Talk:Book - Quain's Elements of Anatomy
EMBRYOLOaY.— THE OYUM. 673
EMBRYOLOGY;
on, DEVELOPMENT OF THE FOETUS AND ITS ORGANS.
It is proposed to bring tog-ether in the present Section a short statement of the manner in which the parts of the body originate in the embryo, and acquire by development in the course oi" foetal life their complete form and structure. The collected facts bearing upon this subject constitute the department of anatomy known as Embryology, Embryological or Foetal Anatomy, or Foetal development, — a knowledge of -which is not only most interesting in itself, but is also of grent importance for the elucidation of adult human anatomy and the whole science of Organic Morphology.
Although much attention has been given to the structure of the human foetus at different periods of its growth, yet the materials are still wanting for a detailed history of its early development ; accordingly it is necessary for further elucidation to have recourse to the information obtained by studying the process in animals. But this illustration from analogy is fully warranted by the general conformity in the plan of development which has been ascertained to prevail among the higher vertebrate animals, and by the agTeement with this plan of the more important phenomena which it has been possible to observe in the human species. In what follows, therefore, while the main object will be to state the more important facts whicli have been ascertained as to the development of the human ovum and embryo, the history of the phenomena as they occur in birds and mammals will also be referred to in so far as it tends to throw light on human embryology.
1. THE OVUM : ITS MATURATION, FECUNDATION AND SEGMENTATION : FORI.IATION OF THE BLASTODERM.
The mature ovarian ovum. — The unfecundated ovarian ovum which is approaching maturity, and is about to be discharged from the Graafian follicle and pass into the Fallopian tube, is composed of the following parts, viz. : — 1. The firm, almost homogeneous external vitelline membrane, which is termed zona pellncida in mammals ; 2. The yolJc substance or vitellus, a mass of soft or semifluid protoplasmic matter, involving numerous granules and oil globules, and containing embedded in it, near the surface ; 3. The germinal vesicle, consisting of a spheroidal delicate enclosing membrane with protoplasmic fluid and fine granules within, and containing in its earlier states if not to the last ; 4. The germinal sjwt or macula.
The ovarian ovum, therefore, may be regarded as a complete organised cell, in which the yolk forms the protoplasmic contents, the germinal vesicle the nucleus, and the macula the nucleolus.
Distinction of the germ.— In the ovum of the mammal tlie whole yolk-substance is so uniformly of the same appearance and structure throughout that, but for the presence of the germinal vesicle, little distinction can be perceived between one part and another ; and, further,
674
THE OVUM AND BLASTODERM.
the greater part of the yolk is immediate!)' engaged in the first or preliminary changes which precede the occurrence of embryonic development. The whole yolk-mass, or its greater part, is therefore directly
formative or germinal, or, as it has been said, the ovum is hoJohJastk.
But in birds and reptiles, in which the ova are comparatively large, the greater part of the yolk forming the yellow yolk substance, takes no immediate part in the first formative processes, and these are restricted to the small whitish flat disc, called, the cicatricula in the fowl's Qgg, which is composed of fine granular protoplasm, occupies a determinate place on the surface of the larger yolk-mass, and, so long as the yolk remains in the ovary, has the germinal vesicle situated in its centre.
Fig. 4S9. — Ovarian Ovuji of a MajiFig. 489. MIFEK. -f
a, The entire ovum, viewed under pressure ; tlie granular cells have been removed from the outer surface, the germinal vesicle is seen in the yolk substance within ; h, the external coat or zona burst by increased pressure, the yolk protoplasm and the germinal vesicle having escaped from within ; c, germinal vesicle more freed from the yolk substance. In all of them the macula is
r- ' . ' ? To the part thus distinguish '■- t^, ..<>" f^ble from the rest of the yolk
■^ the name of germ may be given ;
and it has also been styled the
primary or germinal or formative yoUc, and the protoplasm or protoblast,
while the remainder of the yolk-substance has been called the nutritive
or food yoUc, the secondary yolk or deutoplasm. The oviparous ovum
has therefore been named merodlasfic, or partially germinal.
It is not known whether in the mammals' ovum the whole yolk ought to be considered as purely germinal, or whether, as seems more probable, some nutritive yolk may not be combined intimately with the germinal substance ; but even if so, it is obvious that the germinal bears a much larger proportion to the nutritive yolk than in tlie bird or reptile, and, as will appear more clearly hereafter, there is thus some foundation for the distinction between the holoblastic and the meroblastic forms of ova, although it may be that in different animals these forms pass insensibly into one another.
In both kinds of ova, however, whether holoblastic or meroblastic, the su1)sequcnt phenomena of development show that the spot where the organising process begins, occupies a determinate situation in the ovum, and that the first rudiments of the embryo arrange themselves in a determinate order round a central point in the germ.
There is, therefore, in the ova of birds and mammals, a part of the yolk which is more immediately germinal, and a central point of that germ from which development spreads, to which the name of f/erminal pole may be given. The centre of the germ is probably coincident witli the ])lace last occupied by the germinal vesicle.
Disappea-rance of the germinal vesicle. — The most marked change in the interior of the ovum which is known to accompany its maturation and escape from the ovary is the disappearance of the
MATURATION OF THE OVUM— FECUXDATIOX. 075
germinal vesicle, — a phenomenon which occurs in all vertebrates, and in a large proportion of, but probably not all invertebrate animals. This change is iudependent of fecundation. The details of the process have not been traced in mammals, but from various observations in birds and batrachia, and more especially from the recent minute researches of Oellacher in fishes, it follows that the disappearance of the vesicle really depends on its extrusion from the substance of tlie yolk in which it was imbedded, and is attended with the bursting or breaking down of its delicate outer membrane ; so that when the vesicle is thrust out on the surface of the yolk, and opened out, its fluid contents must be effused in the space intervening between the vitelline membrane and the surface of the yolk. The actual expulsion of the vesicle in the trout's ovum is attributed by Oellacher to the contractions of the yolk protoplasm, and the expulsion of the vesicle in this animal takes place previous to the rupture of its membrane and dispersion of its contents. (Archiv. f. Mikroscop. Anat. vol. viii, p. 24.)
Fig. 490. — Mature Ovarian Ovum op the Pi^f. 490.
GuiNEA-riG (from BischofF. ) ^so
The zona pelhicida is hidden by the adherent A , »
cells of the membrana granulosa, which have as- ^ ^ ->
sumed a pediciilated form next its surface. The finely granular yolk substance fills the cavity of '"
the zona. The germinal vesicle has disappeared.
The time at which the disappearance takes place seems to be subject to some variation. Most frequently it is close . upon the time of the escape of the ^
ovum from the Graafian follicle ; but
sometimes it is several hours later, and ^ -^ [ - -^ "^j"
in other instances it seems to occur previously ; and, indeed, in many cases
preparatory changes in the position, form, and consistence of the vesicle have been observed while the ovum was still within the ovary.
As the mammiferous ovum leaves the ovary it has still adhering to its outer surface one or two layers of the cells belonging to that part of the tunica granulosa with which it was surrounded in the Graafian follicle. These cells assume towards the period of maturation more or less of a pediculated form (see fig. 490), but after one or two days they gradually fall away from the surface of the zona, and leave tliat membrane free in the Fallopian tube.
Fecundation. — Should the ovum not be fecundated it is carried down through the female passages by the ciliary action of the lining membrane, and is lost by absorption or removal. But if seminal matter is present in the tubes, and the ovum is subjected to its influence within a due time, so that fecundation is effected, there immediately follows the commencement of a series of changes in the yolk protoplasm, which result in the formation in a determinate situation of a stratum of organised cells constituting the laminar germ named the hiastoderm, which is the seat of all subsequent processes of development in the ovum.
The encounter of the ovum with the seminal filaments or spermatozoa generally takes place in the upper part of the Fallopian tube or
X X 2
676
THE OVUM AND BLASTODERM.
oviduct, and it is now ascertained that the spermatozoa not only adhere in nnmbers to the external surface of the ovum, but actually penetrate throu2;h the zona, so as to come in contact with and possibly also combine with the substance of the yolk (see figs. 491, 492, and 493). We are, ho^Yever, entirely ignorant of the nature of the operation of the spermatozoa upon the substance of the germ. The shrivelled remains of these particles are seen for days adherent to the ova, and even in the substance of the germ, and though doubtless they at last disappear, it has not been determined whether this is by combination of their substance with that of the germ or in what other way the mutual or reciprocal action of the male and female generative elements may take place.
The fact remains as one of the most remarkable in the whole range of biological phenomena, that by the contact of an inappreciable amount of the male product with the germinal material of the ovum, the latter passes from an apparently inert condition into one of genetic activity,
Fig. 491.
Fig. 491. — Ovum of the Rabbit from the Falloi'Ian Tube, twelve hours after Impregnation (from Bisclioff). =f'^
A few gvaimlar cells adhere to the outer surface of the zona, in which and in the zona itself spermatozoa are seen ; «, zona ; b, t'vvo hyaline globules witliin the cavity left by the shrinking of the yolk.
the ultimate result of which is the accomplishment of a series of the most complicated phenomena of organic formation and growth, giving rise to a new being, which, while it may be of either sex, repeats in all respects the characters of the species, and may inherit in a greater or less degree the minutest peculiarities, whether structural or functional, of either or of both its parents.
There are two changes following impregnation which have been observed in the mammars ovum, and which are deserving of notice, though their import is not yet known. One of these changes consists in a certain contraction or dimiaution in the size, and an increase in the apparent compactness or firmness of the mass of the yolk, so that a larger space than before, filled with clear fluid, comes to intervene between the yolk and the suiTOunding zona. The other change referred to is the appearance in this space of one, or most frequently two. and occasionally of three, clear or hyaline spherules, which are easily distinguished from the surrounding fluid by their peculiar highly refracting outline (Quatrefages, Ed. Van Beneden, Bischoff). These spherules are of variable size, but generally their diameter is from one-tenth to one-fifteenth of that of the mammal's ovum (fig. 4yi, />, and 492. A). They are perfectly hyaline and homogeneous and do not appear to possess any external envelope. They remata visible for some days during the early j)hases of yolk-segmentation, about to be described, and hence by some they have been named seginentation globules. Their source and destination, however, are entnely unknown.
Segmentation of the yolk or germ. — After the disappearance of the germinal vesicle the germinal part of the yolk constitutes for a time a non-nucleated mass of protoplasm ; and if then subjected to the influence of fecundation it undergoes the change of segmentation, which results in the conversion of the germ or germinal part of the yolk into
SEGMENTATIOX OF THE GERM.
677
a layer of organised cells. This new organised structure, the blastoderm of Pander, is the future seat of embryonic development.
A segmenting process of this kind is universal throughout animals as a prelude to the commencement of embryonic development ; but it differs greatly in its extent, and somewhat also in its nature, according to the proportional relation of the directly germinal to the nutritive components of the yolk in different classes of animals. Thus in mammals, the process of cleavage appears to be complete, or to involve the whole mass of the yolk protoplasm, with which the germ is coextensive, at least in the first steps of the process; while in the bird's egg, which is pre-eminently merohlastic, the segmentation is restricted in the first instance to the disc of the cicatricula, and the great mass of the yolk substance takes no share in the change.
In intermediate forms of ova, as in amphibia and osseous fishes the segmenting division extends to a greater or less width over the yolk.
102.
Fig. ■492.— Ova OF THE ILvBBrr uxcEiiGoiNa Segmentation in tueir descent THRouan THE Fallopian Tube. (From Bischoff.) f A, tlie ovum from tlie middle part of the tube twelve or fifteen hours after impregnation, the germinal vesicle has disappeared, the yolk is contracted, and two hyaline globules are seen in the cavity between it and the zona ; rotation of the yolk took place in the direction of the arrows ; B, ovum a little more advanced, the first segmentation has taken place, a clear globule or nucleus is seen in both the yolk spheres : spermatozoa adhere everywhere to the zona ; C, an ovum four hours later than that shown in B, the second segmentation has taken place ; D, ovum from the lower part of the tube in which the third stage of segmentation is completed, and eight yolk spheres are formed, the albuminous covering is increased in thickness : diameter of the whole, j^th of an inch.
678
THE OVUM AXD BLASTODEKM.
just in proportion to the respective limits of the germinal and nutritive parts of the yolk ; but always affecting first the germinal part, and extending subsequently outwards from the germinal pole as a centre.
The process of segmentation has not been seen in the human subject, for the human ovum has not yet been detected in the progress of its descent through the Fallopian tubes ; but the phenomena have been observed with care by Bischoff and others in a variety of maramiferous animals, and as no important differences have been found to occur among them, there is no reason to doubt the similarity of the process in man. The yolk cleavage sets in within a few hours of the entrance of the mammiferous ovum into the tube, and continues to progress regularly during its descent towards the uterus, soon after its arrival in which the process is completed. The duration of this varies in different animals, being not more than from three to four days in the rabbit, in which it is the shortest known, and extending to from seven to eight days in the dog. It probably occupies not less than eight days in the human subject.
In the bird's egg the segmentation of the cicatricula is accomplished between the time of the entrance of the yolk into the oviduct and that of its being laid with its albuminous, membranous, and shell coverings, which may vary from IG or 20 to 24 or 30 hours ; and there may be some difference in the degree of completeness of the segmenting process at the time of the exclusion of the egg, according to the time the egg has taken to pass through the oviduct, the season of the year, and other circumstances.
Fig. 493. Fig. 493.— OvrM of the Rabbit
SIXTY-EIGHT HOURS AFTER IMPREGNATION (Allen Thomson). 2 so 1 ■
This ovimi is probably in tlie sixth stage of segmentation. Spermatozoa were observed within the zona, z, the zona ; a, the thick layer of albumen peculiar to the rabbit's ovum at this stage.
a. Segmentation of the mammaVs ovum. — This process may be shortly described as follows : — First the whole mass of yolkprotoplasm, contracted as before mentioned, splits into two somewhat ovoid or ellipsoid masses, by the formation of a fissure which begins on the surface and speedily runs through the whole thickness of the yolk (fig. 492, b). The two masses so formed lie somewhat pressed together within the vitelline membrane ; each mass presenting nearly the same appearance and structure as the whole yolk did previous to its cleavage. But as soon as this change has taken place, and according to some
GERM-SEGMENTATION IN MAMMALS AND BIRDS,
079
observers even previous to its commencement, there may be seen within each mass a small clear space similar to a nucleus. To these precursors of nuclei the name of hiasfidc has been given.
In the next stage each of the two first segments becomes cleft so as now to form four (c), each one of these having its clear spherule or nucleus within ; a third division resolves the masses into eight, of like composition with those which preceded them (d), the segments becoming of less and less size in successive stages, as meanwhile the bulk of the ovum as a whole undergoes little increase. The fourth stage ends in a division into 16 segments, the fifth into 32, the sixth into G4 (hg. 493), the seventh into 128, and the eighth into 256 (fig. 494). But it is right here to observe that while it is possible in the earlier stages to trace the reduplication of individual masses so that the succession of their numbers, when the division is complete in each stage, follows in the series of the multiples of two, yet, as the division of the different masses in any stage is not simultaneous, other and as they might be termed irregular munbers may be observed, especially in the earlier intervals of division ; as for example, three between the first and second stage, or five, six, or seven masses between the second and third, and so on. In the more advanced stages, from the great increase in number, it becomes almost impossible to follow the division of individual masses.
It is also deserving of notice that while the earlier clefts seem to pass right through the yolk and its first segments, so as to involve in the first four or five stages the whole mass of the yolk, in the later stages they do not do so, and the process seems to be in so far diferent, that the segmenting spheres come to be collected on the surface, and a mass of unsegmented granular and semi-fluid protoplasm or yolk substance remains within. However this may be effected, it is certain that the later division involves only the superficial set of spherules, and when the process is completed, the yolk mass comes thus to be covered by a layer of these protoplasmic spherules or segment globules, each of which possesses a nucleus and may after a time also acquire an external envelope, so as to present in all respects the features of a fully formed organised cell. (See the account of the histological relations of these spherules in the General Anatomy, p. 9).
Fig, 494.
Fig. 494. — Ovum op the Rabbit from the Utekus. (from Kolliker after Bisclioff). ^'^
The whole surface of the yolk is now divided into cellular comiiartments. A dark spot below marks the position of a quantity of granular spheres inside the cellular elements of the blastoderm, a, the albuminous layer, now much thinned out and incorporated with the zona ; b, the cells of the outer layer of the blastoilerm resulting from segmentation ; o, the spot of granular opaque spheres.
I. Segmentation in the iird's ovum. — In the ova of birds the segmenting process is somewhat different from that now described in mammals, seeing
that it is restricted to the germinal disc or cicatricuui. From the researches of Coste and several concurrent observations it appears that
«80
THE OVUM AND BLASTODERM.
the first division is effected by a groove or fissure which passes through the thickness of the germinal disc, having probably a direction at right angles to the long axis of the egg. This is crossed by a second fissure so as to divide the disc into four parts near the centre. A third cleavage or fissuring is still of the same radial character, dividing the disc into eight parts or sections ; but this is succeeded by another iu a different direction, which may be named concentric, and which has the effect of separating from the rest those parts of the radial segments of the disc which are next to the germinal centre : a subsequent alternating succession of radial and concentric fissures ends by dividing the whole disc into organised nucleated cells of a similar kind with those by which the whole of the mammiferous yolk becomes covered. A third set of fissures, which may be termed horizontal, must also occur to complete the separation of the segmented masses from the subjacent material. In this manner the germinal disc or cicatricula of the bird's egg has already, before the commencement of incul)ation, that is, during its descent through the oviduct and previous to being laid, been con verted by the organising process of segmentation into the layer of cells which constitutes the blastoderm. The cicatricula of the laid egg is therefore of quite a different structure from that of the ovarian ovum, though occupying the same place and presenting much the same appearance to the unassisted eye. This layer appears to be double from the first in the bird's egg, or to consist of two strata of cells, differing somewhat in their character.
Fi' VJj.
Fig. '195. — Cicatricula of the
Bird's Egg.
A, diagrammatic section tlirough the cicatricula of a newly laid egg ; a, vitelline membrane ; b, segmented germ disc ; c, below this the germ cavity ; d, the yolk cavity within the white yolk ; c, c, the yellow yolk substance.
B, view from above of the cicatricular or germ disc of a newly laid impreguatetl egg in whicli segmentation has been complete. The opaque area is seen surrounding the central transparent area.
C, cicatricula of an unimpregnated hen's egg, showing the vacuolar .structure produced by incomplete segmentation.
In this process there is much which is obscure and still imperfectly known, and much to excite our curiosity. The source of the first segment nucleus has not been discovered, nor is it known whether or in what way it may be related to the dis]iersed contents of the germinal vesicle or to its macula, and we are equally in the dark as to what may be the influence of the spermatic element upon the germ, and what the forces by which the cleavage and the formation of the multiplying spheres are brought about.
Contractile and other movements in the germ. — With respect to the last
THE BLASTODERM. 681
mentioned topic it may be remarked that certain heaving and rotatory motions which have been observed by several cmbryologists immediately before and during the occui-rence of the cleavage, indicate the play of contractile and it may be°of other forces within the protoplasm ; and these forces have been supposed to have some relation to the nucleus. Recent observations by Flemming in the ovum of Anodonta, of Oellacher in that of the trout, and of Goette in the toad, seem to show that there is some structm-al condition related to the process of division which may have a connection with its occm-rence ; for in the eggs of these animals the space within the domain of each segment sphere about to be formed is occupied by fine filaments radiating from the centre towards the circumference, and preceding the formation of the clear nuclear space within. It is probable that the hyaline globules may be the result of the first yolk contraction.
Secondary Segmentation.— The segmenting process previously described may be called j^ >• i ma /■//, for it is not yet ascertained in how far the whole of the blastoderm, considered as the organised substratum for the development of the new being, owes its origm directly to the first process of germ segmentation, or to what extent a later process of an analogous kind may contribute to the formation of some of its deeper elements. The most recent observations, such as those of Oellacher and Goette on the egg of the bird, of Ray Lankester on the ova of Cephalopoda, and of Balfour on that of sharks, would tend to support the view that in meroblastic ova at least, the process of segmentation, considered as one of conversion of the yolk into blastodermic elements, is not completed in the first series of such divisions, but continues to take place in a modified form for some time afterwards, thus extending the blastoderm over the surface of the yolk more and more by the addition of newly acquired elements. These elements appear to be fonned from nev.^ centres of cell organisation external to the limits of the germinal part of the ovum, by what may be called a process of free cell formation, and to contribute mainly to the production of the deeper part of the blastoderm. (Ray Lankester in Ann. & Mag. of Nat. Hist., 1873, p. 81, and F.M. Balfour in Jom-n. of Microscop. Science, July, 1873 and 1874 ; Goette in Ai-chiv, fiir Mikroskop. Anat., vol. x., 1874.)
Partial seg-mentation in unfectindated ova. — It is proper further to state that although the process of segmentation as now described is the necessary preliminary to the formation of the blastoderm and is only complete in ova which have been perfectly fecundated, yet an imperfect or partial kind of segmentation has been found also to occur in unfecundated ova. This has now been obsei-ved in a variety of animals, such as moUusca, fishes, batrachia. and also in the mammiferous ovum (see Bischofi:, Ann. d. Sc. Nat. 1844. and Miiller's Archiv. 1847 : Leuckart, article '• Zeugung " in Wagner"s Handwoiterbuch der Physiol., 1852). Oellacher has recently investigated these phenomena with care in the egg of the fowl, from which it appears certain that some degree of segmentation of the germ does occur in unfecundated ova, but that it is of an irregular and incomplete kind as compared with that which follows impregnation, that it never goes on to the formation of a complete cellular blastoderm, and that although some of the earlier stages of segmentation are gone through and the g-erm is to some extent divided into segment areas, yet these are afterwards ijroken up by vacuoles and other unnatural processes of development, and no true blastodei-mic layer of cells is fonned (fig. 49.5, c). Enough, however, has been seen to show that some formative power resides in the germinal part of the yolk independently of the concurrence of the male element. It is not improbable that this segmentation in unfecimdated ova may occur to a greater extent in the lower than in the higher animals.
2. THE BLASTODERM ; ITS STRUCTURE AND RELATION TO THE DEVELOPMENT OF THE EMBRYO.
Position and extent. — It has already been stated tliat 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
682 THE OVOI AND BLASTODEEM.
formation, and before any incubation has taken place, already consists of two layers of cellular elements (fig'. 496, s and d).
These two layers diflfer considerably. The cells of the upper layer are of smaller diameter, about a-^Vo"? 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 tlie nucleus, which appears, however, to exist in the greater number, and the whole of tliese 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.
Fig. 496. — JMiciioscopic view of a vertical section thkouoh half the Blastoderm OF A newly-laid Egg. (From Strieker), -f"
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 tlie 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 tlie 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 tlie 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 transjjarent 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 accomjmnying development that there arises a thickened opaque disc, the eml^rijonal spot of Coste, and
LAYEES OF THE BLASTODERM. 683
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 bod\^, 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 growtli 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 qnblast proceed the epidermis and its appendages, the great nervous centres, and the principal parts of the eye, ear, and nose ; one la^'cr of the amnion and yolk-sac, and in mammals, probably the outer layer of the permanent chorion.
'2. From the lijipoUast 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
DISCOVERY OF THE BLASTODERMIC ELEMENTS. tSS
Tarior.s 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-pJcural elements : the space formed by the separation of these two sets of parts is the visceral or pleuro-peritoneal cavitij.
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 tlie 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 17(')9,and -s\-as 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 embrj-o, was, under the influence of DoeUinger'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. Tlie 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 (origin alty. 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. Eathke. 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 gro'n'th of plants and animals," published in 1839, threw new light upon the histological composition of the o^■um and blastoderm and tlieir relation to the phenomena of development (see^ General Anatomy, p. 6 et srt/.) ; and in the years contemporaneous with Yon 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 structiu-e 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 Jil. Foster and F. M. Balfour — the latter of whom is also the author of important original researches quoted in the coui'se 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.
Fig. 497. — Microscopic view of a vertical section through the Blastoderm of THE Bird's Egg after twelve hours op incubation. (From Strieker.) ^f"
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 i^avement-like epithelium. But while this is apparent towards the centre of the blastoderm there is accumulated towards the periphery in a thickened zone (ojiaqiie 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 sliai-e 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 hypolilast 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 modiiication of this \dew may be mentioned that of His, according to which a middle layer (though not distinguished by him as such by namej 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 (Strieker, Waldeyer, Peremeschko, Klein, Ocllacher and Goette), the cells which form the middle layer do not proceed either from the epiblast or hy^joblast in the place which they ultimately occupy, but these cells arise as new products of cell formation below the hy^joblast, pass by migi-atory 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 agi-eed the cells of the mesolDlast first come to be collected in any considerable qiiantity. 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 rapiiUy extend themselves, both by thickening in the centre and by spreading towards the peripliery : 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.
FiiT. 498.
Fig. 498. — Section of a
Blastoderm at right
ANGLES TO THE LoNG
Axis or THE Embryo,
NEAR ITS .MIDDLE, AFTER EIGHT HODlls'
Incobation (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 ftirm. as he has an-ived 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 foi-mative 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 j-olk, to which allusion was previously made.
It is'right. however, to state that on the other side there is the weighty authority of KoUikcr. T\-ho. in association with the younger Vii'chow, 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 enth-ely to the proliferation of cells connected originally with the lower surface of the epiblast.
Difference in Animals. — The foregoing description applies to the sj-mmetrical position and central mode of development of the blastoderm which belong to the ova of reptiles, buxls and mammals : but it Ls 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 ^vhole.
In a carefully conducted series of recent obsez'vations, 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 A-esicular covering of the yolk, but that the hypoblast, lying belo'«- the disc, does not extend over more than a thii-d of the circimiference. Tlie 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 appeai-s bounded superficially by a delicate cuticular film.
SHORT OUTLINE OF THE MORE GEN^ERAL PHEN"OMEK"A OF THE DEVELOPMENT OF THE OVUM.
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 minutel}', they are mainly of the nature of cell multiplication and cell differentiation. Eegarded as a whole tliey 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.
Fiff. 499.
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 peUucida 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 oi? the PRiMiirvE Trace and MEDULLAur Canal in
THE Ovum op the Dog (from Bischofi').
«, Jj, and c, represent the natural size of the ova of wliicli tlie 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 ai-ea, 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.
AXIAL RUDIMENTS OF THE EMBRYO.
691
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.
1.— INTRA-EMBRYONIC PHENOMENA OF DEVELOPMENT.
Axial Utidiment 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. — E.MBRi'0 OF THE DoG SEEN FROM ABOVE, Fig. 501.
WITH A PORTION OP THE BLASTODERM ATTACHED.
Tlie medullary canal is not yet closed, but shows the dilatation at the cephalic extremity with a jjartial 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 ure visible ; so, the upper division of the blastoderm ; sp, the lower division, where they have been cut away from the peripheral x^arts.
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 meduUary 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 Uotochord. — One of the next steps in early development as observed in the bird is the formation ft-om 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 dorsaUs (see fig. 504: and sections in figs. 502 and 503, ch).
dorsalis into its by their
"^^
Fig. 502. Fig. 502.— Tkansverse Sec tion THROUGH THE EjlBRYO OF THE ChICK AND
Blastoderm at the end OP THE First Day (from Kolliker).
/;, epiljlast ; dd, lij'jioblast ;
sp, mesoblast ; Pv, primitive
or medullary groove ; in,.
medullary plates ; ch, cliorda
vwp, primordial vertebral plate ; uwh, commencement of division of mesoblast
upper and lower lamina; ; between Rf and h the dorsal lamiuK or ridges whicb
ajiproximation close in the medullary canal.
(! il.
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 AmfJiioxus and C ijdostomatous fishes, in which the
Fig; 503.
Fig. 503. — Transverse Section through the Embryo of the Chick and UlastoDERM 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 sjpinto h p I, somatopleure, and d f, splanchnopleure ; IX n (/, Wolffiau duct.
CLEAVAGE OF LATERAL PARTS OE THE MESOBLAST.
G93
vertebrae are not developed or are imperfect, it attains much larger proportions, and itself constitutes the principal vertebral axis.
Protovertebrae. — 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 ol 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 vertebra3.
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 s})lanchnopleure portions of the mesoblast, and the space between them is the commencement of the pleuroperitoneal cavity, which afterwards forms by its partition within tl>e embryo the pleurte, 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.
FSo.
Fig, 501. — 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 " paiietaL
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).
Ym. 505.
Fig. 505. — Transyersk Section through the
EMBRro-CnicK before and some time after
THE CliOSURE OF THE MEDULLARY CanAL, TO SHOW THE UPWARD AND DOWNWARD INFLECTIONS 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 thii-d 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 forminc!^ the walls of the body whicli enclose the great nutritive viscera in the thoracic, abdominal and pelvic divisions of the trunk ; — along with wliich 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. 506.
AND Upper Part op the Body op an EnDRro-CHicK OF THE FouRTH Day (adapted from Remak and Huxley).
1, cliorda 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 cerebi-al vesicle ;
C'*, the cerebellum, between which and the cer- C^~[- ^""^^ ^ ] JL'^>C<$J^^!-— ' TI vical 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 aud VIII, the auditory vesicle.
The 23"erves. — 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 labj'rinth originates by invohition 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
Fig. 507. — Section of the Commencing Eve of an Embryo in thkee stages.
A. Commencement of the forniation of the lens by dei^ression of a part of C, the corneous layer ; 2^r, the jjrimitive ocular vesicle now doubled back on itself by the depression of the commencing lens.
15. The lens depression enclosed and the lens beginning to be formed in the inner side, the optic ve.ssel more folded back.
C. A third stage in which the secondary optic vesicle, r, begins to be formed.
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.
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.
as soon as the blastodermic cells „ first sunple tubular form of the organ.
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 establisliment of tlie 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 aortte 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
698 GENERAL THENOMEXA OF E:^rERYOXIC DEYELOPilEXT.
suiTonnding the A'ascular 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 pi'imitive 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 Sectiox through the Ovum of a Mammal in the loxg
AXIS OP 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-intestinai 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, tlie 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 ca^oIution 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 shor't 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 iJrimordkd 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
Fig. 511. — Human Ejibryo op about
FOUR WEEKS (froiu KoUiker after A.
Thomson), f
/, the anterior limb rising as a .semicircular plate from the lateral ridge. (The tigure is elsewhere desciibed).
organs are integumental in their origin, and may be considered as aridng in the epiblast and mesoblast jointly.
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, whch are nearly the same in all vertebrate animals, and receiving prolongations of the 33ones, muscles, nerves, and blood-vessels corresponding to a certain number of the vertebral somatomes in the situation of the anterior and posterior limbs respectively.
2. EXTBA-EMBKYONIC PHENOMENA OF DEVELOPMENT OP THE OVUM.
Foetal Menilbranes. — While the changes before described in the central part of the blastoderm lead to the formation of the rudiments of the embryo, there are simultaneously developed in its peripheral parts, or extended into them from within, certain membranes which lie external to the body of the embryo, but are for a time more or less organically connected with it by the original continuity of the blastodermic elements in which both sets of parts originate.
Of these membranes, the yolk-sac exists in all vertebrate animals ; the aniuion and allantois are common to birds and mammals, bub are absent in amphibia and fishes ; and the chorion, in the sense in which the name will be employed here, may be considered as peculiar to mammals.
Tlie Yolk-sac. — This name is given to an organised and vascular covering formed by the extension of the layers of the blastoderm over the surface of the yolk within the original vitelline membrane. In human embryology it has also received the name of umbilical vesicle. It consists originally of all the layers of the blastoderm, and in fishes and amphibia retains these throughout the whole term of development "' ;
Fig. 512. — Diagrammatic Sectio>'S of the Ovum in different stages of r)EVELOi>
MENT TO SHOW THE PROGRESS OF FORMATION OF THE MEMBRANES (from Kollikcr).
1. Ovum in which the choriou has begun to be formed, with the blastoderm and rudiment of the embryo within. 2. Ovum in which the cephalic and caudal folds have contracted the itmbilical ui)erture towards the yolk-sac, and the amniotic folds are turning towards the dorsal aspect. 3. The amniotic folds being completed have met in the dorsal region ; tiie umbilical opening is more contracted, and the allantois has begun to sprout. 4. The true amnion is detached from the reflected or false amnion which has disai3i3earecl or combined with the chorion ; the cavity of the amnion is more distended ; the yolk-sac is now pediculated, the allantois projects into the space between amnion, choriou, and yolk-sac, and the villi of the chorion begin to ramify. 5. The ovum when it has become embedded in the uterine decidua ; the yolk-sac (umbilical vesicle) is now connected to the fcetus by a long duct, the amnion is increased in volume ; the allantois remains ouly as a pediculated vesicle towards the attachment of the short umbilical cord to the part of the chorion where the placenta is about to be formed. The vascular layer of the allantois has now combined with the chorion, the villi of wliieh have undergone further development.
d, vitelline membrane or ijrimitive chorion ; d', commencing villi of the chorion ; sp, epiblast ; sz, villi of the chorion more advanced ; ch, permanent chorion with which the vascular layer of the allantois is combined : cJi, z, true vascular villi of the chorion ; am, amnion ; uh, its cavity ; Ics, cephalic fold ; ss, caudal fold of the amnion ; a, the ejubryonal rudiment in the epiblast ; m, that in the hypoblast or mesoblast ; st, margin of the vascular area in its early stages : dd, hypoblast ; kli, hollow of the vesicular blastoderm, becoming afterwards d», the hollow of the yolk-sac ; dg, ductus vitellomtestinalis ; al, allantois ; e, embryo ; v, original space between amnion and chorion ; -vl, wall of the thorax in the region of the heart ; M, pericardial cavity.
The batrachia seem to be an exception to this statement, but only in consequence of tlie yolk sac being so extremely limited that it merges in the intestine itself. The yolksac and primitive intestine are in fact combined togethei", there being no umbilical constriction between them.
Fig. 512.
T)ut in the higher animals, as the greater part of the epiblast and
Bomatop eure laver of the mesoWast comes to be detached from e
^urfHce of the yolk by the expansion of the amnion and allantois, the
hypobas^^ layer of the mesobhast are alone the
Xanent constituents of the wall of the yolk-sac ;. -d hiwgh he^e last the membrane of the yolk-sac is continuous with the wall ot the intestine in the vitello-intestmal aperture.
The yolk-sac is the seat of the first circulation of the blood m the
omphalo-mesenteric yessels of its ^^s^^^lf /^•«^' . ^J*^ J^ .^'Jl^^e animals especially these yessels spread at a later period oyer the who e surface of the yolk in the membrane which forms the sac. Ihe food material of the yolk is probably absorbed by these vessels and conveyed by them as nourishment into the system of the embryo. In many of these animals however a quantity of the yolk substance also remains at the end of incubation, and by actual transference into the intestine of the embryo serves for a time as its digestible food.
In mammals the yolk-sac grows for a time with the embryo and other parts of the developing ovum, and the yolk substance within it must undergo a corresponding increase. There are however great cliflFerences among the difierent tribes of mammals in the extent of the development of the yolk-sac during uterogestation. In some it remains large and vascular, while in others it becomes atrophied and its vessels are obliterated at a comparatively early period. In rodentia it attains its highest degree of development, and comes in contact for a space with the interior of the chorion. In ruminants it is very soon elongated into two attenuated tubes which extend towards the ends of the ovum. In carnivora it is of considerable size, stretching through the ovum towards its opposite poles.
Fig. 513.— Magnified View op the Human Embryo of four "sveeks with the
Membranes opened (fi-om Leishman after Coste).
-^iK'ni^^ ™bilical_ vesicle with the omphalo-mesenteric vessels, v, and its long tubular fie ..P^rl n ^ f testiue ; c, the villi of the chorion ; m, the amnion opened ; u, cul oL?! • '"""f °*°i^' ^'"1 on each side of this the umbilical vessels passing out to the
™ « l\ Ti T^?'°' *^^ 'y^ ' ^^^ ^'- vesicle ; h, the heart ; I, tlie itver ; o, the yiMestine rw'^S' ^ W^l^^" ^^dy, in front of which the mesentery arid fold ot intestine. The Wolffian duct and tubes are not represented.
In the human species it retains its vascularity and continues to grow up to the fifth or sixth week, at which time it has assumed a pyriform shape, and is connected by a tubular vitelline duct with the intestine.
But notwithstanding all these varieties of form and development of the yolk-sac in the more advanced stages, we recognise the same fundamental structure and relations to other parts as in oviparous animals. Thus in human embryoes of from two up to four weeks there have been observed the same progressive changes from the wide communication of the yolk-sac with the open primitive intestine, to the narrower vitello-intestinal aperture, and the subsequent elongation of this into a tubular vitello-intestinal duct (figs. 511 and 51a.).
The human yolk-sac or umbilical vesicle, which expands i)roportionally with the early increase of the ovum, never, however, surpasses the size of a small pea ; in an ovum of from five to six weeks it lies loosely in the space between the amnion and chorion. At a later period, the duct elongating with the umbilical cord, the vesicle remains in the same relation to these membranes : it now also becomes flattened and more closely attached in the narrower space remaining between them. In the third month it is found connected with a coil of intestine which in the form of a hernia occupies the umbilical cord outside the abdomen of the embryo. At a later period the much elongated and attenuated duct with the omphalo-mesenteric vessels, now impervious and shrunk, may still be traced through the umbilical cord, while the flattened vesicle may be found, even up to the end of the term of nterogestation, somewhat further removed from the place of implantation of the umbilical cord on the internal surface of the placenta, but still between the amnion and chorion.
The Amnion. — This vesicular covering of the embryo does not exist in amphibia and fishes, but in reptiles, birds, and mammals, it is formed at an early stage of development, and contains a fluid in which the foetus is suspended by the attachment of its umbilical cord or an equivalent uniting medium.
The structure of the amnion is essentially similar in the three classes of animals in which it exists and its mode of formation nearly the same. It is destitute of blood-vessels, and consists of two layers, derived respect ivel)^, the inner from the epiblast, and the outer from the somatopleure layer of the mesoblast ; the first consisting of distinct nucleated cells, the second presenting a fibrous structure. To its external or fibrous layer it also owes the property of muscular contractility, which it possesses in a considerable degree.
The formation of the amnion takes place by the gradual backward inflection from the flat part of the blastoderm adjoining the embryo of the two layers before mentioned, first at the cephalic, and a little later at the caudal extremity and at the sides (see fig. 512, 2, 3, and 4, Jcs, ss, am), so that the two layers of which the amnion is composed are lifted up and separated from the remaining two lower layers (splanchnopleure" and hypoblast) of the blastoderm, by a space which is the same as, or rather a peripheral extension of the pleuro-peritoncal cavity. The embryo thus comes to sink down as it were (the cephalic part before the rest) into the hollow produced by the rising of the amniotic folds round it.
The backward folds deepening more and more, gradually conrerge on the dorsum of the embryo, and at last come together (fig. 512, 3), the margins of the reflection narrowing rapidly and being finally completely obliterated or lost by their convergence and by the subsequent dissociation of the inner from the outer divisions of the folds (fig. 512, 4). The separated inner division now becomes the entire closed sac of the amnion, connected only with the rest of the parts at the umbilical constriction where it is continuous with the integument of the embryo. The outer dissociated division is the false amnion of Pander and Von Baer, passing out into the remaining peripheral part of the blastoderm, and constituting for a time an external covering of tlie ovum, -which in birds and reptiles appears afterwards to be lost by thinning or absorption ; but which in mammals may be connected with the development of the permanent chorion in a manner to be referred to hereafter.
