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The Blastoderm

Its Structure and Relation to the Development of the Embryo

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Sharpey W. Thomson A. and Schafer E.A. Quain's Elements of Anatomy. (1878) William Wood and Co., New York.

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1878 Elements of Anatomy: The Ovum | The Blastoderm | Fetal Membranes | Placenta | Musculoskeletal | Neural | Gastrointesinal | Respiratory | Cardiovascular | Urogenital
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Position and extent

It has already been stated that in the bird's egg the result of segmentation is the conversion of the germinal disc into an organised cellular blastoderm, Avhich, from the time of its first formation, and before any incubation has taken place, already consists of two layers of cellular elements (fig. 496, s and d).

These two layers differ considerably. The cells of the upper layer are of smaller diameter, about 1/2400 inch, more compactly laid together, so as to be slightly compressed, and shortly prismatic, and are all provided with distinct nuclei. Those of the lower layer are of somewhat larger size, and of a more granular aspect, so as to hide the nucleus, which appears, however, to exist in the greater number, and the whole of these cells are rather scattered in reticular groups than united into a distinct and consistent layer (His). Below this layer there is a narrow space occupied by clear fluid between the germ and the surface of the white yolk, to which the name of sulnjerminal cavity is given, and in this space a number of granular spheres or formative cells are found, somewhat similar to the cells of the lower layer.

Fig. 496. Microscopic view of a vertical section through half the Blastoderm of a newly-laid egg. (From Stricker).

S, upper layer of small nucleated cells ; D, lower layer of larger gramilar cells ; M, segment spherules lying in t!ie subgermiual cavity ; A, substance of the white yolk below the germ.

In mammals, too, it would appear from the observations of Bischoff, Coste, Reichert, and others, that the blastoderm which covers the yolk after the completion of segmentation, though not double from the first, comes soon to consist of two layers. The exact time and mode of the appearance of a second layer are, however, still imperfectly known : and, from the difficulty belonging to the question of secondary segmentation in the deeper part of the yolk previously adverted to, it may be doubtful how far the whole blastoderm of mammals is to be regarded as the direct product of a primary segmentation, or a part of it is due to a later organising process.

There is, however, a great difference in the relation of the primitive blastoderm to the rest of the ovum in birds and in mammals. In the former, as already stated, previous to incubation, this organised cellular disc cover.-=; only a very limited part of the surface of the yolk, while in mammals it completely surrounds the yolk from the first, and thus constitutes thQvesimlar blastoderm of Coste, Keichert, and other authors.

From the first the blastodermal disc of birds shows a difference in its centra] and peripheral parts, the former being thinner and more transparent, and thus forming the so-called transparent area, the latter being thicker and more opaque, is the opaque area. But in mammals the central portion of the primitive blastoderm presents no defined transparent area, and differs chiefly at first from the rest by its greater thickness, and it is by later changes accompanying development that there arises a thickened opaque disc, the eml^rijonal spot of Coste, and that still later in this disc, when expanded and altered in shape, there is formed the first trace of the embryo. The same distinction, however, as in birds, a])pears in the end between a transparent or embryonal area, and an opaque peripheral area, a part of which is occupied by the vessels of the first circulation.

In birds the blastoderm spreads itself rapidly during the first stages of incubation by cell-multiplication over the surface of the yolk, until at last the whole is covered by its layers ; but in mammals, as the yolk is still of comparatively small size after segmentation is complete, but undergoes soon afterwards very rapid and great enlargement, and as it is completely covered by the primitive blastoderm, it is obvious that that membrane must undergo corresponding extension, not by marginal, but by interstitial cellular multiplication.

Trilaminar structure

The bilaminar blastoderm which results directly from segmentation soon undergoes farther changes, by which a third most important element is introduced into its composition, so that, at an early period of development and previous to the actual formation of any part of the embryo, it is found to consist of three layers of cellular elements, placed one above the other. These layers may, from their relative position on the yolk, be named the outer, middle, and inner blastodermic membranes, ectoderm, mesoderm, and cndoderm, or, following the nomenclature of Foster and Balfour, opihiasf, mesotlast, and hypoMast, the upper, middle, and lower germs ; and the ovum of birds and mammals may thus, along with that of a considerable number of animals, be styled triplohlasfic.

The origin of the middle layer is still involved in some obscurity. By one set of observers it is considered to be most closely connected with the original lower layer, and while the original upper layer of the primitive bilaminar blastoderm remains undivided, constituting the epihiast, the looser substance of the original lower layer undergoes a differentiating change, by which there is separated from its under part a thin lamina of flattened united cells to form the Injpohlast, while the remaining portion, with rounded cells of a different structure, becomes distinct superiorly, and accumulates between the upper and lower layers, especially towards the centre, to form the foundation of the mesq lfkish which according to this view would result, like the epiblast and hypoblast, from the primary segmentation. But by other embryologists, it is held that a part, if not the whole, of the mesoblast proceeds from a secondary process of segmentation or cell formation occurring below the original blastoderm ; and farther, that the new cells which thus give rise to the mesoblast are carried from below towards the place where they form that layer by migratory movements, the nature of which is not yet understood."

Leaving the question of the origin of the middle layer for farther remark hereafter^fe^-e shall here state in the shortest and most general terms the relation ascertained to subsist between the three several constituents of the organised germ and the origin of the rudiments of the embryo and other parts developed from the ovum. In doing so, if allowance be made for the differences previously noted, the same description may apply to the fundamental formative processes of birds and of mammals.

