Book - An Introduction to the Study of Embryology 8
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Haddon An Introduction to the Study of Embryology. (1887) P. Blakiston, Son & Co., Philadelphia.
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Chapter VIII. General Considerations
Complexity of Embryological Phenomena
The phenomena of Embryology are of a very complex nature, owing to abbreviation or precociousness in the development of certain organs, and in the occurrence of a series of transformations which have reference solely to the ancestry of the individual, the latter often bearing no discernible relation whatever to the adult condition.
The irrelevance of these metamorphoses to the adult state is in some cases emphasised by the fact of their suppression in certain members of a group, as, for example, amongst the Scyphomedusse. The genus Pelagia, although closely related to Aurelia, develops directly from the egg wuthout the intervention of the Scyphistoma larva; and even Aurelia may abnormally have an abbreviated development. The characteristic larval forms of the Echinozoon Echinoderms are omitted in the development of those forms in which the young are reared in brood-pouches or similar protective chambers. The following will serve as types : - Leptychaster kerguelenensis, Ophiacantha vivipara, Hemiaster cavernosus, and Psolus ephippifer. The direct development of Astacus is an example of the suppression of metamorphoses amongst the Crustacea, but in this Decapod a good deal of food-yolk is present.
The passing through of a free larval existence must be considered as constituting a drain upon the energy of the organism, and this loss naturally affects the adult condition. As Sollas points out, when such a larva â€œ finally reaches the adult state, it has already to a considerable extent worn out its machinery and expended its powers of converting energy. A still more important consequence, however, would seem to follow from the premature aging due to a free larval existence, and that is the comparatively early exhaustion of the powder of undergoing transformational change ; the adult or comparatively stable state is reached sooner than it otherwise would be, and the chances of further development are correspondingly diminished.- It has been pointed out by several authors that the individual which is best equipped as an adult is that which has rapidly passed through its embryonic condition under circumstances where it has been extraneously nourished and protected. Again, to quote from Sollas, â€œ The longer life in the mature state, acquired by those forms which are saved from the drudgery of a larval existence, offers increased opportunities for evolution to the adult animals, so that a progressive development, starting from higher and higher platforms, is directly favoured. But not only is a longer existence assured to the adult - existence in the embryonic state is shortened ; and perhaps here the influence of seclusion is most clearly exhibited, for the energy which would be expended in a free larva in activities other than those involved in producing structural change is here solely devoted to that end, and hence the embryonic stages are passed over by secluded forms with comparative rapidity.-
In studying the development of animals, it must always be remembered that what is known as the â€œ struggle for existence - is continually acting upon the larval form as an individual, and that while the larva has to adapt itself to present conditions and to supply its own wants, the rudiments, or the formative tissue (blastema), of future organs may be precociously formed. This is the main reason for the complications and abbreviations which occur so frequently in the development of animals. Occasionally larval forms, so to speak, run wild, and do not develop into their normal adults, the form known as Leptocephalus amongst Teleosts affording a good example of this vagary.
The real nature of many einbryological phenomena must remain unknown until the properties of protoplasm are considerably more elucidated. At present, we can deal only with the results, and not with the causes of changes in organic matter.
In the course of this work attempts have been made to indicate how certain organs may have been developed from pre-existing simpler structures in response to definite stimuli or to the requirements of the organism. The further our knowledge extends the more certain it appears that evolution is mainly the result of a mechanical necessity, or, as James Hinton put it, â€œorganic forms are the result of motion in the direction of least resistance.-
Suggestions as to the possible significance of observed embryological facts must be held only in the most tentative manner. It is easy to frame plausible theories respecting the evolution of organs or of the animals themselves, but great caution is necessary in accepting them, and, at best, they should be regarded as merely working hypotheses.
Sketch of a Possible Evolution of the Metazoa
The Protozoa combine all the essential activities of life within the limits of small independent units of protoplasm, and even in these differentiation may occur to a considerable extent. Those causes which result in the production of complicated organs in the Metazoa also act on unicellular forms, but, having less scope, the result is less evident. The higher organisation of multicellular animals is solely attributable to the large number of aggregated units which constitute their body; the forces acting upon all living beings must be the same.
The formation of masses or colonies of cells (aggregates of protoplasmic units) may possibly be primarily due to imperfect fission. Cell-division itself ( ie ., reproduction) is usually regarded as being primitively due to excess of growth consequent upon excess of nutrition ; Geddes, however, suggests a different interpretation of the origin of cell-division (p. 279). Amongst the Protozoa reproduction results in the formation of distinct and independent organisms, each one of which is unicellular like its parent. In only a few forms are individuals aggregated into colonies, and in these but little co-ordination occurs.