Fig. .^14. — Human Embryo of betwken THE Third and Fourth Week, Magnified ABdUT FIVE DiAMETERS (frOUl
Koiliker after Allen Tliomson).
a, amnion adherent (unusually) to the interior of the chorion in the dorsal region ; h, umbilical vesicle or yolk-sac with a wide communication with the intestine ; c, lower jaw ; d, superior maxillary process ; c, second j)Ostoral plate, and behind it other two, with the pharyngeal clefts behind each ;
f, commencement of the anterior limb ;
g, primitive auditory vesicle ; h, eye ; i, heart.
In the human ovum, as in most mammals, the amnion is formed at a very early period. The membrane lies at first so close to the embryo that it is with difficulty distinguished from the surftice of the body : but after the dorsal closure is completed, it is soon separated by the fluid which accumulates in its cavity.
The muscular contractility possessed by the amnion doubtless resides in its outer layer derived from the somatopleui-e. The contractions appear to be rhythmic, as they may be seen in the opened incubated egg of the fowl, or even in the entire egg. by means of a bright light in a dark chamber, from the sixth or seventh day of incubation ; and it is probable that they are of a similar natui'e in mammals.
The amniotic fluid contains about 1 per cent, of solid matter, consisting chiefly of albumen, but also traces of urea, which is probably derived from the urinary secretion of the foetus.
It would appear that there is a, difference in the structure of the reflected or false amnion in birds and in mammals. In the former it is composed of the same two layers as the amnion itself, but in mammals the development of the jnesoblast appears to cease at the place of reflection of the true into the false amnion, so that the latter consists only of the coi-neous layer or epiblast.
The Allantois : Urinary Vesicle. — Although this membrane "becomes in the more advanced stage of development widely distributed
THE ALLANTOIS. 705
iu the periphery of the ovum, yet in its origin it differs from the other membranes now under consideration in its close connection with one of the internal organs of the embryo. As ah-eady stated, this membrane does not exist as a foetal structure in fishes or amphibia.
In reptiles, birds and mammals, it is formed in connection with the hinder part of the primitive intestine, is the bearer of an extended capillary distribution of the umbilical or hypogastric vessels, and in combination with them performs important functions connected with the nutrition of the foetus and the aeration of the foetal blood.
The recent observations of His and of Dobrynin have shown that it springs from splanchno]:)leure elements of the mesoblast and hypoblast, below and in front of the caudal extremity of the embryo close to the place of division of the mesoblast into its somatopleural and splanchnopleurallaminfe. The former of these is reflected in the caudal fold of the amnion already described ; the latter buds out from the end of the primitive intestine into the pleuro-peritoneal space, and receives within it an evolution or outfolded process of the hypoblastic lining of the alimentary canal. It is placed at first rather behind the part which later becomes the cloaca, the orifice of which is still closed : but very soon it is doubled forwards upon the cloaca, so as to lie below it, and when this orifice is afterwards opened it forms the common outlet of the intestine and the allantois (fig. 510, and fig. 512, 3 and 4).
The blood-vessels, which are developed with great rapidity in the outer layer of the allantois, are formed in connection with those which become the two umbilical arteries and the corresponding umbilical veins, which last, however, do not run entirely in the same course as the arteries, but join the omphalo-mesenteric and pass towards the liver; one of the original veins very frequently becoming obliterated, as occurs in the human subject. The capillary network spread over the surface of the allantois appears almost as soon as the first prominence of the membrane begins to bud out from the wall of the primitive intestine, and the vessels appear at first to be in direct connection with the terminations of the two primitive aortas ; but subsequently, when the two aortge coalesce, the umbilical arteries appear as branches of the iliac arteries (see the Development of the Vascular System).
The allantois in expanding takes the shape of a pediculated flask-like vesicle, extends into the pleuro-peritoneal space, and is filled with fluid like the other membranes of the ovum. It is usually directed towards the right side of the embryo, or the opposite from that on which the yolk-sac is laid. In its subsequent great expansion iu the egg of birds the allantois spreads out in a flattened form over the whole internal surface of the membrane of the shell, thus coming to occupy more and more of the space previously held by the albumen, the rapid liquefaction and disappearance of which are coincident with the greatest expansion of the allantois and other membranes.
The allantois, though greatly flattened out in its most advanced state, still consists of an outer and an inner wall, separated by the fluid, and both bearing the finely ramified blood-vessels, which, however, are most richly distributed on the outer division ; and in these last it is easy to see, on opening an egg during incubation from the eighth day onwards, the marked difl'erence of colour of the blood in the outgoing and returning' vessels fi'om the action of the surrounding air on the blood which has passed through the capillaries.
703 THE FCETAL MEMBRANES.
It is also worthy of notice that from the time Avhen the allantois has attained some size, it, Hke the amnion, is possessed of contractility, ■which probably resides in its external layer ; and accordingly, on opening an incubated egg, from the effect of change of temperature or other stimuli, active motions may be perceived, caused by the alternate contraction and relaxation of different parts.
In mammalia the origin and early development of the allantois are nearly the same as in birds, but in a more advanced stage of development, the important connection which the outer layer of this membrane has with the formation of the vascular part of the chorion and foetal placenta, modifies considerably the relations of the membrane to the other parts of the ovum. In all of them, however, the two layers of the allantois (splanchnopleure and hypoblast) are easily distinguished from each other, the internal being entirely devoid of blood-vessels, of a simple cellular structure, and containing the fluid with which the inner sac of the allantois is filled. The external layer, on the other hand, is highly vascular, and is composed of fibro-cellular and contractile fibrous elements.
In the ruminants, pachydermata and the cetacea, the allantois attains to very large dimensions, extending widely into the greatly elongated ovum. In the carnivora it passes round the middle of the ovum externally in accordance with the zonal form of their jjlacenta. while in the rodentia and in man its vesicular or deeper membrane at least, containing the fluid, has a much more limited expansion, and stops apparently in its growth as soon as it has assumed the flasklike form and has reached the interior of the chorion. This appears to be the most probable explanation of the appearance, described by several embryologists, and observed also more than once by the writer, of a pyriform space extending in early human ova from the umbilicus to the inside of the chorion at the place where the ]ilacenta is beginning to appear or ■ndll afterwards be formed. (.See a recently described case by W. Krause in Reichert and Dubois, Ai-chiv, 187;").) But in this and all other fonns the umbilical vessels which pass out of the embryo are i:)laced externally to the vesicle of the allantois or its continuation by the ui-achus towards the iirinary bladder : and these vessels undergoing an extremely rapid development, pass off into the chorion and placenta, which thus owe their- vascular structures to the outer layer of the allantois.
In the human subject the allantois is both of very early formation, and its non-vascular or internal part ceases to extend itself at a very early period, that is. before the end of the fouith week. But already by this time the blood-vessels of the outer layer, by themselves or more probably in association with a connectivetissue layer in which they were originally situated, have overrun the whole interior of the chorion, and very soon furnish to the developing villi of that structure, the fibrous element with vessels, of which they secondarily become possessed. The manner of the completion of this jirocess will be apparent from what follows, as to the formation of the chorion (Von Baer, Reichei-t, Remak, Ivolliker).
The Chorion. — The ovum of the mammifer when it enters the cavity of the uterus is covered only by the vitelline membrane, or zona pellucida, which is of ovarian origin, and as a rule (notwithstanding the apparent exception of the rabbit to be afterwards referred to) it does not appear that it acquires any other covering for some days after its arrival in the uterus. By the time, however, that it becomes fixed in that part of the uterus which it is to occupy during the subsequent period of its intrauterine life, a great change takes place in the nature of the external covering of the ovum, by its conversion into a new
lBmni8?!4ii«is5<??5s?ir^-^*'?--*':-<i- ♦*>'<' '5""»""
THE CHOEIOX 707
membrane, which acquires more or less of a composite villous structure, becomes vascular throughout the whole or a part of its extent, and which, by its farther development, comes to form the principal means on the side of tlie ovum of establishing an organic connection between the embryo and the uterus. While the name of prochormi, or primitive chorion, might without impropriety be given to the altered and expanded zona pellucida as the sole early covering of the ovum in mammals, the term chorion is most suitably reserved for the newly formed membrane here referred to.
By some aiithors. indeed, the name of chorion has been applied to the external covering of the ovum of all animals without regard to its source or its relations to other parts. Thus by some the vitelline membrane has been regarded as a chorion when it appeared that no other membrane existed external to it ; and by others the name has been given to such adventitious parts as the albumen, shell, or shell membrane of the ovipara : but such a use of the term chorion is liable to create confusion, and it seems more expedient that it should be restricted to the peculiar external covering of the mammiferous ovum, which, as will be shown hereafter, is not an original constituent of the ovum like the vitelline membrane, but a structure of new formation in the course of development.
Fig. 515. —View op the Chorion of the Human Ovum of ABOUT Four or Five Weeks, opened (from KoUiker after Allen Thomson). Natural size.
This figure gives a general view of the villous structure of the chorion previous to the formation of a placenta, and shows the large space which frequently intervenes at an early period between the amnion and chorion.
At a very early period in the majority of mammals, and especially in the human species, tlie chorion acquires numerous villous processes over the whole or a part of its outer surface. These soon undergo a great development, and constitute a peculiar feature in the human ovum, whence the membrane has been known in'human embryology as the chorion frondosum, or shaggy chorion.
Tlie blood-vessels borne by the developed villi of the chorion, and named umbilical m human anatomy, are originally derived from those of the allantoid membrane, and are the seat of an extended circulation of the foetal blood in a system of outgoing arteries and returning veins witli their intervening widely diffused capillary vessels. It is by this system of vascular chorionic villi being brought into contact or close proximity with the blood-vessels of the uterus, that the essential conditions of nterogestation, as regards the continued supply of nourishment to the foetus and the aeration of its blood, are secured in the whole class of mammiferous animals. There is, however, very great difference among these animals in the extent and form of the development of the villous structure of the chorion now referred to, as well as of the concomitant changes which occur in the uterus itself, by which a more or less intimate organic nnion is established between the maternal parent and the offspring. The history of these differences belongs to the account of the structure and formation of the placenta, which will be given hereafter. At this place it will be sufficient to state that, while in some animals, as the pachydermata and cetacea, the connection between the ovum and uterus is reduced to its simplest form, and consists in little more than the implantation of comparatively simple and diffused chorionic villi in minute recesses of the vascular lining membrane of the uterus ; in others there is a greater or less degree of deeper interpenetration of
Fifr. 5in.
Fig. 516. — Surface and Profile Views
OP THE Ovum of the Eabbit at the
TIME OF THE FORMATION OF THE
Chorion (KoUiker after Bischoff ).
A and B, an ovum of 3 lines in diameter ; C, one of 4 lines. «, the chorion, with commencing villi ; h, the vesicular blastoderm ; c, the thickened part forming the embryonic area ; d, the increasing extent in which the blastoderm was found to consist of two layers.
the more highly developed and complex villi with a vascular structure formed from the uterine lining membrane, and which, from its being in whole or in part separated along with the ovum from the uterus in certain animals at the period ol^ birth, has received the name of decidua.
Origin of the Chorion.— The manner in -^-hich the permanent chorion is fii'st formed has not yet Ijeen fully ascertained. The deposit of an albuminous layer on the external surface of the zona pellucida of the rabbit, which takes place in the course of the descent of the ovum through the Fallopian tube, naturally led to the supposition that the chorion might be derived from some external deposit or uterine secretion of this natm-e ; Ijut the fact that a similar deposit from without has not been obseiwed to occur in other animals, and that the albuminous coat in the rabbit very soon thins away like the zona itself, and gives place to other structures, has caused this view to he abandoned. Xor is it probable that the chorion proceeds mainly, as held by some, from a development of the vitelline membrane. For when the rapid expansion of the o\-um occm-s shortly after its arrival in the cavity of the uterus, the zona pellucida becomes proportionally dilated, and is reduced to an extreme degree of thinness, so that at this period it is liable to be ruptured -^'itti the slightest force, and there is thus caused great difSculty in the examination of the o\Tim. After a few days the external covering of the ovum, which was previously smooth on its siu'face. becomes covered with slight projections, which gradually rise in the form of simple villi, and these, according to Bischoff, have at first the same homogeneous stiiicture as the zona originally presented. But according to Kolliker it may be doubted whether these villi are at first entu-ely homogeneous, and. at all events, he has ascertained that in a veiy early stage of their formation in the human ovum, as in the ovum of from fifteen to eighteen days, described by Coste, and which Kolliker had an opportunity of examining microscopically, the simple villi consist of hollow tubular processes, which are entirely composed of nucleated cells, similar to those of the upper layer of the blastodenn. It is, therefore, most i^robable that according to the view first suggested by Reichert. the villous chorion of the mammal's ovum is a product of the development of the blastodenn, and is formed in fact by the extension of its outer layer, now termed epiblast.
Fig. 517.
Fig. 517. — Front and Side Views of an Early Htjjian Ovum Four Tijies the NATURAL SIZE (froiii Keicliert).
This oviim is sujiposed to be of tliirteen days after impregnation. The surface bare of villi is that next the wall of the uterus, showing at e, the opacity produced by the thickened embryonic disc. The villi covered chiefly the marginal parts of the surface.
Villi of the Chorion. — A large part of the external surface of the ovum is ill the earlier stage.? beset "with villi, and these villi acquire
vascularity by the extension into tliem of the blood-vessels of the allantoid membrane from within. In subsequent stages, however, the form and extent of the development of the villi are subject to great variety in different animals, according to the peculiar form which is assumed" in each tribe by the organic connection established in uterogestation between the uterus and the ovum.
In the human species the villi appear to become vascular at a very early period, as ascertained by Kolliker in an ovum of between three and four weeks, in which he found that, while a delicate loop of bloodvessels penetrated into each of the villi, the internal part of the villus, which, as before stated, was previously a hollow cellular tube, was now filled with a fibrous connective tissue bearing the simple blood-vessels, and of a structure precisely similar to that of the outer layer of the allantois. It is therefore extremely probable that the primitive zona of homogeneous structure, after being thinned out to great tenuity by the continued expansion of the ovum, disappears entirely, and is replaced by a cellular membranous structure derived from the upper layer of the blastoderm, while the deeper fibro-vascular part proceeds fi'om the outer layer of the allantois ; and from this it necessarily follows that the chorion is no original component of the ovum, but an acquired or newly-formed structure developed from a union of epiblastic and mesoblastic elements.
Endochorion or Vascular layer of the Allantois. — The separation of the outer vascular layer of the allantois from the deeper layer (hypoblast")
M'liicli contrans the fluid, is sufficiently oljvious in many animals, as. for example, in the sheep or pig. But in the human subject, assuming that the vascular elements of the chorion are derived from the allantois as in animals, which there is no reason to doubt, it has been found difficult to determine the exact manner in which they first pass into the villi, in consequence of the very early time and extreme rapidity of the development of the allantois. But notwithstanding the observations previously mentioned of a nonvascula,r liediculated vesicle in relation with the allantois, passing from the umbilicus of the embryo into the space between the amnion and chorion : yet, in the great majority of instances, so rapid is the expansion of the membrane, that even in ova of from three to four weeks old it has been found impossible to trace more than the connection of the pedicle of the allantois through the urachus with the genito-urinary sinus ; and in all the cases which have been observed, already the umbilical vessels are found detached from the deeper membrane, and passing ■\\'idely over the whole interior of the chorion to penetrate everj'where into its villi. We are led thus to su^Dpose that by the early and rapid expansion of the outer layer, or by some other naode of development of its fibrous and vascular elements, the blood-vessels of the human allantois have been brought into combination with the cellular laj^er of the chorion, and have penetrated everywhere into its villi, into the whole of which blood-vessels and fibrous elements may at first be traced. According to this view it is to be understood that while the vascular layer of the allantois may thus become widely diffused, the vesicular or deeper layer may have only a comparatively restricted range of development.
UTEROGESTATION : PLACENTATION.
Incapsulation of the Ovum in the Becidua. — The further history of the chorion may be best given along with that of the structures by which the ovum is fixed in the uterus, and organic union established between it and the maternal system. This union is efFected by the close interpenetration of the vascular villi covering the surface of the chorion with a soft and spongy layer of substance, Avhicli is the product of a rapid enlargement or a sort of hypertrophy of the lining membrane of the uterus. To the latter substance the names of decidua and cadwa are given, from the circumstance that it is separated from the uterus at birth along with the foetus and its membranes. Not only is the ovum from an early period completely imbedded in a covering of decidua, but there takes place at a somewhat later period of uterogestation, in a limited area of one side, a greater enlargement of the vascular chorionic villi, and in close combination with, and surrounding these villi, a corresponding increased development of the decidual substance, by which there is produced the large discoid mass of complex structure, named the placenta (or uterine cake), through which the nourishment of the foetus and the ai'ration of its blood arc mainly carried on during the latter three-fourths of the period of uterogestation.
This placenta continues to increase in size with the foetus and its membranes, and as pregnancy advances, considerable changes take place in the relations of some of these parts, for the fuller comprehension of which it will be necessary to state the successive steps by which the ovum becomes fixed in the uterine decidua, or mcajisulaied, and the manner in which the development of the chorion, decidua, and placenta proceeds.
Earliest Observed Htinian Ova. — In two distinct cases of younj^
EARLIEST HUMAN OVA. 711
unimpregnated women who died at the time cf the invasion of tiie menstrual flow, Mr. H. Letheby detected an ovum covered by the granuhu- cells in the first part of the Fallopian tube, on the same side on which a ruptured follicle was found in the ovary (Trans. Roy. Soc. Lond. 1852, p. 7). But the fecundated human ovum has not yet been traced in the course of its descent in the Fallopian tube, nor has it, indeed, been seen in a satisfactory manner previous to the time when it is already imbedded in the uterine decidua. The history of the most authentic cases of the earliest ova observed in this state leads to the conclusion that the fecundated human ovum does not reach the cavity of the uterus before the seventh or eighth day after its escape from the ovary ; and that already by the twelfth or thirteenth day, if not even sooner, it has acquired a chorion covered with villi, and has become imbedded in the decidua.
The human ovum, like that of mammals generally, probably undergoes very little enlargement during its descent through the tube and the occurrence of segmentation, and its diameter on arriving in the cavity of the uterus does not probably surpass one hundredth, or at most one eightieth of an inch'. A very rapid expansion, however, of the whole ovum no doubt occurs immediately after its entrance into the uterus, in the same manner as observed in a number of mammals, so that within two or three days, its size may have increased to ten or twelve times its original diameter. Having at first only the zona pellucida for its external envelope (or primitive chorion), and being nearly smooth on the surface, it soon acquires, by a process previously described, its new chorion, on which are formed the permanent villi ; and by the time when it has become incapsulated, according to Reichert's observations in the recently published very careful description of an ovum which was probably of the thirteenth or fourteenth day after impregnation, it has a diameter of nearly one-fourth of an inch, and is beset with villi over a considerable part, but not the whole of its surface (see fig. 517).
Several other examples of incapsulated ova of nearly the same period have been observed, as by Von Baer, Velpeau, Wharton Jones, Coste, and Allen Thomson ; but in all these, with the exception of one observed by Velpeau, and supposed to be of the tenth day, or earlier, the whole surface of the ovum was already uniformly covered with short thick set chorionic villi.
The ova observed by Velpeau and Wharton Jones were like that described by Reichert of a period previous to the formation of an embiyo, and may be stated as probably of from ten to foui-teen days old. (Velpeau, " Ovologie Hiimaine," Paris, 1833 ; WTiarton Jones ia '• Trans. Roy. Soc. of Lond.," 1837, p. 339 ; Reichert, " Beschr. einer Friihzeit. Menschl. Frucht, &c.," Berlin, 1873.) Those of Von Baer, Pockels, Coste, and Allen Thomson are of the period immediately following, or from fourteen to eighteen days after impregnation. (Von Baer, " Entwickelungsgeschichte," p. 270 ; Pockels in "Oken"s Isis," 182.5 ; Coste, " Hist. Gen. et Partic. du Developpement," 1817 ; Allen Tliomson, '• Contrib. to the Hist., &c., of the Human Ovum before the Third Week after Conception," in " Edin. Med. and Sui'g. Joum." No. 140.)
Formation of Decidua. — Before the arrival of the ovum in the uterus, the lining membrane of that cavity undoubtedly undergoes a preparatory change, by which the formation of the decidua is commenced, and this change, in its first stages, or up to the time of the arrival of the ovum in the uterus, corresponds closely with that which takes place at every successive menstrual period in the uterine membrane. This process consists essentially in a thickening or hypertrophy of the lining membrane, and is mainly due to an extremely rapid proliferation of the subepithelial cells and fibro-cellular tissue, and an increased development of the blood-vessels and glands.
Fig. 518.
Fig. 518. — The DEciDrA opened
AND VIEWED FKOil BEFORE (after
W. Hunter).
This is a representation of the thickened membrane of the uterus thrown off as the product of abortion at a very early period of gestation. A and B mark superiorly tue passage of two bristles through the openings from the Fallopian tulies into the cavity of the uterus (cavity of the decidua), and inferiorly the exit of the bristles at the os iiteri. In these three situations the torn edges are seen where the decidua has been separated from the continuous part of the mucous membrane. At 0, where an ovum has probably been lodged, the inner part of the decidua is made to bulge towards the cavity of the uterus, and begins to form decidua reflexa.
The formation of a com^ plete decidua within the
uterus has been observed in several cases in which, although the ovum was not discovered, or had not yet arrived in the uterus, there was reason to believe impregnation had occurred six or eight days previously. (Von Baer, E. H. Weber.) And a similar condition has been observed in several examples of extra-uterine pregnancy (Hunter and others) ; from which ii, appears that the earlier changes connected with the formation of the decidua are independent of the presence of the ovum in the uterus.
When the ovum has been recently imbedded in the decidua, it forms a swelling or projection of the surface within the uterine cavity, on opening into which the villous chorion is found surrounded by the substance of the decidua or thickened mucous membrane ; but the covering of this substance which passes over the free surface of the. ovum, or that which is towards the uterine cavity, is thinner and simpler in its structure than at the place of attachment of the ovum and in other parts of the uterine surface.
The most projecting part or summit of the swelling formed by the imbedded ovum more especially is somewhat different from the rest, and indicates, by a sort of cicatricial mark, a place where the substance of the decidua, as it gradually covered in the ovum, may be supposed to have finally closed.
The decidual thickening of the mucous memln-ane affects nearly equally the whole of the lining of the uterine cavity, but towards the OS internum, and the openings of the Fallopian tubes, the thickening gradually decreases, and the membrane assumes the unaltered condition which is maintained in these passages. By the fifth or sixth week, when the ovum has reached a diameter of from an inch to an inch and a half, and the nterus has nearly donbled the size which ii presents in the nnimpregnated state, the swelling formed by the ovum and decidua projects strongly like a tumour within the uterine cavity. The membrane of the other parts of the uterus has also undergone progressive increase in its thickness by decidual hypertrophy, so that, having become, as it were, too wide for the capacity of the cavitA', it is thrown into a number of grooves enclosing irregular folds and mammillary projections, but still exhibiting throughout the peculiar features of the mucous lining membrane.
Fig. 519.— View of the Interior of the Human Gravid Uterus at the Twenty-fifth Day (from Farre after Coste).
?t,_ uterine wall ; o, villi of the chorion of the ovnm : dv, decidua vera and enlarged iitenne glands ; dr, decidua reflexa, di\-ided round the margin of the ovum, and turned down so as to expose its pitted surface, which has been removed from the ovum. The right ovary is divided, and shows in section the plicated condition of the earlv conius luteum.
Fig. 520.— DiAGRAJiMATic View of a Transverse Section op the Uterus at the Sea'enth or Eighth Week of Pregnakcv.
c, c, c', the cavity of tlie uterus wLich Ijocoraes tlie cavity of tlie deciJua, oiiening at r, c, the cornua, into the Fallopian tubes, and at c', into the cavity of the cervix, which is closed by a plug of luucus ; dr, decidua vera ; the tlat shade indicates the thickened subepithelial structure, the radiated lines the glandular tubes between this and the muscular wall ; (//•, decidua rcflexa with the sparser villi imbedded in its substance ; (Is, decidua serotina, involving the more developed chorionic villi of the commencing placenta, and forming also between these and the muscular wall a layer outside which the glandular tubes are represented ; ch, chorion, with its villi; u, umbilical vessels of the foetus passing into these, and in the umbilical cord ; aJ, remains of the allantoid pedicle ; am, amnion ; i/, und.ilical vesicle ; v/', its duct, connected with /, the intestine of the embrv-o. The placenta is shown as if it were situated at the fundus, but may be supposed to be on the posterior wall of the uterus.
On the side of the ovum which is towards the uterine wall, there is also a layer of decidua, in which the villi of the chorion are imbedded. At this place, from the sixth to the eighth week, these villi begin to be more thickly set and of larger size, aud to undergo a more complex ramification than on the other sides; and as at the same time there is a corresponding increase in the decidual substance, these villi become more and more closely involved in it, and there is thus established the commencement of that more intimate combination of foetal villi and decidual substance, which by its progressive development in the two or three following weeks, gives rise to the formation of the placenta.
By the clianges now described there has become apparent the distinction of the three portions of decidua usually recognised by authors, viz., decidua vera, decidua reflexa, and -decidua serofi/ia. The first of these is that portion of the altered membrane which lines the general cavity of the uterus in every part except that occupied by the attachment of the ovum ; the decidua reflexa is that w'hich covers the ovum as it projects into the uterine cavity, and which is continuous with the decidua vera at the base of the swelling.
The name of decidua serotina has been somewhat variously employed by authors ; but may, in the meantime, be most suital)ly applied to the whole of the decidual substance inteiwening between the ovum and the uterus, and which may include, therefore, both that which is concerned in the formation of the placenta, and the distinct layer of decidual substance which at a later period is found covering the uterine surface of the placenta.
The cavil u of ilie decidua, which intervenes between the decidua vera and decidua reflexa, and which subsists during the first half of the period of pregnancy, is obviously the same as the original uterine cavity, and, so long as it remains open, naturally communicates with the Fallopian tubes at the upper angles, and the canal of the cervix at the lower. In the last three months of pregnancy, however, this cavity is completely obliterated by the union of the decidua vera and reflexa into one layer over the whole of their extent, so that when afterwards much extended and I'educed to a comparatively thin and irregular stratum of substance, they are at birth thrown off as one membrane along with the other envelopes of the foetus.
x\s the human ovum has never been observed in the progress of its mcapsulation, the exact manner in which this occurs is still involved in doubt. From the various observations, however, already referred to on early ova which have undergone recent incapsulation, and the knowledge of what occurs in animals, it may be conjectured, as first suggested by ^Sharpey (Baly's transl. of "Mailer's Physiology," 1842, p" 1580) that the minute ovum when it arrives in the uterus may be sunk or imbedded in the soft or spongy substance of the mucous membrane, and that when it subsequently enlarges it carries with it, or there is formed round it a covering of the membrane, the substance of which is at the same time undergoing a rapid decidual development, and that this substance continuing to grow with the ovum and expanding with it, constitutes the decidua reflexa. The entire similarity of the structure of the decidua reflexa at its base with that of the decidua vera is in favour of the view that it owes its origin to a similar mode of production.
The formation of the decidua is, as has already been stated, to be attributed mainly to a great increase in the development of the subepithelial tissue. Its substance, accordingly, consists in great measure of the cells, round and spindle-shaped, and cell-fibres which belong to that tissue ; but these are mingled with much larger irregularlyformed multi-nuclear cells, which increase in number as pregnancy advances, and which are peculiarly characteristic of the structm'e of the outer layers of the decidua.
The blood-vessels and the glands of the mucous membrane also undergo great enlargement and modification. The whole of the decidua vera and the basilar part of the reflexa are at first penetrated by blood-vessels derived from those of the uterus, more especially in the latter part of the second and first, half of the third months, Avhen the decidual structure may be considered as having reached its highest degree of development. After this time the blood-vessels of the decidua reflexa, and later those of the whole lining decidua of the uterus, except in the immediate vicinity of the placenta, shrink and ultimately disappear, so that the united decidua becomes in the end wholly non-vascular. The same retrograde process, or atrophy and disappearance, occurs in the blood-vessels of the chorionic villi by which the decidua reflexa is penetrated, and, although the villi themselves never entirely disappear, but may be traced even in the advanced stages of pregnancy as sparse and shrivelled irregular arborescent processes, the blood-vessels very soon begin to shrink and disappear from all the villi ^hich do not form part of the placental structure.
The uterine glands also become enlarged during the development of the decidua, being both elongated in their deeper convoluted portions, which are directed towards the muscular wall of the uterus, and undergoing a peculiar change not yet fully understood, in the parts next their openings on the inner surface. Over the surface of the whole decidua vera, as it lines the uterine cavity, and also on the decidua reflexa, except at its most projecting part, a number of irregular pits are visible to the naked eye, which are frequently so numerous as to give the membrane a reticulated or sieve-like appearance (cups of Montgomery). These pits are really the uterine glands enlarged and altered soon after the commencement of pregnancy, as first clearly shown by Sharpey (Midler's Physiology by Baly, 1842, p. 1579), and the fact has since been observed by others (Kolliker, Coste). The villi embedded in the decidua do not, however, occupy the cavities of these pits, but so far as yet ascertained, are rather sunk in the interglandular hypertro])hie*d substance of the decidua between them (Schroder van der Kolk, Kolliker, and Priestley. See the lectures by the latter " On the Development of the Gravid Uterus," 1860, p. 24). Upon the more exact relation of the villi to the uterine glands in the placenta, further remarks will be made hereafter.
PEXETEATIOX OF DECIDUA BY THE VILLI.
717
From what has been previously stated, it will be seen that there is at first a general union or interpenet ration of the villi of the chorion with the vascular decidna, the more extended part of this union taking place in the decidua reflexa, and the remainder in that portion of the decidua which is interposed between the ovum and the uterus, and which has been already referred to as decidua serotina, but which might from its relation to the formation of the placenta with propriety be named decidua ^jJaccnicdls. Already in the latter half of the second month of pregnancy, the villi on the uterine side of the chorion become larger and more ramified than those which run into the decidua reflexa ; and
Fi.-. 52L
Fig. 521. — View of the Dissection of the Pregxant Uterus op Forty Days after
Conception (from Leisliman after Coste).
a, the embryo sho-OTi within the amnion ; c, the chorion opened, the umbilical vesicle seen lying between it and the amnion ; d, deep surface of decidua reflexa where it has been turned back from the ovum ; r, r, remainder of the decidua i-eflexa projecting from the uterine surface ; v, decidua vera, of which the glandular structure is shown in the cut edges ; x x , openings of the Fallopian tubes seen within the uterus. In the uterine wall the distinction is sho-rtai between the outer muscular part with the wide uterine vessels, and the glandular and decidual internal parts.
ris- UTEROGESTATION.
as tliese latter, while they still continue to grow to some extent, do not increase in number, or in the complication of their ramifications, they gradually become more sparse, thin, and elongated, and lose their vascularity.
An active process of increase meanwhile is going on in the villi placed on the uterine side of the ovum, by which, while they become larger and longer, and penetrate more deeply into the uterine decidua, they also become more minutely and extensively ramified. Changes at the same time occur in the disposition of the decidua placentalis, by which it receives into its substance, and is more and more intimately interlocked with the developing chorionic villi. In the earlier stages, as up to the eighth or ninth week, the foetal and maternal structures may be separated by the withdrawal of the villi from the recesses of the decidua in which they are sunk ; but by the middle of the third month, this becomes no longer possible in consequence of the closer combination or interlocking of the two structures ; in the remaining half of the third month the union becomes more intimate, and by the middle of the fourth month the completion of the placenta is etfected by the continued increase in size and modification of the structure of the maternal and foetal elements.
Structure of the Placenta. — At the time when the placenta has attained its characteristic form and peculiar structure, or after the fourth month of pregnancy, it forms a large discoid or lenticular mass interposed in a limited space between the foetal membranes and the uterus. It presents a fcetal and a uterine surface, the former having implanted into it, usually near the middle, the umbilical cord, which carries to the placenta the umbilical arteries and veins of the foetus, and is covered by a tubular prolongation of the amnion, passing over it from that membrane Avhere it lines the placenta to the abdominal integument of the foetus. The placenta continues to increase in size with the foetus, and when it has attained its full dimensions, it has a width of from seven to eight inches, and a thickness of about one inch and a quarter. But towards the circumference it rapidly thins, where it becomes continuous with the chorion and decidua. The foetal surface is covered by the chorion and amnion, and presents the larger divisions of the umbilical vessels before they dip into the substance. The uterine surface shows a subdivision into a number of large lobes, sometimes called cotyledons, which are covered with a layer of decidua (d. serotina) passing over the whole of this surface, and sending septal prolongations into the placenta between the lobes, which in some places run almost as far as the fcetal surface.
The more uniform substance of the placenta (parenchyma) within these lobes, consists, on the one hand, of highly-developed and complicated tufts of foetal villi, which adhere to the chorion by vascular stems of considerable size and strength, and subdivide again and again into very complex ramifications ; and on the other, of certain dilated vascular spaces continuous with the uterine vessels, the outlines of which follow closely the ramifications of the villi throughout every inflection of their surface. These spaces are doubtless to be regarded as belonging to the maternal system, but their exact nature it is very difficult to determine in the fully formed condition. They probably originally. possess walls of their own, and arc contained in abounding substance of uterine or decidual tissue : but this has become so reduced in thickness, or so closely united with, and so nearly assimilated in its structure to the villi, that it has been found difficult to follow it with certainty, and its existence has even by many been entirely denied. The relations of these two parts of the placenta, as ascertained by the observation of their gradual development in the growth of the placenta, and their comparative anatomy in animals, will be referred to hereafter.
The whole of the placental mass, together with the layer of decidua on its external or uterine surface, and the united decidua vera and reflexa are separated at birth along with the foetus and its membranes.
Fig. 522. — Vertical Section thkough the jiiddle part of the Placenta and the
XJterixe Wall (from Farre after Wagner).
Tlie preparation was from a woman who died in the thirtieth week of gestation : the fines V, u, run through the wall of the uterus to the outer surface of the placenta ; d, the decidua serotina ; p, the tufts of foetal vascular villi, of which two larger divisions are separated by decidual septa, as at dp ; /, the placental end of the umhilical cord ; am, the amnion ; cli, the chorion ; vf, divided fcetal blood-vessels ; r, stems of vascular villi ; us, uterine sinuses or veins ; a, a, coiled arteries passing into the placenta.
Circulation of blood in the placenta. — The existence of a distinct circulation of blood in the fcetal and in the maternal vessels of the placenta, discovered by the Hunters, has long been placed beyond doubt by the experimental investigations of all those who have injected the two sets of vessels with sufficient care and success. The nature of the distribution of the vessels is very different in the two parts of the placenta. In the tufts of foetal villi, the umbilical arteries and veins, possessed of distinct coats, undergo gradual subdivision by ramification into smaller and smaller tubes, until they at last reach capillary minuteness, and the terminal capillaries run in long and tortuous loops which pass from one extreme branch of tlie villi to another, within the fibrous core of which they are situated, and SchriJder van der Kolk has described also a tiner superficial network of capillaries distributed below the epithelial covering of the stems and larger branches of the villi. By artificial injections fluids can be made to pass with perfect precision from the umbilical arteries through the capiharies of the villi into the veins, or in the reverse direction from the veins into the arteries. Nor does there ever occur, except from visible accidental rupture of the vessels, either extravasation of the injected material into the intervening tissue, nor any escape into the maternal sinuses.
Fig. 523. Fig. 523.— Small Portion of Tla CENTA SHOWING THE FcETAL ViLLI
Slightly Magnified (from Leishman after Weber).
The uterine blood-passages, on the other hand, are of the nature of irregular spaces, into which the maternal blood is poured directly by numerous small coiled arteries which, as shown by the Hunters, pierce the external decidua at the uterine surface of the placenta, and open into these blood-spaces without the intervention of any capillary sub Tl'j. 524. Fig. 524. — Chorionic Villits from: the Pla centa AT THE Twelfth Week. Enlarged ISO Diameters (from Leisliman after Ecker).
From a to h, the epithelial coveriDg is left entire ; from a to a it ha.s been removed and tlie fibrons core with the capillary blood-vessels is shown.
division. The result of artificial injection of the blood-vessels in the pregnant uterus equally demonstrates the nature of the circulation in the maternal part of the placenta, for it is easy to show by this method, that a fluid thrown into the uterine arteries fills at once all the maternal bloodspaces of the placenta, surrounding everywhere the chorionic or foetal villi, and returns thence into the uterine veins b_y a number of slanting venous channels, the ntero-jilaccntal sinuses, provided with delicate coats, which issue from the placenta at its uterine surface by piercing the decidua serotiua, and which are most numerous towards the circumference of the organ, where they are in communication with the so-called circular vein or circular sinus previously referred to. Some of these veins may even be traced for some distance into the placenta, in the septa of decidual substance, which are prolonged from the external decidiia serotina bet-^veen the lobes. (For an excellent account of the evidence in favour of the foregoing views, supported by original observations, see Professor Turner's observations on the " Structure of the Human Placenta," in Proceed. Eoy. Sec. of Edin., May, 1872, and in Journ. of Anat., vol. vii., p. 120.)
Farther Consideration of the Structure of the Placenta. — Two doubtful points respecting the structure of the placenta still requne consideration, viz.. 1st, the extent to which uterine tissue is included in or penetrates into, or I'emains as a constituent of the maternal part ; and 2nd, the relation of the interpenetration of the fojtal villi and the uterine decidua to the glandular or other structures of the uterus.
In regard to the first of these points, the views of anatomists still differ gi'eatly ; for, on the one hand, some hold that there is no vestige of iiterine tissue left in connection with the maternal blood-spaces, at all events in the deeper two-thirds of the thickness of the placenta, and that consequently the maternal blood cii-culating in the iDlacenta is in direct contact with the epithelial covering of the foetal villi ; while others are inclined to regard that epithelial covering, or some part of the structui-e which appears to belong to the foetal villi, as really containing some of the elements of the decidua.
Goodstr, indeed, described a double cellular covering of the placental villi, regarding the external layer as of uterine, and the internal as of foetal origin. But later anatomists have not succeeded in confirming these observations, and it does not appear certain that there is more than one obvious layer of cells over the surface of the foetal villi. If, therefore, we assume the existence of a layer of uterine cells in the fully formed placenta, we are reduced to the necessity of supposing that it has either replaced that of the chorionic villi, or has become closely incorporated with it (KoUiker).
The observation of the gradual penetration of the decidua by the villi in the earlier stages of placental formation, the possibility of separating the foetal and maternal structui'es from each other during a certain time, and the undoubted presence both of decidual tissue and of uterine blood-vessels possessed of walls of their own in the commencement, — aU supply convincing proof that uterine elements of structure have originally existed in the placenta, and have contributed to its formation along with those derived from the fostus ; but the condition of the uterine elements in the more advanced stages of placental gi'owth, if they really then exist, still requires further investigation.