Relation of the Layers to Development

With respect to the histogenetic changes which accompany the conversion of parts of the blastoderm into the several organs and textures, the reader is referred to the various parts of the section on General Anatomy in -which the development of the textures is treated of. Here it is enough to state, that in the upper layer or epiblast of the bird's ovum it is mainly by endogenous cell-multiplication that the increase of substance and extension of area is effected ; that in the lower layer, there is, according to Foster and Balfour, continued conversion of the cells of tho white yolk into those of the hypoblast ; and that in the mesoblast there is prolonged addition of cellular elements by new production of formative cells from below the germ ; and further, that in all the three layers it is mainly by internal differentiation of the various groups of the cells so formed that are produced the different kinds of formative bases, or initial deposits, whether cellular or extra -cellular, which are converted by farther changes into the rudiments of the several organs and textures of the animal body or its foetal appendages. But, while the formative processes consist essentially in minute histogenetic changes, they are also accompanied by changes of form which are more obvious. Thus the folding or inflection of certain of the layers of the blastoderm which brings about the enclosure of the visceral cavity of the body, or that which accompanies the formation of the amnion ; the progressive rising of the dorsal laminse and their final union, which attends the closure of the canal for the brain and spinal cord ; the increased accumulation of formative cells in one place leading to growth and increase, and their diminution or removal in others leading to atrophy ; the fusion of certain membranes or masses of tissue uniting parts which were previously separate, and the fission or solution of continuity between other masses producing their separation ; the excavation of one set of hollows and the obliteration of others, as in the case of blood-vessels and ducts, — are only a few examples of developmental changes, which are dependent, no doubt, more immediately on textural differentiation, but which indicate different forms and modes in which the constructive processes are brought about.

The following is the general relation of the several germinal layers to the production of different systems and organs of the embryo and its accessory parts in so far as yet discovered.

  1. From the epiblast proceed the epidermis and its appendages, the great nervous centres, and the principal parts of the eye, ear, and nose ; one layer of the amnion and yolk-sac, and in mammals, probably the outer layer of the permanent chorion.
  2. From the hypoblast proceed the epithelial lining of the whole alimentary canal (excepting that of the mouth), and of the lungs, the epithelial lining of the ducts of the glands connected with the alimentary canal, and also the deep layer of the yolk-sac and allantois.
  3. From the mesoblast proceed in general all the parts of the skeleton, the muscles, fascia, and tendons, the peripheral nerves, the true skin, the connective tissue, the vascular system and blood, the muscular and fibrous coats of the alimentary canal and all other visceral passages, the serous membranes, the parenchyma of many glands, and the genito-urinary system, together with the outer layer of the amnion, the vascular layers of the yolk-sac, the allantois and the chorion, and the foetal part of the placenta.

The mesoblast does not, however, serve as the basis of these very various parts indifferently or equally throuyhout its whole extent, but in the following divisions, viz., First, by a central mesial or axial part, out of which proceed the rudiments of the protovertebral segments of the body ; and, Second, by two lateral parts which undergo subdivision into an upper and lower lamina, the first of these subdivisions containing the rudiments mainly of volunto-motory parts, the walls of the body, or somato-plenral elements ; and the seconct forming the involuntomofcory parts, as in the walls of the alimentary canal, heart, &c., or splanchno-pleural elements : the space formed by the separation of these two sets of parts is the visceral or pleuro-peritoneal cavity.

From the foregoing enumeration of the several parts of the embryo which are traceable in their origin to one or other of the layers ot the blastoderm, it must not be inferred that these initial elements remain each distinct or separate from the rest, while undergoing the formative changes of conversion. Some of them, doubtless, do maintain their independence in a remarkable degree, as is the case with most of the parts derived from the hypoblast, and some of those fi'om the epiblast ; but in the case of parts proceeding from the mesoblast, this independence is in a great measure lost ; and notwithstanding the original separation, we see, especially in the vascular and nervous systems and in the connective tissue, that in the course of their farther development, there is a great amount of spreading of one into the other sets of the blastodermic elements.

Discovery of the Blastodermic Elements

We owe to C. F. Wolff, as described in his Theoria Genera, tionis, published in 1759, the first proof derived from observation of the actual new formation of, the organs of the embryo (epigenesis) from the simple granular (cellular) elements of the yolk, and to a later work of the same author (On the Formation of the Intestine, which originally appeared in 1769, and was republished in German by J. F. Meckel in 1812) the first suggestion of the laminar constitution of the germ. The full discovery, however, of the three layers of the blastoderm, and especially their relation to the development of the organs and systems of the embryo, was, under the influence of Doellinger's teaching at Wiirzburg, the work of Pander, and was first published in his inaugural dissertation at that University in 1817. The segmentation of the yolk, noticed by Swammerdam and Spallanzani, was first described in batrachia by Prevost and Dumas in 1823, in a Memoir which was followed by an important series of contriljutions by the same authors to the history of the development of reptiles, birds, and mammals. the discovery of the germinal vesicle of the bird's egg by Purkinje in 1825 led the way to more minute observation of the constitution of the germinal part of the ovum. But the foundation of embryology as a modem science was most surely laid by C. E. von Eaer of Konigsberg (originally the associate of Pander and pupil of Doeilinger), who discovered the ovum of mammals in 1827, and in his work entitled " Die Entwickelungsgeschichte der Thiere, Beobachtungen und Reflexion en," 1829 — 1837, gave the fullest, the most accm-ate, and the most philosoiDh:cal account of the development of animals which has ever appeared. The contemporaneous researches of H. Eatlike. also the pupil of Doellinger along with Pander and Von Baer, contributed greatly to the advance of embryological knowledge.