More precise histological research is now demonstrating that in most, if not in all, animal (and vegetable) tissues the component cells are united together by strands of protoplasm, often of extreme tenuity. There may thus be a protoplasmic continuity extending throughout the whole organism, and possibly all the living cells of an animal are directly or indirectly connected with one another, except the lymph and blood-corpuscles.
The observations of Sedgwick on the syncytial segmentation of Peripatus (fig. 19) are in this respect very suggestive, and it may yet be proved that the complete division of an ovum into distinct segmentation spheres (fig. 12) is apparent rather than real.
It appears that all the cells of adult Coelenterates are connected together by means of protoplasmic processes, and it might fairly be assumed that the cells of a segmented ccelenterate ovum and of the embryo into which it will develop are similarly united ; but there is at present no definite embryological evidence to support this conclusion. The cellular network of the parenchymula larva of Obelia (fig. 46) is, according to Merejkowsky, a secondary condition due to the fusion of the processes of amoeboid cells.
Whether directly continuous or not, all the cells of a Metazoon are so grouped as to constitute a co-ordinated whole, the life of the individual being the sum-total of the activities or lives of the constituent cells. Theoretically each one of these cells possesses all the attributes of protoplasm, as, most probably, was actually the case when the ancestral form was passing from the Protozoon to the Metazoon condition, a stage which is now represented by the blastula larva. We may assume that each cell then possessed nutritive, sensory, metabolic, and reproductive functions ; but in process of time specialisation occurred, and the concurrent limitation of function resulted.
In unicellular animals one pole or aspect of the body is usually concerned in the ingestion of food, and we are justified in assuming the same for the Protozoon ancestor of the Metazoa.
The segmentation of the ovum is stated to occur in two different ways. Either, according to the generally received account, it may from the first divide the cell horizontally into a nutritive (vegetative) and sensory (animal) portion ; or, according to Agassiz and Whitman, the ovum may divide longitudinally (axially), then transversely, and lastly horizontally. In either case a multicellular mass is formed, of which the upper pole is more especially sensory (epiblast) and the lower nutritive (hypoblast). Assuming it to have been flattened, Biitschli has termed this theoretical ancestral form a Plakula (p. 23).
The series of stages from an unicellular form to an organism, consisting of two sets or layers of cells, presents us with no special difficulty, and plausible theories have been framed to account for the formation of a double-layered gastrula from the single-layered blastula. It is a matter of some importance to note that embryological evidence, as a whole, supports the conclusion that the future epiblastic (ectodermic) and hypoblastic (endodermic) cells are already practically differentiated in the blastula stage, and that the gastrula was evolved as a result of that differentiation. It is too often assumed that all the cells of the blastula are identical in every respect.
Brief History of Mesoblastic Tissues
The conversion of a diploblastic form to one with three layers (triploblastic) is readily conceivable. It is possible that the third layer (mesoblast) primitively arose as the result of excessive nutrition of the nutritive cells (hypoblast). The inner moieties of these cells separating themselves as amoeboid cells (mesamoeboids, or archaeocytes of Sollas), which would then crawl about in the space (segmentation cavity) between the two layers. Similar cells arise in some embryos from the epiblast also. These cells would readily assume the amoeboid condition, as they were not subject to pressure and had sufficient space for migration.
Whether originally specially nutritive or not, the wandering cells would readily become modified and change their function ; their contractile power might be emphasised, and thus they might be converted into simple muscle-cells. By the secretion of mineral matter they would form skeletogenous cells. By retaining a free existence others would serve as carriers of matter, or, in other words, become corpuscles of the nutrient fluid.