The actual enlargement of the uterine capillaries of the decidua into the form of vascular spaces has been traced by Virchow (Archiv, vol. iii. p. 450), and Priestley has observed the capillary form of the maternal vessels surrounding the villi in a product of abortion of the 8th week (Lectures on the Development of the Gravid Uterus, 1860, p. 02). Some anatomists, indeed, as Schroder Van der Kolk, afiirm that they have been able to detect the remains of a vascular wall in connection with the blood-spaces of some parts of the placenta (Waamemingen over het Maaksel van de Menschelijke Placenta : Amsterdam. 18G1), so that it may with reason be surmised that the change which takes place in the stnictirre of the parts forming the human placenta, in the course of the fourth month, is mainly of the nature of a rapid thinning and absorption of the elements of the decidual tissue and vascular walls.
The view that the placenta originally consists of uterine as well as foetal elements combined receives the fullest confirmation from the study of the comparsr tive anatomy of the various forms of simple placental structures in animals, as, in the diffyge placenta of the pachydermata, solipeds, cetacea and some other animals, and in the cotylcdonous placenta of most ruminants ; in both of which only the foetal part of the placental structure undergoes separation from the uterus at birth, — constituting the iwn-dccidnate iorm. of placentation ; and in the various forms of more complete union of the foetal and maternal elements, which occur in the zonal placenta of the camivora, and the discoid of the rodentia, in which more or less of the maternal stmcture derived from decidual formation comes away with the foetal product at birth. But in all forms of placentation of animals, -n-itli the exception of tiie Simiac, in -v-\-hich the structure probably agrees closely Avith the human, the elements of uterine structure are very clearly present, and the uterine blood-vessels may be recog-nised as such, not being dilated into vide sinuses or lacv\na3, but retaining more or less the capillary form of distribution : although in some instances the capillaries have undergone considerable dilatation, and geem to be passing into the condition of venous sinuses. (For a very instructive view of the structure of the placenta in these various animals and their bearing upon the nature of the i^lacental structure in general, the reader is referred to Professor Turners Lectures on the Structure of the Diffused, the Polycotyledonous and the Zonary forms of Placenta, as published in the Journal of Anatom}- and Physiology, vol. x.. p. 127. Oct. 1875, and separately, 1870. There may also be consulted Yon Baer's Entwickelungsgeschichte. 1839. and Eschricht, De oi-ganis qua3 Respirationi et Xutritioni Foetus Mammalium inserviunt, Hafnia3, 1837.)
Bclni'wn of the I'frrlnr Glands to tlic Pliiciiital titriictiirc. — It has long been known that in the placentation of various mammals the uterine glands undergo an increased development, and it has been supposed that they enter into structural and functional relation with the fcetal villi ; but more precise knowledge is still wanting as to whether, and to what extent this relation is of the nature of an actual penetration of the cavities of developed and dilated glands hj the chorionic villi. It was shoAAii by Dr. Sharpey (Miiller's Physiology by Baly, vol. ii. p. 1574) that thedecidua which enters into the fonnation of the zonular placenta of the dog. and doubtless of other camivora, consists of a hypertrophical part of the uterine mucous memlu-ane of corresponding figure, in which the main ducts of the largely developed glands become dilated at their orifices into utricular saccules or pouches, that hollow membranous processes containing foetal vessels rise from the chorion, from among the smaller vascular villi, and apply their flattened summits to the widened mouths of the gland ducts, and enter a short way within the saccules : while these receptacles are filled with a whitish semifluid secretion and are lined with an epithelium, which also covers the intriided part of the fcetal process. As pregnancy advances, the chorional and decidual structures are further interlocked, and the arrangement becomes more intricate. Decidual, that is, maternal blood-vessels are abundantly distributed round the villi of the chorion, which they closely cover. These vessels are larger than ordinary capillaries ; they are unsupported by decidual stroma, and are separated along M-ith the rest of the placenta in parturition.
The observations of Shaipey appeared to receive confirmation from the investigations of E. H, Weber (Zusatze zur Lehre vom Baue, kc, der Geschlechtsorgane, 184G,) and Bischoff (Entmck, des Hundeeies, 184.5,), and by many these observations have been held to prove satisfactorily a connection between the glands and placental formation. But Dr. Sharpey has expressed himself cautiously on this general question, and more recent observations tend rather to throw doubts on the penetration of the uterine glands by the villi of the chorion.
More especially the observations of Ercolani (Mem. of the Acad, of Bologna, 1868 and 1870) and of Turner (loc. cit.) seem to show that it is possible that the saccules described by Sharpey may be formed in the uterine decidua of the carnivora independently of the glands ; and they are disposed to think that these saccules are produced rather in the interglandular tissue. From an examination of the whole evidence on this point and an investigation of the structure and formation of the placenta in different animals, contained in the lectiu-es prcAdously quoted. Turner has ascertained that in the diffused fonn of placenta the uterine glands open in the sow into feebly vascular intervals between the vascular crypts in which the foetal chorionic fringes are sunk, and not into these crypts themselves ; that in the mare the glands open on elevated ridges between the vascular crypts which receive the foetal villi, and only in the Cetacea (Orca gladiator) did he find gland apertures in some of the placental crj-pts. Again, the maternal cotyledons of the i-uminants are destitute of utricular glands, and these are confined to the sunk intercotyledonous part of the uterine membrane. In the zonal placenta of the camivora, as previously stated. Turner failed to trace the uterine glands into the recesses of the decidua which receive the prolongations of the chorionic plates and villi, constituting the foetal portion of the placenta ; and he cliilers therefore from Sharpey in regarding the crypts as of new formation and independent of the glands. He is thns led to the general conclusion that m no kinds of placenta do the uterine glands form an essential part of the placental structm'e, and that the uterine crypts which receive the fcetal processes are essentially interglandular in their origin. Nevertheless Turner recognises the existence in all placentas of uterine structiu'al elements of a cellular nature, which he regards as descendants of the epithelial or subepithelial tissue of the uterine mucous membrane, and to which he attributes, as others have done, glandular functions in the preparation of the matter which is absorbed as nourishment by the blood-vessels of the foetal \'illi. Further observations will be requhed to detennine in how far these views admit of application to the stractiu-e of the full}'- formed human placenta. (See also Tm'ner's Memoir on the Placentation of Seals. Trans. Roy. Soc. of Edinbm-gh. Vol. xxvii.. 1875.)
With respect also to the relation of the uterine glands to the penetration of the decidua by the viUi in the fonnation of the human placenta, further observations are stiU requii-ed. As iireviously stated, anatomists have failed to trace the villi into the dilated parts of the enlarged glands, and Kolllker after a careful examination of the whole subject, comes to the conclusion (Lectures, p. 1G2) that the villi in becoming involved in the decidua have no pennanent connection with the glands. He affirms indeed that the glands soon shrmk over the whole extent of the decidua. beg'uining to do so as early as in the second month of j^regnancy, and have in great measiu-e disappeai-ed before the chorionic villi are fully connected with the uterine membrane. Reichert. however, in his recent description of an early human o\Tim affirms that commencing villi actually enter the mouths of uterine glands, and he has given a diagrammatic representation of the prolongation of the tubes of the uterine glands through the decidua to the surface of the membrane, and the small marginal villi of the o\Tim as actually within tenninal portions of the glands, the cavities of which have undergone some degree of ramification. Fui-ther observations will, however, be necessary for the confirmation of a statement so much at variance with the results of most other observers.
General conclusion- — In recapitulation of the preceding description it may be stated that the human placenta is an organ which is formed by the conabination of two different structural elements ; of which one is derived from the foetus or its membranes, and the other from the uterus. The foetal part consists of the developed vascular villi of the chorion, continues to grow and extend itself with the foetus during the whole of uterogestation, and is the seat of a complete circulation of the foetal blood through the capillary ramifications of the umbilical arteries, veins, and capillaries. The uterine element of the placenta originates in a part of the decidua, which is produced by increased growth and transformation of the lining membrane of the uterus and its blood-vessels. "With the hypertrophied structure so produced the villi of the foetal chorion at first interpenetrate, so that in the earlier stages of placental formation the uterine and fcetal elements are for a time separable, and may still be distinguished fi-om each other, at the period even when they have become more intimately united. But in the progress of development the uterine elements are so much modified, and finally so completely attenuated or removed, that they almost entirely disappear ; and as along with them the walls of the blood-vessels are either thinned out to the last degree, or are entirely absorbed, there remain only the vascular spaces through which the maternal blood flows. A doubt, however, may still exist as to whether these spaces are, or are not, entirely deprived of any uterine enclosure.
The maternal blood is introduced into these spaces directly by the small coiled uterine arteries, without capillary intervention, and after moving through the ^vhole of the placental spaces in contact with or in closest proximity to the foetal villi, it is returned by numerous veins through the outer decidua serotina into the vascular channels of the uterus. This blood is essentially arterial in its qualities, and may be supposed to perform a double function, viz., 1. to exert an aerating effect on the blood of the foetus through the tissue of the villi and the walls of their minute vessels, and 2. to supply for absorption by the foetal vessels the new materials required in the continued nourishment of the foetus.
The continuity of the decidua vera and decidua reflexa with the decidua serotina, and of all three with the whole thickness of the placenta at its margin, sufficiently demonstrates the actual coonection of the maternal elements in these several structures, and the existence of that connection is fully confirmed by tracing the steps of the primary formation and subsequent development of the two sets of placental elements, by which are ascertained the actual presence of both in the commencement, and the gradual modification and disappearance of the maternal part. The view thus arrived at receives further support from the result of the observation of the varieties of form and structure presented by the placental organisation in different animals.
Separation at Birth and Restoration of the Mucous Mem"brane of the Uterus. — In the act of birth the whole decidual structures which have been formed in human uterogestation are separated from the uterus along with all those belonging to the ovum, and the placentation is thus said to be completely deciduate. Thus in parturition, fi'om the effect of the contraction of the uterine walls and the abdominal muscles, after the usual rupture of the foetal membranes and the discharge of the amniotic fluid, the foetus is first expelled : the placenta is next detached and pressed downwards, carrying with it the layer of serotina by which it is covered on its uterine surface, and along with it necessarily are broken through the coiled arteries and the slanting veins ; the membranes of the ovum follow, consisting of the amnion and chorion, together with the shrunken covering of decidua which in the last stage of pregnancy remains from the union of decidua reflexa with decidua vera matted together into one, which is finally peeled off the whole of the interior of the uterus.
The uterus having now contracted and its cavity being greatly reduced in size, there remains, probably, on the uterine surface a part of the subepithelial or decidual structure of the mucous membrane, in irregular shreds rather than in one continuous layer, and in the deeper part are imbedded the convoluted uterine glands extending outwards into the layer of fibres formed by the muscularis mucosse. These remains of decidua, with the clots of blood resulting from the rupture of the vessels, are gradually cast off with the lochia, a discharge which is at first of a mixed character, but gradually becomes more and more composed of corpuscles similar to the white blood globules, and this is succeeded by the closure and contraction of the vessels, the prolongation of the gland tubes to the surface, the formation of a complete ciliated epithelial lining to the cavity, and the complete restoration of the natural structure of the whole membrane.
+++++++++++++++++++++++++++++++++++++++++++++++
DEVELOPMENT OF THE SKELETON.
II. DEVELOPMENT OP PARTICULAR SYSTEMS AND
ORGANS OP THE BODY.
THE SKELETON AND ORGANS OF VOLUNTAEY MOTION.
The morphological development of the skeleton and organs of voluntary motion is closely in accordance with the general plan of development which belongs to the whole vertebrate body. The first steps are connected with the formation of the strictly axial part, consisting of the enclosing walls of the cranio-vertebral cavity for containing the rudiments of the brain and spinal marrow, and for the issue of the successive pairs of nerves arising from tliem. These are succeeded by the formation of the walls of the great visceral cavities of the head and trunk, in which the facial and costal arches are to be distinguished ; and lastly, the appendicular parts, or the limbs and limb-arches, are developed. The permanent forms of these parts are only produced in the process of ossification ; but the rudiments of most of them are already to be distinguished in the masses of cartilage or formative tissue which precede the ossifying change.
As the mode of ossification of the several bones has been described in the osteological part of the work, and the histological view of theprocess of formation of bone has been given in the part on General Anatomy, the morphological view of the development will alone be referred to in this place, in which will be included the more important phenomena of the preparation of the matrix or formative material for the various parts of the skeleton.
Fig. 525. — Embryo of the Doo seen from above, Fig. 525.
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 cex'ebral 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.
1. VERTEBRAL COLtJMN AND TRUNK.
Relation of Vertebral Erudiments to the Notochord. — It has already been shown (General Phenomena of Development, p. 692), that all the parts of the skeleton ow^e their primitive formative material to mesoblastic elements, and that the bodies and arches of the vertebras, and the adjacent part of the cranial walls are formed from continuous blastodermic substance lying below and around the primitive medullary canal. A part also of the basis of the cranium has this in common witli the vertebral axis, that its formative substance surrounds the notochord, extending forward from the column of the vertebral bodies into the occipito-sphenoid part of the cranial basis, which is there composed of the formative substance termed the invesiing mass of Rathke.
It is to be remembered, however, that closely as the formative tissue of the bone elements appear to surround the notochord, that structure does not itself, nor by its sheath, contribute to the formation of the vertebral or basi-cranial bones, but merely lies within them ; and the formative material, out of which the bones are produced, is derived from mesoblastic substance which passes inwards from the primordial vertebral plates, and envelopes the chorda external to its sheath. The formation of the notochord, therefore, precedes that of the formative bone-elements which afterwards envelope it, and the remains of the notochord, unaffected directly by any ossifying change, are found in the interior of the commencing bones, and may be traced even for a long time throughout the whole length of the column of the bodies of the vertebrre.
This important fact was first demonstrated by H. Miiller, of Wm-tzbrn-g-, who showed fui-ther that the notochord did not pass through the anterior arch of the atlas, but was traceable directly from the body of the axis vertebra through its odontoid process, and thence into the basi- occipital and basi-sphenoid bones, reaching as far as the pituitary fossa. (Heinrich Miiller, " Ub. d. Vorkommen von Eesten des Chorda Dorsalis b. Menschen nach der Geburt," in " Zeitsch. fiir Rat. Med.," von Henle u. Pfeifer, 1858, b. ii. See also Gegenbaur, " Untersuch. ub. Vergleich. Anat. Das Kopfskelet der Selachier," Leipzig, 1872. W. Miiller, " Bau der Chorda DorsaUs," in '■ Jenasch. Zeitsch." b. vi. E. Dursy, " Zur Entwick. des Kopfes," 1869, and Mihalkovics, '• On the Chorda and Pituitary Body," in "Archiv filr Miki-oscop. Anat.," b. xi., 1875.)
It may be mentioned further, as the result of H. Miiller's observations, that though in general the chorda passes through the middle of the vertebral bodies, the position was found subject to variation in the caudal portion of the column, where it sometimes pa,?sed above, and at other times below, the vei-tebral bodies.
The notochord itself has been generally held to be produced from an intruded central column of mesoblastic cells, and this seems to be the nrode of origin in birds ; but it may be doubtful whether it is the same in all animals. In sharks Balfour finds that ihere is no median column derived from the mesoblast, and attributes the origin of the notochord to the hj^joblast (Quart. Joum. of IMicroscop. Sc, Oct. 1874). The same origin is ascribed to it in mammals by Hensen, who finds that the notochord is late of being formed in the rabbit, — an observation confii-med by KoUiker : Mihalkovics, on the other hand, is inclined to refer it in all animals to the epiblast. However this may be, the tendency of recent research appears to be to show that the notochord may be more nearly alUed to epithelial structiu'es than to cartilage with which it has generally been previously associated. It is at all events important to note that it is in many respects different from the parts ascertained to proceed from the mesoblast, that it never combines with their elements, and that there is no penetration of its substance by connective tissue or blood-vessels, as happens in all other parts derived from the mesoblast.
The interesting observations of Kowalevsky on the existence of a chorda dorsalis in Ascidia (Mem. de TAcad. de St. Petersboiu'g, torn. x. and xi., 1867 and 1868), would appear to show that this structure, and the tj^je of development which accompanies it, are not confined to vertebrate animals, and that in them the notochord may present more of a merely vestigial character than constitute an important element in the formation of the skeleton. The constancy of its position and relations, however, is an important fact regai-ding its history.
The notochord does not undergo transverse segmentation in the same manner as the protovertebral plate does. It remains undivided to the last, but in the course of vertebral ossification it shows alternate diminutions and enlargements of its diameter, corresponding in number and position with the vertebral divisions. One of these enlargements is found between the odontoid process and the basi-occipital bone, and another has been observed by Mihalkovies in the interval l>etwecn the basi- occipital and the basi-sphenoid bones or their cartila[,nnous matrices.
Fig. 526. — View from above of the Embryo-Chick in the Fig. 525.
FIRST HALF OF THE SeCOND DaY.
1, 2, tbe three primary enceplialic vesicles ; iu front and to the sides the cephalic fold ; crossing at 2 the fovea cardiaca ; 3, tlie caudal extremity of the medullary canal dilated into a rhomboid form ; 4, 4, six primordial vertebral divisions.
In front of this last swelling, the chorda is beiit down below the base of the sknll, and tapering to a fine filament, ends or is lost in the floor of the pituitary fossa. The enlargements now mentioned have some interest in connection with the question of the vertebral constitution of the sknll
Segmentation of the Protovertebree. — The transverse vertebrate segmentation which occurs in the primary vertebral plates affects only that part of these plates which is formed of mesoblast. It begins at a very early period, as already stated, even before the close of the primary medullary canal, in the form of one or two, or it may be three short transverse transparent lines which separate a corresponding number of dark or condensed quadrilateral masses of the primitive vertebral plates. These quadrilateral masses, the so-called primordial vertehrcc (Urwirbel of the Germans) (fig. 526, -4, 4), do not, however, correspond merely to the vertebrae of the skeleton, nor are they directly converted into theii- rudiments, but they are rather divisions equivalent
Fi-. 527
Fig. 527. — Cervical part of the Piujiitive Vertebral COLDMX AND ADJACENT PARTS OF AN EmBRYO
OP THE Sixth Day, showing the division of the Primitive Vertebral Segments (from Kolliker after Remak).
1, 1, chorda dorsal is iu its sheath, pointed at its nipper end ; 2, points by three lines to the original intervals of the primitive vertebrffi ; 3, in a similar manner indicates the places of new division into permanent bodies of vertebrte ; c indicates the body of the first cervical vertebra ; in this and the next the pirimitive division has disappeared, as also in the two lowest represented, viz. , d and the one above ; in those intermediate the line of division is shown : 4, points in three places to the vertebral arches ; and 5, similarly to three commencing ganglia of the spinal nerves : the dotted segments outside these i^arts are the muscular plates.
in number and position to the vertebral segments of the body (somatomes of Goodsir) ; each one comprising superficially a segment of the muscular plate, and more deeply a pair of intervertebral ganglia and nerves, as well as the parts of the skeleton which lie before and behind them.
The more obvious protovertebral segmentation docs not extend into
728
DEVELOPMENT OF THE VERTEBRAL COLUMN.
the mesoblastic tissue beyond the commencement of the basis of the cranium, the mass of blastema which there surrounds the prolongation of
Fig. 5'28. — Sections op the Vertebral Column of a Human Fcetus of eight ■WEEKS (from Kolliker).
A, transverse longitudinal section of several vertebrre. 1, 1, cliortla dorsalis, its remains thicker opposite the intervertebral discs ; 2, is placed on one of the bodies of the permanent vertebrce ; 3, on one of the intervertebral discs.
B, transverse horizontal section through a part of one dorsal vertebra. 1, remains of the chorda dorsalis in the middle of the body ; 2, arch of the vertebra ; 3, head of a rib.
the notochord (the investing mass of Rathke) remaininoj one and undivided, or being devoid at least of the marked cleavage which occurs in the strictly vertebral part.
It is fi'om this protovertebral plate on each side, whether in its entire primitive condition, or in its later and divided state, that the material is derived for the formation of the bodies and laminae of the vertebra? and the muscles which cover them. This is effected by the rapid increase of the mesoblast, and by the extension of that structure beyond the immediate confines of the vertebral laminae in an inward and downward direction, so as to throw a quantity of new mesoblastic material round the notochord, and inwards and upwards, so as to pass in between the primary medullary canal and the enveloping layer of epiblast.
The muscular plate. — Shortly after this extension of the mesoblast in the two directions before mentioned, another separation, or rather differentiation, is observed to take place in the direction of its length, in the formation along the dorsal surface, and below the epiblast, of a series of circumscribed plates which form the foundation of the erector muscles of the spine, and the great dorsal muscles of the trunk. These constitute together the muscular, or rather the musculo-cutancovs plate, for it appears also to include the formative rudiment of the true skin.
There is thus deposited the formative material for the vertebral bodies, the vertebral arches, and the muscles which immediately surround them, together with the general integument.
Meanwhile the vertebral segmentation goes on progressing from before backwards, extending through the dorsal, lumbar, sacral and coccygeal vertebrfB, till the process is complete ; but this is accompanied by other changes having reference to the separation of the nerveroots and ganglia from their formative tissue, and the development of the elements of the permanent vertebra?.
In the outer portion of each protovertebral mass a transverse partition arises which separates the anterior part, as ganglion and nerve root, from
DEVELOPMENT OF THE SKELETON.
729
the posterior, as matrix or forerunner of the bone and other structures which belong to the vertebral column. Each nerve then comes thus to be placed in front of the future permanent vertebra with the proto Fig. 529.
V ~
Fig. 529. — Transverse section through the Dorsal region of an Embryo-Chick,
END of Third Day (from Foster and Balfour).
Am, amnion ; mp, muscle plate ; c v, cardinal vein ; Ao, dorsal aorta at the point where its two roots begin to join ; Ch, notochord ; Wd, Wolffian duct ; Wh, commencement of formation of Wolffian body ; tip, epiblast ; so, somatopleure ; liy, hypoblast. The section passes through the place where the alimentary canal Qiy) communicates with the yolk-sac.
vertebral division of which it was originally connected. In the inner or central part of the primordial vertebrae a different kind of change occurs, first, by the amalgamation or fusion of the protovertebral masses, and subsequently by their subdivision in such a manner that the intervertebral disc arises on a level with or opposite the centre of each protovertebral mass, and the blastema, out of which a permanent vertebral body is formed, is made up by the union of two parts, an anterior and a posterior, the first of these being derived from the hinder part of the preceding protovertebral mass of the same number, the other part being supplied by the anterior section of the protovertebral mass immediately following.
This is a somewhat complicated change ; and the more difficult to be followed that it would appear that the original division between the protovertebral masses disappears previous to a new segmentation taking place. Thus it results that, as respecta the centrum or body part, the posterior half of one protovertebra is
730 DEVELOPMENT OF THE VERTEBKAL COLUMX.
tliro'wn into connection w4th the anterior half of the one next following, and tlius each permanent body is formed from imrts of two protovertebral masses ; ■while in respect to the arches, each one proceeds from the hinder segment of the anterior of the two protovertebra3 concerned, the spinal gangiion and root being thrown into connection with the hinder part of the permanent vertebra immediately in front of the protovertebra of which they originally formed a part.
Formation of Vertelbral Matrices. — While the material for the vertebral bodies is laid dowu round, tlie notochord, a further extension of mesoblastic substance from the primordial vertebral plates takes place at the sides and round the medullary cavity for the matrix of the vertebral arches, and in due course, by differentiation of the formative cells, chondrification of the substance occurs in the form of the strips which constitute the first rudiments of the vertebral arches, and the accompanying transverse and other processes. The first ossification of these bones is from cartilage, but doubtless in them, as in other bones, much of the subsequent growth and extension of the bone substance proceeds from sub-periosteal deposit. It is also to be remarked that in some bones originating in membrane, cartilage may subsequently contribute to the growth and extension of the bone, as ajspears to occur in the lower jaw and clavicle.
The chondrification of the formative matrix of the bones in the human embryo takes place chiefly during the fifth and sixth weeks of foetal life, and in the seventh and eighth, ossification has begun in several of the long bones. But even before this time an ossific deposit shows itself in the fibrous matrix of the clavicle and lower jaw. By the ninth week the greater number of the bones have begun to ossify.
The formation of cartilages for the arches of the vertebrae begins first in those of the dorsal region, and spreads from these forwards into the cervical vertebrae and basis of the skull, and backwards * into the lumbar and sacral vertebrae : but the extension of the matrix upwards ceases in the lower sacral and coccygeal region where the arches are deficient.
A small cartilaginous band forms the matrix of the subcentral portion or anterior arch of the atlas vertebra, quite distinct from that of the body of the axis, and out of the line of prolongation of the notochord.
In the lateral plates the cartilaginous matrices of the ribs are formed in connection with those of the transverse processes, and in the vertebral part of the ribs themselves ossification is comparatively early ; but a considerable part remains unossified in the sternal portion, or costal cartilages, in connection with their special use in the mechanism of the respiratory movements.
Certain portions of the transverse parts of the cervical and lumbar vertebrae are undoubtedly homologous with ribs ; but we give the name only to those costal bars which are separately articulated to the vertebra, and the first of the vertebras with which a rib reaching the sternum is articulated is reckoned as dorsal. Among the thoracic ribs a certain number, as elsewhere stated, of the cartilaginous matrices behind the first, are in the commencement united together at their ventral extre Hci'fi aud elsewhere, unless otherwise explained, the terms used to indicate position npply to the primitive proue position of the embryo as it lies in the blastoderm, the dorsal aspect upwards aud the veutral dowuwiu-ds.
DEYELOPMEXT OF THE CEAXIULL 731
mities into a strip of cartilage on each side, and thus the matrix of the sternum is at first cleft in two behind the pre-sterual portion. Subsequent fusion of these two lateral strips unites them into one ; and the transverse division of the bone only appears from the result of ossification in successive distinct centres. This fact possesses an interest in connection with the tendency of the meso-sternuui and xiphi-sternum to divide and to produce various degrees of the malformation termed fissura sterni.
In the lumbar region there is reason also to look upon part of the transverse processes as representing costal elements, but it is only in cases of abnormal formation that they are found distinct from the rest of the vertebra. (See the Descriptive x\natomy, Vol. I., p. 22.)
The sacrum is peculiar in presenting, thi'ust in and compressed between its strictly vertebral elements and the iliac bone with which it is united, several bony pieces which may be regarded as interposed ribs. The ossification of two of these occurs as early as the fifth or sixth month of fostal life.
2. THE HEAD.
The head of the embryo consists at first, as already stated, of the cranial part alone, the face, nose, and mouth being absent. Below the cranium, and extending as far forward as the point of junction of the anterior with the middle encephalic vesicle, is situated the pharyngeal portion of the primitive alimentary canal, closed in anteriorly by the inflection of the blastodermic layers. It is at this place that subsequently the opening of the alimentary canal to the exterior takes place in what constitutes ultimately the isthmus of the fauces ; and in front of this the buccal cavity, not yet existing, is afterwards formed.
In the progressive development of the head the principal changes by which its fundamental parts come into shape may be enumerated shortly as follows, viz., First, increase of deposit and textural differentiation of the mesoblastic substance for the formation of the cranial walls in their basilar, lateral, and upper portions ; second, the interpolation of the sense-capsules as connected with the formation of the rudiments of the nose, eye, and ear ; third, the development of the cerebral hemispheres and other parts of the brain from the three primary encephalic vesicles; fovrtli, the occurrence of the several cranial inflections ; and fifth, the new formation of outgrowths for the development of the parts of the face.
1. The Craiiiuiii. — The basal portion of the cranium consists primarily of two fundamental parts. Of these the i^osterior is distinguished by the presence of the prolongation of the notochord within it as far forward as the part of the skull which afterwards becomes the pituitary fossa. This portion, which may be named occipito-sphenoid, is originally formed by the undivided investing mass of Rathke, which surrounds the anterior extremity of the notochord, and contains the matrix of the future basi-occipital and basi-sphenoid cartilages. By its later extension to the sides, it forms the matrix of the exoccipitals and the periotic mass of cartilage which surrounds the primary auditory vesicles. The main part extends forward below the posterior and middle primary encephalic vesicles, ending at the pituitary fossa.
The anterior portion of the basis cranii may be called sphcno-ethmoid, as containing the matrix of the pre-sphenoid, and the ethmoid
732
DEVELOPMENT OF THE HEAD.
cartilages. It is mainly produced in connection with the trabeculae cranii, which contain between their separated limbs the pituitary fossa. This part of the cranial basis contains no prolongation of the noto
Fig. 500.
Fig. 530. — The Lower or Cartilaginous part op THE Cranium op a Chick op thk Sixth Dat (from Huxley).
1, 1, cliorda dorsalis ; 2, the shaded portion here and forwards is the cartilage of the base of the skull ; at 2 the occipital part ; at 3 the prolongations of cartilage into the anterior part of the skull called traheculce cranii ; 4, the pituitary space ; 5, parts of the labyrinth.
chord ; it lies below the anterior encephalic vesicle (thalamencephalon), and becomes greatly modified in connection with the expansion of the cerebral hemispheres and primary ocular vesicles, and the development of the nasal fossae and mouth, together with the other parts of the face.
The primary parts of the three principal sense organs, it may here be stated, the nose, eye, and ear, formed in connection respectively with the cerebral hemispheres, the thalamencephalon, and the third primary vesicle, are interpolated between the rudimentary parts of the head as follows, viz., 1. The nose between the frontal, intermaxillary and ethmoid ; the eye between the frontal, sphenoid, ethmoid and maxillary ; and the ear between the basi-occipital, exoccipital and alisphenoid. "While the base of the cranium, to the extent already mentioned, is
Fig. 531. Fig. 53L — View from below op the Carti
laginous Base of the Cranium with
its Ossific Centres in a Human Foetus
op about Five Months (from Huxley, slightly
altered).
The bone is dotted to distinguish it from the cartilage, which is shaded with lines. 1, the basilar part, 2, the condyloid or lateral pvarts, and 3, 4, the tabular or superior part of the occipital surrounding the foramen magnum ; 5, centres of the pre-sphenoid on the inside of the optic foramen ; 6, centres of the post-sphenoid ; 7, centres of the lesser wings or orbito-sphenoid ; 8, septal cartilage of the nose ; 9 & 10, parts of the labyrinth.
cartilaginous in its origin, the lateral and upper walls are chiefly of membranous formation, as in the squama occipitis, the squamo-zygomatic of the temporal, the parietal and the frontal bones.
The trabeculai stretch forward to the anterior extremity of the head, and maintain the foremost place as the seat of the nasal cartilages and external apertures of the nose. Behind these the coalesced trabeculae form a narrow cthmo-vomerine cartilage, the nasal septum, round the
DEVELOPMENT OF THE CRANIUM.
733
back of which the vomer is formed as a bony splent covering ; while in the hinder lyre-shaped interval of the separated trabeculge is placed the infundibulum in connection with the pituitary body.
Fig. 532.
Fig. 532. — Basilar Part op the Primordial Cranium OF A Human Fcetus of three months seen from ABOVE (from Kolliker).
a, iipi)cr half of the squama occipitis ; h, lower half of the same ; c, cai-tilaginous plate extending into it ; d, (in the foramen magnum) the exoccipital ; c, basi-occipital ; /, peti'ous, with the meatus anditorius interuus ; g, dorsum selliB, with two nuclei belonging to the basi-sphenoid bone ; /;, nuclei in the anterior clinoid processes ; i, great wing nearly entirely ossified ; I; small wings ; I, crista galli ; m, cribrethmoid ; n, cartilaginous nose ; o, strip of cartilage between the sphenoid and the parietal ; p, osseous plate between the lesser wings and the cribriform plate.
From the side of the presphenoid cartilage the matrix of the orbitosphenoids or lesser wings, containing the optic foramina, is developed ; and from the sides of the basi-sphenoid proceeds the matrix of the greater wings, which are also cartilaginous in their origin.
In the periotic or cartilaginous rudiment of the temporal bone three centres of formation are distinguished by Huxley, viz., 1. OpisthoUc, or that surrounding the fenestra rotunda and cochlea ; 2, prootic, or that which encloses the superior semicircular canal ; and 3, epiotic, or that which surrounds the posterior semicircular canal and extends into the mastoid portion. They soon unite into one so as to form the petromastoid bone.
Fig. 533. — Longitudinal Section through the Head op an Embryo op Four Weeks (from Kolliker). -j" V, anterior encephalic vesicle, cerebral portion ; z, interbrain ; m, midbrain ; h, cerebellum ; n, medulla oblongata ; no and a, optic vesicle ; o, auditory depression ; t, centre of basi-cranial flexure ; t', lateral and hinder parts of tentorium ; p, the fold of epiblast which forms the hypophysis cerebri.
The styloid process and the auditory ossicles are of cartilaginous origin.
The squamo-zygomatic and tympanic are produced from membrane.
Tlie Cranial Flexures. — The earliest and the most important of the cranial flexures is that which takes place at the anterior extremity of the notochord and in the region of the mid-brain or middle encephalic vesicle. Here, as previously stated, the notochord extends into the substance of the basis of the cranium as it is prolonged forwards in the line of the vertebral bodies. At this place the medullary tube, and the substance forming the wall of the cranium especially, undergoes a sudden bending downwards and forwards, so as to cause the projection of the thickened cranial base in a marked manner upwards. This coincides with the place where the investing mass and the trabe
roi
DEVELOPMENT OF THE HEAD.
cula3 meet, and where inferior]? tlie })ituitary body, and superiorly the infnndibnlum are afterwards formed. The investing mass of blastema, in which the anterior extremity of the notochord is enclosed, and the notochord itself, terminate here behind the pituitary fossa, or what afterwards becomes that part, in a place corresponding to the dorsum selliB of human anatomy. Above and behind this, the middle cerebral vesicle forms the most prominent part of the cranium, which remains a characteristic feature of this part of the embryo head for a consideral^le time. Another early flexure of the cranium accompanies the development of the cerebellum from the third primary vesicle, a cleft now appearing behind and below the rudimentary cerebellum, in the region of the fourth ventricle, and above the medulla oblongata, and this flexure is necessarily attended with a convexity forwards, or another flexure in the place of the pons Varolii.
Fig. 534.
Fig. 534. LONC.ITUPTXAL
Section of the Human Embryo at the sixth or
SEVENTH ^VEEK
^^ ~'~"' """ ^ '"' 1, cerebral hemispheres
2, vesicle of the third venti'icle ; 3, mid-hrain ; 4, cerebellum ; 5, medulla oblongata ; c7i, notochord passing up through the bodies of the vertebrae into the basis cranii and terminating in the head between the infundibulum and the sac o£ the hypophysis cerebri ; s, the vertebral spines ; n, the spinal cord ; p, the phannix ; 7i, the heart ; I, the liver ; ?', the stomach and intestine; cl, the cloaca ; r, the urinary bladder and pedicle of the allantois ; it, u', the umbilicus containing the vitellointestinal duct, urachus and vessels ; between i, and /, superiorly, the Wolffian body is shown.
The great cranial flexure thus marks the division between the strictly basi-cranial, or occipito-sphenoidal, and the basi-facial, or sphenoethmoidal part, the chorda terminating between those two portions of the cranial base, with a conical and sharp point. Here the chorda is itself lient downwards and forwards, and terminates in a s]3ot which corresponds to the post-sphenoid body, or dorsum sellas. According to Mihalkovics, who has recently investigated the subject with care (see Archiv filr j\Iikroskop. Anat., vol. xi., 1875,) in connection with the formation of the pituitary gland in mammals and birds, the chorda tapers off to a fine point in front of this spot, but presents a slight swelling just at the place of the future occipitosphenoidal suture.
The formation of the mouth, and its opening by the fauces into the pharyngeal or first part of the primitive alimentary canal, are phenomena of development intimately connected with the formation of the central part of the cranium and sella turcica, but they are also associated with the development of the face, which is next to be considered.
Formation of the Moutli and Hypophysis cerebri. — Along with the changes which accompany the formation of the principal cranial flexui-e, is
THE PITUITARY BODY. 735
•associated in a remarkable manner the origin of a body (tlie pituitary gland or hypophysis cerebri) the nature and uses of A\'hich in the adult are entii'cly
Fiff. 535.
Fig. 535.— Vertical Section of the Head in Early Ejibryoes of the Rabbit,
Magnified {from Mihalkovics).
A. From an embryo of five millimetres long.
B. From an embryo of six millimetres long,
C. Vertical section of the anterior end of the notochord and i^ituitary body, &c., from an embryo sixteen millimetres long.
In A, the faucial opening is still closed ; in R, it is formed ; c. anterior cerebral vesicle ; mc, meso-cerebrum ; mo, medulla oblongata ; co, corneous layer ; to, medullary layer ; if, infundibuhim ; avi, amnion ; spe, spheno-ethmoidal, be, central (dorsum sellte), and spo, sj^heno-occipital parts of the basis cranii ; /;, heart ; /, anterior extremity of primitive alimentary canal and opening (later) of the fauces ; i, cephalic jrortion of primitive intestine ; tha, thalamus ; p', closed opening of the involuted part of the pituitary body (py) ; ch, notochord ; pk, pharynx.
imkno-mi, but the constancy of whose presence, and the imiformity of its connections in the whole series of vertebrate animals, points to some important morphological relation.
The general nature of this body, in its joint connection with the infundibulum of the brain on the one hand, and a diverticulum of the alimentary canal on the other, was first pointed out by Eathke (Miiller's Archiv, 1838. p. 482), although he afterwards abandoned the view there set forth. It was, however, fully confirmed by others ; and, among recent observers, we owe more especially to William Miiller an
736
DEVELOPMENT OF THE HEAD.
elaborate investigation of the whole subject (Jenaische Zeitschr., vol. vi., 1871), who traced most carefully the nei-vous and diverticular elements in their development, and their union with mesoblastic elements in the formation of the gland, Goette next ascertained that the diverticulum from below is connected with the buccal cavity and epiblast, and not with the pharynx and hypoblast, as was previously supposed (Archiv fur Miki-oscop. Anat., vol. ix., p. 397). The observations of Mihalkovics on Mammals complete the history of this point in development, and will be mahily employed in the following description.
The formation of this body may be shortly described as consisting in the meeting and combination of two outgrowths from very different fundamental parts ; one cerebral or medullary from above, and the other corneous or epiblastic (glandular), from below, in a recess of the cranial basis which afterwards becomes the pituitary fossa (fig. 53.5, B, <f,'py). The cerebral outgrowth, the posteiior of the two parts, takes place by the fonnation of a pointed projection do\\Tawards of a portion of the lower medullaiy waU of the vesicle of the third ventricle, and its firm adhesion to the base of the cranium. This is the commencement of the infundibulum. Meanwhile, a little in front of the same place, there is projected upwards from below a part of the basilar surface of the cranium, so as to form a deep recess lined by the corneous layer from the back and upper part of the future mouth. This recess is the commencement of the hypophysis or pituitary- body in its glandular portion, which is not, as has been supposed, a recess from the pharynx, seeing that it is in front of the opening which is afterwards formed for the fauces. The depressed and sharpened out anterior part of the notochord is directed downwards and forwards, while the sac of the hypophysis is can-ied upwards and backwards, and, according to Mihalkovics, the attenuated end of the chorda soon disappears from between the infundibulum and the hypophysis, previous to the occurrence of the intimate union which follows between these two bodies. The anterior extremity of the chorda, therefore, is lost in the floor of the pituitary fossa, and the swollen or dilated portion of the chorda which succeeds, and which comes then to form the apparent termination, occupies the interval between the basi-occipital and the basi-sphenoidal cartilages. The chorda traced back from this point, presents another swelling at the junction of the basi-occipital cartilage with that of the odontoid process, into which last it passes. The third swelling of the chorda lies between the odontoid cartilage, and that of the body of the axis vertebra.