The investigations of Schwann "On the conformity in the structure and growth of plants and animals," published in 1839, threw new light upon the histological composition of the ovuum and blastoderm and their relation to the phenomena of development (see General Anatomy, p. 6 et seq) ; and in the years contemporaneous with Von Baer's researches, and following their publication, many important contributions appeared which greatly extended the scientific knowledge of the subject ; among the authors of which may be mentioned here, as the most prominent, the names of Yalentin, Rusconi, R. Wagner, Reichert, Kolliker. M. Barry, Bischoff, Coste, and Kemak. The knowledge of the development of the ovum and embryo of mammalia was especially advanced in the succeeding decennial period by the valuable memoirs of Bischoff on the rabbit, dog, guinea-pig, and roe-deer, published successively between 1847 and 1854, and an important addition was made to the history of human development and that of some animals by the publication of the elegant and elaborate work of Coste in 1847 and several following years.

To the careful observations of Remak more particularly, as described in his work on the development of the fowl and the frog, published in 1851-54, we owe the fullest and most consistent account of the structure and formation of the blastoderm and of the relation of its several parts to the earlier phenomena of embryonic development. The Lectures of Kolliker, published in 1861, formed the most valuable addition to the history of development in the ten years succeeding the publication of the researches of Remak. In 1868 the blastoderm and its early transformations were subjected to renewed examination in the elaborate researches of His (Untersuch. lib. die erste Anlage des Wirbelthierleibes). In the succeeding years appeared the varied researches of Dursy on the development of the head, Waldeyer on the ovaries, Oellacher on birds and fishes, and Goette on batrachia and birds, and numerous others, so that every year brings forth numerous original contributions to different departments of the subject. In 1874 there appeared the first English treatise on the development of the embryo since the time of Harvey, in the excellent " Elements of Embryology" by M. Foster and F. M. Balfour — the latter of whom is also the author of important original researches quoted in the course of this section. In the same year a short and useful systematic work on the Embryology of Vertebrate animals has appeared by Dr. Schenk of Vienna.

Origin of the Mesoblast

Although there is the general agreement before indicated among embryologists as to the trilaminar structure of the blastoderm in the ovum of the higher vertebrates, when it has made some progress in development, and as to the general relation of the several layers to the production of the systems and organs of the embryo, there is by no means the same unanimity of views as to the manner in which the different layers, and more especially the lower and middle layers, come into existence.

Fig. 497. Microscopic view of a vertical section through the blastoderm of the bird's egg after twelve hours of incubation. (From Stricker.)

S, upper layer of cells or epiblast ; D, lower layer now forming a single continuous layer of flat cells, or hypoblast ; M, large formative cells beginning to form the middle layer, or mesoblast ; A, subgerminal part of the yolk.

In the egg of the fowl the cells of the middle layer begin to make their appearance in the central part of the blastoderm between the two original or primitive layers from the eighth to the twelfth hour of incubation, while about the same time a lower layer becomes distinct, as before stated, by the arrangement in a single layer of the lowest cells, their assumption of the flattened form, and their mutual union somewhat after the manner of a pavement-like epithelium. But while this is apparent towards the centre of the blastoderm there is accumulated towards the periphery in a thickened zone (opaque area) a quantity of cells of larger size and granular aspect in which no division into an under and middle layer is yet to be perceived. According to most observers the original upper layer takes no share in these changes, but remains distinct and undergoes the changes which belong to its own phases of development.

With respect to the formation of the hypoblast it would appear to be no more, at least in its central part, than a differentiation or change of form occurring in cells existing from an earlier period in the primitive lower layer ; while its peripheral extension is probably owing to the conversion into its pavement-like cells of the subjacent elements of the white yolk. But as to the manner in which the mesoblast takes its origin, two distinct kinds of views exist among embryologists. According to one of these, following the suggestion of Remak. the cells of the mesoblast take their rise by a process of separation from the cells of the primitive lower layer by changes which are coincident with the conversion of the deepest set of those cells into the continuous lamina of hypoblast. And as a modification of this view may be mentioned that of His, according to which a middle layer (though not distinguished by him as such by namely arises in common from the formative cells of both upper and lower primitive layers through an axial plate, into which he holds they unite.

According to the other view, which originated in the Vienna school, and has received much support from a number of observers emanating from it (Stricker, Waldeyer, Peremeschko, Klein, Ocllacher and Goette), the cells which form the middle layer do not proceed either from the epiblast or hypoblast in the place which they ultimately occupy, but these cells arise as new products of cell formation below the hypoblast, pass by migratory movement into the seat of the mesoblast, either through the hypoblast, or, as most hold, round its peripheral margin, and thence into the central part of the blastoderm, where all are agreed the cells of the mesoblast first come to be collected in any considerable quantity. Having once gained this position, or. in other words, a certain portion of mesoblast having been thus formed in the axial or central plate of the embryonic area, its cells speedily multiply and rapidly extend themselves, both by thickening in the centre and by spreading towards the periphery : other mesoblastic cells continue to be introduced from below at the margin of or through the hypoblast, so as to complete the formation of a middle layer by the eighteenth or twentieth hour.

Fig. 498. Section of a Blastoderm at right angles to the long axis of the embryo near its middle after eight hours incubation. (from Foster utrI Balfour).

A, epiblast formed of two layers of cells ; B, mesoblast thickened below the primitive gi'oove ; C, hypoblast formed of one layer of flattened cells ; pr, primitive groove ; vie, mesoblast cell ; bd, formative cells in the so-called segmentation or subgerminal cavity. (The line of separation between the epiblast and mesoblast below the primitive groove is too strongly marked in the figure.)