Most of the internal supporting (endoskeletal) elements, with the exception of the notochord of the Chordata and the connective tissues, are, together with the blood-corpuscles and vascular system, developed from the mesoblast. Lankester has associated these series of tissues under the common designation of â€œ skeletotrophic.- This he regards as a â€œ natural group of tissues which is divisible into - (i.) Skeletal, including fibrous, adenoid, adipose, bony, and cartilaginous tissues. (2.) Vasifactive, including capillaries and embryonic blood-vessels. (3.) Haemolymph, including the haema or haemaglobinous element and lymph, the colourless element of vascular fluids.-
Lankester further points out that â€œ the mother-cells of all tissues are either â€˜ entoplastic - or â€˜ ectoplastic, - or both - that is to say, the metamorphosis of their protoplasm is either essentially one occurring at the surface of the protoplasmic corpuscle, or one occurring deeply within its substance, or the two processes may go on in connection with the same cell.- Thus hyaline cartilage is essentially ectoplastic, while notochordal tissue results from a metamorphosis of the cells and is essentially entoplastic. â€œ Fibrous tissue generally is ectoplastic, as the protoplasmic corpuscles remain more or less intact whilst surrounded by the fibrous and lamellar masses to which they have peripherally or laterally given origin. This is true of ordinary subcutaneous areolar tissue, of tendon, of mucous tissue (umbilical cord), and of corneal tissue. At the same time we find in various Invertebrate groups (not in the Vertebrata) an entoplastic form corresponding chemically aud functionally to the ectoplastic forms just cited. This is the vesicular connective tissue so abundant in the Mollusca, in the Nemertines, and other Invertebrates. The only tissue which in form represents this among the connective tissues of Vertebrates is adipose tissue.- The vesicular cells of Mollusca contain glycogen ; indeed, a glycogenetic function is now known to be widely distributed in various mesoblastic tissues.
Yet further, the tissues of the connective group which are specially related to the nutrient fluids (such as blood and lymph), and which form the wall of the coelom or of blood-channels, may be entoplastic when they give rise, by internal metamorphosis (liquid vacuolation), to capillary vessels ; or ectoplastic when they constitute spongy or lacuniferous cell aggregates, the cells separated by intercellular channels, such as we find in the â€˜ pulp * of lymphglands and the spleen, and in the lacunar tissue of Molluscs.
The formation of gastric pouches (archenteric diverticula) appears to have resulted from the disproportional growth of the hypoblast. In forms higher than the Ccelenterates these pouches were constricted off from the central cavity and formed a true body-cavity or coelom. A nutritive fluid might collect by osmosis within the body-cavity.
The progression of the organism in a determinate direction would ensure a bilaterally symmetrical arrangement of the organs of the body, and, consequently, of the archenteric diverticula. A dorsal and ventral mesentery would result from the appression of the inner walls of the confluent lateral coeloms, while transverse mesenteries or septa would occur if the coeloms of the segments remained distinct.
The primitive nutritive corpuscles (mesamoeboids) lie within the blastocoel (or, as Hubrecht proposes to term it, the archicoel), and consequently outside the archenteric diverticula. When the coelomic walls were approaching one another, many of the corpuscles would be enclosed between them ; and if a small space was left between the walls of the coeloms, a tube would be formed, lying within the mesentery, containing amoeboid corpuscles. The walls of the coeloms possess actual or incipient muscle-fibres, and are therefore contractile. The contractility of the walls of the mesentery would thus result in a longitudinal contractile tube containing corpuscles, in other words, a vascular system would be initiated. The development of the heart in both Vertebrates and Chsetopoda appears to support this hypothesis of its evolution.
Hubrecht claims for the blood- vascular system of the ISTeinertiue Worm Linens that it arises merely by the â€œ connective tissue- not obliterating the archiccel in these places, and that the indifferent mesoblast is modified in situ into the endothelium and walls of the vessels. In most other animals the smaller vessels are formed by the hollowing out of solid cell-rows and cell-groups.
It would be rash to hazard a conjecture concerning the evolution of the excretory organs until we have more precise information concerning their development in the lower Metazoa. It is not improbable that there is no genetic connection between the excretory organs (nephridia) of certain groups ; thus it is difficult to see the homology in such organs as the green gland of Decapod with the excretory tubes of Amphipod Crustacea, or these again with the nephridia of Peripatus and the Malpighian tubules of Insects. The Vertebrate excretory organs appear almost certainly to have been evolved from some primitive form of nephridium, from which the nephridia of the Segmented Worms were independently differentiated.
But little is known concerning digestion and assimilation in embryos. The actual processes must be assumed to be essentially similar to those occurring in adults. The following general features, which alone can be dealt with here, are worthy of notice.
As was mentioned very early in this work, an oosperm must be regarded as an amoeboid Protozoon, which multiplies by fission very rapidly, but which is precluded from obtaining fresh nutriment directly. The energy requisite for this enormous activity is provided by the breaking down, through digestion, of the highly nitrogenous food-yolk which is stored up within the body of the ovum.