FiK. 53G.
B
Fig. 536. — Cranium and
Human Embryo seen
(from Ecker).
Face of the from before
A, from an embryo of about three weeks : 1, anterior cerebral vesicles and cerebral hemispheres ; 2, interbrain ; 3, middle or fronto-nasal process ; 4, superior maxillary plate ; 5, the eye ; 6, inferior maxillary or mandibular plate (first postoral) ; 7, second plate ; 8, third ; 9, fourth, and behind each of these four plates their respective pharyngeal clefts. B, from an embryo of five weeks : 1, 2, 3, and 5, the same as in A ; 4, tlie external nasal or lateral frontal process ; 6, the superior maxillary plate ; 7, the mandibular ; x , the tongue ; 8, the first phaiyngeal cleft, which becomes the meatus auditorius extenius.
The base of the skull, therefore, consists of two parts, one the posterior, in which the chorda is imbedded, and corresponding to the futui-e basi-occipital and basi-sphenoidal parts, the other in front of this, into which the chorda does not penetrate, the sphcno-ethmoidal, and which, according to the researchee of Parker and Gegenbaur, is of a later fonnation, and is more immediately related to the development of the face.
THE CEANIAL BASIS.
737
The flask-like outgrowth of the buccal epiblast which gives rise to the hypophysis cerebri, is now gradually shut off from the corneous layer and cavity of the mouth, iirst by the constriction, and subsequently by the closure of its place of communication. There remains however, for a considerable time, a longish.
Fig. 537.
Fig. 538.
Fig. 537. — OcTLiXE Plan ViKW OF THE Upper Part of the Body of an Embryo Pig, two-thirds
OF AN INCH IN LeNGTH.
Magnified seven diameters (from Parker).
Fig. 538. — Plan op the Skull, &c., of the same Ejibryo seen from below. Magnified ten diameters (from Parker).
In this a7id the preceding figure the letters, where j)resent, indicate the following parts : —
c' to 0^ , the five primary divisions of the brain ; a, the eye ; n, the nose ; m, the mouth ; <?•, cartilage of the trabeculEe ; ctr, cornua trabecularum ; pn, prenasal cartilage ; 2}p[/> pterygo-palatine cartilage ; mn, the mandibular arch with Meckel's cartilage ; te, first visceral cleft which becomes the tympanoeustachian passage ; au, the auditory vesicle ; hi/, the cerato-hyoid arch ; Ir, the branchial bars and clefts, 1 to 4; tItJi, the thyro-hyoid ; py, the pituitary fossa ; ch, the notochord in the cranial basis, .surrounded by the investing mass (ii') ; vii, facial nerve ; ix, glosso-pharyngeal ; X, pneumog;istric ; xii, hypoglossal nerve.
thread of union between the two (fig. 535, C, 7/). The epithelium of the enclosed portion subsequently undergoes development into glandular coeca and cell-cords, and its internal cavity becomes gradually obliterated. This fonns the anterior part or lobe of the pituitary body. The posterior part owes its origin to the combination with mesoblastic tissue of a. widened extension of the infundibular process of the brain, which is thrust in between the sac of the pituitary body and the dorsum sella;. The nervous structure of this posterior lobe afterwards disappears in the higher animals, but in the lower it retains its place as a part of the brain.
vol.. ir. 3 B
738
DEVELOrMENT OF THE HEAD.
2. Subcranial, Facial, or Pharyngeal Plates or Arches. —
In man, and all vertebrates, there are developed below and on the sides of the cranial part of the head, a series of processes or bars in pairs, which contribute to the formation of the subcranial structures constituting the face and jaws, and the hyoid and other parts intervening between the head and trunk. These bars first received attention
Fig. 539.
Fig. 539. — Outlines showing the early changes in the form of the Head op
THE Human Embryo.
A, profile view of tlie head and fore i)art of the body of an embryo of about four weeks (from nature, '^^) : the five primary divisions of the brain are shown, together with the primai-y olfactory and optic depressions, and a, the auditory vesicle ; 1, marks the mandibular plate, and behind this are seen the three following plates with the corresponding pharyngeal clefts. B, from an embryo of about six weeks (from Ecker, f ) : the cerebral hemispheres have become enlarged and begin to spread laterally ; 1, the lower jaw ; 1', the first pharyngeal cleft, now widening at the dorsal end, where it forms the meatus externus ; the second cleft is still visible, but the third and fourth clefts are closed and
- he corresponding plates have nearly disappeared. C, from a human fojtus of nine weeks
(from nature, \) ; the features of the face are now roughly formed ; the first pharyngeal ■cleft is now undergoing conversion into the meatus, and the auricle is beginning to rise at its outer border.
from their discovery by Eathke in 182G, published in the Isis of that year, and were named by him the branchial arches, from the relation ■which some of them bear to the gill bars of branchiate vertebrates. Their nature and transformations were fully investigated by Eeichert in 1837 (Miiller's Archiv, 1837). From later researches it appears that other processes, with somewhat similar relations to the cranium, occurring further forward, may be associated with those described as branchial by Eathke, and it will be expedient therefore to describe the whole of the subcranial outgrowths together at this place. In this the views of Huxley and Parker will be chiefly followed. (See "On the Stracture and Development of the Skull in the Pig," by W. K, Parker, in Trans. Eoy, Soc. 1873, p. 289 ; Huxley, in Elements of Compai\ Anat., 1864, and Manual of Compar. Anat., 1871 ; also Gegenbaur, Das Kopfskelet, &c., 1872). According to these views the parts of the head situated in fi*ont of and above the future mouth, are formed from two pairs of plates, w^hich may thence be called preoral, in one pair of which the bars are the same "with the trahecukc cranii of Eathke surrounding the pituitary gland, and are the basis of formation of the pre-sphenoid, ethmoid, nasal, and pre-maxillary portions of the skull, while in the other pair, consisting in each of a deeper and a superficial part, the bars form the foundation
SUBCRANIAL AND FACIAL STRUCTURES. 7.VJ
■of the pterygo-palatine wall of tlie nose and mouth, and the superior maxillary bone. The nasal pits or primary nasal depressions, which extend themselves afterwards into the nasal fossae, and remain permanently open to the exterior, are formed by a depression of the surface of the epiblast in the anterior prolongation of the head as constituted by the ends of the trabeculas, and the harder structures of the septum and walls of the nasal cavities, as well as the turbinated structures on which the olfactory nerves are distributed, are all derived from the anterior parts of these trabecule ; — a mesial union giving rise to the nasal septum, while lateral parts circling round the nasal pits, form the alar enclosures of these depressions (see figs. 537 and 538).
The second preoral subcranial plates have received the name pterygopalatine from the nature of their deeper connection with the bar in which the pterygoid and palate-bones are afterwards formed. These enclose the posterior nasal apertures, and advancing from the two sides, at last meet each other in the palate, and in front meet the pre-maxillary process to complete the palate and upper jaw.
But these are only the deeper parts of the structure out of which the upper jaw is formed, there being on the surface of the head, and behind the depression of the eye on each side, a bulging process known as the superior maxillary process, in which the upper jaw and malar bone are formed, and which has externally the appearance of bending round the angle of the mouth in continuity with the mandibular or inferior maxillary.
The formation of the superficial parts of the face, as seen from before, may be described as follows, viz. : — In the middle, there descends in what now forms the region of the forehead, a mesial portion, the fronto-nasal plate, which forms the integument of the nose, as far as the inside of the nostrils, and the columella of the nose, together Avith the mesial part or lunula of the upper lip, that is, all the part lying inside the depression of the nostrils. On the outside of these depressions, a short lappet surrounds the orifice of the nostrils, as wings forming the external nasal processes. It is towards the outside of these last plates that the ocular depression is situated, that depression being thus interposed between the lateral or external nasal plate,- and the maxillary plate, and forming the fissure which has been tailed the ocular fissure, but which afterwards becomes more fetrictly the lachrymal in its anterior part (see figs. 536 and 539).
The great buccal aperture now passes across the face, having the middle and external nasal with the superior maxillary plates above and in front, and the inferior maxillary plates below and behind. It will be remembered, however, that the cavity of the mouth is thrown forward by the outward development of the subcranial plates, which deepen more and more the buccal cavity as they grow outwards from the primitive cranium.
The POSTOKAL pairs of pharyngeal visceral plates are four in number ; the first being that already mentioned as following immediately behind the mouth, and forming the mandibular or inferior maxillary.
At an early period of foetal life in the human foetus, and in that of all vertebrates, plates of this description are found, but in the lower vertebrates a greater number exists than in the higher. The number of four pairs belongs to man in common with all the non-branchiate vertebrates. Behind each of these plates there are formed from within,
3 B 2
740 DEVELOl'MENT OF THE HEAD.
in the course of development, clefts which penetrate the wall of the pharynx ; these clefts, or so-called branchial apertures, running completely through the wall of the pliarynx and the external wall of the body of the embryo (see figs. 537 and 538).
The auditory pit, or primary depression from the epiblastic surface which forms the rudiment of the labyrinth of the ear, is situated immediately above the upper or proximal end of the two first postoral plates, and consequently on a level with the first postoral cleft. And this proximity of position is connected with the intimate relation in which two sets of parts stand to each other : for the part called the first branchial cleft is afterwards converted into the external and middle passages of the ear, (meatus, tympanum, and Eustachian canal), the membrana tympani being at a later period thrown across the passage. It forms thus the tympauo-eustachian cleft or canal. The tympanic bono is of membranous origin and is formed round the first cleft. The external auricle is of integumental origin, and is formed in the second postoral bar posteriorly and externally to the aperture of the first cleft.
The second postoral cleft is the first true water passage, or the first of those which serves as a gill aperture in branchiate vertebrates, and which may in the lower classes be increased to a greater number.
Although the description of some of the changes which the several pharyngeal plates or branchial arches undergo in the further process of development, belongs to a different part of the subject, yet it may be useful to describe sliortly the more important of them in this place.
In the first or mandibular arch a strong cartilaginous bar is formed known as the cartilage of Mcclcel, on the exterior of which, but not in its own substance, throughout a considerable part of its distal length, the lower jaw-bone is afterwards developed. The proximal part next the cranium, which comes later to be connected with the auditory capsule, becomes in mannnalia the malleus, in birds and reptiles chiefiy the OS quadratum. (Seefigures in connection with development of the ear.)
In the second or hyoid arcli are developed the styloid process, the stylo-hyoid ligament, the lesser or upper coruua of the hyoid bone, the series of parts which connect them with the basis of the skull, being united like the first to the auditory capsule : but the proximal part of this arch would appear also to have the incus formed in it, and to be connected with the stapes and stapedius muscle.
The third arch' is the thyro-hyoid, and is related to the formation of the lower or great cornua, and the body of the hyoid bone. It corresponds with the first true branchial arch of amphibia and fishes, in which animals the clefts and bars behind this arch become more numerous than in the higher vertebrata.
The fourth arch, which has no special name, but might be called suhhyoid or cervical, does not seem to form the basis of any particular organ, but is situated exactly at that part of the body which becomes elongated as the neck, — a part Avhich may be considered as absent in the foetus, and the formation of which by a simple process of elongation gives ^""^e to some peculiar features in the anatomy of the parts composing it.
Relations of Cranial ITerves. — The rudiments of four cranial nerves, besides the optic and auditory afterwards to be referred to, are found at a very early period in connection with the plates now under consideration, and the following is the relation in which, according to Parker, they stand to these plates in all vertebrate animals. These nerves are the
EELATIONS OF THE CRAXIAL XERYES.
ni
fifth pair or trip-eminus, the facial, the g-losso-pharyngeal, and the l^neumo-gastric. The two first are situated iu front, and the two latter behind the auditory sac. These nerves all divide or fork above a visceral cleft, one division going to the posterior face of the arch in front of the cleft, the other to the anterior face of the arch behind it.
540.
Fig. 540. — Embryo of tue Ciiioii at tub end of the Fouhth Day (from Foster and
Bcalfour).
The amnion has been removed ; Al, allantois ; Clf, cerebral hemispheres ; FS, thalamencephalon, with Pn, the pineal gland projecting from its summit; MB, midbrain ; Cb, cerebellum ; IV, V, fourth ventricle ; Z, lens ; chs, choroid slit ; C'en V, iiuditory vesicle ; sm, superior maxillary process ; IF, 2F, &c., first, second, third and. fourth visceral folds ; V, fifth nerve in two divisions, one to the eye, and the other to the first visceral arch ; VII, seventh nerve passing to the second visceral arch ; Gph, glosso-pharjmgeal nerve passing to the third visceral arch ; Pg, pneumo -gastric nerve passing to the fourth arch ; / V, investing mass ; ch, notochord ; Mt, the heart ; HIP, muscle plates ; W, wing ; HL, hind limb.
The orbito-nasal and the palatine divisions of the trigeminus belong to the trabecular arch, the former above, the latter below the optic nerve. Of the other division, one part (the superior maxillary nerve) follows the palato-pterygoid arch, the other (inferior maxillary nerve) accompanies tlie mandibular arch.
The facial nerve (portio dura of seventh pair) divides above the tympano-eustachian passage, its anterior part (chorda tympani) going to the posterior side of the mandibular arch, and its posterior part (descending branch of facial) to the outer or anterior side of the hyoid arch.
The glosso-pharyngeal nerve, by a similar division, goes by its inner or anterior branch (lingual) to the inner or posterior side of the hyoid arch, and by its other division (pharyngeal) to the front of the first branchial or thyro-hyoid arch.
742 DEVELOPMENT OF THE LIMBS.
In the higher animals the pneumo-gastric nerve shows no close relation to the clefts, but in branchiate vertebrates it is continued past the gills, and sends forked branches to the gill arches in front and behind each of the clefts.
3. ORIGIN AND FORMATION OF THE LIMBS.
The close connection of the limb-arches with certain vertebral segments of the trunk has been previously referred to in the morpholoo-ical remarks, given under the description of the bones and muscles m the first volume ; and although the vertebral homology of the parts of the limb proper is not so apparent, at least in the proximal segments, yet in the quinquifid division of the more remote parts, in the preaxial and postaxial arrangement of these divisions, and in their relation to the nerves and some other circumstances, we can scarcely fail to perceive some very near relationship between the structure of the limb as a whole, and a certain number of the vertebral segments of the trunk.
Fi£,'. 541.
Fig. 541. — HujiAN Embryo of about
FOUR WEEKS (froiu Kijllikei', after A.
Thomson). |
/, the anterior limb rising as a semicircular plate from the lateral ridge. (The figure is elsewhere described. )
The limbs do not exist from the earliest time of the formation of the cranio-vertebral part of the trunk, but only begin to be formed when the development of the axial part of the body has made some advance, as in the first half of tlie fourth day of incubation in the chick, and at the commencement of the fourth week m the
human embryo. . „ -, t r> ,, • -■
They first make their appearance as two pan-s of buds from the side of the vertebral part of the trunk, in the form of flattish lateral elevations with curved free margins projecting from the exterior of the body, outside the thickened ridge (sometimes called the Wolffian ridgc) where the division of the mesoblast into somatopleure and splanchnopleure take place, and near the outer margins of the muscular plates. The anterior pair of limbs appears earlier than the posterior, and for a long time is always more advanced in the development of its parts.
The place of formation of the auterior and posterior limbs does not Tary to any great extent throughout the vertebrate animals,— and this fact may be looked upon as one of the most marked features ot vertebrate organisation. . ^■ ^ x. •:. •
The thickened plate which forms the commencing limb, by its increased growth, projects still more from the side, so as to take the form of a flattened lappet with a semicircular free margin ; presenting then two surfaces which may be named dorsal and ventral with reterence to their correspondence to the like surfaces of the trunk, constituting respectively the primitive extensor and flexor surfaces of the limb ; while the anterior margin of the semicircular lappet corresponds to the preaxial and the posterior margin to the postaxial borders of the future limbs.
DEVELOPMENT OF THE LIMBS.
743
The whole thickness of the somatopleure division of the mesoblast is
involved in this primary limb-bud, and it is of course also covered
with the epiblast or cuticular layer, in the substance of which there
is considerable increase of thickness at the most prominent part of the
margin.
As the limb-buds increase in size, the lateral limb-plate, or Wolffian ridge, which is at first very prominent in its whole length, becomes less, and gradually flattens down into the more even surface of the wall of the trunk.
The part of the limb which appears first, corresponds more immediately with the hand or foot than with the other divisions of the limb. Along with this, however, at a very early period there is an indication of the formation of the limb-girdles as folds passing off from the side
Fig. 542. — Diagrammatic Outline of the Profile VIEW OF the Human Embryo OF ABOUT seven weeks, TO SHOW THE PRIMItive relations of the Limbs to the Trunk. (Allen Thomson.) f
r, the radial (preaxial), and u, the iilnar (postaxial) border of the hand and forearm ; t, the tiljial (preaxial), and /, the fibular (postaxial) border of the foot and lower leg. (The foot is represented at a somewhat more advanced stage than the rest of the embryo).
of the trunk. As the projection of the limb increases from the side of the body, the distal or terminal segment becomes slightly notched off from the part next the trunk. This terminal part, forming nearly three-fourths of a somewhat circular flattened plate, contains the rudiments of the hand or foot. The next change which takes place is in the division of the proximal part, or rather the preaxial border and ventral surface, by a notch which separates the fore-arm and lower leg from the upper arm and thigh at the elbow and knee joint respectively. In the third stage the notched division of the free lateral curved margin, with intermediate slightly tubercular projections of the substance, shows the commencement of digital development, in which it soon becomes apparent that the pollex and hallux occupy the preaxial position in their respective limbs, and are followed by the series of other fiugers to the fifth, which is placed on the postaxial border. From these it is easy to trace, by reference to the simple original position of the limbs, the preaxial position afterwards held by the radius in the one and the tibia in the other, and the postaxial position of the ulna or fibula. In the meantime the internal differentiation of texture takes place, by which is brought out the more complete distinction of the segments of the limbs, and the various component parts of each, which gradually appear in the cartilages for the bones, muscular plates extended from the general muscular sheath of the trunk, prolongation of the cutaneous layer of the integument, the formation of nerves, blood-vessels, &c., the consideration of all which belongs to the history of more advanced development.
744
DEVELOPMENT OF TEE MUSCLES.
In order also to complete the history of the formation of the limbs, it is necessary to take account of the changes of attitude the anterior and posterior respectively undergo, as compared with the primary embryonic position. In this the elbow comes first to be turned outwards and then directed backwards, bringing the flexor surface of the upper arm forwards, while the position of the flexor surface of the fore-arm and hand, though generally and naturally inwards, may, by supination, be brought forwards, and by pronation backwards, the "latter being the permanent position given to the manus in most animals. In the hinder limb, again, the thigh is turned inwards, so that in the higher animals the flexor surface looks backwards, and in all animals the lower division of the limb is turned inwards and the sole of the foot downwards, so that the extensor surface and dorsum look forwards. (See vol. i., p. 122.)
4. DEVELOPMENT ^ OP THE MUSCLES.
The muscles of the trunk derive their origin from the muscular plates previously referred to as lieing separated by difierentiation of the
Fig. 543.
WJ?
Fig. 543. — Section through the Lumbar IIkgion of an EiiBRYO-CnicK of Four
Days (from Foster and Balfour).
nc, neural canal ; jir, posterior root and ganglion of a spinal nerve ; ar, anterior root ; ■»«/>, muscle-plate; cfi, notochord ; WB, Wolffian ridge; AO, aorta; Vca, cardinal vein ; Wd, Wolffian duct ; Wb, Wolffian body with glomeruli ; ge, germinal epitlielium ; Md, depression forming the commencement of the Miillcrian duct ; d, alimentary canal ; M, mesentery ; >S'0, somatopleure ; SP, splanchnopleure ; 1', blood-vessels ; P2h pleuro-peritoneal space.
FORIvIATIOX OF THE JOINTS. 7J5
formative cells in the outer or superficial part of the protovertel)raI masses. Some difference of o]iinion exists, however, among embryologists, as to how far the hypaxial (hyposkeletal of Huxlc}') as well as the epaxial muscles, proceed from this source alone, or whether only the latter are traceable to the muscular plate formed by the protovertebral differentiation, and the hyj^axial may be supposed to proceed from a deeper source.
Eeccnt observations seem to show that a downward extension of the mesoblast from the protovertebrge may also give rise to the hypaxial muscles.
Being developed from the segmented protovertebral elements, the muscular plate shows at first the same division into segments, which ai-e separated for a time by intermuscular septa (myotomes) as occurs during life in a considerable number of them in fishes and amphibia.
The formation of the longer muscles of the trunk proceeds from the disappearance of the septa, and the longitudinal union of the fasciculi of successive myotomes. In the trunk the direction of these remains for the most part chiefly longitudinal, but those connected with the limb-girdles change their direction with the development of the limb.
The formation of the muscles of the limbs themselves has not been traced in detail. The greater number of these muscles appear rather to arise independently in the blastodermic tissue of the limb-bud, than to be prolonged from the sheets of trunk-muscles (Kiilliker).
The facial muscles and the platysma, belong to the subcutaneous system, and are developed along with the skin.
The diaphragm is at first wanting. It arises soon after the formation of the lungs, fi'om two parts which spring from above and the sides, and which divide the pleural and peritoneal cavities, which were jDreviously in one, from each other.
The muscles begin to be formed in the human embryo in the sixth and seventh week.
Formation of the Joints. — With regard to the formation of the joints, very little is known. It would appear that the cavities of the synovial joints are not yet formed at the time when chondrification has taken place in the matrix of the bones. It is therefore by a secondary process of solution of continuity that these cavities are produced. The articular cartilages remain as the coverings of the opposed surfaces of the bones, and the various ligamentous and other parts belonging to the joints arise by processes of textural differentiation which it is unnecessary to particularise here.
Distinction of Bones according to their Cartilaginous or Membranous
Origin. — There is here appended for the assistance of the reader a note of the distinction as regards their origin from cartilage or fibrous membrane of the .several pennanent bones of the skeleton.
1. Boiu'.s- (irisiiif/from Cartilage: —
fl, III the Head.
Basi-occipital. ex-occipital, and part of the supra-occipital or squama occipitis.
The whole sphenoid except the cornua sphenoidalia.
The p.etro-mastoid or periotic portion of the temporal bone.
The mes-ethmoid and ethmo-turbinal.
The pterygo-palatine.
The malleus (quadi-ate of animals) with Meckel's cartilage.
The incus and stapes, with the stylo-hyoid.
The thyro-hyoid.
746
DEVELOPMEXT OF THE NERVOUS SYSTEM.
b. Ill the Trnnh.
The bodies, arches, and processes of the vertebraa. The ribs and sternum.
c. In the Limbs.
The scapula and coracoid. The clavicle in part, and all the other bones of the Tipper limbs (excepting sesamoid).
The ilium, ischium, and pubis, and all the other bones of the lower limbs, including the patella, but excepting sesamoid of toe.
2. BoJies arittiiig from Fihvcms Memirane : —
a. In the Head.
Pai-t of the squama occipitis.
The frontal.
The parietal.
The squamo-zygomatic and tympanic of the temporal.
The nasal and lachrjTual.
The maxillaries and i3re-maxillaries.
The vomer and comua sphenoidalia.
The inferior or maxillo-turbinal.
The malar or jugal.
The inferior maxillary or mandibular.
i. Ill the Trunh. None.
c. In the Limbs.
The clavicle in part.
(The marsupial bone of animals.)
The smaller sesamoid bones of tendons.
DEVELOPiUENT OF THE NERVOUS SYSTEM.
The Cerebro-spinal Centre. — From what has been previously stated it will have been seen that the rudiment of the cerebro-spinal
Fig. 514.
Fig. 545.
Fig. 544. — Embryo of the Doo seen from above, ■with a portion of the Blastodekm attached (from Bischofi").
Tlie medullary canal, not yet closed, shows at the cephalic extremity a partial division into the three jirimary cerebral vesicles ; and at the posterior extremity a rhoniboidal enlargement. Six proto-vertebral divisions are visible ; so, the upper division of the blastoderm, sp, the lower division.
THE SPIXAL MARROW. 747
Fi"-. 545. — Embkyo of the Dog more advanced, seen from above (after Bisclioff ).
The medullaiy canal is now closed in ; c, the anterior encephalic vesicle ; o, the primitive optic vesicle in communication with the anterior encephalic ; au, the primitive auditory vesicle opposite the third encephalic vesicle ; am, the cephalic fold of the amnion enclosing the anterior third of the embryo ; ov, the omphalo-mesenteric vein entering the heart posteriorly ; in-, the proto- vertebral divisions, now become numerous.
nervous centre is formed more immediately from the thickened medullary plates of the involuted epiblast, the ridges of which, rising from the surface of the blastoderm, become united dorsally along the middle line into a hollow medullary tube of a cylindrical form. This tube becomes dilated at its anterior or cephalic extremity, and this dilated portion becomes divided by two partial constrictions into the three primary cerebral or encephalic vesicles, which, as representing fundamentaf portions of the brain, have been termed the fore-brain, mid-brain and hind-brain. The spinal portion retains its more uniform cylindrical shape, escepting towards the caudal extremity, where it is longer of
Fi<r. ZiPj.
Fig. 546. — Transverse Section THROUGH THE EjlBRYO OF THE ChICK, AND
■^^A, Blastoderm at the end
OP THE First Day. Magnified FROM 90 TO 100 TIMES (from Kolliker).
d'/> tUuh d d i,a-p c/i /i, epiblast ; (?c?, hypoblast ;
sp, mesoblast ; Pv, medullary
groove ; m, medullary plates ;
c7i, chorda dorsalis ; utrp, proto-vertebral plate ; uw7i, commencement of di^'ision of
mesoblast into its upper and lower laminte ; between i?/ and h the dorsal laminte or
ridges which by their approximation close in the medullary canal.
closing, and forms for a time a flat open rhomboidal dilatation. The continuous cavity enclosed within the primitive medullary tube is the same with that which, variously modified, afterwards constitutes the central ventricles of the brain and canal of the spinal cord.
The formative cells composing the medullary substance are at first spherical, but they afterwards become elongated and spindle-shaped, and increase rapidly by multiplication. They represent at first the
Fig. 547.
/-r
Fig. 547. — Transverse Section of the Embryo Chick on the Second Day. Magnified FROM 90 to 100 times (from Kolliker).
The explanation of the letters is in part the same as in the foregoing figure, mr, the medullary tube now closed along the dorsal line and covered in by continuous epibkist ; nwh, hollow of the proto-vertebral mass ; mp, mesoblast external to the protovertebroe dividing into hpl, somatopleure, and df, splanchnopleure ; ao, one of the primitive aortas ; iinff, intermediate mass connected with the origin of the Wolffian body.
7-18 DEVELOPMENT OF THE NERYOUS SYSTEM.
grey substance, or the nerve-cells and non-medullated fibres. The cylindrical cells which, from the first, line the whole canal, remain permanently in the part of it which forms the central canal of the spinal marrow, and frequently present the ciliated structure.
THE SPINAL MARROW The internal prey substance of the spinal marrow is first formed; the white substance is produced later on the exterior. The sides acquire considerable increased thickness, while the dorsal and ventral parts remain comparatively thin, so that the cavity assumes the appearance in section of a slit, which becomes gradually narrower as the lateral thickening increases ; and at last the opposite surfaces uniting in the middle divide the primary central canal into an anterior or lower and posterior or upper part (see figs. 547 and 548).
The lower of these divisions becomes the permanent central canal, the upper or dorsal is afterwards so far obliterated that it is filled with a septum of connective tissue belonging to the pia mater, and becomes the posterior fissure of the cord (in human anatomy). (Lockliart Clarke, Phil. Trans. 1862.)
In birds and mammals there is no distinction to be seen at first between the outer or corneous layer of the involuted epiblast and the cells which by their increase more immediately constitute the medullary plates. In batrachia, however, the dark colour of the corneous layer shows it to be distinct from the more strictly nervous layers. In osseous fishes there is no medullary groove or canal at first, but an involution of a solid column of cells, which is subsequently hollowed out for tlie formation of a ventricular cavity.
The masses of grey matter first formed in the spinal marrow correspond chiefly to the anterior columns ; these are succeeded by lateral masses or columns, and somewhat later by small posterior columns. There are at first no commissures except by the passage of the deepest
Fig. 548. — Transverse Section of the CervicaIi Part of the Spinal Cord of a Human Ejibryoop Six Weeks (from Kolliker). 'f
Tins and the following figure are only sketched, tJie white matter and a part of the gre.y not being shaded in. r, central canal ; e, its epithelial lining, at e (inferiorly), the i)art which becomes the anterior comniissure ; at c (superiorly), the original place of closure of the canal ; a, the white substance of the anterior columns, beginning to be separated from the grey matter of the interior, and extending round into the lateral column, where it is crossed by the line from [/, which points to the grey substance ; p, posterior column ; ar, anterior roots ; j5r, posterior roots.
layer of cells across the middle line, but the fibres from the roots of the nerves when formed are traceable into the grey substance of their rcs])cctive anterior and posterior columns.
The white substance is formed external to or on the surface of the deeper grey substance j but it is not yet determined whether it is
THE SPINAL MARROW.
749
developed out of the cells composing the grey matter or from separate blastema to which the mesoblast may in part contribute. It is combined with connective tissue elements, and its structure is <lifferent from that of the grey substance, which is undoubtedly produced by multiplication and differentiation of the involuted epiblastic cells.
Flj:. 549.
Fig. 549.— Traksvekse Section op Half
THE Cartilaginous Vertebral Column
AND THE Spinal Cord in the Cervical
Part of a Human Embryo op from
NINE to ten weeks (from Kolliker) '~
c, central canal lined with epithelium ; a, anterior column ; ^j, posterior column ; p', band of Goll ; [/, ganglion of the posterior root ; p r, posterior root ; a r, anterior root passing over tlie ganglion ; d m, dura-matral sheath, omitted near p r, to show the posterior roots ; b, body of the vertebra ; cl), chorda dorsalis ; n a, neural arch of the vertebra.
On the fifth and sixth days in the chick, according to Foster and Balfour, the white columns increase rapidly in size, and the anterior median fissure begins to be formed between the anterior columns by their swelling outwards and leaving its interval between them. It is at first wide and shallo^v, and soon receives a lining of vascular connective tissue or pia mater. The commissures are now also formed ; the anterior grey commissure first, then the posterior grey, and somewhat later the anterior white commissure.
In the further increase of the anterior and lateral white columns as they thicken, they become more united together on each side, so that they can only be arbitrarily distinguished ; the fibres of the roots of the nerves are traced through them into the grey matter ; the cornua of grey matter become more and more developed, and the fissures between the white columns deepen, while the connective tissue or pia-matral septa run more completely inwards through the white substance.
Angular cells with radiating processes make their appearance in the grey matter, and the nerve-fibres both of the grey and white matter become more distinct.
The cylindrical cells lining the central canal retain their distinctness, and they are more completely separated from the grey matter by the delicate tissue of the ependyma. Throughout the greater part of the spinal marrow the dorsal part of the primary medullary hollow is obliterated to form the fissure, but in the sacral region of birds it opens out in the rhomboidal sinus, and in the filum tcrminale of the human spinal marrow the whole primary medullary cavity remains.
The SPINAL COED has been found by Kolliker already in the form of a cylinder in the cervical region of an embiyo four weeks old. Un-united borders have been seen by Tiedemann in the ninth week towards the lower end of the cord, the perfect closing of the furrow being delayed in that part, which is slightly
750
DEVELOrMENT OF THE ^'EPLVO^S SYSTEM.
enlarged, and presents a longitudinal median slit, analogous to the rliomboidal sinus in bii-ds.
The anterior fismirc of the cord is developed very earlj', and contains even at fii-st a process of the pia mater.
Fin
<-/
icw'^'
Fig. 550. — Transverse Section OF Half of the Spinal Cord of the Chick OF Seven Days (from Foster and Balfour). Magnified.
pciv, posterior, lew, lateral, and acw, anterior •white columns ; pc, jjosterior cornu of grey matter with small cells ; ac, anterior grey cornu with large cells ; ep, epithelium of the canal ; c, the upper part now open and filled with tissue in the posterior fissure ; S2')c, the lower division of the primitive medullary cavity, which remains as the permanent canal ; «/, anterior fissure left between the ]3rojecting anterior columns ; ajc, anterior grey commissure.
- aye
The cervical and lumhar cnlarricmcnts, opposite the attachments of the brachial and crural nei-ves, appear at the end of the third month : in these situations the central canal, at that time not filled up, is some'ndiat larger than elsewhere (see figs. 556 and 5.58).
At first the cord occupies the whole length of the vertebral canal, so that there is no Cauda equina. In the fourth month the veitebraB begin to grow more rapidly than the cord, so that the latter seems as it were to have been retracted within the canal, and the elongation of the roots of the nerves which gives rise to the Cauda equina is commenced. At the ninth month, the lower end of the cord is opposite the third lumbar vertebra. (Kolliker, Ent'wickelungsgeschichte ; Lockhart Clark in the Phil. Trans. 1862; Bidder und Kupfer, Untersuch. iib d. Eiickenmark, Leipz., 1857. Foster and Balfom-, Elements of Embryology.)
Till lately it was believed that the roots and ganglia of the spinal nerves are at first distinct from the medullary substance of the cord,
- and that they originate by differentiation of cells in the mesoblastic
f5ubstance of the protovertebral plate. But recent observations, to bo more particularly referred to hereafter, have shown that they arise in 2)art at least in close connection with the spinal cord itself.
THE BRAIN OR ENCEPHALON.
1. — General phenomena of development as ascertained in birds and ■onammals. — A reference has previously been made to the simple form in which the brain at first presents itself in the anterior dilated portion of the primitive medullary tube, and its partial division into the three primary cerebral vesicles. This is placed within simple cranial walls formed by the cephalic inflection of the blastoderm, without face or
PEIMITIVE FOEM OF THE BRAIN.
(51
any other parts ; so that the head of the embryo consists at first of no more than the wider part of the medullary tube and the simple enclosing wall.
Fig. 55L
Fig. 551. — Four Views of the Brain of an Embrto-kitten in the Stage of First Division into the Five Cerebral Eudiments, magnified Three Diameters (from Reicliert).
A, from above ; B, from the side ; C, vertical section sliowiug the interior ; D, from helow.
1, Cerebral hemisphere, prosencephalon ; 2, thalamencephalon ; 3, mesencephalon, still single ; 4, cerebellum, epencephalon ; 5, mj-elencephalon, medulla oblongata ; o, optic nerves ; V, fifth ^pair ; "VIII, eighth pair or glossopharyngeal and pneumogastric ; i, infundibulum ; v, v', general ventricular cavity, opening at v, into the lateral ventricle by the foramen of Monro.
In the base of this wall, it will be remembered that the notochord extends forward beneath the posterior and middle of the vesicles, and occupies, therefore, the part of the cranium corresponding to the occipito-sphenoidal basis, while the trabecute cranii, developed forwards
Fig. 552.
Fig. 552. — Vertical Sections of Embryonic Brains in two Stages of Transition from the Rudimentary Condition, Magnified THREE Diameters (from Eeichert).
A, Brain of the embryo pig in commencing state of transition. 1, Right cerebral hemisphere ; 2, thalamencephalon and position of the pineal gland ; 3, midbrain, with a large cavity ; /, foramen of Monro ; i, infundibulum ; 4, cereliellum ; 5, medulla oblongata.
B, Brain of the embryo of the cat more advanced, c, Cerebral hemisphere passing backwards so as to cover the other parts in succession ; I, olfactory bulb ; II, optic ner^'e ; th, thalamus opticus ; /, foramen of Monro ; cc, cor25US callosum ; ^3, pineal gland ; i, infundibulum ; cq, corpora quadrigemina, not yet divided ; 3, third ventricle ; cr, crura cerebri, the aqueduct of Sylvius, now reduced in width ; c', cerebellum ; 4, fourth ventricle ; jpv, Pons Varolii ; mi, medulla oblongata.
from below the anterior vesicle, are prolonged in the anterior or sphenoethmoidal part. The latter cerebral rudiment, therefore, which corresponds to the thalami optici and third ventricle, and which may with Huxley be conveniently called thaJamcncepiialon, is at first the foremost part of the brain, and the region of the ijituitary fossa lying below it is the foremost part of the cranial basis. The manner in which the development of the trabecule and other elements of the face modifies the
752 DEVELOPMENT OF THE XERVOUS SYSTEM.
form of this re,e,'ion of the head has already been adverted to, aud need not be repeated here.
As regards the earh'est phenomena of development in the brain itself, there are three changes which mainly tend to modify its form in the most marked degree, viz., 1st, the development from the anterior vesicle on each side of the primitive ocnlar vesicle ; 2nd, the expansion from another part, somewliat later, of the vesicles of the cerebral hemispheres; and 3rd, tlie formation in the forepart of the posterior encephalic vesicle of a new cerebral rudiment corresponding to the cerebellun).
Fig. r)53. Fig. 553. — Sketches op the Peimitiye Parts
I „T ., OF THE Human Brain (from Kolliker).
1, 2, and 3 are from the human embryo of about seven weeks. 1, view of the wliole embryo from behind, the brain and spinal cord exposed ; 2, the posterior, and 3, the lateral view of the brain removed from the body ; h, the cerebral hemisj)here (prosencephalon) ; i, the thalamenceiihalou ; i', the infundibulum at the lower part of the same ; m, the middle ^jrimary vesicle (mesencephalon) ; c, the cerebellum (epencephalou) ; m 0, the medulla oblongata. Figure 3 shows also the several curves which take place in the development of the parts from the primitive medullary tube. In 4, a lateral view is given of the brain of a human embrjo of three months : the enlargement of the cerebral
hemisphere has covered in the optic thalami, leaving the tubercula quadrigemina, ?;?,
apparent.
The formation of the primitive ocnlar vesicles, by an evolution of the lateral wall of the primitive medullary tube, gives to the first vesicle and the adjacent part of the head a much greater lateral width ; but the cranial wall, though pushed out by the enlarging oculai: vesicles, does not follow closely the inflection of their surfaces. As the subsequent contraction of the stalk of the ocular vesicles progresses, these vesicles are thrown more backwards and downwards by the change next to be described.
Tills is the evolution or expansion of the wall of the anterior encephalic vesicle into the two cerebral hemispheres, which takes place in front and at each side, so that the vesicles of the right and left hemispheres are from the first separate and distinct. As these vesicles become dilated, the cranial wall undergoes a corresponding expansion in the forepart of the head, and the vesicle of the thalamencephalon, which was at first the foremost part of the embryo-head, is thrown downwards and backwards into a deeper position.
The middle encephalic vesicle, increasing greatly in size, takes the most prominent part of the head superiorly, both from its own greatei relative magnitude, and from the sudden bend which the head now takes below this vesicle in the great cranial curvature.