Among the most recent observers. Klein and Balfour favour the migratory view : the latter, however, in a somewhat modified form as he has arrived at the conclusion that the mesoblast takes its origin not directly from the epiblast or hypoblast, but in part from cells which are included (as the result of the first segmentation) in the blastoderm between those of its upper and lower layers, and partly from the larger spheres or formative cells which are the product of a later process of cell production occurring in the lower part of the germ, and which migrate from the place of their fonnation in the germ cavity, round the margin of the hypoblast into the space above it. The researches of Goette lead nearly to the same conclusion as those of Balfour, and if confirmed would go far to prove the occuiTcnce of a secondary or jirolong-ed segmentation in the subgerminal yolk, to which allusion was previously made.

It is right however, to state that on the other side there is the weighty authority of Kolliker. T\-ho. in association with the younger Virchow, has recently soiight in vain for the evidence of such migration as has been described by the observers previously referred to and attributes the formation of the mesoblast entirely to the proliferation of cells connected originally with the lower surface of the epiblast.

Difference in Animals

The foregoing description applies to the symmetrical position and central mode of development of the blastoderm which belong to the ova of reptiles, buxls and mammals : but it is right to state that in the lower vertebrata, or in amphibia and in osseous and cartilaginous fishes, there are several remarkable differences. Among these may be particularlj' noticed the non-symmetrical development of the blastodenn, and the excentric position of the commencement of the embryo : the involution of the epiblast at the aperture of the blastodenn termed ' anus " by Rusconi. or blastopore, at which the cells of the epiblast become continuous with the larger deeper cells from which the mesoblast and hypoblast originate. (See F. M. Balfour, " On the Development of the Elasmobranch Fishes,"" and " Comparison of the Development of Vertebrates,"" in the Quart. Joum. of Jlicroscop. Science for Oct. 1874. and July, 1875 ; E. Eay Lankester. '• On the Primitive Cell Layers of the Embryo." Sec, in the Ann. and Jlag. of Nat. Hist, for 1871.

The Blastoderm of Mammals

A variety of observations have shown that the blastodenn of mammals consists, when fully fonned. essentially of the same kinds of elements arranged in three layers, as previously described in birds ; but the mode of fonnation of these layers has not yet been fully investigated. By the observations of Bischoff. Coste, and Reichert, it was ascertained that as the result of the first segmentation the yolk became invested with a comjilete superficial covering of distinct nucleated cells, which may be looked upon as coiTesponding to the ui)i)er or oiiter layer, or epiblast. "Within this there remains for a time a thick plate or rounded mass at one side of opaque spherules, which seemed to be segment spherules not yet converted into cells, and the interior of the yolk was elsewhere filled with a graniilar fluid. Some time later, or about the fifth day in the rabbit"s ovum, a thickened spot, the germinal area of Bischoff. or tache embiyonnau-e of Coste, gi'adually made its appearance in the place where the primitive trace of the embryo is afterwards formed. This consisted in a thickening of the layer already fonned. and of an accumulation of a layer of new cells below it. which, gradually extending itself over the sui'face of the yolk, gives a second covering of cells to the whole.

In a carefully conducted series of recent observations, Hensen finds (Zeitsch. filr Anat. u. Entwick. Leipz. Xov. 187o) that in the rabbit's ovum, at the time when the germinal disc is still round {'> days 4 hours) the epiblast. with its central thickening, fonns a complete Vesicular covering of the yolk, but that the hypoblast, lying below the disc, does not extend over more than a thii-d of the circimiference. the cells of the middle layer are at this time restricted to the liinder part of the genu disc, in which place the primitive trace of the embryo first appears. Kolliker also, in the same animal (Verhandl. d, Physik. Med. Gesellsch. z. "Wiirzbiu-g, Kov.. 187."j) describes the inner layer (h\']3oblast) as spreading rapidly over the inner surface of the oriter layer or epiblast, so as at last to give a complete double covering to the yolk.

In sections of a vesicular blastoderm of the cat, prepared by Mr. Schafer, but not yet described, two layers may. as he has pointed out to me. be seen, the outer of which (epiblast) lies immediately within the primitive chorion and is coextensive with it, whilst the inner layer (hypoblast), although also complete, forms a smaller ring than the outer, and is in contact with the latter at one place onl3'. Both layers, although elsewhere fonned of a single stratum of cells, are here slightly thickened, but especially the outer (as if a mesoblast were about to be developed from it) : the hypoblast at this place appears bounded superficially by a delicate cuticular film.


Distinction of Embryonic and Peripheral Phenomena

From what has gone before it will be seen that the fundamental phenomena of development in the ovum consist essentially in changes which take place in the several layers of the blastoderm. Considered individually and minutely, they are mainly of the nature of cell multiplication and cell differentiation. Eegarded as a whole they may be placed under two divisions, according as 1st, they have their seat in the parts fi'om which the future embryo is formed, and are therefore intra-emlnjonic, or, 2nd, are extra-emhrnonic, and connected with the production of other parts, having usually a membranous form, which surround the embryo within the ovum, and form principally the amnion, yolk-sac, allantois, and chorion. It is to be remarked, however, that although in the progress of development all these membranes are mainly peripheral or extra-embryonic in their situation, they are not entirely so in their origin, for one of them — the allantois — springs originally from a part within the body of the embryo ; and all of them, in mammals at least, by the original continuity of the blastoderm, are necessarily united at certain places with parts of the embryo. Hence they have been called foetal appendages or foetal membranes.

Fig. 499. Ovum of the Rabbit from the Uterus (from Kolliker after Bischoff). The ovum was about one seventh of an inch in diameter ; a, the remains of the zona pllucida or external membrane ; b, the vesicular blastoderm ; c, the germinal area ; d, the outer limb of the double layer.