In many cases the stored-up nutrient material, yolk, is really derived from neighbouring ovarian cells which the ovum has swallowed. (This process, which is simply a case of feeding, must not be confounded with the formation of a plasmodium or syncytium by the fusion of previously distinct protoplasmic units.) The ovum has, in fact, gorged itself preparatory to entering upon a stage of rapid cell-division. The telolecithal and centrolecithal distribution of the yolk in the ovum and developing embryo has been already referred to. In the former case the yolk is actually stored up within the primitive hypoblast cells, that is, within those very cells whose function is to digest it. In the second case the yolk is afterwards transferred to those cells.
1. Hypoblastic Digestion
The act of digestion is almost entirely performed by the hypoblast. From the nature of the case all Protozoon digestion must be intracellular, that is, must be effected within the cell itself. It is now proved that the digestion of the Coelenterates and of some Turbellarian Worms is largely intracellular, although extracellular digestion also occurs to some extent. Even in some of the lower Vertebrates the epithelial cells of the intestine may send out pseudopodia for the purpose of ingesting fragments of partially digested food. In other words, the lower Metazoa have not yet broken away from the traditions of Protozoon digestion. In this respect early embryos of higher Invertebrates reproduce the ancestral condition ; for we find in the Crustacea (fig. 22) that the hypoblast of the gastrula stage feeds upon the yolk by means of pseudopodia, and the digestion is intracellular.
Caldwell states that throughout larval life intracellular digestion occurs in the first stomach of Phoronis, but that this mode of digestion ceases when the metamorphosis takes place.
Kollmann has recently shown that in the meroblastic ova of the Lizard and Fowl (fig. 66) the primitive cells of the germinal wall, in the equivalent of the gastrula stage, engulf and digest the yolk spheres and granules like an Amoeba eating its prey.
It is probable that extracellular digestion, as it occurs in the more specialised Metazoa, does not take place till â€˜â€˜hepatic- or other secretory cells make their appearance. Most Prosobranch Molluscs, such as Purpura and Fusus, possess a large quantity of food-yolk which is stored up within the hypoblast cells (fig. 18), and the digestion of which is consequently intracellular. It is well known that during the veliger stage these Molluscs are truly cannibals and devour their weaker brethren. This new food passes into the mesenteron (archenteron), and certain of the hypoblast cells acquire a very different appearance from the remainder and constitute true digestive cells. Food in process of digestion is seen within the cavity of the mesenteron. As a matter of fact, the two modes of digestion take place simultaneously until the yolk is quite absorbed.
This view is rendered the more probable from the fact that in the Ichthyopsida the distinctive complex digestive glands are either not at all or only slightly developed. Each individual cell of the mesenteron may be regarded as individually digestive, and thus in these forms hypoblastic intracellular digestion occurs.
Temporary pseudopodia, for the seizure of food particles, are very generally emitted by the cells of the intestinal epithelium in the lower Vertebrates. Such highly differentiated glands as the peptic glands and the glands of Lieberkiihn are found, from the Reptilia upwards, in an increasing degree. Their secretion acts chemically upon the whole or a portion of the food, and digests it within the cavity of the alimentary canal. The liver has been omitted in this connection, as it is not, in the true sense of the term, a digestive gland. As Wiedersheim has pointed out, there is a well-marked correlation between the folds of the mucous membrane and the development of intestinal glands. At first, as in the Cyclostomes, the folds have only a longitudinal direction, but afterwards transverse folds appear and crypts are formed in order to increase the secretory surface of the alimentary canal.
In Mammals the embryo is nourished directly by the blood of its mother, and the hypoblast of the foetus has never been functional in digestion ; it consequently requires what Sollas has termed a gastric education before it can digest the food of the adult. (This argument does not apply to those Sharks and Lizards in which there is a slight connection between the yolk-sac of the embryo and the blood-vessels of the wall of the oviduct, as in these forms a large amount of food-yolk is always present.) The secretion of milk by the mother supplies a readily assimilable pabulum, and the peculiar character of the first-formed milk probably renders the education still more gradual. A somewhat similar digestive education occurs in some Birds, such as Pigeons, the Flamingo, and others.