The formation of the cerebellum begins by a thickening in the upper and lateral walls of the part of the posterior primitive vesicle which is next to the midbrain, and is accompanied by a deep inflection of the medullary tube between it and the remaining part of the vesicle which forms the medulla oblongata.
FIVE TRIMARY DIVISIONS OF THE BEAIN.
753
There are thus distinguished the rudiments of five fundamental constituents of the brain, under which it will be found convenient to bring the notice of the development of the several parts forming the full grown organ, and which m^y in this association be shortly enumerated as follows, viz. : —
1. The cerebral hemispheres, with their ventricular hollows or lateral ventricles, the corpora striata, and the olfactory lobes, — a set of parts to which, as a whole, the name of proceredrum or j)rosejice2}haIo?i may be Q-iveu.
Fig. 554. — Sketches of the early form Fig. 554.
OF THE PARTS OF THE CeREBRO-SPINAIi
Axis in the Human Embryo (after '* Tiedemann).
A, at the seventh week, lateral view ;
1, spinal cord ; 2, medulla oblongata ; 3, cerebellum ; 4, mesencej^halon ; 5, 6, 7, cerebrum. B, at the ninth week, posterior view; 1, medulla oblongata; 2, cerebellum ; 3, mesencephalon ; 4, 5, tlialami optici and cerebral hemispheres. C and D, lateral and posterior views of the brain of the human embryo at twelve weeks, a, cerebrum ; b, corpora quadrigemina ; c, cerebellum ; d, medulla oblongata ; the thalami are now covered by the enlarged hemispheres. E, posterior view of the same brain dissected to show the deeper parts. 1, medulla oblongata ;
2, cereliellum ; 3, corjDora quadrigemina ; 4, thalami optici ; 5, the hemisphere tuined aside ; 6, the corpus striatum embedded in the hemisphere ; 7, the commencement of the aorpus callosum. F, the inner side of the right half of the same brain separated by a vertical median section, showing the central or ventricular cavity. 1, 2, the spinal cord and medulla oblongata, still hollow ; 3, bend at which the pons Varolii is formed ; 4, cerebellum ; 5, lamina (supierior cerebellar peduncles) passing up to the corpora quadrigemina ; 6, crura cerebri ; 7, corpora quadrigemina, still hollow ; 8, third ventricle ; 9, infundibu lum ; 10, thalamus, now solid ; 11, optic nerve ; 12, aperture leading into the 1 ventricle ; 13, commencing corpus callosum.
F
Literal
2. The tlialamencephalon with its cavity or third ventricle, the primary ocular pedicles, and the infundibulum.
3. The mcsenceplialon, which is the same with the original middle vesicle, and comprises the corpora quadrigemina and crura cerebri with its contracted internal hollow, the iter a tertio ad quartuni ventriculum of human anatomy.
4. The next part in succession is the cerehellimi, along with which is included the pons Varolii and the fourth ventricle.
5. The hinder part, which passes into the spinal marrow, is the medulla oblongata, with the continuation of the medullary cavity in the fourth ventricle and into the central spinal canal.
In these five fundamental parts or rudiments of the brain, arising out
VOL. II. 3 c
754
DEYELOrMENT OF THE NEHVOUS SYSTEM.
of very simple modifications of the primitive medullary tube, it is mainly by an increased thickening of the medullary wall in some of the parts, and the relative thinning, or even the removal of the substance in others, that the changes accompanying the formation of the cerebral masses are effected, while as a consequence of these and other modifications of form, the several parts of the internal cavity or ventricles of the brain acquire the different degrees of expansion and contraction, or the comparatively closed or open condition which they exhibit in after life. Thus the cerebral hemispheres, and corpora striata are the main masses formed by the lateral thickening and expansion of the medullary walls of the procerebrum, while the corpus callosum and fornix are formed later by a deeper median development in connection with these parts : the thalami optici are the most solid parts of the lower and lateral region of the second rudiment, while in the upper wall the pineal gland, and in the lower the infundibulum with the hypophysis cerebri, are added : the corpora quadrigemina are thickenings of the upper wall of the third rudiment, while the crura cerebri arise by increased deposit in its lower part ; the cerebellum is a large deposit in the upper wall of the fourth radiment, while the pons Varolii is a thickening of its lower wall ; and the parts composing the medulla oblongata are principally formed by increased deposit in the lower and lateral wall of the fifth rudiment.
Thus, also, the lateral ventricles are two lateral expansions of the forepart of the original ventricular cavity which follow the dilatation of the vesicles of the right and left cerebral hemispheres, and communicate with the central or third ventricle by the common foramen of Monro. The central or third ventricle, originally the foremost part of the medullary hollow, is narrowed on the sides by the increased development of the thalami optici, while inferiorly it is prolonged and projects downwards as infundibulum into the pituitary fossa ; and above the wall of this ventricle comes to be opened up by the thinning aAvay of its medullary substance, excepting at the place where it is crossed by the pineal gland. The continuation backwards of the original ventricular hollow, greatly narrowed by the ultimate thickening of the substance of the corpora quadrigemina and crura cerebri, forms the aque
Fis. 555.
Fig. 555. — Vertical Section of thr Brain of a
Human Embryo of fourteen weeks, Magnified
THREE Diameters (from Reicliert).
c, cerebral bemispliere ; cc, carpus callosum bebeginning to pass back , /, foramen of Monro ; p, membrane over tbe third ventricle and the pineal gland ; tk, thalamus opticus ; 3, third ventricle ; I, olfactory bulb ; cq, corpora quadrigemina, mesencephalon : cr, crura cerebri, and above them the aqueduct of Sylvius still wide ; c', cerebellum, and below it the fourth ventricle ; _/)c, Pons Varolii ; m, medulla oblongata.
dnct of Sylvius, or the iter a tertio ad quartura ventriculmn, and is succeeded by the more expanded cavity of the fourth ventricle, lying between the cerebellum and the lower wall. The upper wall of the latter cavity undergoes great thinning, like that of the third ventricle, so as to be reduced m the part before the cerebellum to the valve of Vieussens,
FUNDAMENTAL PARTS OF THE BRAIN. 755
and in the part behind it to be covered only by membrane, and to present an opening from the cavity into the posterior sub-arachnoid space.
From what has before been said of the relation of the fundamental parts of the brain to the basis of the skull, it will be seen that the cerebral development is intimately connected also with the great cranial flexure which occurs at the pituitary fossa ; for while the infundibular prolongation of the thalaraencephalon projects down into this fossa, and the lamina terminalis rises in front in the position of the original foremost part of the encephalon, certain parts of the brain may be considered as situated posterior to this point, viz., the mesencephalon with crura cerebri, cerebellum with pons Varolii, and medulla oblongata, while the cerebral hemispheres, with the corpora striata, corpus callosum, and fornix, notwithstanding their enormous proportional development, may be considered as formed by forward expansion, and as situated in front of this turning point. But though the connections of the cerebral hemispheres with the rest of the brain may thus be considered as anterior to the cranial centre, and while in their earlier stages, and still of small size, they are actually placed as in the lowest Vertebrates, entirely in front of it, yet by the later great proportional development in the higher animals, and especially in man, the cerebral hemispheres come to progress backwards, and successively to cover superiorly the thalami, corpora quadrigemina, the cerebellum, and the medulla oblongata.
The connection of the several parts of the brain, with the five fundamental parts respectively, may be stated in the following tabular form :
I" Cerebral Hemispheres, Corpora Striata,
{1. Prosencephalon.* s Corpus Callosum, Fornix, Lateral Ven(. tricles, Olfactory bulb (Rhiuencephalou). 2. Thalamencephalon. f Thalami Optici,Pinealglaud,Pituitai-y body, (Diencephalon.) ( Third Ventricle, Optic nerve (iirimarily).
ir Middle urimarv Vesicle •? Mesenceiilnlon J Corpora Quadrigemina, Crura Cerebri, Aque11. -MKime primal y vesicle, o. Jiebencepiiaion. j duct of Sylvius, Optic nerve (secondarily).
III. Posterior primary Vesicle,
< Cerebellum, Pons Varolii, anterior part of f- 4. Eiicncephalon. \ the Fom-th Ventricle.
"j ( Medulla Oblongata, Fourth Ventricle, Au 1.5. Meteneephalon. ( ditory nerve.
The changes which take place in the growth of the brain were first elaborately described by Tiedemann ; they have been investigated by Von Baer, Bischoff, Remak, Reichert, KoUiker, and others. (Tiedemann. " Anatomic und Bildiingsgeschichte des Gehu-ns," Nilremberg, 1816 ; Reichert, Bau des Menschlichen Gehirns," Leipzig, 1859, 1861 ; F. Schmidt, '• Beitragez. Entwick. des Gehims," in " Zeitschr. f. Wissen. Zool.," 1862 ; KoUiker, " Entwicklmig,sgeschichte," 1861,)
FARTHER DEVELOPMENT OF THE BRAIN IN MAN AND MAMMALS.
The medulla oblongata is not completely closed in above bj^ nervous matter. The open part of the medullary tube constitutes the floor of the fourth rent ride, which communicate.^ below with the canal of the spinal cord at the place where the calamus scriptorius is eventually formed, and opens posteriorly into the subarachnoid space.
- TliLs and the four following terms are adopted as applicable to the principal secondary
divisions of the primordial medullary tube, and a.s corresponding to the commonly received names of the German embryologists, viz., Vorderhirn, Zwischenliirn, Mittelhirn, Hinterhirn, and Nachhirn ; or their less used English translations, viz., forebrain, interbraiu ('tweenbrain), midbrain, hiudbrain, and afterbrain.
3 c 2
756
DEVELOPMENT OF THE NERVOUS SYSTEM.
The three constituent parts of the medulla oblongata beg-in to ]je distrnguisheJ about the thii-d month ; first the rr.stiform bodies, which are connected with the commencing cerebellum, and afterwards the anterior pjTamids and olives. The anterior puminidx become prominent on the surface and distinctly defined in the fifth month ; and by this time also their decussation is evident. The oUranj fasciculi are early distinguishable, but the proper olirary hochj. or tubercle, doe's not appear till about the sixth month. The fam'wlai c'merea; of the fourth ventricle can be seen at the fomth or fifth month, bub the nvhltc strue not until after birth. •
Cerebellum. — In the human embiyc the cerebellum exists at the end of the second month, as a delicate medullary lamina, forming an arch behind the corpora quadrigemina across the widely open primitive medullary tube.
According to Bischofi", the cerebellum does not commence, as was previously supposed, by two lateral plates which grow up and meet each other in the middle line ; but a continuous deposit of nervous substance takes place across this part
Fig. 556. Fig. 556. — Brain and Spinal Cord exposed feom behind i:v
A FcETus OF THREE MONTHS (from Kolliker).
h, the hemispbei-es ; in, the mesencephalic vesicle or corpora qnadrigemina, c, the cerebellum ; below this are the medulhi oblongata, mo, and fourth ventricle, with remains of the menibrana obturatoria. The spinal cord, s, extends to the lower end of the sacral canal and presents the brachial and crural enlargements.
of the medullary tube, and closes it in at once. This layer of nervous matter, which is soon connected with the coi-pora restiformia, or inferior peduncles, increases gradually up to the fourth month, at which time there may be seen on its under sui'face the commencing corpus th-ntatvm. In the fifth month a division into five lobes has taken place ; at the sixth, these lobes send out folia, which are at first simple, but aftei-wards become subdivided. Moreover, the hem i. '<])?! eres of the cerebellum are now relatively larger than its median portion, or worm. In the seventh month the organ is more complete, and thejfocoihis and 2Mt<terior rein in, with the other parts of the mferior vennifoiin process, are now distinguishable, except the amyrjdalo', which are later in theii' appearance. Of th.ej}eil-incles of the cerebellum, the inferior pair (corpora restiformia) are the first seen — viz., about the thii-d month ; the middle peduncles are perceptible in the foui-th month ; and at the fifth, the superior pedmicles and the Vieussenian valve. The pons VfiroHi is formed, as it were, by the fibres from the hemispheres of the cerebellvun embracing the pyramidal and olivary fasciculi of the meduUa oblongata. According to Baer, the bend which takes place at this part of the encephalon thrusts down a mass of nervous substance before any fibres can be seen ; and m this substance transverse fibres, continuous with those of the cerebellum, are afterwards developed. From its relation to the cerebellar hemispheres the pons keeps pace with them in its gi'owth ; and. in conformity with this relation, its transverse fibres are few, or entirely wanting, in those animals in which there is a con-esponding deficiency or absence of the lateral parts of the cerebellum.
Parts connected with, the Mesencephalon. — The eorjioni qvndricfemina are formed in the upper part of the middle cephalic vesicle ; the hollow in the interior of A^diich communicates with those of the first and thhd vesicles. Tlie coi-pora qnadrigemina, in the early condition of the human embryo, are of great proportionate volume, in harmony with what is seen in the lower vertebrata ; but subsequently they do not grow so fast as the anterior parts of the encephalon, and are therefore soon overlaid by the cerebral hemispheres, which
FARTHER DEVELOPMENT OF THE EXCEPHALON.
757
at the sixth month cover them in completely. Moreover, they become gradually solid by the deposition of matter within them : and as, in the meantime, the ccrchval jwdunclcs are increasing rapidly in size in the floor of this
Fig. 557. — Brain of the Human Em- Fig. 557.
BRYO OF THREE MONTHS. NATURAL /, ^
SIZE (from KoUiker).
In 1 the view is from above, the Upper fiart of the cerebral hemispheres and mesencephalon having been removed. /, fore-part of the divided wall of the hemisphere ; /', hind part of the same turned in which becomes the hippocampus ; est, corpus striatum ; tha, thalamus opticus.
In 2 the lower surface is represented \ to, ti'actus opticus ; and in front of
this the olfactory bulbs and tracts ; c m, single mass of the corpora mammillaria not yet divided ; p, pons Varolii. The cerebellum and medulla oblongata are seen behind and to the sides in both iigures.
(A a.
middle cephalic vesicle, the cavity in its interior is quickly filled up, with the exception of the narrow passage named the SijJrian aqueduct. The fillet is dis
Fig.. 558. — Brain and Spinal Cord op a F(etus of four Fig. .558
MONTHS, seen from BEHIND (from Kolliker).
h, hemispheres of the cerebrum ; m, corpora quadrigemina or mesencephalon ; f, cerebellum ; m o, medulla oblongata, the fourth ventricle being overlapped by the cerebellum ; s s, the spinal cord with its brachial and crural enlargements.
tmguishalble in the fourth month. The corpora quadrigemma of the two sides are not marked off from each other by a vertical median groove until about the sixth month ; and the transverse depression separating the anterior and posterior pau-s is first seen about the seventh month of intra-uterine life.
Thalamencephalon. — From the sides of this vesicle, as has already been described, the optic vesicles are fonned. and from its forepart on the two sides the vesicles of the cerebral hemispheres are developed. Reichert first pointed out that there is left between the hemu2)?ier(Tcslclcs of opposite sides a wedge-shaped interval, which forms the thu-d ventricle. He points out that the tenninal extremity (lamina terminalis) of the cerebrospinal tube is at the tip of this wedge, and is placed immediately in front of the optic commissure, at the lamina cinerea ; and that therefore the infundibulum is not that extremity, as had been previously supposed by Baer, but is an expansion of the vesicle downwards. The formation of the pituitary body has akeady been described. The infundibulum of the thalamencephalon
becomes connected with it superiorly, and seems for a time even to form a part of it.
The pineal gland, according to Baer, is developed from the back part of the thalami, where those bodies continue joined together ; but it is suggested by Bischoff that its development may be rather connected with the pia mater. It was not seen by Tiedemann until the fourth month in the human foetus ; subsequently its growth is very slow : and it at first contains no gritty deposit : this, however, was found by Sommeiiing at birth.
758
DEVELOrMEXT OF THE NERVOUS SYSTEM.
The two optic thalamic fonned from the posterior and outer part of the anterior vesicle, consist at first of a single hollow sac of nervous matter, the cavity of ■which communicates on each side in front with the anterior of the commencing
Fig. 559.
Fig. 559. — Remimagrammatic Views of the Inner Surface op the Right Cerebraii
Hemisphere of the Fcetal Brain at various stages of development (from
Schmidt).
1, 2, and -3, are from foetuses of the respective ages of eight, ten, and sixteen M'eeks ; 4, from a foetus of six months, a, lamina terminalis or imrt of the first primary vesicle which adheres to the sella turcica ; b, section of the cerebral i)eduncle as it passes into the thalamus and corpus striatum ; the arched line which surrounds this bounds the gi-eat cerebral fissure ; c, anterior part of the fornix and the sej)tum lucidum ; d, inner part of the arch of the cerebrum, afterwards the hippocampus major and posterior part of the fornix ; (, corpus callosum very short in 3, elongated backwards in 4 ; in 4, /, the superior marginal convolution ; /', fronto-parietal fissure ; g, gyrus fornicatus ; p', the internal vertical fissure descending to meet the fissure of the hippocampus ; I, olfactory bulb ; F, P, 0, T, frontal, parietal, occipital and temporal lobes.
cerebral hemispheres, and behind with that of the middle cephalic vesicle (corpora quadrigemina). Soon, however, by increased deposit taking jilace in
Fig. 560. B
560. — The Surface of the Fcetal Brain at Six Months (from R. Wagner).
This figure is intended to show the commencement of the formation of the principal fissures and convolutions. A, from above ; B, from the left side. F, frontal lobe ; P, parietal ; O, occipital ; T, temporal ; a, q, a, slight appearance of the several frontal convolutions ; s, the Sylvian fissure ; s', its anterior division ; within it, C, the central lobe or convolutions of the island ; r, fissure of Rolando ; p, the vertical fissure (external part).
FARTHER DEVELOPMENT OF THE EXCEPHALON.
iod
their interior behind, below, nnd at the sides, the thalami become solid, and at the same time a cleft or fis&ure appears between them above, and jjenetrates down to the internal cavity, which continues open at the back part opposite the entrance of the Sylvian aqueduct. This cleft or fissure is the tJiird rentriclc. Behind, the two thalami continue united by the jyoxfrrinr C(i/ii»iix.\ure, -which, is distinguishable about the end of the third month, and also by the ped7i?icle.i of the 2>i»ii glnnrl. The Koft commissure probably exists from an early period, although it could not be detected by Tiedemann until the ninth month.
At an early period the ojriie tracts may be recognised as hollow i^rolongations from the outer part of the wall of the thalami while they are still vesicular. At the fourth month these tracts are distinctly fonned. They subsequently are prolonged backwards into connection with the coi-pora quadiigemina.
Prosencephalon. — Each /iemisj>he/T-rcsicIc becomes divisible into two parts : one of these is the part which from the interior appears as the corpus striatum, and from the exterior as the island of Eeil, or central lobe ; the other fonns the expanded or covering portion of the hemisphere, and is designated by Reichert the mantle. The aperture existing at the constricted neck of the licmispherevesicle, Schmidt and Reichert have recognised as the foramen of Monro.
The corpora striata, it will be observed, have a different origin from the optic thalami ; for, while the latter are formed by thickening of the cu-cumferential wall of a part of the first cerebral vesicle, and thus correspond in their origin with all the parts of the encephalon behind them, which are likewise derived from portions of the cerebro-spinal tube, the corpora striata appear as thickenings of the floor of the hemisphere-vesicles, which are lateral ofl:'-shoots from the original cerebro-spinal tube. On this account, Reichert considers the brain primarily divisible into the stem, which comprises the whole encephalon forwards to the taenia semicircularis, and the hemisphere-vesicles, which include the corpora striata and hemispheres.
Fig. 561. — View of the Inner Fig. 56!
Surface of the Right Half
OF THE FffiTAL BrAIN OF ABOUT
SIX MONTHS (from Reichert).
F, frontal lohe ; P, parietal ; 0, occipital ; T, temporal ; I, olfactory bulb ; II, right optic nerve ; f p, fronto-parietal fissure ; p, vertical fissure ; ;/, internal vertical fissui'e ; /(, hippocampal fissure ; ff, gyrus formicatus ; c, c, corpxis callosum ; s, septum lucidum ; /, placed between tlie middle commissure and the foramen of Monro ; V, in the upper part of the third ventricle immediately below the velum interpositum ami fornix ; v', in the back part of
the third ventricle below the pineal gland, and pointing l)y a line to the aqueduct of Sylvius ; v" , in the lower part of the third ventricle above tlie infundibuhim ; )•, recessus pinealis passing backwards from tlie tela choroidea ; p r,pons Varolii ; C c, cerebellum.
The cerebral hemispheres enlarging, and having their walls increased in
thickness, fonn, dming the fourth month (Tiedemann), two smooth shell-like
lamellfe. which include the cavities afterwards named the lateral ventricles, and
the i^arts contained within them. FoUowdng out the subsequent changes affecting the exterior of the cerebral hemispheres, it is found that about the foiuth.
month the first traces of some of the conrolvtions appear, the intermediate svlci
commencing- only as very slight depressions on the hitherto smooth sm-face.
Though the hemispheres continue to grow quickly upwards and backwards, the
convolutions at first become distinct by comparatively slow degrees ; but towards
760 DEVELOPMENT OF THE NEUVOCJS SYSTEM.
the seventh and eighth months they are developed with gi-eat rapidity, and at the beginning of the last month of intra-uterine life, all the principal ones are marked out.
The Sf/!r!fi)i _p\\:finv, which afterwards separates the anterior from the middle lobe of each hemisphere, begins as a depression or cleft between them about the fourth month, and, after the great longitudmal, is the first of the fissures to make its ajipearance. It is followed by the fissure of Rolando, and the vertical or parieto-occipital fissure, and somewhat later by the internal fronto-parietal fissui-e. After this, the various subordinate fissures dividing the convolutions gradually make their appearance. By the end of the third month the hemispheres have extended so far backwards as to cover the thalami ; at the fourth they reach the corpora quadrigemina ; at the sixth they cover those bodies and great part of the cerebellum, beyond which they project still fiuther backwards by the end of the seventh month.
During the growth of the hemisphere the aperture of the foramen of Monro is extended backwards ; the arched margin of this opening is curved downwards at its extremities, and forms anteriorly the fornix, and posteriorly the corpus fimbriatum and hippocampus major ; above the margin a part of the wall of each hemisphere comes into contact with its fellow, and in the lower part forais the septum lucidum, while above this the hemispheres are united by the development of the great commissure, the coipus callosum.
The corpus callo.sum is described by Tiedemann as being first seen about the end of the third month, as a narrow vertical band, extending across between the forepart of the two hemispheres, and subsequently growing backwards. With this view the observations of Schmidt coincide. Reichert, however, maintains that the commissural structure seen at the forepart of the hemispheres is the anterior white commissure, and that the coiiaus callosum appears in its whole extent at once.
The corpora alhtcantui at fixst form a single mass : so also do the anterior pillars of the fornix, which are distinguished before the posterior pillars. The posterior pillars are not seen until the fourth or fifth month. At that period the hippocampus minor is also discernible.
DEVELOPMENT OF THE NERVES.
Spinal Nerves. — Very little is yet known as to the mode of origin of the nerves. In their peripheral extension the great majority of them seem to arise more immediately from mesoblastic formative elements, and the manner in which this takes place has been adverted to in the General Anatomy at p. 161. The ganglia and roots of the spinal nerves are first seen to make their appearance in some very close association with the protovertebral segments. In this the ganglion comes to be distinguishable as a mass by itself, and the anterior and posterior roots"follow, ■with their jimctiou in the part forming the nerve-trunk beyond the ganglion. But according to recent observations by Balfour in ElasmoBranch fishes (Scyllium and Torpedo), it would appear that both the anterior and posterior roots may arise iu these animals in a closer connection with the nervous centre than was previously believed, and as independent outgrowths from the involuted epiblast of tlie neural canal. The posterior roots are the first to appear, and commence by an outgrowth at the summit (dorsal median groove) of the neural canal, and gradually pass outwards from thence to reach their permanent place of origin in a posterior lateral furrow. A subsequent division of the nerve rudiment takes place into root, ganglion, and a part of the nerve beyond.
The anterior roots spring by an outgrowth from tlie antero-lateral angles of the cord, one for each muscular plate nearly in the place
DEVELOPMENT OF THE NERVES. 761
•which they permanently occupy, and after they have attained some size they iniite with the posterior roots beyond the ganglion.
But although the roots of the nerves may thus be traced in their commencement to elements of epiblastic nature, it is probable that their sheaths and blood-vessels arise from mesoblastic tissue introduced later into them. The ganglia are at first of proportionally very large size, causing even a considerable part of the projections on the surface of the body, as in the human embryo of from four to seven weeks, ■which are usually attributed to the vertebrate segmentation (Kolliker). The union of the roots with the grey matter soon becomes apparent, being most obvious in the anterior roots.
The nerves, like the other elementary parts, are at first composed entirely of cells, but fibres are soon formed by transformation of the cells. On the sixth day in the chick, Foster and Balfour found the fibres developed, but were unable to trace them into connection with the ganglionic cells, but at a later period the connection was observed by Lockhart Clarke to be formed.
The very early development of the trunks of the nerves of the limbs, and their progress outwards into the first part of the commencing limb, were observed and figured by Remak.
Cranial Nerves. — The optic nerve and the retina, arise from epiblast by an extension of the primary medullary wall of the brain, as already described, and may therefore, in some sort, be regarded as an extension of the brain itself. The olfactory tract and bulb are still more to be looked upon as constituent parts of the cerebrum ; but the manner in which the peripheral olfactory nerves which pass through the cribriform plate into the nose are formed has not been yet observed.
Among the other cranial nerves there are four important ones of which the rudiments are seen to be formed at an early period ; taking their origin as has been supposed in the mesoblastic wall of the cranial cavity, and extending thence into the facial or visceral plates. These are the fifth pair or trifacial, the facial or portio dura of the seventh pair, the glossopharyngeal and the pneumogastric nerves. The two first of these are situated in front, and the two last behind the otic vesicle and tympanoeustachian passage ; and according to Parker each of these nerves forks or divides into two above one of the visceral clefts. Thus the fifth pair gives its naso-maxillary division in fi-ont, and its infero-maxillary division behind the oral cleft ; the facial gives its vidian or superior petrosal before and its descending part behind the tympano-eustachian passage ; the glosso-pharyngeal has its lingual and pharyngeal branches divided by the first branchial cleft, and in animals the pneumo-gastric is similarly divided at the second and succeeding clefts.
The third, fourth and sixth pairs of nerves are of subordinate importance, and may be considered as related, the two first to the fifth pair, and the last to the facial nerve. Their peripheral parts are developed in connection with the muscles of the eyeball, but the mode of the formation of their roots in connection with the nervous centres has not been ascertained.
The hypoglossal nerve, although it passes through the exoccipital bone in man, may be compared to a spinal nerve, and probably takes its origin much in the same manner.
The Sympathetic Nerves. — Remak observed the development of the great sympathetic nerves from the lateral plates in loops or arches
762 DEYELOPMEXT OF THE ORGAXS OF SEXSE.
connecting them with the spinal nerres ; wlnle the great sympathetic itself consisted at first of a chain of rounded masses representing; the ganglia, connected together, but so closely set that scarcely any intervening nervous cord was at first perceptible. He also observed the separate formation in the mesentery of birds of the large visceral nerve which he discovered in these animals. The gangliated cord of the sympathetic has been described and figured by Kiilliker in the human foetus of eight or ten lines long. The peripheral sympathetic nerves are also formed at a very early period, and are perceptible in a foetus of three months. In the hinder part of the abdomen their origin appears to be intimately connected in some way with the formation of the suprarenal bodies.
DEVELOPMENT OP THE EYE.
The embryonic structures forming the eyeball and its contents may be considered as proceeding from three sources, viz., 1st, by evolution or expansion from the medullary wall of the first encephalic vesicle (thalamencephalon), giving rise to the retina, in its nervous and pigmental structure and optic nerve ; 2nd, by involution or depression and development of a part of the cuticular epiblast, forming the foundation of the lens and the epithelium of the conjunctiva ; and 3rd, by the intrusion of mesoblastic elements between and around the other parts, so as to furnish the materials out of which are formed the general coverings of the eyeball, cornea and sclerotic, the fibrous and vascular choroid, the ciliary apparatus and iris, the capsule of the lens and the capsulo-pupillary membrane, the vitreous humour, and all the fibrous and vascular parts of the organ.
The very early formation of the primary optic vesicles by the expansion of the lower and anterior parts of the wall of the anterior primary encephalic vesicle has already been described, and the manner in which each of these vesicles forms a hollow pediculated chamber communicating by its stalk with the general ventricular cavity of the primitive brain. The first important change which the primary optic vesicles undergo is connected with the depression of the rudimentary lens from
Fir,'. 562. Fig-. 562. — Section of the Head through the Primitive
Optic Capsule uf one side in an Embryo-Calf of 9 mji. LONO, MAGNIFIED (from Julius Arnold).
To the right is seen the optic capsule with its contracted pedicle and its outer wall depressed by the thickening of the corneous laj'er which forms the commencement of the formation of the lens. The optic stalk is in communication with the thalamencephalon. Mesoblast is seen between the optic capsule and the lens rudiment.
without, and consists in a doubling back or inwards of the medullary wall of each vesicle, so as
to form a depression or cup at its lower and
anterior part, into which the commencing lens
in part sinks. This depression has been called the
secondary optic vesicle, or the opfic cup (Foster and
Balfour). From a very early period the outer fold of this cup undergoes a much greater thickening by the rapid development of its
PEIMAEY DEVELOPMENT OF THE EYE.
■63
cellular constituents than the inner or that towards the brain, and from this, as well as the increase of the inward folding, the original cavity of the primary optic yesicle becomes in a great measure obliterated or narrowed, and the outer and inner folds are closely approximated, while the stalk or pedicle becomes i^roportionally much diminished. The continued increase of cellular development in the outer fold of this cup leads to the formation of the various elements composing the retina ; while in the thinner inner fold only pigment cells are formed. The transition at the bend from the thick nervous part tc
Fi- 5G3
g^.5^18 ^
Fig. 563. — Section thkottgh the Eye op an Embryo-Calf OF TWELVE 3IM., OR HALF AN INCH,
LONG, MAGNIFIED (froui Julius Amold).
The lens follicle Ls now closed in and detached fi'ora the corneous laj'er, and its cavity contains loose cells which are the remains of the superficial corneous cells. The oj^tic vesicle or cajisule is now completely doubled back, so as to present towards the lens side the secondary ocular capsule or cup ; its outer wall now much thickened by the commencement of the development of retina. Mesoblastic tissue is seen to have passed in from the periphery between the optic capsule and the lens, as well as in front of the lens. The commencement of a vascular circle is shown round the exterior of the ocular capsule, and extending also between the lens follicle and the cuticle.
the thin pigmental part is quite sudden, and as soon as pigment cells begin to be developed, a very marked distinction is perceptible between it and the nervous structure of the retina. These cells were formerly regarded as a part of the choroid membrane, but they are now looked upon as belonging rather to the retina, — a view which is supported by the mode of development now described.
Fig. 56i. — Diagrammatic Sketch op a Vertical Longitudinal Section
THROUGH THE EyEBALL OP A HuMAK
FcETus OF FOUR WEEKS (after KoUiker). ^^
The section is a little to the side so as to avoid passing through the ocular cleft, c, the cuticle, where it becomes later the cornea ; I, the lens ; o ij, optic nerve formed by the pedicle of the primary oj)tic vesicle ; v 2'>, primary medullary cavity of the optic vesicle ; p, the pigment-layer of the outer wall ; r, the inner wall form- . ing the retina ; v s, secondary optic vesicle containing the rudiment of the vitreous humour.
The fold which produces the optic cup proceeds from above downwards, and Avhen the lens is formed it seems as if it enclosed the lens
but left for a time an aperture or depression below. This is the
choroidal fold or fissure, which may easily be distinguished in the
emliryo-head after pigment has been deposited, fi'om the circumstance
that the pigment is absent from the cleft, which thus appears for a
764
DEVELOPMENT OF THE ORGANS OF SENSE.
time as a broad white line, particularly obvious in the embryo bird, running from the circumference in upon the lens.
The lens is developed in the part of the cuticle opposite to the -most projecting part of the primary optic vesicle, or at the place where this vesicle comes in contact with the surftice of the head. In this situation there is seen from a very early period a thickening of the epiblast, which seems to reside chiefly in its deeper layer of cells, and in birds and mammals it would appear that an actual involution of the cuticle takes place, so that first an open follicle and next an enclosed ball of cuticle is formed. Although, however, both the corneous and the deeper layer (sensory of Strieker) of the cuticle are enclosed, it is only the cells of the deeper layer which undergo development into the fibres of the lens. The ball of the lens separating from the external cuticle, which passes freely over the surface, a cavity filled with loose cells, the remains of cells of the corneous layer, exists for a time within the lens ball. Then the cells of the hinder or inner wall are seen to rise from the bottom by their elongation, and thus a rapid growth of fibres from that side of the ball takes place, while the anterior or outer wall undergoes no similar change, but retains its simply cellular structure. Figures 565, and 566, show sufficiently clearly the manner in which the fibres thus developed from cells rise fi'om the bottom of the lens ball and come to constitute the solid part of the lens.
The optic cup receives the enlarging lens in its anterior and lower opening, and the reflected margins of the cup closely embrace the margin of the lens ; but there is a considerable space intervening between the lens and the hollow of the optic cup (or secondary ocular vesicle), which comes to be occupied by the vitreous humour. Into this space connective tissue and blood-vessels developed from mesoblastic
Fig. 565. — Sectiojt of the Eye in an Embryo-C^lf
OF 18 MM. LONG, MAGNIFIED (from JuUus Amold).
The posterior or inner v.-all of the lens follicle is now much thickened by the elongation of its fibres, each of which presents a nucleus, and the whole causing a bulging of the posterior wall. The outer la^'er of the lens capsule consists of columnar cells. The cavity of the lens follicle is still visiljle, but is now widened and flattened. Two layers of mesoblastic tissue are now visible between the lens and the cuticle, viz., a deeper vascular layer and a superficial non-vascular one containing nuclei. The secondary optic capsule is now occupied bj' connective tissue nuclei and numerous blood-vessels. The retinal section of the primitive ocular vesicle is now thicker. Pigment begins to ajapear in the choroidal section, and numerous bloodvessels surrovmd the whole exterior.
elements are projected from below, so as ..'^,|y| to furnish the materials for the formation
^ '"^r of the vitreous humour and the blood-vessels
-W which pass through it to the lens, and also
to surround the lens with vascular and fibrous elements, out of which are produced the capsulo-pupillary membrane, and probably also the capsule of the lens. It results from tlie
FORMATION OF THE LEXS.
765
observations of Lieberkiihn that in mammals the fold -n'hich produces the ocular cup or secondary vesicle runs back into the stalk so as to fold in the optic nerve for a considerable space, and by the simultaneous intrusion of mesoblastic tissue, thus to lead to the introduction of the central blood-vessels of the retina within the nerve. But in birds,, according to the same observer, no such infolding of the stalk occurs, so that in them the vessels are excluded from thenerve. The malformation termed coloboma iridis is to be attributed to a persistence of the choroidal cleft or fold, and the pecten of birds, close to the optic nerve, with the vascular fold farther forwards, and the falciform fold of the eyes of fishes are to be regarded as fibro-vascular structures formed by original projection through the same fold.
The further development of the parts of the eye may be briefly stated as follows : —
The expansion of the ocular cup continuing to proceed, the chamber for the vitreous humour enlarges, and that structure gradually comes to occupy its space between the retina and the lens.
The marked distinction between the nervous and the pigmental portions of the primitive ocular vesicle goes on increasing by the continued deposit of pigment in the latter, and its proportional thinning, and by the great addition to the thickness and the textural differentiation of the substance of the former. Thus the cells in the retinal or nervous portion, by their rapid multiplication, soon l^ecome several layers thick ; certain parts of these cells assume the spindle shape, and exhibit elongation into fibres, while others retain the nuclear form, and thus there is foreshadowed the division into the fibrous, ganglionic, and nuclear layers of the retina. On the exterior a limiting membrane
Fig. 5C>6.
>>>.
\
\
Fig. 566. — Section of the Ei'e of
AN EjIERYO-CaLP of 30 MM. LONG,
MAGNIFIED (froiu Julius Arnold).
Tlie cavity of the lens is much reduced in size from the increased development of fibres from behind. The intersection space begins to be formed posteriorly, and the zone of nuclei is thrown forward. The cornea is now formed, covered externally by the cuticular epithelium, and with the separation of the aqueous chamber internally. Close to the lens is the caiDsulo-pupillary membrane, which is continuous with the vascular structure occupying the secondary ocular capsule or cavity for the vitreous humour, and with the choroid membrane round the mai-gin of the ocular capsule, where
iris and ciliary processes will afterwards . ■-^"
be formed. The layer of pigment is ■
now more developed, and the tissue of the sclerotic is begun to be formed. ' •
The eyelids are beginning to project as '
folds of the skin.
makes its appearance, and in connexion with it the rudiments of the cells composing the layer of rods and cones. The space between the
766
DEVELOPMENT OF THE ORGANS OF SENSE.
retinal and pigmental layers rapidly contracts, and finally the rods and cones are closely united ^Yith the layer of pigment cells.
The optic nerve, as already described, is at first connected by its origin with the vesicle of the third ventricle or thalamencephalon, and for a time it retains its original hollow form. But as the cerebral hemispheres are developed forwards, the eye and the optic nerve are thrown backwards and downwards, and a new connection is estal^lished between the optic nerve (or tract) and the vesicle of the midbrain (mesencephalon) : the rudiment of the optic commissure is at the same time formed by the median approximation of the stalks and the growth of one over the other. Each stalk then becomes more and more solid by the development of nerve fibres along with the intruded connective tissue which forms the sheath substance of the nerve.
Lens. — The development of fibres from the hinder wall of the primitive lens-follicle continuing to take place, the cavity of the follicle is first greatly narrowed and then completely filled up by the lengthening fibres, and the lens takes more and more of its fall spherical shape. The new fibres continue to be formed towards the margin of the lens ; each fibre retaining its nucleus, so as to produce the "nuclear zone which runs through the whole lens. This zone is at first situated far back in the lens while the fibres are still short, but as they elongate, its place is advanced, so that it comes to be situated considerably in front of the equatorial plane of the lens. It is most distinct towards the margin where the fibres are newly formed. The anterior wall of the lens-follicle remains as a simple cellular layer. The greater number of the fibres now follow the general curve of the surface of the lens, presenting therefore their concavity towards its centre, but the curvature gradually diminishing in those nearest the middle, where they are straight or nearly so. Only the external short and recently formed fibres present a concavity towards tlie exterior. The intersecting stars of the anterior and posterior poles of the lens now make their appearance by the collection of cells in the peculiarly shaped triradiate space in these two situations, and the ends of the fibres are now traceable to the edges of these spaces, so that the fibres gradually take the arrangement round the poles of the lens which belongs to the adult.
Fig. 567.— Transverse Vertical Section of the Eyeball op a Human Embryo of four weeks (from Kolliker). '""
The anterior half of the section is represented. pr, the remains of the cavity of tlie primary optic vesicle ; ii, the inflected part of the outer layer, forming the retinal pigment ; r, the thickened inner part giving rise to the columnar and other structures of the retina ; r, the commencing vitreous humour within the secondary optic vesicle ; v', the ocular cleft through which the loop of the central blood-vessel, a, projects from below ; 1, the lens with a central cavity.