It is also to be held in remembrance that in birds, the blastoderm, which is originally restricted to the comparatively narrow limits of the cicatricula, extends itself rapidly in the earlier periods of incubation over the surface of the yolk ; while in mammals, the whole yolk is from the first covered by the vesicular blastoderm directly resulting from segmentation. In both, however, there may be distinguished a central and peripheral region of the blastoderm, and to the central part, as being the more immediate seat of the development of the embryo and its organs, without attempting to define very closely its limits, the name of embryonic area may be given. From this area, as from a centre, the changes of development in some measure emanate or spread towards the periphery. In birds the central area is from the first distinguished from the surrounding part by greater transparency and thinness of the bUistoderm, and thus (as ah-eady described) arises the distinction of the transparent and opaque areas. In mammals, on the other hand, the germinal part of the blastoderm is at first entirely opaque, forming the embryonic disc of Costc, Bischoff, and others ; and it is by a subsequent change that a part of this disc clears up or becomes thin and transparent, and that an opaque area is formed in the peripheral part. In both birds and mammals the embryonic area, from being simply round at first, becomes soon somewhat pyriform, and subsequently oval or contracted in the centre, like the body of a violin.

Fig. 500. First appearance of the primitive trace and medullary canal in the Ovum of the Dog (from Bischoff).

a, b, and c, represent the natural size of the ova of wliicli the several germinal area; are represented in A, B, and C. In A the germinal area is pyriform, and the primitive trace occupies two-thirds of the narrow hinder end. In B the trace is elongated and on the two sides are the raised medullary plates, mp, with the primitive groove between. In C the distinction between transparent area, at, and opaque area, ao, is marked by the outline.

It is in the hinder narrower part of this embryonic area, when it has assumed the pyriform shape, that the earliest trace of the embryo can be discerned. This forms the well known primitive streak and groove of authors, but it appears from the observations of Dursy and Balfour in the chick, and of Hensen in the rabbit, that the primitive trace and groove, which are the first indications of embryonic formation, are only transitory and evanescent, and that they are succeeded by the medullary groove and dorsal plates, which commence beyond the cephalic end of the primitive trace, and grow backwards towards the caudal extremity, so as gradually to thrust out as it were at the end the shrivelled remains of the primitive trace. The anterior extremity of this medullary groove becomes afterwards the cephalic, and the posterior extremity or that towards the primitive trace becomes the caudal part of the craniovertebral axis of the embryo.

This primitive axis constitutes in some measure the centre of subsequent changes of development. It consists mainly of a thickening produced by the accumulation of blastodermic cells.

Intra-Embryonic Phenomena of Development

Axial Rudiment of the Embryo

(Cerebro-spinal Axis)

The genetic changes which lead to the first formation of the rudiments of the embryo may be briefly sketched as follows : —

The longitudinal thickening of the blastoderm, which forms the primitive trace, belongs at first chiefly to the upper layer or epiblast, but soon extends to the central part of the middle layer or mesoblast. The hypoblast takes no share in its production.

The elongated plate or thickening is very soon separated towards the cephalic end of the primitive trace by a median groove or linear depression into two lateral plates, which, thickening to some extent, rise into ridges and thus constitute the laminaa dorsales, or dorsal ridges. The groove deepening, and the ridges becoming more elevated, there is then formed a canal, and by the further elevation of the ridges, their approach to each other, and their final coalescence in the middle line, the canal is gradually closed in along the dorsal line. The part of the upper layer which has undergone this inflection and enclosure acquires considerable increased thickness, but still a cavity remains in its interior. The part where it was closed dorsally now becomes separated fi-om the upper layer or epiblast with which it was originally continuous, and the latter passes subsequently free and entire across the dorsal line.

Fig. 501. Embryo of the dog seen from above, with a portion of the blastoderm attached.

The medullary canal is not yet closed, but shows the dilatation at the cephalic extremity with a partial division into the three primary cerebral vesicles ; the posterior extremity shows a rhomboidal enlargement. The cephalic fold crosses below the middle cerebral vesicle. Six primordial vertebral divisions are visible ; so, the upper division of the blastoderm ; sp, the lower division, where they have been cut away from the peripheral parts.

This canal is wider at the cephalic extremity in which the rudiment of the brain is situated, it is of uniform diameter in the succeeding or middle part, and at the caudal extremity remains open for a time, but is closed in at a later period like the rest.

The rudiment of the great nervous centre arises in a thickening of the central portion of the enclosed epiblast which is originally continuous with the rest of the upper layer ; but this part which forms the brain and spinal marrow exhibits considerable thickening at an early stage, thus constituting what by some have been called the medullary plates, while the canal is still open, and subsequently folded round dorsally and closed in the form of a medullary tube, within which is situated the medullary cavity or common ventricle of the brain and spinal marrow. There is at first no distinction between the medullary or nervous structure and the containing walls : these last, including the dura-matral sheath, are derived later from development of mesoblastic elements.

In birds and mammals it does not appear that there is at first any line of separation or distinction between the medullary part and the rest of the epiblast, but in Imirachia a difference of colour in the corneous layer marks a distinction between it and the deeper part which forms the medullary rudiments.

The Notochord

One of the next steps in early development as observed in the bird is the formation from a central columnar portion of the mesoblast of an axial cord occupying the future place of the bodies of the vertebra and basis of the cranium. This constitutes the notochord or chorda dorsalis (see fig. 504: and sections in figs. 502 and 503, ch).

Fig. 502. Transverse section through the embryo of the chick and blastoderm at the end of the first day (from Kolliker). h, epiblast ; dd, hypoblast ; sp, mesoblast ; Pe, primitive or medullary groove ; in,. medullary plates ; ch, chorda vwp, primordial vertebral plate ; uwh, commencement of division of mesoblast upper and lower lamina; ; between Rf and h the dorsal lamiuK or ridges which approximation close in the medullary canal.