2. Epiblastic Digestion
The epiblast very rarely appears to have a digestive function. Metschnikoff, however, has observed intracellular ingestion by the ectoderm cells of larval Actiniae, and Kollmann states that the epiblastic cells of the blastoderm of certain Sauropsida can take up food by means of pseudopodia and digest it in the intracellular manner. It has been previously noted that villi develop from the epiblast which underlies the yolk-sac in Birds (fig. 75, B, v ), and also from the epiblast of the allantoic folds (c, v) } which absorb the remaining albumen of the egg. In the lower Mammals (fig. 8o) similar villi occur, which must be the means of absorbing nutriment from the uterine wall.
3. Mesoblastic Digestion
The undifferentiated wandering mesoblast cells may also be concerned in digestion, but their ingestion of foreign particles may be due in many cases to the mechanical properties of their protoplasm rather than to an actual selection. Our knowledge concerning the behaviour of these cells in embryo Invertebrates is almost entirely due to Metschnikoff, who has proved that the mesamoeboids are of great physiological importance from their first appearance, and in this respect they offer a marked contrast to the mesothelial mesoblast.
In Echinoderm larvae, for instance, which undergo rapid metamorphoses, the disappearing organs break down into albuminoid globules, which are devoured and digested by the mesamoeboids, or phagocytes, as Metschnikoff terms them. The latter also ingest small foreign particles which may be forced into the segmentationcavity. In many cases the mesamoeboids fuse to form a plasmodium or giant cell, in order to effect this more readily ; in some cases the mesamoeboids merely collect round the foreign body in order to isolate it.
The lymph-corpuscles (leucocytes) have been shown by Wiedersheim and by Schafer to perform an important part in digestion in adult Vertebrates. These cells have been proved to force their way through the mucous membrane into the cavity of the intestine, and there to devour fat, and probably amyloid particles ; they then return, and, crawling between the epithelium cells, pass into the lacteals. Others, again, merely ingest the food particles which have penetrated through the intestinal epithelium. In all cases, probably, the leucocytes pass into the lymphatics, where their contents are discharged by the disintegration of the cells themselves. The lymphatic fluid or chyle then passes into the general circulation, carrying with it the digested food which has been conveyed from the intestine by the leucocytes, and a large amount of proteid material derived from their dissolved protoplasm and nuclei.
It is still an open question whether digestion may not be performed in Sponges by the ectoderm as well as by the endoderm. The wandering mesoderm cells are probably concerned in the conveyance of nutriment and the removal of waste products, in addition to those functions which are more generally regarded as typical of that layer.
Closely associated with the subject of embryonic digestion is the part which the foetal membranes of Amniote Vertebrates play in nutrition. The reader is referred to the section which deals with these structures (pp. 78-96) for a summary of the evolution of the foetal membranes of the Amniota. The gradual Requisition by the allantois of the whole of the nutrition of the embryo is especially noteworthy.
The function of respiration must of necessity occur throughout the whole of embryonic and larval life. As a rule, it is more active in larvae than in adults ; at all events, the former always speedily succumb to a deficiency in the supply of oxygen.
The true respiratory process, i.e., the assimilation of oxygen and the excretion of carbon dioxide, occurs in the ultimate tissues ; it is only the exchange of the latter gas for the former of the external medium which occurs in what are termed respiratory organs.
As Dohrn points out, it is the vascular system which is really respiratory, and the pressure of a blood-vessel against an epithelium would cause an evagination of that tissue, be it epiblastic or hypoblastic. Of course the whole skin of the body and the alimentary tract were the primitive respiratory surfaces. The production of gill-filaments on a given area is the result of the presence of blood-vessels ; it is the latter after all, and not merely epithelial prolongations, which constitute gills.
It often happens that in embryos and larval forms the delicacy of the tissues suffices for the interchange of the gases, so that special respiratory surfaces are not required. When protective envelopes are present, they are usually very permeable to gases.
The proctodaeum serves as a special respiratory organ in certain larval Arthropoda; as, for instance, in the ISTauplius larvae generally, and in the aquatic larvae of Dragon-flies.
The higher organisation of the embryos of Vertebrates necessitates a large supply of oxygen, and, consequently, special provision has to be made by the development of larval respiratory organs, especially in those forms which undergo a secluded development. These may either be (i) the phylogenetic respiratory organs, which are utilised in the ontogeny of the individual, or (2) they may bear no relation either to the ancestral or to the adult respiratory organs. A pair of examples of each of these two cases will illustrate the general principle.
1. Utilisation of Phylogenetic Respiratory Organs in Ontogeny
The ordinary hypoblastic gills of Elasmobranchs appear early in the embryo, but the filaments on the posterior aspect of the archs are greatly elongated, so as to form a very characteristic fringe of gills, which have even been regarded as belonging to a different category from the normal filaments.