The capsule of the lens, according to Lieberkiihn's and Julius Arnold's most recent observations, owes its origin to the thin innermost pellicle of mesoblast which is introduced atan early period of development between the lens and the secondary ocular vesicle.
CORNEA AXD YITEEOUS HOIOUR.
707
Cornea. — There is at first no aqueous chamber in the eye, and even after the solution of continuity which gives rise to this space lias occurred, the cavity is not dilated with fluid, till near the time of ])irth. Even then it is very shallow and the lens is placed very near to the cornea. The formation of the cornea is due to a differentiation of the tissue in the layer of mesoblast wliich is introduced from the neighbouring wall of the head, between the primitive lens-follicle and the corneous epiblast, the cavity of the aqueous humour arising by the separation of the corneous part from a layer of the mesoblastic tissue lying within it. The latter gives rise to the anterior part of the vascular capsulo-pupillary membrane, while a still deeper lamina closely embracing the lens, remaining non-vascular, is converted into the lens capsule. Along with the latter is also formed the suspensory ligament of the lens.
Vitreous humour. — The enlargement of the space for the vitreous humour progTessing, the cells of the mesoblast which form its foundation become stellated and very sparse from the effusion of a large quantity of fluid, and the hyaloid membrane surrounding this structure takes its origin from the same mesoblastic elements.
Choroid and other membranes. — The mesoblastic substance which surrounds the ocular vesicle externally is the source of a number of
Fiir. 568.
Fig. 568. — Blood-vessels of the Cap SULO-PUPILLARY MEMBRANE OF A
New-born Kitten, magnified (from Kolliker).
The drawing is taken from a preparation injected by Tiersch, and shows in the central part the convergence of the net-work of vessels in tiie pupillary membrane.
important parts. Among these may be mentioned first the choroid membrane, the cellular (membrana fusca), fibrous, and vascular layers of which are developed out of the deeper division of the mesoblastic substance, and to the same source may be traced in a later stage of
development the ciliary processes, ciliary muscle and iris ; while the zonula ciliaris may be regarded as a part of the deeper mesoblastic tissue connected with the formation of the hyaloid membrane and membrana capsulo-pupillaris. The folds of the ciliary processes gradually increasing, encroach upon the space outside the margin of the lens and indent the zonula ciliaris and canal of Petit.
The sclerotic coat is due to a process of differentiation occurring in the outer layer of the enveloping mesoblastic tissue, which occurs considerably later than those which bring the choroid and the cornea into existence, but there is from the first continuity between the corneal tissue and that of the sclerotic coat.
The capsulo-pupillary membrane, already referred to, may be looked upon as at first a complete fibro-vascular investment of the lens, which owes its origin
768
DEVELOPMENT OF THE OKGANS OF SENSE.
to the deepest pcart of the intruded mesoblast. The vessels of this membrane are svipplied by a branch of the central artery of the retina, which passes foi"n-ards in the axis of the globe, and breaks up at the back of the lens into a brush of rapidly subdividing twigs. The forepart of this tunic, adherent to the pupillary margin of the iris, forms the jnipUhir// mcmhrane by which the aperture of the pupil is closed in the middle periods of foetal life. In the human subject, the whole tunic, to<^ether with the artery which supplies it, becomes atrophied, and is lost sight of before birth, but in some animals it remains apparent for a few days after birth. According to Kolliker, the anterior chamber expands only a short time before birth by the intervention of the aqueous humour between the iris and cornea.
The eyelids make their appearance as folds of integument, subsequently to the foi-mation of the globe. "When they have met together in front of the eye, their edges become closely glued together ; and they again open before birth.
The lachrymal canal may be regarded as a persistently open part of the fissure between the lateral frontal process and maxillai-y lobe of the embryo.
The iii'st discovery of the mode of development of the eye as it is now generally understood was made by Huschke in 1832, and was published in Meckel's Archiv for that year. In addition to the vaiious systematic works on Development previously quoted, the reader is refen-ed to the following, viz., Lieberkiihn, Uber das Auge des Wirbelthier-enibryo, 1872 ; and Julius Arnold, Beitrage zur Entwick. de^ Auges, Heidelberg, 1874.
DEVELOPMENT OF THE EAR.
The first origin of the organ of hearing as an involuted follicle from the superficial epiblast of the head, constituting the primary auditory
Fig. 569.
Fig. 569. — Outlines showing the Formation of the External Ear in the Fietus.
A, head and upper part of the body of a human foetus of about four weeks (from nature). \' Four branchial plates (the first, forming the lower jaw, is marked 1), and four clefts are shown ; the auditory vesicle (a), though closed, is visible from the transparency of the parts, and is placed beliind the second branchial plate.
B, the same parts in a human fcetus of about six weeks (from Ecker). \ The third r.nd fourth plates have nearly disappeared, and the third and fourth clefts are closed ; the second is nearly closed ; but the first (1') is somewhat widened posteriorly in connection with the formation of the meatus externus.
C, human fcetus of about nine weeks (from nature). \ The first branchial cleft is more dilated, and has altered its form along with the integument behind it in connection witli the formation of tlie meatus externus and the auricle.
or otic vesicles, has already been shortly described. From numerous cbservations there is now no doubt that both in birds and mammals
DEVELOPMENT OF THE EAR.
709
Fig. 570.
Fig. 570. — Transverse and slightly Oblique Section of the Head op a Fietal
iShkep, in the Eegion op the Hind Erain (from Foster and Balfour after
Boettclier).
HD, inner surface of the tliickened walls of the hind brain ; RL, recessus vestilnili ; VB, commencing vertical semicircular canal ; CC, canalis cochlew, Avith the cavity of the primitive otic vesicle. On the left side parts only of these structures are seen ; GC, cochlear ganglion of the right side ; on the left side, Gr', the ganglion, and N, the auJitoiy nerve connected with the hind brain.
the otic vesicle forms at first for a time a follicle open to the surface, ami that it has therefore no original connection with the nervous centre. Its position is at the side of the medulla oblongata, and in a l)lace opposite to the interval between the first and second postoral visceral arches. The outer opening of the follicle very soon contracts
Fig. 571.
Fig. 571. — Labyrinth of tee Human Fcetus of FOUR aveeks, magnified (from Kolliker).
A, from behind ; B, from before ; v, the vestibule ; rv, recessus vestibuli, giving rise later to the aqueduct ; r.s', commencement of the semicii'cular canals ; a, upper dilatation, belonging perhaps to another semicircular canal : c, cochlea.
and becomes entirely closed. The follicle
sinks down towards the basis of the cranium,
and becomes imbedded in the formative
mesoblastic tissue lying between the basi occipital and alisphenoid matrices, undergoing chondrification and
ossification at a very early period, as has been already described under
the development of the head.
There are therefore to be distinguished from an early period a part corresponding to the internal membranous labyrinth proceeding from
VOL. II. 3 D
70
DEVELOPMENT OF THE ORGANS OF SENSE. Fig. 572.
Fig. 572. — Transverse Section of the Head op a Fcetal Sheep of four-fifths of
AN INCH IN LENGTH (from Foster and Balfour after Boettclier).
RV, recessus vestibuli ; VB, vertical semicircular canal ; CC, cochlear canal ; G, cochlear ganglion ; HB, horizontal canal.
Fi-. 573.
Fig. 573. — Tr.ANsvERSE Section of THE Cochlea in a Fcetal Calf, magnified (from Kolliker).
C, the wall of the cochlea, still cartilaginous ; c c, canalis cochlear ; I s, placed in the tissue occupying the place of the scala vestibuli, indicates the lamina spiralis ; n, the central cochlear nerve ; (/, the jjlace of the spiral ganglion ; S, the body of the sphenoid • ch, remains of chorda dorsalis.
the epiblast, and an outer cartilaginous or bony and fibrous wall, together with other adventitious structures arisingin the mesoblast. Labyrinth. — In the development of the primary otic vesicle after it becomes completely closed, a series of very remarkable changes by extension of its cavity in different directions gives rise to the formation of the different parts of the labyrinth. The first complication which
FORMATION OF THE EAR-LABYEINTII.
771
the vesicle exhibits is by the extension of a process upwards and backwards, which remains permanent in the lower vertebrata, but in mammals is obliterated, its vestifi^es remaining in the aqueduct of the vestibule. The semicircular canals next appear as elongated elevations of the surface of the primary vesicle : tlie middle portion of each elevation becomes separated from the rest of the vesicle by bending in of
Fig. 574.
Fig. 574. — Views of the Cartilage of Meckel and parts connected WITH THE First and Second Branchial Plates.
A (after Kolliker), bead of a fcetus of about eighteen weeks, showing the cartilage of Meckel in connection with the malleus and the surrounding parts.
M, placed upon the lower jaw indicates the cartilage of IMeckel of the right side.
B (from nature). An enlarged sketch explanatory of the above view ; z, the zygomatic arch ; ma, the mastoid process ; ml, jDortions of the lower jaw of which the parts near the angle and the symphysis have been removed ; M, the cartilage of Meckel of the right side ; M', a .sratdl part of that of the left side, joining the left cartilage at s, the symphysis ; T, the tympanic ring ; m, the malleus ; i, the incus ; s, the stapes ; sta, the stapedius muscle ; st, the styloid process ;p,A,<7, the stylo- j)haryngeus, stjdohyoid, and stylo-glossus muscles ; stl, stylo-hyoid ligament attached to the lesser cornu of the hyoid bone ; hij, the hyoid bone ; th, thyroid cartilage.
its walls under it, and thus the elevation is converted into a tube open at
each end, vyhich subsequently becomes elongated and acquires an ampullar
dilatation. The cartilage wliich forms the osseous labyrinth is continuous with that of the rest of the primordial cranium. The cartilaginous walls of the cavity are united by connective tissue to the
vesicle ; this connective tissue, according to Kolliker, becomes divided
into three layers, of which the outer forms the lining periosteum, the
3 D 2
772 DEVELOPMENT OF THE ORGANS OF SENSE.
inner forms the external walls of the membranous labyrinth, while the interveninp; layer swells up into gelatinous tissue, the meshes of which become wider and wider, till at last the space is left which ultimately contains the perilymph.
The cochlea appears at first as a prolongation downwards from the auditory vesicle, but afterwards becomes tilted forwards. This prolongation of the auditory vesicle is the rudimentary canalis membranacea. Close to it is placed the cochlear nerve, with a gangliform extremity. The canal becomes elongated in a spiral direction, and the ganglion, which is elongated with it, becomes the ganglion spirale. Between the canal and the cartilaginous wall which afterwards surrounds it a large amount of connective tissue intervenes, and in this tissue the cavities of the scala vestibuli and scala tympani gradually appear at a later period, precisely as does the space for the perilympli, in the vestibule. The modiolus and spiral lamina, according to Kulliker, are ossified without intervention of cartilage. Within the canalis membranacea Kulliker finds in the embryo a continuous epithelial lining, thin on the membrane of Reissner and on the outer wall, but forming a thick elevation in the position of the rods of Corti, and a large'r elevation more internally, filling up the sulcus spiralis. On the surface of this latter elevation he has observed a transparent body, the membrane of Corti.
The auditory nerve is said to be developed, separately from both the brain and the labyrinth, in the intermediate mesoblastic wall of the head ; the canal termed meatus auditorius internus being left in the bones round it and the facial nerve. The auditory nerve becomes secondarily connected with the medulla obloug-ata in a manner not yet ascertained, and its fibres are extended into relation with the delicate terminal structiu-es formed in the membranous labyrinth.
Middle and External Cavities of the Ear. — It has been already explained that the external meatus, the tympanic cavity, and the Eustachian tube, are formed in the posterior or upper part of the first postoral visceral cleft, which remains permanently open as the tympano-eustachian passage, except at the place where it is interrupted by the formation of the membrana tjmipani ; and also that the malleus is formed in the first visceral plate from the proximal part of Meckel's cartilage, and the incus, stapes, and stapedius muscle and the styloid process probably in the second plate. It is pointed out by KoUiker that during the whole period of foetal life the tympanic cavity is occupied by connective tissue, in which the ossicles are imbedded ; and that only after respiration has been established this tissue recedes before an expansion of the mucous membrane.
The pinna is gradually developed on the posterior margin of the first visceral cleft. It is deserving of notice that congenital malformation of the external ear, with occlusion of the meatus and greater or less imperfection of the tympanic apparatus, are observed in connection with abnormal development of the deeper parts of the first and second visceral plates and the intermediate cleft ; while cases have been observed of the persistence in the neck of the adult of one or more of the branchial clefts situated behind the first. (Allen Thomson, Proceed. Eoy. Soc. of Edin. 1844, and Edin. Journ. of Med. Sc. 1847.)
DEVELOPMENO? OF THE NOSE.
The organ of smelling, as was first pointed out by V. Baer, owes its origin, like the primary auditory vesicle and the crystalline lens of the eye, to a depression of the integument, or what may be more precisely designated as epiblast. This depression, the primary olfactory groove, is at first encircled by a uniform wall, and is unconnected with the
DEVELOPMENT OF THE NOSE.
773
mouth. This staple has been observed by Kolliker in the human embryo of four weeks. The olfactory groove is enclosed in the anterior extremity of the nasal cartilages prolonged forward from the trabeculas cranii (Parker). Soon, however, by the unequal growth of the surrounding parts, the groove so formed, descending from above, passes into the mouth. Thus the middle frontal process is isolated between the grooves of opposite sides, while the lateral frontal process separates the nostril from the eye. The maxillary lobes, growing forwards from behind the eyes, complete the boundaries of the nostrils, which then open into the fore part of the mouth. Kolliker observed this stage in the latter half of the second month. The palate subsequently grows inwards to the middle line, as has been elsewhere stated, and separates the nasal from the buccal cavity ; leaving only the small communication of the incisor foramen. Meanwhile, with the growth of the face, the
Fig. 575.
- Views op the Head op Human Embryoes, illustrating the Development
OF the Nose.
A, Head of an embi^o of three weeks (from Ecker). ^^ 1, anterior cerebral vesicle ; 2, middle vesicle ; 3, nasal or middle frontal process ; 4, superior maxillary process ; 5, eye ; 6, inferior maxillary process or first visceral plate, and below it the first cleft ; 7, 8, and 9, second, third, and fourth plates and clefts.
B, Head of an embryo of about five weeks (from Ecker). f
1, 2, 3, and 5, the same parts as in A ; 4, the external nasal or lateral frontal process, inside which is the nasal groove ; 6, the superior maxillary process ; 7, the inferior maxilla ; x , the tongue seen within the mouth ; 8, the first visceral cleft which becomes the outer part of the meatus auditorius externus and tynipano-eustachian passage.
C, View of the head of an embryo of eight weeks seen from below, the lower jaw having been removed (from Kolliker). *
n, the external nasal apertures ; ?', premaxillary or incisor process, and to the outer side of this the internal nasal aperture ; m, one of the palatal processes of the upper jaw, which, advancing inwards from the sides, form the partition between the mouth and nose ; p, common cavity of the nose, mouth, and jiharynx.
nasal fossa3 deepen, and the turbinated bones make their appearance as processes from their walls. The ethmo-turbinal cartilages are at first simple, but rapidly extend themselves to take the more or less complex shape which they present in different animals or in man.
Observations are still -^anting to determine whether the olfactory nerves are developed from the bulbs, and have thus a cerebral origin, or are separately
77i DEYELOPMEXT OF THE ALIMEXTAEY CAXAL.
formed from peripheral blastema like all other n-erves, with the exception of the optic.
DEVELOPMENT OF THE ALIMENTAEY CANAL AND ORGANS ARISING FROM THE HYPOBLAST.
The whole alimentary canal, from the fauces to the anus, together with the rudiments of certain organs associated with it in their commencement, viz., the thyroid gland, lungs, trachea and larynx, the liver, and pancreas, as well as the allantois, owe their origin more immediately to inflections of the hypoblast layer of the germinal membrane, which supplies the epithelial lining of their principal cavities ; but in all these organs parts of their structure are supplemented, and some other organs, such as the mesentery and spleen, are wholly formed from the mesoblast, whence proceed the vascular, fibrous, and parenchymatous elements, and also the serous coverings of the organs, where these exist.
ALIMENTARY CANAL,
The primary digestive cavity of birds and mammals, as it extends from one end of the embryo to the other below the vertebral axis, presents at first a manifest division into three parts. One of these^ occupying the part of the embryo which is enclosed by the cephalic fold, and which may be named the foregut, comprises the rudiments of the pharynx and gullet, the stomach and duodenum. The posterior division, which is comparatively short, occupies the caudal fold of the embryo, and corresponds mainly to the lower part of the colon and rectum. Both of these parts have from the first a tubular form, and are closed respectively by the Inflection of the whole blastodermic layers at the anterior and posterior extremities of the body. The middle division has primarily the form of a long and wide groove, lying close below the corresponding part of the vertei)ral bodies, leading at its opposite ends into the cephalic and caudal portions of the gut, and is freely open throughout on its ventral aspect into the cavity of the yolk-sac, with the blastodermic walls of which, as formerly described, the constituents of the intestinal walls are directly continuous (see fig. 576).
The mouth, as elsewnere explained, is no part of the primitive alimentary canal, but is formed by involution of parts of the face, and receives, therefore, its lining membrane from epiblast. It is separated for a time from the pharynx, which is the foremost part of the primitive alimentary canal, by the reflection of the layers of the blastoderm, and the communication which is later established between the mouth and pharynx at the posterior arch of the fauces, is due to a solution of continuity in these layers, which occurs in the chick on the fourth day of incubation, and has been traced at a corresponding period of development in several mammals. The aperture has at first the form of a vertical slit, which widens later as it becomes the opening from the pharynx into the common cavity of the nose and mouth. The diverticulum of the pituitary gland, it will be remembered, occupyingthe place which becomes the top of the permanent pharynx, is formed in connection with the epiblastic or buccal, and not the hypoblastic or pharyngeal division of the alimentary passage (see fig. 535, A and B, py).
The hypoblastic layer of the germinal membrane, from which is derived the epithelial lining of the whole alimentary canal and pas
PKIMAEY FORM OF THE ALIMENTARY CANAL,
■To
sages communicating witli it, is at first extremely thin and simple, and is composed of flat cells ; but as soon as this layer comes to form a part of the. inflected alimentary tube, its character is completely altered, its cells become cylindrical, and it attains a great proportional thickness, which it preserves for a considerable time.
Fig. 576.
Fig. o7i3. — Early form
OF THE Alimentary'
Canal (from Kolliker
after Bischotf).
In A a front view, and in B an antero-posterior section are represented.
a, four pliaryngeal or visceral plates ; h, the phai-ynx ; c, c, the commencing lungs ; d, the stomach ; /, /, the diverticula connected with the formation of the liver ; g, the yolk-sac into which the middle intestinal gi'oove opens ; h, the posterior part of the intestine.
The outer surface of the inflected hypoblast of the alimentary tube is more or less in contact with the splanchno-pleure division of the mesoblast. In the head no marked separation of the splanchnopleure and somatopleure divisions of the mesoblast takes place, but the elements of the former are no doubt combined with the hypoblast in the walls of the pharynx, and the formation of the tympano-eustachian and following pharyngeal clefts is therefore due to the perforation of both epiblastic and hypoblastic layers with intervening mesoblastic tissue, just as occurs in the formation of the opening of the fauces. But in the thorax and abdomen, the primitive alimentary canal is brought into relation with the pleuro-peritoneal cavity, and receives in various parts a serous investment from the lining membrane which becomes developed in that space. In the thorax the right and left cavities remain distinct as the two pleura, while a central portion is separated for the formation of the pericardium, and thus the gullet, as well as the lungs, is brought into relation with the pleura, and receives partial covering from them. The formation of the diaphragm, which does not at first exist, and which grows down from the vertebral column as a partition between the thorax and abdomen, leads to the ultimate separation of the peritoneum from the pleurte. Some examples of diaphragmatic hernia may be considered as arising from the persistence of the original connection between the two cavities. In the abdomen, also, the right and left peritoneal cavities are at first distinct, but when the intestine assumes a tubular form, the right and left cavities are thrown into one across the middle plane of the body.
As the development of the alimentary canal proceeds, the middle open part shortens, more and more of it being converted into the tubular intestiue, and at last, as before explained, there remains only
776
DEVELOrMEXT OF THE ALIMEXTxiRY CANAL.
1
the narrow opening by which the gradually lengthening ductus vitellointestinalis leads into the umbilical vesicle. The middle part of the intestinal canal has, when first produced, more or less the form of a straight tube lying close to the vertebral column ; but as it increases
Fi?. 577.
Fig. 577. — Human Embryo of TniRxr-FiVE DAYS SEEN FROM BEFORE (from Kcilliker after Coste).
3, left external nasal process ; 4, superior maxillary process ; 5, lower maxillary jirocess ; z, tongue ; b, aortic bulb ; b', first perrmmeut aortic arch, which becomes ascending aorta ; b" , second aortic arch ; b"' , third aortic arch or ductus Botalli ; ?/, the developing pulmonary arteries ; c, the superior cava and right azygos vein ; c', the common venous sinus of the heart ; c", the common stem of the left vena cava and left azygos ; o', left auricle of the heart ; v, right v', left ventricle ; a e, lungs ; c, stomach ; j, left omphalo-mesenteric vein ; s, continuation of the same behind the pylorus, which becomes afterwards the vena portaj ; x, vitello-intestinal duct ; a, right omphalo-mesenteric artei-y ; m, Wolffian body ; ?', rectum ; n, umbilical artery ; Vi umbilical vein ; 8, tail ; 9, anterior, U', jjosterior limb. The liver has been removed.
in length, it is thrown into the shape of a loop bent downwards to the umbilicus, — a change which is accompanied by the formation of the mesentery. The latter structure is undoubtedly entirely due to splanchnopleure mesoblastic elements, which, extending themselves between the proto-vertebral masses and the elongating gut, become developed into the vascular and other parts of the mesentery, as was long ago shown by Yon Baer. But the mesoblast, also, by its splanchnopleure division, furnishes the contractile fibrous, vascular, and connective tissue elements of the intestinal walls. The extent to which the glandular elements of the alimentary canal are supplied by the hypoblast, to which their origin was entirely attributed by Remak, or furnished rather by mesoblast from the proto vertebral mass, as held by Schenk, is not yet determined.
As develo[)ment proceeds in the forepart of the alimentary canal, a change in its form manifests itself, by which one part, becoming dilated, forms the commencement of the stomach, while the others remain of smaller diameter as gullet and duodenum ; and in connection with different parts of these 'the rudiments begin to appear of the lungs, liver, and pancreas.
When the tubular parts of the gut have attained to some length, a change of position gradually accompanies their further development. While the oesophageal part remains comparatively straight, the dilated
STOMACH AXD IXTESTIXE.
777
portion of the tube which forms the stomach turns over on its right side, so that the border, which is connected to the vertebral cohminby the membranous fold (or true mesogastrium) comes to be turned to the left, — the position of the tube being still vertical, like the stomach of some animals. By degrees it becomes more dilated, chiefly on what is now the left border but subsequently becomes the great curvature, and assumes first an oblique and finally a transverse position, carrying with
Ficr. 578.
K'fr^
Fig. f'78. — Outline of the Form ,and Position of the Alimentary Canal in
Successive Stages of its Development.
A, alimentary canal, &c., in an emln-yo of five weeks ; B, at eight weeks ; C, at ten ■weeks ; D, at twelve weeks ; I, tbe primitive lungs connected with tlie pharynx ; s, the .stomach ; d, duodenum ; /, the small intestine ; i', the large ; c, the ciecum and vermiform appendage ; r, the rectum ; c /, in A, the cloaca ; a, in B, the anus distinct from s /, the sinus uro-genitalis ; r, the yolk-sac ; v i, the vitello-intestiual duct ; u, the urinary bladder and urachus leading to the allantois ; g, the genital ducts. In B, and 0, the thickness of the colon is erroneously represented as greater than that of the ileum.
it the mesogastrium, from which the great omentum is afterwards produced. A slight indication of the pylorus is seen at the third month. Upon the surface of the part of the canal which immediately succeeds the stomach, and which forms the duodenum, the rudiments of the liver, pancreas, and spleen are simultaneously deposited, in the manner to be stated in the description of the development of these organs.
The place of transition from the small to the large intestine, which is soon indicated by the protrusion of the cfECum, is at a point just below the apex or middle of the simple loop already mentioned, as accompanying the first elongation of the tubular gut. As the small intestine grows, the part below the duodenum forms a coil which at first lies in the commencing umbilical cord, but retires again into the abdomen about the twelfth week ; aftei'wards it continues to elongate, and its convolutions become more and more numerous.
778 DEVELOPMENT OF THE ALIMENTARY CANAL.
The large intestine is at first less in calibre than the small. In the early embryo there is at first no csecum. This part of the bowel gradually grows out from the rest, and in the first instance forms a tube of uniform calibre, without any appearance of the vermiform appendix: subsequently the lower part of the tube ceases to grow in the same proportion, and becomes the appendix, whilst the upper portion continues to be developed with the rest of the intestine. The caecum now appears as a protrusion a little below the apex of the bend in the primitive intestinal tube, and, together w'ith the commencing colon, and the coil of small intestine, is at first lodged in the wide part of the umbilical cord W'hich is next the body of the embryo. The ileo-cascal valve appears at the commencement of the third month. When the coils of intestine and cjecum have retired from the umbilicus into the abdomen, the colon is at first entirely to the left of the convolutions of the small intestines, but subsequently the first part of the large intestine, together with the meso-colon, crosses over the upper part of the small intestine, at the junction of the duodenum and jejunum. The ca3cum and transverse colon are then found just below the liver ; finally, the ca3cum descends to the right iliac fossa, and at the fourth or fifth month the parts are in the same position as in the adult. At first, villous processes or folds of various lengths are formed throughout the whole canal. After a time these disappear in the stomach and large intestine, but remain persistent in the intermediate portions of the tube. According to Meckel, the villous processes are formed from larger folds, which become sen-ated at the edge, and divided into separate villi.
The formation of the hinder part of the gut is complicated with the development of the allantois, which arises as a projection or outgrowth of the hypoblast and mesoblast from the lower wall of its terminal portion. This part rapidly buds out in the pleuro-peritoneal space, having from a very early period a rich network of the branches of the umbilical vessels in its outer layer. The anal or cloacal portion remains behind the allantoid pedicle, and by the fifth day in the chick the whole of the tissues which close the terminal fold thin rapidly away, and become perforated so as to form the primitive anal, or rather the cloacal opening. The separation of the permanent anus fi'om the urogenital orifice is the result of a later process of development.
The mode of development of the alimentary canal explains, in some measure, the complicated folds of the peritoneum. The stomacii being originally more nearly in the line of the alimentary canal and mesial in position, the small omentum aad gastro-phrenic ligament are originally parts of a mesial fold with the free edge directed forwards, which afterwards forms the anterior boundary of the foramen of Winslow. Thus the anterior wall of the sac of the omentum, as far as the gi'eat curvature of the stomach, may be considered as formed by the right side of a mesial fold, while the peritoneum in front of the stomach belongs to the left side of the same, and a sac of the omentum is a natural consequence of the version and disproportionate growth of the tube between the duodenum and the cardiac orifice of the stomach. It is obvious that the view of the omental sac, according to which its posterior layers are held to return to the duodenum and posterior wall of the body before proceeding to form the transverse meso-colon (p. 484) is more consistent with the phenomena of development now described.
FOLDING OF THE PERITONEUM.
7&
than tliat which would make them directly enclose the colon. On the
other hand, the farther elongation of the omental sac and the whole
disposition of the peritoneum, with respect to the colon, must be
regarded as having taken place after the assumption by the great intestine of its permanent position.
Fig. 579. — Sketch op the HujijiN Fig. 57
Embryo of the Tenth Week, showing THE Coil of Intestine in the Umbilical Cord. (A. T.)
Tlie amnion and villous chorion have been ojiened and the embryo drawn aside from them ; r, the umbilical vesicle or yolk-sac placed between the amnion and chorion, and connected with the coil of intestine, i', by a small or almost linear tube ; the figure at the side represents the first part of the umbilical cord magnified ; i, coil of intestine ; v i, vitello-intestinal duct, alongside of which are seen omphalo-mesenteric blood-vessels.
The occuiTence of umbilical hernia in its various degrees may be referred to the persistence of one or other of the foetal conditions in which a greater or less portion of the intestinal canal is contained in the umbilical cord ; and it has been shown that the most common form of abnormal diverticula from the small intestine is connected with the original opening of the ductus vitello-intestinalis into the ileum.
DEVELOPMENT OP THE LIVER, PANCREAS, AND SPLEEN.
The Liver. — The liver is one of the earliest formed abdominal organs. It consists at first, according to most observers, of two solid masses in connection with the lower surface of the duodenal portion of the alimentary canal. Schenk, however (Lehrbuch, p. 93), states that the blastemic mass of the liver is single. A hollow cavity soon appears within the mass, which is the commencement of the main excretory duct (ductus choledochus communis). This cavity is lined by hypoblastic epithelium ; and, according to the commonly received view, is produced as a diverticulum of the hypoblast of the intestine. Through the mass, but at first unconnected Avith its substance, there passes the main stem of the veins from the umbilical vesicle and allantois (umbilical vein or meatus venosus).
In the rudimentary mass composing the liver there are soon observed a number of solid cylinders of blastemic cells which branch out from the hypoblast into the mesoblast, and as these come to unite together by their ends, they at last form a network of solid cords with which the hypoblastic diverticula are connected. In the meantime bloodvessels are developed in the mesoblast lying between the cylinders, which vessels become united as branches with the umbilical vein passing through the liver. Hollow processes also extend themselves from the hypoblastic diverticula and stretch into the solid cylinders of the hepatic parenchyma ; but the greater part of this remains solid for a time, consisting of reticulated strings of cells between which there is nothing but blood-vessels.
According to some embryologists, as Schenk, the hypoblast forms no
7S0
DEVELOPMENT OF THE ABDOMINAL GLANDS.
more than the lining epitlielium of the bile-ducts and gall-ljladder, and the hepatic or glandnlar cells are entirely derived from mesoblast ; but, according to Foster and Balfour, following Reiuak and the earlier observers, the cellular elements of the gland are stated to derive their origin from the h3'poblast, and the mesoblast is mainly converted into blood-vessels and the fibrous tissue of the ducts.
Fig. 580.
Fig. oSO. — E.VRLT Condition OF THE Liver
IN THE Chick on thk
Third Bxy of Incubation (from J. ]\IiilLr.) \"
] , the lieai-t as a .simple curved tube ; 2, 2, tlie intestinal tube : 3, conical protrusion of the coat of the commencing intestine, on which the blastema of the liver (4) is formed ; 4, portion of the layers of the germinal membrane, passing into the yolk-sac.
The gall bladder is formed by extension from the wall of the main duct.
The blood-vessels formed in the liver become branches of the main vein, which passes tlumigh the cellular mass. These are distinguishable as an anterior and posterior set, the arrangement of which is such that the blood flows from stem to branches in the anterior, and from branches to stem in the posterior. Thus the distinction is established between portal and hepatic veins (sec the Development of the Yeins).
The solid cylinders of the blastema represent the hepatic lobular tissue, the hollow processes the hepatic ducts ; but the origin of the finest ducts is not known. Perhaps each cellular cylinder may be looked upon as a collection of hepatic cells, in the centre of which is the minute duct, according to the view now taken of the structure of the adult liver (Foster and Balfour).
The gall-bladder is at first tubular, and then has a rounded form. The aU'eoli in its interior appear about the sixth month. At the seventh month it first contains bile. In the fcetus its direction is more horizontal than in the adult.
The following are the principal peculiarities in the liver of the foetus : —
Size. — In the human fcetus, at the fifth or sixth week, the liver is so largo that it is said to constitute one -half of the weight of the -n-hole body. This proportion gradually decreases as development advances, until at the full pei'iod the relative weight of the foetal liver to that of the body is as 1 to IS.
In early foetal life, the right and left lobes of the liver are of equal, or nearly equal, size. Later, the right preponderates, but not to such an extent as after birth. Immediately before birth the relative weight of the left lobe to the riglit is nearly as 1 to I'G.
Position. — In consequence of the nearly equal size of the two lobes, the position of the foetal liver in the abdomen is more symmetrical than in the adult.
THE FCETAL LIVER.
781
In the very yoimg foetus it occupies nearly the whole of the abdominal cavity ; at the full period it still descends an inch and a half IdcIow the margin of the thorax, overlaps the spleen on the left side, and reaches nearly down to the crest of the ilium on the right.
Form, Colour, (]'<"■ — The foital liver is considerably thicker in proportion from above downwards than that of the adult. It is generally of a darker hue. Its consistence and specific gravity are both less than in the adult.
During foetal life, the umbilical vein runs from the umbilicus along the free margin of the suspensory ligament towards the anterior border and under surface of the liver, beneath which it is lodged in the umbilical fissm-e. and proceeds as far as the transverse fissure. Here it divides into tn-o branches ; one of these, the smaller of the two, continues onward in the same du'ection, and joins the vena cava ; this is the ductus rciumix. which occupies the posterior part of the longitudinal fissure, and gives to it the name of the fossa of the ductus venosus. The other and larger branch (the tnink of the umbilical vein) turns to the right along the transverse or jjortal fissure, and ends in the vena portaj, which. })roceeding from the veins of the digestive organs, is in the foetus comparatively of small dimensions. The umbilical vein, as it lies in the umbilical fissure, and before it joins the vena porta3. gives off large lateral branches, which pass dii-ectly into the right and left lobes of the liver. It also sends a few smaller branches to the square lobe and to the lobe of Spigelius.
Fig. 581. — Under Surface of the FcETAL Liver, with its gre.\t Blood-vessels, at the full Period.
a, the iimhilical A-ein, lying in the umbilical fissure, and turning to the ri^dit side, at the transverse fissure (o), to join the vena portte (p) : the branch marked d, named the ductus venosus, continues straight on to join the vena cava inferior (c) : some branches of the umliilical vein pass from a into the substance of the liver ; >j. the gall-bladder.
Fig. r.8i.
The blood which leaves the liver by the hepatic veins, and is carried into the
heart along with that of the vena cava inferior, consists of the following parts,
viz. ; 1. That of the mnbilical vein, which passes on directly by the ductus
venosus ; 2, that portion of the blood which is distributed to the liver by branches
proceeding immediately from the trunk of the umbilical vein ; and 3, the blood
from the digestive organs of the foetus arriving by the vena porta3.
After birth the umbilical vein becomes obliterated from the umbilicus up to the point of its giving off branches to the liver. The ductus venosus is also obliterated, but the veins which were given as l^ranches from the umbilical vein to the liver remain in communication with and appear as branches of the left division of the portal vein.
The Pancreas. — Tliis organ takes its origin in a mass of mesoblastic tissue, which thickens the wall of the duodenum close to the place where the rudiment of the liver is first seen, but placed more to the left side. This mass may be seen on the third day in the chick. There is, however, also a diverticulum from the primary wall of the intestine or hypoblast. The same doubt prevails, as in regard to the
782 DEVELOPMENT OF THE ABDOMIXAL GLAXUS.
liver, ^vith respect to the exact share of the hypoblastic and mesoblastic elements in the formation of the glandular cells. The main duct and its branches undoubtedly owe their origin to diverticula proceeding from the intestinal hypoblast, and the epithelial lining of the ducts is doubtless derived from that source. By those who consider that the glandular cells also arise from the hypoblast, solid processes of that layer are described as stretching into the mass of mesoblast. Into these the diverticular cavities subsequently extend in more than one main division. The blood-vessels and the connective tissue of the gland are undoubtedly due to the mesoblastic elements, and these are very soon combined with the imrts proceeding from hypoblast.
The Spleen. — This organ appears soon after the pancreas as a thickening of the mesogastrium, and is therefore entirely mesoblastic in its origin. (Peremeschko, Vienna Acad., 18G7, and W. Mliller in Strieker's Histol.) The gradual formation of the trabecular structure from one set of cells and of the blood-vessels and cellular elements of the organ from tlie blastemic substance has been traced. The pulp is formed in connection with the veins, and the arteries are formed along with the Malpighian corpuscles. The spleen is closely related to the pancreas in its origin, but it is later of being formed and contains no hypoblastic elements. In the human foetus of about twelve weeks its shape can be recognised, but the Malpighian bodies are not visible till the middle of foetal life.
Lymphatic Glands. — The development of the mesenteric lymphatic glands has been observed by Sertoli in mammals. (Vienna Acad., 18G6.) The blastema from which they are produced is imbedded in the mesentery, and is therefore entirely mesoblastic. The gradual differentiation of the blastema gives rise in succession to the lymph spaces, the trabecuke and the lymph cells, and the distinction follows between inferent and efferent lymphatic vessels. The development of lym])hatic vessels has been described in the General Anatomy, p. 191.
The Thymus and Thyroid Glands.— The development of these bodies has been described in an earlier part of this volume, pp. 297 and 299. The thymus gland proceeds entirely from mesoblastic tissue ; but, according to the researches of W. Miiller (Jenaisch. Zeitsch., 1871), it would appear that the thyroid body arises at first as a diverticulum from the pharynx, and it therefore contains some hypoblastic elements.
DEVELOPMENT OF THE LUNGS AND TRACHEA.
The lungs first appear as two small protrusions upon the front of the oesophageal jiortion of the alimentary canal, completely hid by the rudimentary heart and liver. These primitive protrusions or tubercles are visible in the chick on the third day of incubation, and in the embryoes of mammalia and of man at a corresponding stage of advancement. Their internal cavities communicate with the oesophagus, and are lined by a prolongation of the hypoblast. At a later period they are connected with the oesophagus by means of a long pedicle, which ultimately forms the trachea, whilst the bronchia and air-cells are developed by the progressive ramification of the internal cavity in the form of cffical tubes, after the manner of the ducts of glands.
The diverticulum of hypoblast is surrounded by a mass of mesoblastic cells, so that the pulmonary parenchyma, like that of the glands,
FOEMATION OF THE LUXGS. 733
owes its origin to both hyiioblastic and mesoljlastic elements. The substance of the mesoblast, thickening round the primary diverticula, becomes penetrated by secondary diverticula formed from the hypoblast processes ; these are succeeded by tertiary branches which develop the bronchia, and ultimately have the air-cells formed as their terminations. The formative process consists essentially in the budding of hypoblastic into mesoblastic substance ; the hypoblast furnishing the lining epithelium of the tubes, and the mesoblast the other tissues, such as muscular fibres, cartilage, blood-vessels, elastic tissue, c^-c.
In the formation of the trachea and bronchi the wall of the primitive oesophagus is projected downwards (or forwards), and by the gradual folding in of the sides a second median tube is separated from the primitive alimentary canal. This new tube grows out at its hinder end so as to bulge at the two sides, and thus the commencement of a right and left bronchus is formed. The subsequent division of the diverticular hollow goes on by budding of the hypoblast from within into the masses of pulmonary blastema. The division into larger lobes externally, three in the right and two in the left lung, may be seen at a very early period in the human foetus. As the bronchial subdivision extends within the lungs, a tubercular or coarsely granular appearance is seen over the outer surface, as observed by Kiilliker in the human foetus in the latter half of the second montli. This is produced by the primitive air-cells placed at the extremities of ramified tubes, which occupy the whole of the interior of the organ : the ramification of the bronchial twigs and multiplication of air-cells goes on increasing, and this to such an extent that the air-cells in the fifth month are only half the size of those which are found in the fourth month.