Its structure is simply cellular, and it takes no direct part in the formation of the bodies of the vertebra or cranial basis, but comes later to be surrounded by the formative substance out of which these parts of the skeleton are developed. In mammals and in cartilaginous fishes its formation appears to be later than in birds. In man and the higher vertebrates its remains are to be seen for a longer or shorter period of foetal life within the cranio-vertebral osseous axis, but in the lowest vertebrates, as Amphioxus and Cyclostomatous fishes, in which the vertebrae are not developed or are imperfect, it attains much larger proportions, and itself constitutes the principal vertebral axis.

Fig. 503. Transverse Section through the embryo of the chick and bastoderm on the second day (from Kolliker).

d d, hypoblast ; ch, chorda dorsalis ; u w, primordial vertebra; ; m r, medullary plates ; h, corneous layer or epiblast ; u w h, cavity of the primordial vertebral mass ; in p, mesoblast dividing at sp into hpI, somatopleure, and d f, splanchnopleure ; ung, Wolffian duct.


On either side of this axial cord a thick mass or plate of mesoblast is collected along its whole length, and very soon there appear several transverse clefts in these plates forming the commencement of protovertebral segmentation. The first formed of these divisions is near the anterior or uppermost of the future cervical vertebra3, and they rapidly extend backwards in the posterior or lower cervical and dorsal region (fig. 501, jw, and fig. 503, uic). The divisions becoming more distinct, separate small quadrilateral masses, which have received the name of protovertebra, by which it is meant to indicate that they are not the same with the permanent vertebral pieces of the skeleton, but rather correspond to embryonic somatomes, or mciameric segments, corresponding closely in number with the permanent vertebral divisions, but including the rudiments of other parts, such as those of the spinal nerves, along with those of the vertebrae.

The basis of the cranium, into which the notochord extends, does not at first present any transverse division similar to that of the vertebral portion of the axis, and the notochord itself is at first without segmentation, and forms therefore a simple and entire cylinder.

Pleural Cleavage of the Lateral Parts of the Mesoblast

Together with the formation of the protovertebral plates and their transverse segmentation, another important change begins in the lateral part of the mesoblast external to these plates, which consists in its cleavage into an upper or outer and a lower or inner lamina, and the consequent formation between them of an interval or space (figs. 503, sji, and 50-1, i^p). The two laminae thus separated constitute respectively the somatopleure and splanchnopleure portions of the mesoblast, and the space between them is the commencement of the pleuroperitoneal cavity, which afterwards forms by its partition within the embryo the pleurae, pericardium, and peritoneum, and which beyond the embryo extends into the space between the amnion and the other developed membranes of the ovum.

Fig. 504. Diagrammatic longitudinal section through the Axis of an Embryo (from Foster and Balfour)

The section is supposed to be made at a time when the head-fold has commenced, but the tail-fold has not yet appeared. A , epiblast ; B, mesoblast ; C, hypoblast ; FSo, folil of the somatopleure ; FSp, fold of the splauchuopleure ; Am, commencing (head) fold of the amnion : NC, neural canal, closed in front, but still open behind ; Oh, notochord, — in front of it, uncleft mesoblast in the base of the cranium ; D, the commencing foregut, or alimentary canal ; Ht, heart ; pp, pleuro-peritoneal cavity.

  • "Pleural" is here used in the sense " parietaL

Inflection of the Walls of the Body of the Embryo

The first rudiments of the embryo, as before described, lie prone and flat on the surface of the yolk, consisting almost entirely in thickenings, with some incurvations, of certain parts of the blastoderm. In the formation of these parts the two upper layers, epiblasfc and mesoblast, are alone concerned, and the hypoblast takes no part in them, but passes thin and flat across the space occupied by the embryonic rudiments.

In the further progress of development a great change of form is now produced by the downward inflection of the whole three layers of the blastoderm, in consequence of which the embryo rises, as it were, out of the plane of the rest of this membrane, and begins to be notched off" from its peripheral parts. The first of these folds, termed crplmlic, (fig. 504) takes place at the extremity of the embryo which contains the rudiment of cranium representing the head, and precedes by a considerable interval the other folds. A similar downward fold subsequently follows at the caudal extremity, and there is also between the cephalic and caudal folds a simultaneous depression of the layers of the blastoderm in lateral folds, so that the embryo takes in some measure the form of an inverted boat, with its keel upwards, and its hollow side opening* towards the yolk cavity, and the fore part being, as it were, partially covered in by the deck of the cephalic fold. Thus are produced the downward ventral or visceral plates which form the side walls of the head and trunk ; and at a later period, by the increased constriction or convergence of the folds round the place of communication between the embryo and the peripheral parts of the blastoderm, there is formed the umbilicus (see figs. 510 and 512).

Fig. 505. Transverse Section through the Embryo ChicK before and some time after the closure of the Medullary Canal, to show the upward and downward inflection of the Blastoderm (after llemak).

A, At the end of the first day. 1, notochord ; 2, primitive groove in the medullary canal ; 3, edge of the dorsal lamina ; 4, corneous layer or epiblast ; 5, mesoblast divided in its inner part ; 6, hypoljlast or epithelial layer ; 7, section of protovertebral plate.

B. On the third day in the lumbar region. 1, notochord in its sheath ; 2, medullary canal now closed in ; 3, section of the medullary substance of the spinal cord ; 4, corneous layer ; 5, somatopleure of the mesoblast ; 5', splanchnopleure (one figure is placed in the pleuroperitoueal cavity); 6, hyjioblast layer in the intestine and spreading over the yolk ; 4x5, part of the fold of the amnion formed by epiblast and somatopleure.