Both external and internal, i.e., epiblastic (?) and hypoblastic gills occur in the newly hatched tadpoles of Frogs. Whatever may be the exact significance of the former, the latter certainly are an example of the utilisation by a larva of the ancestral mode of respiration, as the respiratory organs of the adult, in this case the skin and lungs, have no connection whatever with the former.
2. Secondarily Acquired Larval Respiratory Organs
The embryonic respiration of the Arnniota affords a good example of the second proposition. In none are the walls of the visceral clefts functional as gills at any time, and, as the lungs are only functional after birth, accessory respiratory organs must be provided.
In Sauropsida the area vasculosa of the yolk-sac forms the first respiratory surface, this function is next shared with the rapidly developing vascular allantois, and lastly, owing to its enormous size, the allantois becomes the sole respiratory organ. As has been mentioned above (p. 259), the allantois is probably the hypertrophied and precociously developed urinary bladder, and we may assume that the ancestral forms of the Arnniota, like the Amphibia, had a large membranous vascular urocyst, which was capable of being early utilised as a respiratory organ. The topography of the allantoic blood-vessels, and the fact that the proximal portion of the allantois actually persists as the adult bladder, support this view. It is well known that egg-shells are very porous to gases.
Respiration in the embryos of the Prototherian Mammals is doubtless perfectly comparable with that in Reptilian embryos, whereas, in the Eutheria, aerial respiration is impossible owing to the embryos being included within the uterus. The foetus in utero has, however, no need for special organs of respiration, as it is supplied with aerated arterial blood direct from the main arterial trunks of the parent. The carbonic acid and other waste products of the embryo are carried away by the maternal venous circulation.
Evolution of Nervous System and Sense Organs
The epiblast naturally forms the protective covering of the organism, and would readily be modified to meet special requirements. From its position it would be directly subjected to every vibration in the external medium, and would therefore be continually receiving numerous stimuli, which would call into play the sensibility of the protoplasm of the cells. It is, then, no wonder that sense-cells originated, or that these became grouped together to form sense organs, or that a further differentiation occurred which resulted in the evolution of a highly specialised nervous system.
All these obvious facts were sufficiently noticed in the section on the â€œ Organs derived from the Epiblast,- and therefore need not be reiterated here.
Continuity of Germ-Producing Tissue
The germ-producing tissue is to be regarded as the direct product of the similar tissue of its parent, that is to say, a portion, however minute, of germinal substance is transmitted by the parent to its offspring. The germinal substance in the latter is increased by the ordinary method of nutrition and growth, but it still has the same essential character that was transmitted to it. The offspring in its turn passes on this germinal substance. There is thus a continuity of germinal matter, which, since it is transferred in an extremely minute quantity, must have an inconceivably complex structure, as it possesses the power of transmitting hereditary characters even of the most trivial nature.
It is maintained by some that the nucleus is the essential element in the germ-cell, whether ovum or spermatozoon, and that the cell-protoplasm is merely a nutritive basis. The structure of the ovum has already been stated to be similar in many respects to that of ordinary undifferentiated tissue-cells (fig. 5*). The distinguishing feature of the nucleus over the rest of the protoplasm of the cell consists in its possession of chromatin. As the chromatin always takes a conspicuous part in segmentation, we are justified in assuming that the chromatin or nuclein is concerned in the reproductive function. Fertilisation appears to be mainly the fusion of the nuclein elements of a pair of cells which are liberated from usually two parents. The resulting compound oosperm develops by segmentation and ulterior differentiation into an organism resembling, and at the same time differing from, each of its parents both in feature and in inherited tendency.
Weismann recently proposed the view that the nucleus of every germ-cell contains â€œ germ-plasma ,- or that substance which enables the germ-cell to build up a new individual ; and â€œ histogenetic plasma,- or that substance which enables the germ-cell to accumulate yolk, secrete membranes, or, in short, to develop itself into its characteristic structure as a ripe ovum or spermatozoon.