Fig. 582. — Sketch iLLUSTRATiNa the Deve- Fig. 5S2.
LOPMENT Oi THE RESPIRATORY ORGANS
(from Fathke). A. B O
A, oesophagus of a cliick, on the fourth day of incubation, with the rudimentary lung of the left side, seen laterally ; 1, the front, and 2, the hack of the oesophagus ;
3, rudimentary lung protruding from that tube ; 4, stomach. B, the same seen in front, so as to show both lungs. C, tongue and resjjiratory organs of embryo of the horse; 1, tongue; 2, larjmx ; 3, trachea j
4, lungs seen from behind.
In birds the principal air-sacs, three in number, are formed in direct connection with the lung in the course of its early development, and the riidiments of these sacs may be seen at an early period, as bulging constituent parts of the rudimentary lungs.
Pleurae. — Each lung receives a covering externally from the lining' membrane of the common pleuro-peritoneal cavity of its own side. This is at first only on the outer side; but, as the lungs enlarge, a fissure separates their solid substance from the outer wall of the oesophagus, and the pleura is carried round the lung-mass so as to encircle the gradually narrowing root of each lung. The two pleurse remain separated by the mediastinum and heart.
Pulmonary Vessels.— The blood-vessels of the lungs which arise in the mesoblastic tissue seem to be of comparatively late formation.
784
DEVELOPMENT OF THE VASCULAR SYSTEM.
penetrating into the mesoblast only on the twelfth day in the chick. The pulmonary arteries are developed in mammals in connection Avith the fifth branchial arch of the left side, but the manner in which they become connected with the vessels formed in the lung-substance, and the manner in which a union is established between the pulmonary veins and the left auricle have not yet been ascertained.
DEVELOPMENT OF THE HEART AND BLOOD-VESSELS.
In the account of the general phenomena of development the establishment of the first circulation of blood, by the simultaneous formation of the simple heart and of the first blood-vessels and blood in the body of the embryo and in the vascular area of the blastoderm, has already been described, and in the General Anatomy (p. 180) an account has
Fig. 583.
Kg. 58S. — Outlines of the anterior
HALF OF THE EjIBRYO ChICK A'IEWED FROM BELOW, SHOWING THE HeART IN ITS EARLIEST STAGES OF FORMATION
(after Kemak). "»
A, Embryo of ab(jut 20 to 30 lioxirs ; B, of about 36 to 40 hours ; a, anterior cerebi'al vesicle ; h, j)roto-vertebral segments ; c, amniotic fold ; 1, 1, primitive omphalo-mesenteric veins entering tlie Iieart posteriorly ; 2, their union in the auricle of the heart ; 3, the miLldle part of the tube corresponding to the ventricle ; 4 (in B) the arterial bulb.
been given of the histological changes occurring in the first development of the blood-vessels and blood.
DEVELOPMENT OF THE HEART.
Origin of the Heart — Simple Tubular Foi'in. — The heart takes its origin in the form of an elongated sac or dilated tube in the substance of a thickening of the splanehuo-pleure layer of the mesoblast, in the ventral aspect of the cephalic portion of the primitive alimentary canal, immediately in front of the fovea cardiaca. Doubts have existed as to the exact mode of production of the cavity of the organ, but the observations of Aflanasieff and Klein, and especially those of Foster and Balfour, appear to show that the substance in which the first rudiments of the heart arise is produced by a thickening of the lower wall of the mesoblastic layer of the primitive intestine, and that the cavity is formed by a solution of continuity or liquefaction of tliis substance in such a manner that, while the outer cells constitute the foundation of the commencing fibrous walls,, a deep set of cells very soon or from the first arrange themselves in the form of an endo-vascular lining of the cavity. The oi-gan has at first the form of an elongated sac or dilated tube of symmetrical shape, widening out behind into two lateral orifices, each of which is connected with an omphalo-mesenteric vein of its own side bringing the nascent blood back from the vascular area, while the anterior part of the rudimental heart leads into two arterial
^ PRIMITIVE FORM OF THE HEART. 785
vessels, one of which arches over each side of the primitive pharynx and turns backwards below the proto-vertebrge to form one of the two primitive aortic tubes. From each of these last the omphalo-mesenteric arteries pass off into the vascular area.
According to recent observations by KoUiker and by Hensen (loc. cit.) a still earlier condition of the heart has been perceived in the embryo of mammals, in which there are two separate tubes hollowed out of the lateral parts of the cephalic fold. Each of these tubes is connected with a vein or entering vessel posteriorly, and an artery or out-going vessel anteriorly : these slowly come together and unite by fusion in the middle, in a limited space at first, and then more and more till the single tubular heart results. Each tube is in relation with the isleui-operitoneal cavity of its own side, and when the median fusion takes place the union of these two becomes the pericardium.
Fig. 584.
r.So.
Fig. 584. — Diagrammatic longitcdixal section through the Axis op an Embrto.
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, fold of the somatopleure ; Sp, and FiSp, fold of the splanclmopleure ; Avi, commencing (head) fold of the amnion ; NO, neural canal, closed in front, but still open behind ; C/i, notochord, in front uncleft mesoblast in the ba«e of the cranium ; D, the commencing foregut, or alimentary canal ; Ht, heai-t ; jU^, pleuro-i>eritoneal cavity.
The rudimental heart in the form now described, exists in the chick at the thirty-sixth hour of incubation, and already, while still consisting of formative cells not differing greatly from those composing the other parts of the mesoblast, begins to exhibit motions of alternating systole and diastole, by slow contractions which begin behind and pass forward to the anterior extremity of the tube ; and a small quantity of imperfectly formed blood is propelled through the cavity.
The elongation which the tubular heart now undergoes causes it to lose the symmetrical form ; and its middle part now becomes detached from the lower side of the alimentary canal, and projects downwards (or forwards in the body) with an inclination to the right side of the embryo.
The heart is now found to be surrounded on the ventral aspect by a median cavity, which is a part of the pleuro-peritoneal space intervening between the wall of the heart as splanchno-pleure, and the somato-pleure forming the thoracic wall. This cavity becomes the pericardial sac.
As the development of the tubular heart progresses, the bend
VOL. II. 3 E
786
DEVELOPMENT OF THE VASCULAR SYSTEM.
increases, and the venous is doubled back upon the arterial end. The tube also becomes divided by two slight constrictions into three
Fig. 585. — Human Embryos
AT DIFFERENT EARLY STAGES OF DEVELOPMENT, SHOWING
THE Heart in its tubular
CONDITION.
A, upper half of the body of a human embryo of three weeks, viewed from the abdominal side (from Coste) ; «, frontal plate ; h, protovertebrse, on which, the primitive aortiB are lying ; 3, the middle of the tube of the heart, below it the place of entrance of the great veins, above it the aortic bulb.
B, lateral view of a human embryo more advanced than that
last referred to, and somewhat imperfectly developed (from A. Thomson) ; a, the frontal part of the head ; h, the vertebral column ; v, the wide communication of the umbilical vesicle or yolk-sac with the intestine ; u, communication with the allantois or urachus ; 2, auricular part of the heart connected with the veins posteriorly ; 8, ventricular part of the bent tube ; 4, the aortic bulb ; near the extremities of the tube the divided pericardium is seen.
portions, of which that originally posterior and receiving the veins is the widest, and constitutes the primitive auricle ; the middle one, next
^\ ;a"
Fig. 586.— Diagrammatic Outlines of the Heart and First Arterial Vessels
OP THE Embryo, as seen from the Abdominal Surface.
A, at a period corresponding to the 36th or SStli hour of incubation in the chick • B, and C, at the 48th hour of incubation ; 1, 1, primitive veins ; 2, auricular part of the heart ; 3, ventricular part ; 4, aortic bulb ; 5, 5, the primitive aortic arches and their continuation as descending aorta ; these vessels are still separate in their whole extent in A, but at a later period, as shown more fully in C, have coalesced into one tube in a part ot the dorsal region ; in B, below the upper 5, the second aortic arch is formed, and
DIVISION OF THE HEART'S CAVITIES. 7S7
farther clown the dotted lines indicate tlie position of the succeeding arches to the number of rive in all ; 5', 5', the continuation of the main vessels in the body of the embryo ; 6, 6, the omphalo-mesenteric arteries imssing out of the body of the embryo into the vascular area of the germinal membrane.
in width and most strongly bent upon itself, becomes the ventricular portion ; and the third, situated anteriorly and retaining most the simple tubular form, is the arterial or aortic bulb. This tubular stage of the rudimental heart has been observed in the human embryo by Coste and Allen Thomson (see fig. 585, A and B).
Division into Single Auricle, Ventricle, and Arterial Bulb. — By a continued increase of the inflection of the heart-tube, a change in the relative position of the several parts is effected, so that the auricular cavity comes to be placed above or behind (dorsally) and to the left of the ventricular part, the veins being carried forwards along with it, while the arterial bulb is attached by its extremity in front to the neck of the embryo immediately behind the lower visceral plates. There is as yet only a single passage through the heart, but the distinction of the auricular and ventricular cavities becomes more apparent, both by an increase in the diameter of each, and by the constriction which separates them, and by the much greater thickness acquired ]:»y the walls of the ventricular and bulbous parts as compared with the auricular portion.
The three parts of the heart have now the appearance of being very closely twisted together. The ventricular part becomes considerably wider transversely, and the auricular part shows two projecting pouches, one on each side of the arterial bulb, which are the first indications of the future auricular appendages. At the same time the constriction between the auricular and ventricular parts increases consideral:)ly, and the constricted part elongating produces what has been called the canalis auricular is.
Division of the Cavities. Ventricles.— The next series of changes in the developing heart consists in the division of each original single cavity of the ventricle, auricle, and arterial bulb into two compart Fig. 587. — Head op the Embrto of the Dog with the Fig. 587.
Heart seen from below (from Kolliker, after BischofF).
Magnified. <*
a, cerebral hemispheres : h, eyes ; c, midbrain ; d, inferior maxillary plates ; e, superior maxillary processes ; /, /', /", second, third, and fourth branchial or visceral plates ; g, right, h, left auricle of the heart ; h, right, i, left ventricle ; 1, aortic or arterial bulb, with three paii's of aortic or vascular arches protruding from it.
ments, so as to form the right and left ventricles and auricles, and the stems of the -pulmonary artery and aorta. The first of these changes occurs in the ventricular portion, and is to be seen in progress on the fourth day in the chick, and the sixth and seventh week in the human embryo. The ventricular chamber of the heart, increasing considerably in breadth, that part of it which ultimately becomes the apex of the heart is thrown towards the left side, and in most mammals, and
3 E 2
788 DEVELOPMENT OP THE VASCULAR SYSTEM.
especially in the human embryo, a blmit cleft or depression appears betAveen this and the right part of the ventricle, which causes an external division into two portions corresponding to the future right and left ventricles ; and if the interior of the ventricular cavity be examined at this time, there is perceived a crescentic partition rising from the anterior or lower border of the right wall and projecting into the cavity, at first narrow and placed opposite the external notch, but gradually growing more and more towards the auriculo-ventricular aperture. As development progresses the external division becomes more or less effaced, when the apex of the heart formed by the left ventricle becomes more pointed, and the whole heart takes more of the conical form which belongs to its more advanced condition ; but the depression is still perceptible as the interventricular groove of the adult heart, which, as is well known, varies considerably in depth in different cases. In some animals, as the rabbit, the temporary external division of the ventricles is greater than in the human embryo, while in others, as in ruminants, there is very little of the external notching, and in them, as in birds, the heart very early assumes the conical form. The dugong presents a remarkable example of the persistence of the complete external separation of the ventricles, and there appears to be a tendency to the occasional occurrence of the same in the seal.
The internal septum of the ventricles continuing to rise between the right and left divisions of the cavity, reaches at last the base where it is placed in relation with both the auriculo-ventricular orifice and the root of the arterial bulb ; but at this place there remains for a time a communication over the still free border of the septum between the right and left ventricles, wdiich is interesting, as this is the seat of the abnormal communication between the right and left ventricles in almost all cases of malformation of the heart presenting that condition.
Division of the Auricles. — Although the auricular cavity presents externally some appearance of being divided into two at a period antecedent to the partition of the ventricles, in consequence of the formation of the right and left auricular appendages before mentioned, the internal division of the cavity does not take place till some time later, as on the fifth and sixth days in the chick, and in the eighth week in the human embryo. The auricular septum commences as an internal fold proceeding from the anterior wall of the common cavity, and starting from the septum of the ventricles, it grows backwards towards the entrance of the common vein or sinus, but stops short of it some distance. For a time, therefore, the veins enter the back part of the auricular cavity in common. It is proper to explain, however, that, by the time at which the auricular septum is forming, the venous sinus has been modified so as to produce three veins entering the auricle at its .back part. Of these, two correspond with the right superior cava and the inferior cava veins, and the third to the left superior cava and connected with what afterwards becomes the coronary sinus. For a time, all the three vessels open so as to communicate freely with the whole auricular cavity. But changes now occur which cause the left superior cava and the inferior cava to be directed towards the left side, Avhile the right superior cava is placed more immediately in connection with the right part of the auricular cavity.
The auricular septTim,in extending itself backwards, is not completed, but leaves an oval deficiency in its lower and middle part, as the
DIVISION OF THE AUKICLES.
rs9
foramen ovale, and the inferior cava opens immediately behind this. Some time later in the human embryo, or in the course of the tenth or eleventh weeks, two new folds make their appearance in the auricles posteriorly. One of these constituting the Eustachian valve, of a
Fig. 588. — Shows the position and
FORM OP THE HeART IN THE HuMAN
Embryo from the Fourth to the Sixth
WEEK.
A, upper half of tlie body of a human embryo of Dearly four weeks old (from Kolliker after Coste) ; B and C, anterior and posterior views of the heart of a human embryo of six weeks (from Kolliker after Ecker) ; a, frontal lappet ; h, mouth ; c, below the lower jaw and in front of the second and third branchial arches ; d, upijer limb ; e, liver ; /, intestine cut short ; 1, superior vena cava ; 1', left superior cava or brachio-cephalic connected with the coronary vein ; 1", opening of the inferior vena cava ; 2, 2', right and left auricles ; 3, 3', right and left ventricles ; 4, aortic bulb.
crescentic form, is placed to the right of the entrance of the inferior
vena cava, and in the angle between it and the orifice of the left superior cava (or great coronary sinus), and besides separating these two
veins, and thus throwing the opening of the left superior cava into
communication with the right auricle, this fold, as it runs forward into
the annulus ovalis or border of the anterior auricular septum, has the
effect of deepening the entrance of the inferior cava into a groove
which lies close to the foramen ovale, and directs the blood entering by
that vessel through the passage into the left auricle.
The other fold referred to advances from the posterior wall of the common auricle to meet the anterior auricular septum, but yet to the left of the border of the foramen ovale. To this border, however, it adheres as it grows forwards, and thus gradually fills up the floor of the fossa ovalis. Up to the middle of foetal life, this posterior septum being incomplete, there is a direct passage from right to left through the foramen ; but, after that period, the fold in question, having advanced beyond the anterior border of the annulus ovalis and lying to the left, it does not adhere to this or the fore part of the annulus, but leaves a passage between, and appears as a crescentic fold in the left auricle, which, as it passes beyond the annulus, constitutes a very perfect valve against the return of blood from the left into the right auricle.
Division of the Arterial Bulb. — The third important change occurring in the heart belongs to the arterial bulb, by which there are developed from this tube the first parts or main stems of the pulmonary artery and the ao'-ta. Within the thick walls of this arterial tube there is at first only a single cylindrical cavity, continued from the originally single ventricle ; but, a short time after the partition of the ventricular cavity has commenced, or in the seventh week of the human embryo, a division of the bulb by an independent process begins to
790
DEVELOPMENT OF THE VASCULAR SYSTEM.
take place. This consists in the projection inwards of a lateral fold of the wall on the two sides, affecting, however, only the inner and middle coats, and not perceptible externallj^ ; so as to divide the cavity of the bulb into two channels, which may be described as respectively anterior and posterior, but which from the spiral direction taken by the folds are somcAvhat twisted on each other, so that the channel which at the ventricular end is placed anteriorly becomes connected with the right ventricle and forms the pulmonary stem, and that which is placed posteriorly becomes connected with the left ventricle and forms the commencement of the aorta. In the distant portion of the bulb, however, the pulmonary channel is situated to the left and posteriorly, and the aortic channel is to the right and most forwards, and at this end these channels are respectively connected with different aortic arches, giving rise to the permanent pulmonic and systemic vessels in the manner afterwards described.
It is further to be noted that the partition of the bulb begins at the remote extremity, and progresses towards the ventricles. There is a time, therefore, during which the ventricular septum, and the septum of the bulb, advancing towards each other, are incomplete and disunited ; and from the difference in their direction it is obvious that there must
Fig. 5S9.
Fiff. 590.
Fig. 589. — View op the Front and Right Side of the Fcetal Heart, at four
MONTHS, THE RIGHT AuRICLE BEING LAID OPEN (from Kilian).
a, the right auriculo-ventricular opening ; h, a probe passed iip the vena cava inferior and throiigh tlie fossa ovalis and foramen ovale into the left auricle ; r, vena cava inferior ; c, Eustachian valve ; v, valve of the foramen ovale ; s', vena cava superior.
Fig. 590. — A'iew of the Posterior and left surface of the Heart of a F(ETUs OF four months, the left Auricle being opened (from Kilian).
o, left auriculo-ventricxilar orifice ; c, inferior vena cava, through which a probe b, is passed from below, and thence by the foramen ovale into the left auricle ; e, left auricular appendage laid open ; o, valve of the foramen ovale seen to be attached to the left side of the annulus ovalis of the septum.
FORMATION OF THE VALVES. 791
be a peculiar twist of one or both, in order that they may finally unite so as to become continuous.
The completion of the partition of the aortic and pulmonary stems is afterwards effected by the progress of the division from within outwards through the external walls of the tubes ; but the two vessels remain united externally by a common envelope of pericardium.
The remarkable cases sometimes observed of abnormal transposition of the two great arterial stems from their natural connection with their respective ventricles may be explained by reference to the history of the development of the parts of the heart before given.
Pormation of the Valves. — The formation of the auriculo-ventricular and semilunar valves begins during the time of the changes previously described by the projection of thick folds from the inner wall of the heart. In the case of the semilunar valves the trifid division is early perceived, but the cavities or sinuses within the valves are late of being developed. In the auriculo-ventricular valves there is at first an entire or annular projecting fold of the inner substance round the orifice, and this becomes gradually divided into segments, and the chordse tendincEe arej gradually produced by perforation of the valve plate. (See Tonge in Proceed. Eoy. Soc, 1868.)
The manner in which the pulmonary veins, which are formed separately in the lungs, come to be connected with the left auricle has not yet been ascertained.
No further important changes occur in the internal structure of the heart, but there are some w^hich affect the external form and thickness of its walls. In early foetal Hfe the size of the heart bears a considerably greater proportion to that of the body than at a later period. At birth it is still proportionally large. For some time the auricular portion remains more voluminous than the ventricular, but in the latter half of foetal life the permanent proportion is more nearly established. The walls of both ventricles are also thicker than in after life, and it is especially deserving of notice that the wall of the right is up to near the time of birth quite as thick as that of the left, — a peculiarity which may be connected with the ofiice of the right ventricle to propel the blood of the foetus through the extended course of the ductus arteriosus, the descending aorta and the placental circulation.
DEVELOPMENT OF THE BLOOD-VESSELS.
The Principal Arteries. The Aorta. — The most interesting part of this history is that relating to the development of the aortaand the larger vessels'^ arising from it. The double condition of the main trunk of "the aorta has already been referred to as existing in the chick up to near the end of the second day. About the fortieth hour the inedian fusion or coalescence of the two vessels begins to take place in the dorsal region, by their external union, at first in a very limited space, and very soon afterwards by the formation of a perforation through their united walls. The union of the two vessels which begins in the dorsal region extends itself backwards towards the lumbar vertebrae, and when it reaches the place where the omphalo-mesenteric arteries pass out on each side, these vessels, each of which was originally the continuation merely of one of the aortas, appear now as branches of a single and median aorta. The iliac vessels are the next large vessels formed
792
DEVELOPMENT OF THE VASCULAR SYSTEM.
from the hinder part of the aorta. The first vessels belonging to these trunks are not, however, those of the lower limbs, for these are not yet
Fig. 591.
Fig. 591. — Transverse Section through the Dorsal Region op an Embryo-Chick
OP THE Second Day (from Foster and Balfour, after His).
M, medullary canal ; Pv, proto -vertebral column ; w, rudiment of Wolffian duct in the
intermediate mass; Ch, notocliord ; Ao, one of the two aortas ; A, epiblast ; C, hypoblast ; BC, splanchnopleure ; Pp, pleuroperitoneal space.
formed ; but the umbilical or hypogastric arteries, developed at a very early period in connection with the allantois, and subsequently attain Fi£r. 592.
Fig. 592.- — ^Transversb section through the Dorsal region op an Embryo Chick,
END op Third Day (from Foster and Balfour).
Am, amnion ; m p, muscle plate ; CV, cardinal vein; Ao, dorsal aorta at the point where its two roots begin to join ; Ch, notochord (the line does not quite reach it) ; Wd, Wolffian duct; Wb, commencement of formation of Wolffian body ; ep, epiblast ; so, somatopleure ; Sp, splanchnopleure ; hy, hypoblast. The section passes through the place where the alimentary canal {hy) communicates with the yolk-sac.
AORTIC ARCHES. 793
ing to a large size along with the growth of the placenta. As the limbs are formed, arteries are developed in them, and these are branches of the main aorta ; but they are for a long time comparatively small, w'hile the umbilical arteries speedily attain a large size, so that, even up to the conclusion of foetal life, they appear to form the principal part of the two large vessels into which the aorta divides at its lower extremity. The middle sacral artery may be looked upon as the continuation of the median stem of the aorta, and probably originates from a double vessel in the same manner as the aorta itself.
The double state of the main aorta when first formed in the foetus was discovered by Serres, and described bj him in his 4tli Memoir on Transcendental Anatomy (Annal. des Scien. Nat., 18130), but was doubted by Von Baer, as Serres's observations did not show the relation of the primitive trunks of the aorta to the pharyngeal vascular arches. The fact of the original double condition was, however, placed beyond doubt by Allen Thomson (Edin. New Philos. Journal, ISoO) by the method of tranverse sections, then fii-st employed as a means of embryolog'ical investigation, and the process of median union was traced in full detail. The relation of this process to the occurrence of a permanent double canal in the aorta as a malformation, as described by Vrolik. Schroder van der Kolk and Cruveilhier. and obseiwed in at least one case by Allen Thomson, has already been refeiTed to in vol. i., p. 350.
According- to Serres, the vertebral arteries within the cranium are originally separate, and the basilar artery results from their mesial union or fusion in the same manner as occurs in the aorta, and the median union of the anterior cerebral arteries in the forepart of the Cu'cle of Willis is another example of the same process. It seems probable that the internal cross band obser%^ed liy John Davy in the interior of the basilar artery (Researches Physiol, and Anatom., lSo9, p. 301) may be a remains of the septum or united walls of the two vertebral arteries.
Aortic OX' Branchial Arclies. — The two primitive arterial arches which lead into the dorsal aorta from the arterial bulb of the rudimentary heart, at the time of the establishment of the first circulation, are the most anterior of a series of five pairs of vascular arches which are developed in succession round this part of the pharynx ; and which, since their discovery by Eathke in 1S25 (Oken's Jsis, 1825) have been regarded with much interest, as coiTcsponding with those vessels which are the seat of development of the subdivided blood-vessels of the gills in fishes and amphibia. These vascular arches thus exhibit in the amniota, along with the branchial or pharyngeal clefts and visceral plates, a typical resemblance to the structure of gills, although no full development of these respiratory organs occurs in such animals, but they furnish by their various transformations the basis of formation of the permanent pulmonary and aortic stems and the main vessels to which they give rise.
The form and position of the primitive aortic arches, up to the time of their transformation into permanent vessels, is nearly the same in reptiles, birds and mammals ; and the main differences in the seat and distribution of the large permanent vessels are to be traced to changes in the openness and extent of growth of the several arches. The five pairs Of arches do not all co-exist at the same time, for they are developed in succession from before backwards, and by the third day of incubation, or by the corresponding period of the fourth week in the human embryo, when the posterior arches have been formed, already a part of the anterior arches, beginning with the first one, has become
794
DEVELOPMENT OF THE VASCULAR SYSTEM.
obliterated. Each of tlie first four branchial arches occupies a place in the substance of the pharyngeal or visceral plates, and in front of one of the pharyngeal clefts. The first or anterior is therefore situated in the inferior maxillary plate, and in front of the tympano-Eustachian, or first pharyngeal cleft ; and the fifth arterial arch is placed behind the fourth pharyngeal cleft, and in the substance of the neck, in which there is no distinct bar or plate in the higher animals, but which is the seat of a developed branchial bar in some aquatic animals.
The vessels forming the arterial arches are given off on each side in succession from two short canals, into which the primitive arterial bulb divides immediately in front of the place where it joins the neck. These may be named the lower (ventral) or anterior aortic roots ; and similarly, when they have passed round the wall of the pharynx, the branchial arches unite in succession into a vessel on each side, thus forming the upper (dorsal) or posterior aortic roots.
On the third and fourth days in the chick, and from the fourth to the sixth week in the human embryo, there are still three complete pairs of arterial arches passing round the pharynx, and connected both before and behind with the anterior and posterior aortic roots previously mentioned. The transformations of these arches were in part traced by Von Baer and various other observers, but the fuller knowledge of their changes is due to the later researches of Eathke (Mem. of
Fig. 593. Fig. 593. — Diagram of the Aortic or
., Branchial Vascular Arches op the
Mammal, with their transformations
GIVING RISE TO THE PERMANENT ArTERIAL
Vessels (accordiug to Ilathke, slightly altered).
A, P, ijrimitive arterial stem or aortic Lnlb, now divided into A, the ascending part of the aortic arch, and P, the pulmonary ; a, the right ; a', the left aortic root ; A', the descending aorta. On the right side, 1, 2, 3, 4, 5, indicate the five Ijranchial primitive arterial arches ; on the left side, I, II, III, IV, the four branchial clefts, which, for the sake of clearness, have been omitted on the right side. It will be observed, that while the fourth and fifth pairs of arches rise from the part of the aortic bulb or stem, which is at first undivided, the first, second, and third pairs are branches above c, of a secondary stem on each side. The permanent systemic vessels are represented in deep shade, the pulmonary arteries lighter ; the parts of the primitive arches, which have only a temporary existence, are drawn in outline only, c, placed between the permanent common carotid arteries ; ce, the external carotid arteries ; ci, c'l, the right and left internal carotid arteries ; s, the right subclavian rising from the right aortic root beyond the fifth arch ; r, the right vertebral from the same opposite the fourth arch ; v', s', the left vertebral and subclavian arteries rising together from the left or permanent aortic root opposite the fourth arch ; P, the pulmonary arteries rising together from the left' fifth arch ; d, the outer or back part of the left fifth arch, forming the ductus arteriosus ; j)n, 2^n , the right and left pneumogastric nerves, descending in front of the aortic arches, with their recurrent branches represented diagrammatically as passing behind, with a view to illustrate tlie relations of these nerves respectively to the right subclavian arteiy (4) and the arch of the aorta and ductus arteriosus {d).
FORMATION OF PERMANEXT "VESSELS. 795
Vienna Acad., 1857), and although some 'points are still left in doubt, their history may now be given from these observations, and the supplemental illustration derived from the investigation of the various examples of congenital malformation, the greater number of which are manifestly related to variations in the natural mode of transformation. This will be explained by reference to the diagram in fig. 593.
From these researches it appears that the permanent vessels owe their formation to the persiscence of certain of tlie foetal arches or parts of them, while other arches or portions of them become obliterated and disappear. Thus it is ascertained that in mammals the main aortic arch, which in the adult passes to the left of the trachea and gullet, is formed by the persistence of the fourth embryonic arterial arch of the left side, which not only remains patent, and becomes connected with tlie aortic stem of the arterial bulb, but keeps pace by its increased width and the development of its walls with the rate of growth in the other parts of the body, so that it soon surpasses all the rest of the arches in its width of calibre and thickness of its walls. In birds, however, the permanent aortic arch passes to the right of the trachea and gullet, and it is formed by the persistence of the fourth embryonic arch of the right side ; while, in all reptiles, as there are two permanent aortic arches, it is by the persistence of both the right and left foetal arches that the two aortas are produced, the right being that which is most directly connected with the systemic or left ventricle.
The pulmonary arteries of mammals would appear by Rathke's observations to be developed in connection with only one foetal arterial arch, viz., the fifth of the left side, from the middle part of which they appear as branches, and the whole fifth arch of the right side undergoes rapid atrophy and ultimate obliteration. The first part of the left fifth arch, becoming the common pulmonary artery, is connected with that division of the arterial bulb which is separated as the pulmonary stem ; but the remote part of this arch also remains fully patent, and undergoing equally with the rest of it full development, continues to lead into the left root of the aorta as ductus arteriosus Botalli, which serves to convey the blood from the right ventricle of the foetal heart into the descending aorta, but becomes obliterated at the time of birth.
This duct is therefore in mammals due to a persistent condition of the fifth left branchial arch. But, in birds and reptiles, it appears that the process of transformation is somewhat different, for in them the right and left pulmonary arteries (excepting in those serpents in which there is .only one lung developed) are formed in connection with the respective right and left fifth branchial arches, and there are thus two ductus arteriosi during foetal life, the short one of the right side corresponding to that which is left in mammals, and the longer one of the left side passing round the pharynx into the left aortic root. Both of these arches are obliterated at the time of the exclusion of the bird from the eg^ ; but in some reptiles the ductus arteriosi remain permanently open during life.
The subclavian and vertebral arteries were shown by Rathke to spring from the posterior aortic roots at a place between the junction of the fourth and fifth arches. In mammals, the vessels on the left side are from the first in direct connection Avith the aortic root at the place
79G DEVELOPMENT OF THE VASCULAR SYSTEM.
which they permanently occupy ; but upon the right side, as the fourth arch and the aortic root are obliterated posteriorly, the passage for blood from the aortic stem into the subclavian trunk is formed by the persistence of the forepart of the fourth right arch as far as the place where it meets the origin of the subclavian and vertebral arteries.
The common carotid trunks, occupying the region which afterwards becomes the neck, but which is at first absent or extremely short, are formed by the anterior divisions of the aortic roots ; while the external carotid artery is due to the persistence of a channel in the continuation of each anterior aortic root, and the internal carotid artery arises from the persistence of the crossing third arch and the upper part of the posterior aortic root.
Thus it falls out that, in man and a certain number of mammals, an innominate artery is formed on the right side by the union of the first part of the fourth right aortic arch leading into the right subclavian, and the right anterior aortic root which forms the common carotid ; while, on the left side, the carotid and subclavian vessels rise separately from the permanent aortic arch in consequence of the distance lying between them in the original foetal condition.
It does not come within the scope of this chapter to describe the further steps of development of these vessels, nor to enter into an explanation of the manner in which abnormal position of the arch of the aorta and its branches, or of the pulmonary arteries, may be supposed to arise. For further information on this subject the reader is referred to the short account of the varieties given in the description of the blood-vessels in the first volume of this work, as well as to the third volume of Henle's Handbuch, and to the special works of Tiedemann and Eichard Quain on the Arteries.
DEVELOPMENT OF THE GREAT VEINS.
In the early embryo, before the development of the allantois, a right and a left omphalo-mesenteric vem bring- back the blood from the walls of the nmbilical vesicle, and unite to form a short trunk, the meatus venosus, which is continued into the auricular extremity of the rudimentary heart.
In the first commencement of the placental circulation, or in the fourth week of foetal life, two umbilical veins are seen coming from the placenta, and uniting to form a short trunk, which opens into the common omphalo-mesenteric vein. Very soon the right omphalo-mesenteric vein and right umbilical vein disappear. In connection with the common trunk of the umbilical and omphalo-mesenteric veins, two sets of vessels make their appearance in the yomig liver. Those furthest from the heart, named vena; Iwpatlca advehentcs, become the right and left divisions of the portal vein ; the others are the hepatic veins, vencv kcpaticce. Tcvclwnifs. The portion of vessel intervening between those two sets of veins forms the ductnn rcnoKiix, and the part above the hepatic vein, being subsequently joined by the ascending vena cava, forms the upper extremity of that vein. Into the remaining or left omphalo-mesenteric vein, open the mesenteric and splenic veins. The part above the latter forms the trank of the portal vein ; and the portion of vessel between the union of this with the umbilical vein and the origin of the venaj hepaticaj advehentes is so altered that the portal tiimk opens into the commencement of the right vena advehens.
At the time of the commencement of the placental cu-culation, two short transverse venous trunks, the ductii of Ci/ricr, open, one on each side, into the auricle of the heart. Each is formed by the union of a superior and an inferior vein, named the itrimitive jugular and the cardinal.
The primitive jugtdnr vem receives the lilood from the cranial cavity by channels in front of the ear, which are subsequently obliterated : in the greater
FORMATION OF THE GEEAT VEIKS,
797
part of its extent it becomes the external jugiilar vein : and near its lower end it receives small brandies, which, grow to be the external jugular and subclavian
Fig. 594. — Diagrams illustrating
THE DEVELOPMENT OF THE GrEAT
Veins (after KiJlliker).
A, plan of the principal veins of the fretus of about four weeks, or soon after the first formation of the vessels of the hver and the vena cava inferior.
B, veins of the liver at a somewhat earlier period.
C, principal veins of the foetus at the time of the first estabhshment of the placental circulation.
D, veins of the liver at the same period.
dc, the right and left ducts of Cuvier ; ca, the right and left cardinal veins ; j. j,ihe jugular veins ; s, the subclavian veins ; az, the azygos vein ; u, the umbilical or left umbilical vein ; «', in B, the temporary right umbilical vein ; o, the omphalo-meseuteric vein ; o', the right omphalo-mesenteric vein ; m, the mesenteric veins ; p, the jjortal vein ; p', p', the vente advehentes ; I, the ductus venosus ; V, I', the hepatic veins ; c'l, vena cava inferior ; il, the division of the vena cava inferior into common iliac veins ; cr, the external iliac or crural veins ; h, the hypogastric or internal iliac veins, in the line of continuation of the jirimitive cardinal veins.
In C, li, in dotted lines, the transverse branch of communication between the jugular veins which forms the left innominate vein ; ri, the right innominate vein ; ca, the remains of the left cardinal vein by whish the superior intercostal veins fall into the left innominate vein ; above lo, the obliquely crossing vein by which the hemiazygos joins the azygos vein.
veins. The cardinal veins are the primitive vessels which return the blood from
the Wolffian bodies, the vertebral column, and the parietes of the trunk. The
inferior vena cava is a vessel of later development, wliich opens into the trunk
of the umbilical and omphalo-mesenteric veins, above the vense heijaticas revehentes. The iliac veins, which unite to form the inferior vena cava, communicate with the cardinal veins. The inferior extremities of the cardinal veins are
persistent as the internal iliac veins. Above the iliac veins the cardinal veins
are obliterated in a considerable part of theii' course ; their upper portions then
become continuous with two new vessels, fhe j;osfe?-ipr vertchral veins of Rathke,
which receive the lumbar and intercostal twigi-'.
As development proceeds, the direction of the ducts of Cuvier is altered by the descent of the heart from the cervical into the thoracic region, and becomes continuous with that of the primitive jugular veins. A communicating branch makes its appearance, directed transversely from the junction of the Jeft subclavian and jugular veins, downwards, and across the middle line to the rig-ht jugular ; and further down in the dorsal region between the posterior verteljral veras a communicating branch passes obliquely across the middle line from right
7PS
DEVELOPMENT OF THE VASCULAR SYSTEM.
to left. The communicating branch between the primitive jugular veins is converted into the left innominate vein. The portion of vessel between the rio-ht subclavian vein and the tennination of the communicating branch becomes
Fisr. 595.
Fig. 595. — A and B. — Diagrammatic Outlines op the Vestige of the Left Superigr
Cava and of a Case of its Persistence (sketched after Marshall), h,
A, brachio-cephalic veins with the superior intercostal, azygos, and principal cardiac veins.
B, the same in a case of persistence of the left superior cava, showing its communication with the sinus of the coronary vein. The views are supposed to be from before, thep)arts of the heart being removed or seen tlirougli.
1, 1', the internal jugular veins ; 2, 2', subclavian veins ; 3, right innominate ; 3', right or reguhir sujjerior cava ; 4, in A, the left innominate ; in B, the transverse or communicating vein between the right and left superior vemg cavte ; 5, in A, the oj)ening of the superior intercostal vein into the innominate ; 5', vestige of the left superior cava or duct of Cuvier ; 5, 5', in B, the left vena cava superior abnormally persistent, along with a contracted condition of 4, the communicating vein : 6, the sinus of the coronaiy vein ; 6', branches of tlie coronary veins ; 7, the superior intercostal trunk of the left side, or left cardinal vein ; 8, the principal azygos or right cardinal vein ; 7', 8', some of the upper intercostal veins ; 9, the opening of the inferior vena cava, with the Eustachian valve.
the right innominate vein. The portion of the primitive jugular vein below the communicating vein, together with the right duct of Cuvier. forms the vena cava superior, while the cardinal vein opening into it is the extremity of the great vena azygos. On the left side, the portion of the primitive jugular vein placed below the communicating branch, and the cardinal and i^osterior vertebral veins, together wdth the cross branch between the two posterior vertebral veins, are converted into the left superior intercostal and left superior and inferior azygos veins. The variability in the adult arrangement of these vessels depends on the various extent to which the originally continuous vessels are developed or atrophied at one point or another. The left duct of Cuvier is obliterated, except at its lower end, which always remains pervious as the coronaiy sinus. Even i^i the adirlt, traces of the existence of this vessel can always be recognised in the form of a fibrous band, or sometimes even a narrow vein, which descends
THE FCETAL CIRCULATIOX. FORAMEN OVALE.
i99
obliquely on the left auricle ; and in front of tlie root of the left lung there remains a small fold of the serous membrane of the iiericardium, the re.sfii/ial fold of the pericardium, so named by Marshall, to whom is due the first full elucidation of the natui'e and relations of the left primitive vena cava.
The left duct of Cu%'ier has been observed persistent as a small vessel in the adult. Less frequently a right and left innominate vein open separately into the right am'icle. an arrangement which is also met witli in birds and in certain mammals, and which results from the vessels of the left side being developed similarly to those of the right, while the cross branch remains small or absent. (Quain on the Arteries, plate 58, figs. 9 and 10.)