The fundamental steps, therefore, in the development of the vertebrate embryo result in the formation in the axial part, or head and trunk of the body, of two cavities, of which one is situated above and the othor below the notochordal axis ; the upper constituting the cranio-vertebral canal, and containing the rudiment of the cerebrospinal nervous centre ; the lower forming the walls of the body which enclose the great nutritive viscera in the thoracic, abdominal and pelvic divisions of the trunk ; — along with which may be associated the parts which form the face and jaws, and Avhich enclose the cavities of the nose, month, and pharynx, incUiding in their substance the hyoid bone and its accompanying branchial arches.

The cerebro-spinal nervous centre

The brain and spinal cord have at first together the form of an elongated tube, of which the primary wall is of nearly equal thickness throughout. The cjdindrical portion in the region of the protovertebraa forms the spinal cord. In the dilated cephalic portion, constituting the rudimentary brain, there is from a very early period a partial division into three portions by slight intervening constrictions of the wall of the medullary tube. These constitute the three jn-imarij enrpphalic resides, and give rise in the next stage of development to the five fundamental portions of the brain usually recognised by embryologists and comparative anatomists, viz., forebrain, interbrain, midbrain, hindbrain and aftcrbrain. The general cavity enclosed by the inflection and union of the medullary plates constitutes the mesial ventricles of the brain and the canal of the spinal cord.

Fig. 506. Magnified side view of the Head Fig. and Upper Part of the Body of an embryo chick of the fourth day (adapted from Remak and Huxley).

1, chorda dorsalis ; 2, tlirce of the upper priDiitive cervical vertebne ; C\ one of the vesicles of the prosencephalon, with the nasal fossa below ; C-, vesicle of the thalameacephalon, with thg eye below it ; C'*, the middle cerebral vesicle ;

C'*, the cerebellum, between which and the cervical vertebra is the medulla oblongata. At the anterior extremity of the chorda dorsalis, where it reaches the ijost-spheuoid, is seen the rectangular bend of the middle of the cranium, which takes i^lace at the sella turcica ; and in front of tliis, towards the eye, the pointed infundibulum ; V, the rudiment of the trigeminus nerve ; VII, the facial ; VIII, the vagus ; IX, the hypoglossal ; in front and below these numbers respectively, first, the upper and lower jaw, with the first cleft, which becomes the meatus auditorius externus ; and lower down the second, third, and fourth arches and clefts in succession ; in front of these the aortic bulb attaches the heart ; between VII and VIII, the auditory vesicle.

The Nerves

The peripheral nerves are formed, quite independently of the nerve centres, in mesoblastic elements along with the vascular and other tissue composing the parts in which they are distributed. The anterior aud posterior roots of the spinal nerves and the roots of the cranial nerves (excepting the optic, which has a special connection with the brain) probably arise as outgrowths from the medullary wall of the cerebro-spinal centres.

Organs of the Senses

To the earliest period also belongs the formation of the rudiments of the principal organs of the senses, viz., the eye, ear and nose. The mode of origin differs, however, in the three. In the eye, which is the earliest to appear, the retina, or nervous part, is an extension from the anterior encephalic vesicle, while the lens is derived by development from an involuted portion of the epiblast, and other ocular structures proceed from the mesoblast. In the ear the labyrinth originates by involution of its cavity from the epiblast in the neighbourhood of the third encephalic vesicle, and the auditory nerve growing out from the medullary wall of the third encephalic vesicle, is subsequently extended into the ear vesicle ; ■while the middle and outer ear cavities are developed from mesoblastic elements in connection with the first and second post-oral subcranial plates and the intervening pharyngeal cleft. In the nose likewise the open cavity afterwards occupied by the distributed extremities of the olfactory nerves originates by depression or involution from the epiblast in front of the first encephalic vesicle of the cranium.

Fig. 507. Section of the commencing Eye of an Embryo in three stages.

A. Commencement of the formation of the lens by depression of a part of C, the corneous layer ; 2^r, the primitive ocular vesicle now doubled back on itself by the depression of the commencing lens.
B. The lens depression enclosed and the lens beginning to be formed in the inner side, the optic vessel more folded back.
C. A third stage in which the secondary optic vesicle, r, begins to be formed.

Vascular system

The next important series of changes by which the foundations of the great organic systems are laid consists in the formation of the rudiments of the heart, blood-vessels and blood, and in the establishment of the first circulation. The several part of the sanguiferous system all originate in the deeper or splanchnopleural division of the mesoblast, but once formed in this section of the blastoderm the blood-vessels very soon extend into all other parts which are vascular.

Fig. 508. Outlines of the anterior half of the embryo chick viewed from below, showing the heart in its earlier stages of formation (after Remak).

A, Embryo of about 28 to SO hours ; B, of about 36 to 40 hours ; a, anterior 'cerebral vesicle ; h, proto-vertebral segments ; c, cephalic fold ; 1, 1, primitive omf)halo-mesenteric veins entering the heart posteriorly ; 2, their union n the auricle of the heart ; 3, the middle l^art of the tube corresponding to the ventricle ; 4 (in B) the arterial bulb.

The formation of the heart, blood-vessels and blood is nearly simultaneous, and the rhythmic contractions of the heart begin have arranged themselves in the While the heart or propelling organ is being formed within the body of the embryo, the greater number of the primitive blood-vessels are developed in the peripheral part of the blastoderm in the vascular and transparent areas, and comparatively few arise in the embryo ; these last consisting at first only of the two vessels, the primitive double aorta, which carry the blood from the heart to the arteries distributed in the perijiheral area, and the corresponding venous trunks which return the blood from the area to the centre of the circulation. These primitive vessels become afterwards the omphalo-mesenteric arteries and veins of the yolk-sac.

