism includes the chemical changes in the developing organism
In the normal development of an embryo these fundamental ontogenetic processes are all closely interlinked, constituting an integrated system, “They fit in with each other in such a way that the final product comes into being by means of a precise co-operation of reactions and events” (Needham, 1942).
Stages in Embryology
From a descriptive point of view the principal stages in metazoan embryological development are —
(1) Maturation. This is the process associated with the formation of mature female and male germ cells (gametes — ova and sperms) from the undifferentiated germinal epithelium (oogonia and spermatogonia) of the female or male gonads. During maturation of both oocyte and sperm a reduction of the chromosomes to one-half of the somatic number occurs. This reduction results from a specialized mode of nuclear division called meiosis. In this stage the female sex cell grows to a relatively large size due to storage of yolk. The male sex cell remains small but undergoes changes in shape and internal structure which make it a motile orgamsm Consequently the mature gametes are highly specialized cells which when fully differentiated do not usually live long unless they take part in fertilization
(2) Fertilization. This is the fusion of a female and a male gamete. It results in the formation of the zygote or fertilized ovum. This process has two fundamental objectives; Increase in number of nuclei and number of cells is not necessarily growth in the sense of size expansion. Cell dmsion, for example, occurs without size expansion in the earlier stages of cleavage (Chapter IV) Nevertheless nuclear and cell division are so inumately bound up with growth that Needham classifies them with true growth processes from the initntion of cm])rVQmcJc>_clapmcDt^ second the reitoration of the iromon some number of the species and hence the achievement of biparentnl mhentance with all its important implications The z\^.otc ahhoiii.h rcsullini; from the fusion of two biqlil) specialized cells is regarded as being the most tmspeciahzed (umlifTefentiaitd) of all metazoan cells
(3) Cleavage The zvgotc soon undergoes repeated subdisision In mitosis so that a number of cells Uasiomrt} each mnch smaller than tlic o%um itself is produced Tims the unicellular z>gotc becomes a multicellular organism
(4) Blastula The blastomctes at the end of clcwagc arc cscntualK grouped to form a hollow sphere of cells the hlisiiila or in mammals the hlastocjst Uxpcnmenis on and intrasiiam staining of the hlastulae of lower \crtchrites especially Amphibia hate made U possible to delimit the future fate of all regions of the blastiila and thus to ascertain their potency ic what localized areas become 111 normal development The dincrcnt areas of the blastula can be referred to as presumpine organ regions thus one region is presumptive notochord another presumptive neural plate etc
{5) Gastrula The blastula stage is succeeded In the gastnila stage which results from changes m position and displacements (morphogenetic movements) of the v anouv presumptive regions of the blastula gastrulation results in the establishment of the three pnmarv germ lavers trvladtm and rcWrrwi and brings the presumptive orgam of the emhrso into the positions m which they will undergo their sulwetjuent development In reptiles birds and mammals this gastrulaiion period is represented bv the embryonic disc and primitive streak stages
(6) Neurula The gastruh sta^e iv followed In one m which the neural plate and the axial embryonic structures are elaborated In Amphibia this » known as the ncurula This stage corresponds roughly to the somite stages in human tlevrlopment (Chapter IV) At the end of the ncurula or somite stage of development the general pattern of the embryo is well established and later embryos ire said to be in the so-called functional period of development.
Functional Period of Development
The earlier embryonic stages which have been described above result in the appearance of the general embryonic pattern before the onset of specific function m the pnmordia of the different organs and (issues which arc diffrentiated in these stages Functions in the general sense are carried out at all times as all the cells are undergoing mctaljohc changes and must work to live Hut with the onset of specific functions such as beating of the heart contraction of muscles, secretion by glands etc the embryo enters on what may be called the functional period of development Different organs of course commence to function at different times and no sharp distinction can be made between pre functional and functional stages growth and differentiation proceed in both Ncvertlielevs it is useful to consider the processes of earlier stages as blocking out the mam cmbrvonic organ systems which will subsequently be elaborated under the influence of the specific functions which they perform The functional influence does not by any means replace the genetically determined general pattern of development, but It will be seen m later chapters that m the dev clopment of many organs and tissues (e g , the heart and blood vessels and the skeletal system) the effect of the function of an organ on its development is considerable.
This functional stage of development results in the different organs and tissues coming into physiological relationship with each other and therefore in a degree of integration of total function which cannot exist m earlier stages The integration is facilitated and indeed rendered possible bv the differentiation of the vascular and nervous systems and the onset of function in the endocrine glands The endocrine glands arc of special importance in the later stages of embryological and in postnatal development The growth hormone of the pituitary the thyroid hormone and the hormones produced by the gonads afford particularly good examples of endocrine influence on development, but all the endocrine glands are probably concerned in the regulation of normal growth and differentiation
References
Adelmann, H. B (1942) The Embryological Treatises of Fabncius of Aquapendente Cornell Umv Press, Ithaca, N Y
Castle, W E (1940) Mammalian Genetics Harvard Univ Press, Cambridge
Child, C M (1941) Patterns and Problems of Development Umv Chicago Press, Chicago
Cole, F J (1930) Early Theories of Sexual Generation Clarendon, Oxford
Ford, E B (1942) Genetics for Medical Students Methuen, London
Goldschmidt, R (1938) Physiological Genetics McGraw-Hill, New York
Gruneberg, H (1947) Animal Genetics and Medicine Hamilton, London
Harvey, E B (1942) Maternal inheritance in cchinoderm hybrids J Exp ,^00/, 91 , 213-235
Huxley, J S (1942). Evolution The Modern Synthesis Allen & Unwin, London,
and de Beer, G R (1934) Elements of Experimental Embryology. Cambridge Umv Press, London
Meyer, A W (1936) An Analysis of the De Generatione Ammahum of William Harvey Stanford Umv. Press, California
(1939) The Rise of Embryology Stanford Umv Press, California
Needham, J (1933) On the dissociabihty of the fundamental processes in ontogenesis Biol Rev , 8, 180-223
(1934) A History of Embryology Cambridge Umv Press, London
(1942) Biochemistry and Morphogenesis Cambridge Umv Press, London
Nordenskiold, E (1929) The History of Biology Knopf, New York du Nouy, L (1936) Biological Time Methuen, London ^
Roux, W (1888) Beitrage zur Entwickelungsmechanik des Embryo. Arch f Path Anal, u Phys {Virchow's) 114 , 1 13-153
(1895) Gesammelte Abhandlungen uber Entwicklungsmechamk der Orgamsmen Engelmann, Leipzig
Russell, E S (1 J16) Form and Function Murray, London.
(1930) The Interpretation of Development and Heredity Clarendon, Oxford.
Streeter, G L (1942) Developmental horizons in human embryos description of age group XI, 13-20 somites, and age group XII, 21-29 somites Contrib Embryol, Carnegie Inst Wash, 30 , 21 1-245
Waddington, C H (1939) An Introduction to Modern Genetics Allen & Unwin, London
Walton, A , and Hammond, J (1938) The maternal effects on growth and conformation in Shire horse Shetland pony crosses Proc Roy Soc Lond , B 125 , 311-335
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Hamilton WJ. Boyd JD. and Mossman HW. Human Embryology. (1945) Cambridge: Heffers.
Cite this page: Hill, M.A. (2024, April 25) Embryology Book - Human Embryology (1945) 1. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Book_-_Human_Embryology_(1945)_1
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