It is the germ-plasma alone that is required for the development of the embryo. The histogenetic plasma, having performed its function of building up the germ-cell, is useless, and has to be got rid off ; so it is extruded as the polar-cells, or as the passive element in the male germ-cells. If the germ-plasma left in the ovum has sufficient vigour (which would probably depend upon its quantity), there is nothing to prevent its further development into a new individual - that is, nothing to hinder the occurrence of parthenogenesis. As a matter of fact, however, this is rarely the case, and it requires the sudden accession of fresh energy in the shape of a spermatozoon to enable the germ-plasma of the ovum to further develop. In this view there is no essential distinction between the nucleus of the ovum and that of the spermatozoon ; the latter, like the former, is merely germ -plasm : the difference being that, as a rule, the germ-plasm of the male cell has an entirely different series of inherited characters, which it can transmit to the segmentation nucleus in the same manner as those of the female cell are transmitted.
The essential act of fertilisation, therefore, does not consist in the fusion of elements which differ in kind, but merely in the sudden accession of a store of energy which will enable the ovum to segment and build up a new individual. This brings fertilisation to resemble conjugation yet more closely ; and it further explains how it is that, in those forms in which parthenogenesis is not known to occur, the ovum may segment, and proceed a short way on its development.
This theory also agrees well with certain facts concerning the asexual reproduction of animals or plants. During segmentation there is formed in the nuclei of the segmentation-cells fresh histogenetic plasma, which is more especially concerned in the differentiation of the tissues ; but the germ-plasma may be generally diffused, or it may be early localised within certain segmentation-cells. Sponges, the Hydra, some Sea- Anemones, may be taken as examples of the former condition, as in these animals apparently any portion of the body containing ectoderm and endoderm will serve to produce a new individual ; and in the case of the two first-named, the germ-cells themselves appear to arise indiscriminately from the mesoderm in the former, and from the ectoderm (?) in the latter. In other Ccelenterata the germ-cells are of hypoblastic origin. In the second case, where the germ-plasma is localised to a special tissue, those segmentation-cells which will form the epiblast possess no germ-plasma, and, consequently, they can only build up specialised tissue. On the other hand, in most cases at all events, the germ-plasma, which at first is restricted to the nuclei of the hypoblast cells, becomes, as development takes place, still further localised until it is situated solely in that tissue which has for its especial function the reproduction of the individual. In other words, it is restricted to the generative gland. Asexual reproduction in such groups as the Polyzoa and Ascidians, and certain "Worms, is rendered possible by the retention of germ-plasma within certain undifferentiated tissue (funicular tissue, stolon, budding zones, &c.), from which the whole or part of the new individual may be formed ; but it is impossible to reproduce a perfect individual from any fragment containing epiblastic and hypoblastic tissue, as can be done in the case of Sponges or the Hydra. In this connection it is interesting to find, as Gruber has shown, that if an Infusorian be artificially divided, each portion will become a perfect individual. But if the dismembered portion does not possess a fragment of the original nucleus, the animal thus produced lacks the power of reproduction. It is perfect in every respect, except that it is deprived of the germ -plasma, which alone possesses the reproductive function.
Geddes has recently discussed the theory of growth, reproduction, sex, and heredity in terms of the metabolism of protoplasm. â€œ Protoplasm is regarded as an exceedingly complex and unstable compound, undergoing continual molecular change or metabolism. On the one hand, more or less simple dead matter or food passes into life by a series of assimilative ascending changes, with each of which it becomes molecularly more complex and unstable. On the other hand, the resulting protoplasm is continually breaking down into more and more simple compounds, and finally into waste products. The ascending synthetic constructive series of changes are termed anabolic, and the descending disruptive series catabolic.