Fig. 509. Diagrammatic Outlines op the Heaut and Peimitive Vessels op the Embryo Chick as seen from below and enlarged.

a, soon after the first establishment of the circulation ; B, c, at a somewhat later period ; 1, 1, the veins retiiruing from the vascular area ; 2, 3, 4, the heart, now iu the form of a notched tulje ; 5, 5, (upper) the two primitive aortic arches ; 5, 5, (lower) the primitive double aorta ; A, the single or united aorta ; 5', 5', the continuation of the double aortaj beyond the origin of the large omphalo-mesenteric arteries, 6, 6.

The first rudiment of the heart consists of an elongated tubular contractile chamber hollowed out of a mass of mesoblastic cells in front of the reflection of the cephalic fold into the flat part of the blastoderm. This tube is divided into two at its anterior and posterior extremities, and perhaps it is originally entirely double. Posteriorly the heart-chamber receives the nascent blood from the entering venous channel on each side, and anteriorly it opens into two arterial vessels, which passing one on each side of the primitive pharyngeal cavity, and turning backwards below the protovertebral plates, form the two primitive aortae before mentioned, from each of which by a sudden bend outwards, as observed in birds, the omphalo-mesenteric arteries pass off into the vascular area. There is, however, some difference in the number and form of these arteries in birds and mammals, but in all of them the first circulation begins in a similar vascular area, and among the earliest veins formed is a circular or terminal sinus suiTonnding the vascular area and receiving the blood from the capillary or subdivided vessels of the area within.

Alimentary Canal

The formation of the rudiment of the alimentary canal or primitive intestine takes place below or within the boat-shaped part of the embryo previously described by the folding in, soonest at the cephalic and later at the caudal extremities, and subsequently along the sides, first of the hypoblast, from which the epithelial lining only of the intestine is formed, and afterwards of certain parts of the splanchnoplenre section of the mesoblast which give rise by their meeting in the middle to the mesentery, and furnish in their extension over the intestinal tube the muscular and peritoneal coats and the connective-tissue and vascular elements of the gut.

The primitive alimentary canal is thus constituted in its early form by an anterior and posterior ca3cal tube, of which the anterior is the first produced, — both of them closed at the extremity by the reflected • layers of the blastoderm, — and by a wide middle part between the tubular portions, which at first has the form of a groove or gutter running under the vertebral axis of the embryo, and completely open below into the cavity of the yolk-sac. As development proceeds, the intestinal folds involve m.ore and more of this central open part and convert it into the tubular form ; and the opening into the yolk is thus gradually narrowed, while the reflected part of the blastodermic layers which pass between the yolk-sac and the intestine becomes lengthened out so as to take the form of an elongated duct known as the ductus vikUoinfest inalis (see figs. 510 and 512).

Fig. 510. Diagrammatic Section through the Ovum of a Mammal in the long axis of the Embryo.

e, the cranio-vertebral axis ; , the cepbalic and cauflal portions of the primitive alimentary canal ; a, the amnion ; a', the point of reflection into the false amnion ; v, yolk sac, communicating with the middle part of the intestine by v i, the vitello-intestinal duct ; M, the allantois. The ovum is surrounded externally by the villous chorion.

As the parts constituting the face are at first entirely absent, there is necessarily no cavity corresponding to the mouth, but as the structures which give rise to the jaws and face come to be developed below the cranium, the buccal and nasal cavities are generally deepened by the increasing projection of these parts, and the mouth at last communicates with the forepart of the primitive alimentary canal by an opening formed into it at the fiiuces. The mouth, therefore, derives its lining from the epiblast and forms no part of the original hypoblastic inflection Avhich gives rise to the pharyngeal cavity.

The posterior opening of the alimentary canal is formed at a considerably later period than that of the fauces. When first produced by the solution of continuity in the posterior reflection of the blastodermic layers, it represents in mammals as well as in birds a cloaca, or part of the primitive intestine common to the alimentary canal and the genitourinary passages.

Reproductive and urinary organs

As completing the present short notice of the development of the rudiments of the principal organs of the embryo, there may, lastly, be mentioned the temporary organs named the Wolffian todies, with their ducts and associated parts, which are the precursors of, and are very constantly and intimately associated with, the first origin and subsequent evoIution of the reproductive and urinary organs. These bodies, when fully formed, constitute a pair of symmetrical organs which occupy nearly the whole extent of the abdominal cavity, and consist mainly of short transverse tubes presenting a glomerular vasctilar structure very similar to that which exists in the glandular structure of the permanent kidneys. They have thus been named the primordial kidneys (see fig. 513, W, p. 702).

The Wolffian bodies arise mainly in connection with the central portion of the mesoblast, and as the permanent kidneys and their ducts, the testicles and ovaries, and the respective male and female passages are in their origin all intimately connected with the Wolffian bodies, we may look upon the urinary organs and the internal reproductive organs as equally products of the middle layer. The external sexual organs are integumental in their origin, and may be considered as arising in the epiblast and mesoblast jointly.

Fig. 511. Human Embryo op about four weeks (from Kolliker after A.Thomson), f/, the anterior limb rising as a .semicircular plate from the lateral ridge. (The figure is elsewhere described).

The Limbs

The limbs do not commence till after the rudiments of all the organs already referred to have made their appearance. They are to be regarded as out-growths from the lateral part of the trunk, and take their origin by a sort of budding out or extension of the elements composing the wall of the trunk in two determinate places, which are nearly the same in all vertebrate animals, and receiving prolongations of the bones, muscles, nerves, and blood-vessels corresponding to a certain number of the vertebral somatomes in the situation of the anterior and posterior limbs respectively.

1878 Elements of Anatomy: The Ovum | The Blastoderm | Fetal Membranes | Placenta | Musculoskeletal | Neural | Gastrointesinal | Respiratory | Cardiovascular | Urogenital

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