Herbert Spencer first pointed out that in the growth of- similarly shaped bodies the mass increases as the cube of the dimensions, the surface only as the square, and applies this conception to express the occurrence of cell-division. â€œThus,- as Geddes expresses it, â€œ in the growing cell the nutritive necessities of the increasing mass are ever less adequately supplied by the less rapidly increasing absorbing surface. The early excess of repair over waste secures the growth of the cell, but the necessarily disproportionate increase of surface implies less opportunity for nutrition, respiration, and excretion ; and waste thus overtakes, balances, and threatens to exceed repair. Three alternatives are then possible - (i) a temporary equilibrium may be established and growth ceases, or (2) the increase of waste may bring about dissolution and death, or, still more frequently, (3) the balance of mass and surface may be restored by the division of the cell.-
Continued surplus of anabolism involves growth ; this growth is sooner or later checked by the preponderance of katabolism, and the most frequent alternative is the restoration of the balance by cell-division. Thus arises discontinuous growth or asexual reproduction. Budding, simple-division, and spore formation, like continuous cell-division, are simply different forms of the necessary separation which must occur at the limit of growth if the continuity of life is to be preserved. Like continuous cell-division, asexual reproduction occurs when waste or katabolic processes are in the ascendant. But what holds true in the growth of the individual cell is valid also in regard to the aggregate. There, too, a limit of growth must eventually be reached, when discontinuous growth in some form becomes inevitable. The essential difference is simply that at first in the unicellular individual the disintegration and reintegration entirely exhaust the organism and conclude its individual existence, while in higher forms the process becomes more and more localised.-
(b.) Sexual Reproduction
A comparative study of the methods of reproduction which occur amongst the lower plants and Protozoa will demonstrate that â€œthe almost mechanical flowing together of exhausted cells, as illustrated in plasmodia, is connected through the known surviving cases of multiple conjugation with normal conjugation ; - the dimorphism which marks the transition from conjugation to fertilisation, making the latter indispensable, appears very gradually. â€œThe very gentleness of the gradation leads one to regard the two processes as analogous responses to the same physiological necessities. The same disturbance of the balance between anabolism and katabolism which results in the occurrence of asexual reproduction leads in more developed forms to the separation of the dimorphic and mutually dependent elements of sexual reproduction. As asexual reproduction occurs at the limit of growth, so a check to the asexual process involves the appearance of the sexual, which is thus still further associated with katabolic preponderance.- The following illustration will suffice : - Under conditions of favourable temperature and abundant food the parthenogenetic reproduction of female Aphides can be indefinitely prolonged, while a lowering of the temperature and diminution of the food at once reintroduce sexual reproduction.
Nature of Sex
In attempting to define the distinctive characteristics of male and female, it is necessary to begin with the sexual elements themselves. The difference between male and female is there exhibited in its fundamental and most concentrated expression. It is in the sexual elements, indeed, that the continuity ot organic life is secured, the vegetative organs being but appendages to the direct immortal chain of sex- cells. The large quiescent ovum is the result of a continued surplus of anabolism over katabolism, while the growing preponderance of katabolism must find its outward expression in increased activity of movement and in diminished size ; and the natural result is the flagellate sperm-cell.-
In multicellular organisms sexual reproduction makes its appearance when nutrition is checked. â€œ Some of the cells are seen differentiating at the expense of others, accumulating capital from their neighbours ; and if their area of exploitation be sufficiently large, emphatically anabolic cells or ova result ; while if their area is reduced by the presence of numerous competitors struggling to become germ-cells, the result is the formation of smaller, more katabolic, and ultimately male cells. In the same species distinct organisms may, in the same way, become predominantly anabolic or katabolic, and may be distinguishable as completely female or male organisms.-
The numerous facts which have now been accumulated prove that â€œ such conditions as deficient or abnormal food, high temperature, deficient light, moisture, and the like, are obviously such as would tend to induce a preponderance of waste over repair - a Jcatabolic diathesis ; and we have just seen that these conditions tend to result in the production of males. Similarly, such factors as abundant and rich nutrition, abundant light and moisture, must be allowed to be such as favour constructive processes and make for anabolism ; and we have just seen that these conditions result in the production of females .-
Oogenesis and Spermatogenesis
In the maturation of the ovum, the formation of polar cells seems rightly interpreted as an extrusion of the katabolic or male elements from the preponderatingly anabolic ovum ; the converse occurs in spermatogenesis.
According to this view of Geddes -, li fertilisation is comparable to mutual digestion, and the reproductive process has arisen from a nutritive want. The essentially katabolic male cell, getting rid of all accessory nutritive material contained in the sperm-blastophore, brings to the ovum a supply of characteristic katastates, which stimulate the latter to division. The profound chemical differences surmised by some between the male and female elements are intelligible as the outcome of the predominant anabolism and katabolism in the two elements. The union of the two sets of products restores the normal balance and rhythm of cellular life.-
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|Introduction to Embryology 1887: Chapter I. Maturation and Fertilisation of Ovum | Chapter II. Segmentation and Gastrulation | Chapter III. Formation of Mesoblast | Chapter IV. General Formation of the Body and Appendages | Chapter V. Organs from Epiblast | Chapter VI Organs from Hypoblast | Chapter VII. Organs from Mesoblast | Chapter VIII. General Considerations | Appendix A | Appendix B|
Cite this page: Hill, M.A. (2021, May 7) Embryology Book - An Introduction to the Study of Embryology 8. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Book_-_An_Introduction_to_the_Study_of_Embryology_8
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