Fig. 596. — View op the F<etal Heart AND Great Vessels, from the left
SIDE, TO SHOW THE VeSTIGE OF THE
Left Superior Cava VEI^f in situ. (This figure is j)lanned after one of Marshall's, and slightly altered according to an original dissection.)
a, right auricle ; b, left auricle and pulmonary veins ; c, the conns arteriosus of the right ventricle ; d, the left ventricle ; c, descending aorta ; + , vestigial fold of the loericardium ; /, arch of the aorta, with a part of the pericardium remaining superiorly ; r/, main pulmonary artery and ductus arteriosus ; [/', left pulmonary artery ; 1,1', right and left internal jugular veins ; 2, 2', subclavian veins ; 3, 3', right innominate and superior vena cava ; 4, left innominate or communicating vein ; 5, 5', remains of the
left superior cava and duct of Cuvier, passing at + in the vestigial fold of the pericardium, joining the coronary sinus, 6, below, and receiving above the superior intercostal vein, 7 ; 7', 7', the upper and lower intercostal vein, joining into one.
A case is recorded by Gruber, in which the left vena azygos opened into the
coronary sinus, and was met by a small vein descending from the union of the
subclavian and jugular. (Reichert and Dubois. RejTnond's Archiv, 18(54, p. 729.)
In this case, the jugular veins had been developed in the usual manner, while
the left vena azygos continued to pour its blood into the duct of Cuvier.
(Consult KoUiker, Entwickelungsgeschichte, p. 41-1, et seq. ; J. Marshall on the Development of the great Anterior Veins in Man and Mammalia, in Phil. Trans., part i., 1850 ; and Wenzel Gruber, Uber die Sinus Communis und die Valvulaj der Vense Cardiaca3, &c., in Mem. de I'Acad. imper. des Scien. de St. Petersbourg, 1SG4 ; and in Virchow's Archiv, Jan. 1SG6.)
PECULIARITIES OP THE FCETAL ORGANS OP CIRCULATION.
It may be useful here to recapitulate shortly the peculiarities of structure existing in the advanced stage of the formation of the foetal organs of circulation with reference to their influence in determining the course of the blood during intra-uterine life, and the changes which occur in them in consequence of the establishment of pulmonary respiration at birth.
The so-called foramen ovale retains the form of a free oval opening in the septum auricularum up to the fourth month, but in the course o"f that month and tlie next there takes place the growth from below and on the left side of a flat plate or curtain, which advancing upwards fills up the floor of the fossa ovalis, adheres to its left borders as far as its anterior part, and then becoming free and passing beyond the anterior
800 THE F(ETAL CIRCULATION.
border of the fossa, converts the aperture into an oblique passage or slit over the valvular margin of the fold, so that in the last three and a half months the arrangement is completed, by which blood may readily pass from the right into the left auricle, but not in a contrary direction.
The Eustachian Valve constitutes a crescentic fold of the lining structure of tlie heart, which is situated to the right of the opening of the inferior vena cava and fossa ovalis, deepens that fossa, and directs the blood entering it from the inferior cava towards the opening of the foramen ovale ; while it throws the opening of the great coronary vein into connection with the right auricle, into which the superior vena cava also opens.
The formation at an early period of foetal life of the transverse vein of the neck uniting the left with the right brachio-cephalic veins, carries the whole of the blood returning from the head and neck, together with that from the main azygos, into the stream entermg the heart by the superior cava.
The ductus arteriosus passes from the main pulmonary artery into the aorta, at the hollow part of the arch, a short distance beyond the place of origin of the left subclavian artery. It is nearly of the same width with the pulmonary stem, while the right and left pulmonary arteries are of comparatively small size, so long as the lungs have not been expanded by air in rfespiration.
TTmbilical Vessels. — Besides tlie usual branches of the descending aorta intended to supply the abdominal viscera and the lower limbs, two large vessels, named hypogastric or iimhilical arteries, are prolonged from the common iliacs, and passing out of the abdomen, proceed along the umbilical cord, coiling round
Fig. 597. — Semi-diageammatic view of the Organs of Circulation in the Fcetus FROM BEFORE (from Luschka, modified, and from Nature). |
a, front of the thyroid cartilage ; 6, right side of the thyroid body ; c, trachea ; d, surface of the right lung turned outwards from the heart ; e, diai^hragm below the apex of the heart ; /, right lobe of the liver, dissected to show ramifications of the portal and hepatic veins ; /', the middle part and left lobe of the liver in the same manner, .showing branches of the umbilical veins and ductus venosus ; g, right, fj', left kidney ; ij" , suin-arenal bodies ; h, right, li, left ureter ; i, portion of the small intestine tunied towards the side, to show the veins from it going to the portal vein ; Ic, urinary bladder ; I, is placed below the umbilicus, which is turned towards the left of the fcetus, and points by a line to the urachus ; m, rectum, divided and tied at its upjier part.
A, A. right auricle of the heart opened to show the foramen ovale : a probe, introduced through the large divided right hepatic vein and vena cava inferior, is seen passing through the fossa ovalis into the left auricle : at the lower part of the fossa ovalis is seen the Eustachian valve, to the right and inferioi'ly the auriculo-ventricular orifice ; B, the left auricular appendix ; C, the surface of the right ventricle ; D, placed on the inner surface of the left lung, i)oints to the left ventricle.
1, ascending part of the arch of the aorta ; 1', back part beyond the ductus arteriosus ; 1", abdominal aorta; 2, stem of the pulmonary artery ; 2', the place of division into right and left pulmonary arteries and root of the ductus arteriosus : the left pueumo-gastric nerve is seen descending over the arch of the aorta ; 3, superior vena cava ; 3', right, 3", left innominate vein ; 4, stem of the inferior vena cava, between the junction of the hepatic vein and the right auricle ; 4', continuation of the vena cava inferior below ; 5, umbilical vein within the body of the fojtus ; 5 x , without the bo'ly, in the umbilical cord ; 5', 5', ductus venosus ; between 5 and 5', the direct branches of the umbilical vein to the liver ; 5", 5", hepatic veins, tlu-ough one of which a probe is passed into the fossa ovalis and through the foramen ovale ; 6, vena portD3 ; 6', its left branch joining the umbilical vein; (i", its right branch; 7, placed on the right iliac vein, points to the right common iliac ai-tery; 7', left common iliac artery; 8, right, 8', left umbilical arteries coming from the internal iliac arteries ; 8 x , umbilical arteries without the body, in the umbilical cord ; 9, 9', external iliac arteries ; 10, placed below the right renal vessels ; 11, inferior mesenteric artery, above the root of which .ore seen the two spermatic arteries.
THE FCETAL CIRCULATION.
Fig. 597,
801,
3 F
802 TIIE FCET-iL CLRCULATIOX.
the umbilical vein to reach the placenta. The commencement of each of these vessels afterwards forms the tmnk of the corresponding- internal iliac artery, but, from their size, they might be regarded in the fojtus as the continuations of the common iliac arteries into which the aorta divides. From the placenta the blood is returned by the umbilical vem. which, after entering the abdomen, communicates by one branch with the portal vein of the liver, and is continued by another, named ductus rcnosus, into one of the hepatic veins, through which it joins the main stem of the vena cava inferior.
Course of the Blood in the Poetus. — The right auricle of the foetal heart receives its blood from the two venre cavfe and the coronary vein. The blood brought by the superior cava is simply the venous blood returned from the head and upper half of the body ; whilst the mferior cava, which is considerably larger than the superior, conveys not only the blood from the lower half of the body, but also that which is returned from the placenta through the umbilical vein. This latter stream of blood reaches the vena cava inferior, partly by a direct passage — the ductus venosus, and partly by the hepatic veins, which bring to the vena cava inferior all the blood circulating through the liver, whether derived from the supply of placental blood entering by the umbilical vein, or proceeding from the vena portfe or hepatic artery.
The blood of the superior vena cava, descending in front and to the right of the Eustachian valve, and mixed with a small portion of that from the inferior cava, passes on into the right ventricle, and is thence propelled into the trunk of the pulmonary artery. A small part of it is then distributed through the branches of that vessel to the lungs, and retuins by the pulmonary veins to the left auricle ; but, as these vessels remain comparatively undilated up to the time of birth, by far the larger part passes through the ductus arteriosus into the dorsal aorta, entering that vessel beyond the place of origin of the arteries of the head and upper limbs, and, mixed perhaps with a small quantity of the blood flowing into the aorta from the left ventricle, is distributed in part to the lower half of the body and the viscera, and in part is conveyed along the umbilical arteries to the placenta. From these several organs it is returned by the vena cava inferior, the venaj portsB, and the umbilical vein : and, as already noticed, reaches the right auricle through the trank of the inferior cava.
Of the blood entering the heart by the inferior vena cava, only a small part is mingled with that of the superior cava, so as to pass into the right ventricle ; by far the larger portion, directed by the Eustachian valve through the foramen ovale, flows from the right into the left auricle, and thence, together with the small quantity of blood returned from the lungs by the pulmonary veins, passes into the left ventricle, from whence it is sent into the arch of the aorta, to be distributed almost entu-ely to the head and upper limbs. A small portion of it, may, however, flow on into the descending aorta, and join the fuller stream of blood from the ductus arteriosus. From the upper half of the body the blood is returned by the branches of the superior cava to the right auricle, from which its course into the right ventricle and pulmonary trunk has been already traced.
There is probably a considerable difference in the early and more advanced stages of foetal life, m the distribution of the stream of blood entering the heart by the vena cava inferior. In the early stages, a lai-ge part of the cuiTcnt being directed into the left, but in the three last months, and as the foetus approaches maturity, more and more of the blood of the inferior cava joins the stream from the superior cava ; and, indeed, the coui-se of the blood, and the relative position of the veins, as well as other original peculiarities of the foetal heart, become gradually altered, in preparation, as it were, for the important changes which take place at birth. It seems also probable that very little of the blood propelled from the left ventricle passes into the descending aoi-ta beyond the ductus arteriosus diu-ing those months of foetal life in which the peculiarities of the circulation are most complete.
From the preceding account of the course of the blood in the foetus, it will be seen that, whilst the modified blood from the placenta is principally conveyed to the upper or cephalic half of the foetus, the lower half of the body is chiefly supplied with the blood which has already circulated through the head and upper
CHANGES AT BIRTH. 803
limbs. The larger portion of the blood, however, which passes into the descending aorta, is sent out of the body to the placenta. This duty is principally performed by the right ventricle, which after birth is charged with an office somewhat analogous, in having to propel tlie blood through the lungs. But the passage of the blood through the vessels of the umbilical cord and placenta is longer and subject to greater resistance than that of the pulmonary chculation, and the right ventricle of the foetus, although probably aided by the left in the placental circulation, also takes a large shave in the systemic through the lower half of the body ; and this, ijerhaps, may be the reason wliy the wall of the right equals in thickness that of the left ventricle in the fa^tus.
Sabatier was the first to call attention particularly to the action of the Eustachian valve in separating the currents of blood entering the right auricle by the superior and inferior venaj cavse. (Traite d'Anat., vol. ii., p. 22i.) This separation, as well as that occm-ring between the currents passing through the aortic arch and the ductus arteriosus into the descending aorta, were illustrated experimentally by John Reid. (See art. '• Heart," in Cyclop, of Anat. and Phj'siol., and Edin. Med. and Sui-g. Journal, ISo.j.) A striking confirmation of the extent to which the last mentioned division of the two currents of the foetal blood may take place, without distm'bance of the chculation up to the time of bii-th, is afforded by the examples of malformation in which a complete obliteration has existed in the aortic trunk immediately before the place of the union of the ductus arteriosus with the posterior part of the aortic arch.
CHANGES IN THE CIRCULATION AT BIRTH.
The changes which occur in the organs of circulation and respiration at birth, and lead to the establishment of their permanent condition, are more immediately determined by the inflation of the, lungs with air in the first respiration, the consequent rapid dilatation of the pulmonary l)lood-vessels with a greater quantity of blood, and the interruption to the passage of blood through the placental circulation. These changes are speedily accompanied by shrinking and obliteration of the ductus arteriosus, in the space between the division of the right and left pulmonary arteries and its junction with the aorta, and of the umbilical arteries from the hypogastric trunk to the place of their issue from the body by the umbilical cord ; — by the cessation of the passage of blood through the foramen ovale, and somewhat later by the closure of that foramen, and by the obliteration of the umbilical vein as far as its entrauce into the liver, and of the ductus venosus within that organ.
The process of obliteration of the arteries appears to depend at first mainly on the contraction of the coats, but this is very soon followed by a considerable thickening of their substance, reducing ra^mlly their internal passage to a minute tube, and leading in a short time to final closure, even although the vessel may not present externally any considerable diminution of its diameter. It commences at once, and is perceptible after: a few respirations have occurred. It makes rapid progress upon the first and second days, and by the third or fourth days the passage through the umbilical arteries is usually completely interrupted. The ductus arteriosus is rarely found open after the eighth or tenth day, and by three weeks it has in almost all instances become completely impervious.
The process of closure in the veins is slower, there not being the
same thickening or contraction of their coats ; but they remain empty
of blood and collapsed, and by the sixth or seventh day, are generally
closed.
3 F 2
S04
DEYELOPMEXT OF THE UEO-GENITAL ORGANS.
Although blood ceases at once to pass through the foramen ovale from the moment of birth, or as soon as the left auricle becomes filled with the blood returning from the lungs, and the pressure ^Yithin the two auricles is equalised, yet the actual closure of the foramen is more tardy than any of the other changes now referred to. It is gradually effected by the union of the forepart of the valvular fold forming the floor of the fossa ovalis with the margin of the annulus on the left side ; but the crescentic margin is generally perceptible in the left auricle as a free border beyond the place of union and not unfrequently the union remains incomplete, so that a probe may be passed through the reduced aperture. In many cases a wider aperture remains for more or less of the first year of infancy, and in certain instances there is such a failure of the union of the valve as to allow of the continued passage of venous blood, especially when the circulation is disturbed by over-exertion, from the right to the left auricle, as occurs in the malformation attending the morbus coeruleus.
DEVELOPMENT OE THE GENITAL AND TJRINAEY ORGANS. The development of the permanent genital and urinary organs in
Fig. 598.
Fig. 598. — Enlarged View from before op the Left
Wolffian Body before the Establishment op the
DISTINCTION op Sex (from Farre after Kobelt).
a, a, h, d, tubular structure of the WoliBan bodj ; e, Wolffian duct ; /, its upper extremity ; <y, its termination in .r, the iiro-genital sinus ; h, the duct of Miiller ; i', its upper still closed extremity ; I:, its lower eud terminating in the uro-genital sinus ; I, the mass of blastema for the reproductive organ, ovary or testicle.
birds and mammals, is preceded by the formation of a temporary glandular organ with which the principal parts of both these sets of organs are in their origin connected. These bodies are named the Wolffian bodies, after their discoverer, C. F. Wolff". From this close association of these organs, it becomes necessary to describe their development together.
PRIMARY FORMATION OP THE URO-GENITAL SYSTEM.
Wolifian bodies. — The Wolffian bodies occupy a considerable space in the abdominal cavity of birds and mammals from an early period of fcetal life, extending at first from the fifth or sixth protovertebral segments to near the caudal extremity, in the form of two reddish prominent ridges, one on each side of the primitive intestine, and below the protovertebral columns and primitive aortaa. They are thickest in the middle of their length, and taper somewhat at their upper and lower extremities. They consist, when fully formed, of short slightly convoluted tubes running transversely, connected on the inner side with vascular glomeruli, very similar to the Malpighian corpuscles of the permanent kidneys, and leading along the outer border into a tube named the
ORIGIN OF THE URO-GENITAL SYSTEM.
805
Wolffian duct, which terminates on each side in the cloaca. The Maipighian glomeruli were first discovered by Kathke, who pointed out
Fig. 599. — Human Embryo of from 25 to 28 days, viE\yED
FROM BEFORE, THE ThORAX AND AbDOMEN OPENED (frOlll
KoUiker after Coste).
0, tlie eye ; in, the maxillary plate ; mn, the inferior maxillary plate ; h, the second postoral plate ; h, the heart ; w, Wolttiaii bodies and ducts on their outer borders ; I, the liver ; c', the upper and t", the lower limbs ; a, the allantoid pedicle, and on each side of it the umbilical arteries ; i, i, tue upper and lower parts of the intestine of which the middle parts with the vitello-intestinal duct have been removed, leaving the mesentery sti'etched between.
Fig. 599.
their vascular structure, and their vessels derived from neighbouring branches of the aorta. The ducts of the Wolffian bodies are found to contain a whitish fluid, and the bodies are believed to perform the glandular office of kidneys during a part of foetal life in the higher vertebrata, and they have accordingly received the name of irrimonlial Iciihieys, a designation which is quite appropriate, as it appears that in fishes and amphibia, they remain as the whole in some, and a part in others, of the permanent kidneys.
In the human foetus they begin to be formed along wuth the allantois, at a very early period, probably before the third week, as they are already very apparent in the fourth. They have attained their full size by the sixth week, and in the seventh and eighth are rapidly diminishing in size, in connection with the changes which accompany the development of the genital organs and the permanent kidneys.
Fig. 600.
Fig. 600. — Transverse Section through: the Embryo of the Chick and Blastoderm ON THE Second Day (from Kolliker).
d d, hypoblast ; ch, chorda dorsalis ; u w, primordial vertebrae ; m r, medullary
plates ; h, corneous layer or epihlast ; u lo h, cavity of the primordial vertebral mass ;
in p, mesoblast dividing at s p into h p I, somatopleure, and d f, splanchnopleui'e ;
un g, Wolffian duct, beginning in the intermediate cell-mass.
As development advances the "Wolffian bodies rapidly become proportionally shorter and thicker : they shrink towards the lower part of the abdominal cavity, and soon become almost entirely wasted. By
806
DEVELOPMENT OF THE TJRO-GENITAL OEGANS.
tne middle of tl\e third month only traces of them are visible in the human embryo.
First origin of the Wolffian bodies. — Difference of opinion has for some time existed among embryologists as to the exact source of
3 "-■
fig. 601. — Tkaxsverse Section THRotJcn the Abdominal Region of the Chick ok THE Third Day (from Kolliker).
The explanation of tlic letters is the same as that in the previous figure.
the rudiments of the uro-genital system, but it now appears to be fully ascertained by the concurrence of a variety of observers, more especially of Waldeyer, Scheuk, and Balfour, that the Wolffian duct, which is the iirst part formed, and the formative substance of the AVolffian tubes
Fisr. 602.
d<l'
Fig. 602. — Transverse Section of the Embryo-Chick of the Third Day (from Kolliker).
mr, medullary canal and medulla of the spinal cord ; en, notochord ; mch, primordial •vertebral mass ; 7n, muscle-plate ; dr and (//, groove of the primitive intestine as formed by the hyijoblast and splanchnopleure ; cio, one of the two aoi-tte : mw, Wolffian body ; unff, Wolffian duct ; re, vena cardinalis ; h, epiblast ; hp, somatopleure and its reflection into the amnion ; ^^j the p] euro-peritoneal space.
and glomeruli, proceed from the mesoblast, and as these form the foundation of the principal urinary and genital organs, it follows that this
THE WOLFFIAN BODIES.
807
system as a wliole has its Inundation in the mesoblastic layer. In birds
and mammals the duct, which is first formed, appears in its commencement as a sohd cord in the upper part of a group of cells, projecting
below the epiblast, in the interval between the protovertebral mass and
Fig. 603. — Kidneys, Wolffia>- Bodies, Wolffian Fi^r. 603.
AKD MtJLLERIAN DuCTS OP A FtETAL BiRD.
Magnified (after J. Miiller).
a, kidney ; 5, tuliular part of Wolfflan body : c, the ovary ; d, suprarenal body \ e, ureter ; /, Wolffian duct ; g, duct of Miiller.
the united somatopleure and splanchnopleure of the mesoblast, and thence called the mtermcdiate cell mass (fig. GOO, vnfi). This cord becomes hollow, and gradually changes its place by sinking downwards in the cellular mass in which it is imbedded, towards the pleuro-peritoneal cavity, while the tubular and glomerular structures of the Wolffian body are developed as diverticula from the duct in connection with the neighbouring cellular blastema.
The intermediate cell-mass now forms a considerable projection to the outside of the mesentery, which occupies a median
position (figs. 602 and C04), and the epithelium on its surface exhibits a considerable thickening in two places, first, along the inner side, where it becomes columnar, and forms an opaque whitish ridge, the //«'we^;//7itflium, the seat of after formation of the primitive ovigerms ; and second, along the outer side in a line inside the seat of the Wolffian duct, where, by a process of grooved involution, there is gradually formed the duct named Miiller ian, after its discoverer, Johannes Miiller. It is now fully ascertained that both the Wolffian and Miillerian ducts are constantly present in all embryoes of birds and mammals, whatever the sex they may be destined afterwards to assume ; but the respective ducts have a difterent sexual destination, for the duct of ]\lUller becomes converted into the oviduct of the female, while in the male the Wolffian duct forms the vas deferens, or main seminal duct of the testicle ; and while vestiges of the duct of Miiller are perceptible in the developed male, remains of the Wolffian duct are almost always present in the female in a manner afterwards to be described.
The permanent kichicijs of birds and mammals take their origin in connection with the Wolffian duct and formative substance deposited near the Wolffian bodies. Their first rudiments consist in a diverticulum from the upper or dorsal aspect of the Wolffian duct near its posterior extremity, which constitutes the commencement of the ureter ; and from this the tubular and glandular parts of the kidney are formed by extension into the neighbouring mass of blastema at a period somewhat later than that of the development of the Wolffian body itself.
The researches of Waldeyer and others have shown that the procluciive glands of the generative organs in the two sexes, ovary and testis, arise from nearly the same part of the intermediate cell mass, but in
808
DEVELOPMENT OF THE UEO-GEXITAL OEGAXS.
a manner somewhat different. Both are mainly produced in the substance which lies along the inner border of the blastemic mass alreadyreferred to, and which may therefore be named the common reproductive blastema ; but with this important difference between them, that in the
Fijr. C04.
Fig. G04. — Transverse Section op the Wolffian Body and Rudiment of the Ovary
AND THE Duct op Muller in an Embryo Chick at the end of the fourth
DAY (from Waldeyer).
WK, Wolffian body ; y, section of the Wolffian duct ; a, germ epithelium with, o, o, cells enlarging into ovigerms ; a', epithelium near the place of involution of MviLler's duct, z ; E, stroma of the ovai-y ; m, mesentery ; L, lateral wall of the abdomen.
female the primitive ova originate more immediately from the cells of the surface in the germ ejntlieUum, and become afterwards imbedded as Graafian follicles in the deeper substance of the mass which forms a stroma round the ova ; while the glandular substance of the testicle is apparently developed within the cell mass, without any direct concurrence of the superficial or germ epithelium, — which, though at first existing in male as well as in female cmbryoes, and even exhibiting some tendency to the enlargement of some cells as ovigerms (Waldeyer), soon becomes atrophied and reduced in thickness in the male as the structure of the testicle becomes developed.
The ducts of Muller open at their anterior extremities into the pleuro-peritoneal cavity by the orifice which ultimately becomes the infundibulum and fimbriated ostium abdominale ; and, as their lining membrane has originally been formed by an involution of the epithelium (germ-epithelium) of that cavity, it follows that the lining membranes of the female passages (Fallopian tubes and uterus) which in their later development assume the characters of mucous membrane, and are de
THE WOLFFIAN AND MULLERIAN DUCTS.
809
scribed as such, have in reality the same origin as the lining membrane of the pleuro-peritoneal cavity.
Fig. 605.
Fig. 605. — DiAGRAiniATic Outline OF THE Wolffian Bodies
IN THEIR RELATIONS TO THE EUDIJIENTS OF THE RErRODUC TivE Organs (A. T.).
ot, Seat of origin of the ovaries or testes ; W, Wolffian bodies ; 11', w, Wolffian chicts ; m, m, jVIiillerian ducts ; r/c, genital cord ; ■iig, sinus iirogeuitalis ; /, intestine ; cl, cloaca.
These ducts at first unite with the Wolffian ducts on each side separately, but later they become separated from them and conjoined at their lower or posterior extremity, and in the development of the female type the uterus results
from the further growth of this median or united part, while in the male sex the prostatic vesicle and gland may be looked upon as its nearest representative, and other partial vestiges of the female passages are to be found in the human species and in various degrees in different mammals.
. The Wolffian duct, as has already been stated, becomes the vas deferens of the testicle, while the secreting part of the gland, comprising the tubuli seminiferi and the rete testis, are developed in the reproductive blastema of the intermediate cell mass. The union of these two parts of the male organs through the coni vasculosi and the epididymis is brought about by the development of the eflFerent vessels in the upper part, or what may appropriately be termed the sexual part of the Wolffian body, as this structure has been shown by Banks and others to differ from the lower and larger part of the organ by the absence of the vascular tufts or glomerular arrangement in connection ^vith its tubes. The convoluted tubes forming the efferent vessels, which fi-om the time of their first production are in communication with the upper part of the Wolffian duct, become subsequently connected with the vessels of the rete testis, and thus the original Wolffian duct becomes in its upper part the tube of the epididymis, and in its lower the main excretory duct or vas deferens of the testis.
Homologies of theWolffian body; — An interesting view of the correspondence of the urino-genital organs in different animals is presented by the recent observations of embryologists on the formation of the Wolffian bodies. It was ascertained by His, Bornhaupt, Rosenberg and Goette, that in the lower vertebrates a second body similar to the Wolffian was formed later in connection with its main duct ; and the researches of Balfour and Semper have shown that in the selachians the permanent kidneys, which had long been believed to be the same
810
DEYELOPMEXT OF THE UEO-GEXITAL OEGANS.
with the Wolffian bodies, consist in reality of two sets of tubular organs, of which one corresponds to the Wolffian bodies of the embryoes of the amniota, while the other tubnlar body, already referred to as being of later formation and as connected with the main Wolffian duct, corresponds to the permanent kidneys of the higher animals. Balfour
Fig. 606.
Fig. 606. — Diagram op toe
Primitive Uro-genital Organs IN THE Embryo previous
TO Sexual Distinction.
The parts are shown chiefly in profile, but the MuUerian and Wolffian ducts are seen from the front. 3, ureter ; 4, urinary bladder ; 5, urachus ; ot, the mass of blastema from which ovary or testicle is afterwards formed ; W, left Wolffian body ; X , part at the apex from which the coni vasculosi are afterwards developed ; w, w, right and left Wolffian ducts ; m, on, right and left Miillerian ducts uniting together and with the Wolffian ducts in g c, the genital cord ; ug, sinus urogenitalis ; ?', lower part of the intestine ; cl, common opening of the intestine and urogenital sinus ; cp, elevation which becomes clitoris or penis ; Is, ridge from which the labia majora or scrotum are formed.
has also ascertained (Jour, of Anat. and Physiol., yoI. x., 1875) that in the selachians both the ducts are found which exist in the amniota, viz., both the Wolffian and the Miillerian ducts, but that they arise in a somewhat different manner from that by which they are produced in birds and mammals, inasmuch as in the selachians the duct of Miiller arises by the formation of a septal partition which divides the original duct through a considerable part of its length into two canals : one of these, the Miillerian duct, is in communication with the pleuro-peritoneal cavity in front, and opens into the cloaca behind as a separate tube ; the other corresponding with the Wolffian, besides being the excretory duct of the primordial kidneys, becomes the vas deferens of the testicle. In the selachians, therefore, the permanent kidneys consist of two parts, of which one, the anterior, is homologous with the temporary kidneys or Wolffian bodies, while the other, or posterior part, corresponds with the permanent kidneys of birds and mammals.
Balfour and Semper have made further the interesting discovery that the transverse tubes of the two parts of the primordial kidney of the lower animals correspond in number and position with the vertebral segments of the region of the embryo in which they are situated, — a fact of great interest in vertebrate morphology, and, according to the authors, leading also to important views of the morphological correspondence of the organs in question with similar organs in the anne
OEIGIN OF THE EXTERNAL OEGANS.
811
lida. The tubes of the kidneys in the lower vertebrata are therefore named segmental tiilios, and their common duct (Wolffian), the segmental duct. lu the amniota, however, the same correspondence between vertebrate segments and Wolffian body tubes no longer exists. The External Organs. — The existence in the embryo at first of a single outlet or cloaca, for the urogenital passages and the alimentary canal in common, has already been referred to. This condition of the
Fig. 607. — Development of the External Sexual Organs in the Male
AND Female from the Inbifferext
Type (from Ecker).
A, the external sexual organs in an embryo of about nine weelis, in which external sexual distinction is not yet established, and the cloaca still exists ; B, the same in an embryo somewhat more advanced, and in which, without marked sexual distinction, the anus is now separated from the urogenital aperture ; C, the same in an embryo of aljoutten weeks, showing the female tj^se ; D, the same in a male embryo somewhat more advanced. Throughout the figures the following indications are employed ; }■)€, common blastema of penis or clitoris ; to the right of these letters in A, the umbilical cord ; p, penis ; c, clitoris ; cl, cloaca ; \({f, urogenital opening ; «, anus ;
I s, cutaneous elevation which becomes labium or scrotum ; I, labium ; caudal or coccygeal elevation.
s, sci'otum ; co^
parts connected with the surface continues even beyond the time when the sexual distinction has begun to become manifest in the deeper organs, as up to the seventh day in the chick and the end of the eighth week in the human foetus. Previous to this time the cloaca presents itself in the form of a Avide cavity, into the middle of which the intestine descends on the dorsal aspect. The pedicle of the allantois opens by a deep groove or recess anteriorly or on the ventral aspect, and on each side there is a widening, into wliich, in succession from the ventral to the dorsal aspect, open the Mullerian and Wolffian ducts and the ureters. The external opening has the form of a vertical slit wider above and below, and is situated in a raised portion of the common integument, from which all the other parts retire more and more within the cavity of the pelvis as it gradually deepens.
The first change which takes place in the rudiments of the external organs, and which is common to all embryoes, and therefore to botli sexes, consists in the advance from the sides and behind of the partition which separates the intestinal portion from the rest, thus throwing the urogenital ducts into connection with a wide ventral part ot the lower aperture, urogenital sinus, while the intestine is left in communication with the narrower dorsal section. The anus, strictly so called, now appears as the opening of the alimentary canal, and in front of it the urogenital aperture forms a narrow vertical slit wider behind than before, and leading into the urogenital sinus.
In front of the last-named aperture there now rises a well-marked prominence of the integument, the rudiment of the still indifferent
812 DEVELOPMENT OF THE UEO-GEXITAL OEGANS.
organ rei:)resenting the clitoris or penis. Into this jirominence the nrogenital groove runs forward, and surrounding the prominence in front, and continued downwards on each side of the urogenital opening, there is a raised ridge of integument, which is the foundation of the future lal)ia majora in the female, and of the two halves of the scrotum in the male.
The description of the later changes which occur in these parts in the development of fuller sexual differences will be given hereafter. Here it will be sufficient to state their general nature. In embryoes which are assuming the male type, the common eminence becomes gradually longer, more cylindrical and deeply grooved along its lower surface. The lateral ridges of the urogenital opening become united from behind forwards along the middle line, and this union is gradually continued into the ridges of the groove below the penis, so as to enclose a canal which becomes the urethra with its tegumental and spongy vascular coverings, and to form below this the scrotum, in which the raphe is the remains of the median union of the integmnent.
In female embryoes, on the other hand, the cylindrical eminence remains comparatively small, and the groove along its lower surface ■widens into two folds, forming the laljicc minorcc or nymphae ; while the larger lateral integumental folds, retaining their prominence and remaining separate, constitute the labia majora. The groove is not closed, but widened and shortened so as to become the vulva, while more deeply the sinus urogenitalis shortens itself considerably so as to form the limited atrium vagiiuc, into which open the urethra from the urinary bladder and the now united lower portion of Midler's ducts forming the yagina.
From the previous statement, it appears that both the urinary and the reproductive organs take their origin in symmetrical pairs from the intermediate cell-masses of the mesoblast, which are situated to the outside at first, and subsequently below, the protovertebral columns. The earliest formed of these organs are the Wolffian bodies, by which the others are all intimately connected together in their development, so as, to form one great system. It further appears that, while the urinary organs are developed in an entirely similar manner in all embryoes, there are in the sexual organs certain departures from the common type b}^ which the peculiarities of the male and female are established. The general plan of development of these organs having been previously described, the history of the process will now be completed by an account of the further changes which they undergo.
FURTHER HISTORY OF THE3 DEVELOPMENT OF THE UROaENITAL
ORGANS.
Tlie Kidneys and their Ducts. — -These organs are developed togetlier from a mass of formative cells situated posteriorly on the dorsal aspect of the Wolffian bodies, their first hollows being formed as diverticula from the Wolffian duct.
The formative blastema of the kidney, as observed 133- Eathke in the foetal calf, soon contams a series of club-shaped bodies v^'hich have their larger ends free and turned outwards, and their smaller ends or pedicles directed inwards towards the futiire hilus, where they are blended together. As the organ grows these bodies increase in number, and finally, becoming hollow, form the uriniferous tuhes. At fii-st, short, wide, and dilated at their extremities, the tubuli soon become elongated, narrow, and flexuous, occupying the whole mass of the kidney, which then appears to consist of cortical substance only. At a subsequent period, the tubuli nearest
FAETIIEE DEVELOPMEXT OF THE URINARY ORGAXS. 813
the hilus become straighter. and thus fomi the medullary substance. The tubuli, as sho-wTi by Valentin, are absolutely, as 'u-ell as relatively, wider in the early stages of formation of the kidney. The Malpighian corpuscles have been seen by Rathke in a sheep's emin-yo, the kidneys of which measured only two and a half lines in length. Koliiker observed the kidneys already foi-med in tlie human embryo of between six and seven weeks, the ureter being hollow, and communicating with dilated cavities within the rest of the blastema. At eight weeks they had assumed their characteristic reniform shape, and about the tenth week they are distinctly lobulated. The separate lobules, generally about fifteen in number, gi-adiially coalesce in the manner already described ; but at birth, indications of the original lobulated condition of the kidney are still visible on the surface, and the entire organ is more globular in its general figui-e than in the adult. The kidneys are then also situated lower do-mi than in after-life.
In the advanced fcetus and in the new-born infant, the kidneys are relatively larger than in the adult, the weight of both glands, compared with that of the body, being, according to Meckel, about one to eighty at bu-th.
The Suprarenal Boclies. — These organs have their origin in a mass of blastema, placed in front of and between the kidneys and the Wolffian bodies. They appear to originate in a single mass, and afterwards to become divided. Koliiker has also observed them in close connection with the substance in which the large sympathetic plexus of the abdomen is produced, but it is not ascertained that they have a common origin.
In the human emlnyo the suprarenal bodies are at the seventh or eighth week larger than the kidneys, and quite conceal them, but after that time their relative size diminishes, so that at about the tenth or twelfth week they are smaller than the kidneys. At six months, according to Meckel, the proportion of the suprarenal bodies to the kidneys is as 2 to 5 ; at birth the proportion between them is 1 to 3, whilst in the adult it is about 1 to 22. They diminish much in aged persons, and are sometimes scarcely to be recognised.
The Urinary Bladder and Urachus. — It lias elsewhere been stated that in the human eniljiTO the vesicular part of the allantois extending beyond the umbilicus is closed at a very early period. Its pedicle, however, remains in communication with the urogenital sinus, and receives the ureters from the developing kidneys. The lower part of the pe'dicle undergoes a gradual dilatation to form the urinary hladder, while at the connection of this part with the urogenital sinus a constriction occurs in the part which gives rise to the urethra. Tliis in the female opens at once into the original urogenital sinus, but in the male the passage is continued onwards through the penis by the median union of the parts below that organ.
The part of the allantois situated above or in front of the bladder within the abdomen remains very much narrowed as the urachus, a tapering process of the upper extremity of the bladder into which at first the internal cavity is prolonged, but which later consists only of the muscular and fibrous coats. This process may for a time be traced for a short distance within the umbilical cord, but at an early period all vestiges of its farther prolongation disappear.
Genital Cord; — In both sexes, as was first fully shown by Tiersch and Leuckart in 1852, the two Wolffian ducts become united by surrounding substances into one cord behind the lower part of the urinary bladder ; but retaining internally their separate passages until they reach the sinus urogenitalis. With this cord the Miillerian ducts are incorporated posteriorly, so that at one time there are four passages through the whole of the genital cord. The Miillerian ducts next coalesce into one at some little distance from their lower ends, and this fusion, progressing upwards and downwards for a considerable space, a
814
DEVELOPMENT OF TEE UEO-GENITAL OEGAIs^S.
single median cavity is produced which lies between the still separate canals of the Wolffian ducts. A large accumulation of tissue in its walls gives to the genital cord great thickness as compared with the neighbour
Fig. GOS.
Pig. 608. — Transverse Sections of the Genital Cord in a Female Calf Embryo.
Magnified 14 diameters (from Kolliker).
1, near the upper end ; 2 and 3, near the middle ; 4, at the lower end ; a, anterior, p, posterior aspect ; m, Miillerian ducts, united or separate ; W, Wolffian ducts.
ing parts of the ducts where tliey emerge from its enclosure. The lower ]3art of the united Miillerian ducts thus comes afterwards to form the foundation of the vagina and lower part of the uterus in the female, and the corresponding prostatic vesicle with its occasional accompaniments, or the uterus masculinus of the male.
REPRODUCTIVE ORGANS.
In the farther history of the development of the genital organs it will be expedient to consider them in the two sexes in succession under
Fig. 609. Fig. 609. — Internal Genital Organs op a Male
Human Embryo of S^ inches long (from Waldej'er).
t, body of the testicle with seminal canals formed ; c, epididymis, or upper part of Wolffian body ; w, Wolffian body, lower part, 1>ecoming paradidymis or organ of Giraldes ; w', Wolffian duct, becoming vas deferens ; ff, gubernaculum.
the three heads of 1st, the productive organs ; 2nd, the conducting passages ; and 3rd, the external organs.
Keproductive Glands. — It has already been explained that although the male and female productive organs lake their origin fi'om a mass of blastema which is on the whole identical in the two sexes, yet there are such differences in the development ol' the essential parts of the respective structures of the ovary and testicle as almost to warrant the conclusion that these organs are from the firsfc in some measure distinct.
FAETHER DEVELOPMENT OF THE TESTICLE. 815
The distinction of sex begins to be perceptible in the internal organs of the human eminyo in the seventh week, and becomes more apparent in the eighth. The reproductive gland is from the first connected with the Wolffian body, of which its blastema seems to be actually a part ; and it remains attached to it, or after its disappearance to the structure wliich occupies its place, by a fold of the peritoneal membrane, constituting the mesorchiura or mesovarium. Upper and lower bands fix the Wolffian body ; the upper passing to the diaphragm may be named the diaphragmatic ; the lower running down towards the groin from the Wolffian duct, contains muscular fibres and constitutes the future gubemaculum testis and round ligament of the uterus.
The Testicle. — In male embryoes at the tenth week already seminal canals are visible, being at first, according to Kolliker, entirely composed of cells, but by the eleventh and twelfth weeks the tubes have become somewhat smaller, longer, and are now branched and possess a membrana propria. There is also by the end of the third month a commencement of lobular division, and the body of the testis is now covered with a condensed laj-er of fibrous tissue which forms the tunica albuginea.
In connection witli the development of the spermatic filaments or spennatozoa, the essential part of the male reproductive element, previously referred to at p. 448 of this volume, it may here fui'ther be stated that renewed researches by Neumann (Ai'chiv filr Microsc. Anat., vol. xi., p. 292), appear to show that the doubts thrown by Sertoli and Merckel on the statements of V. Ebner are not well founded, that there really exist within the seminal ducts protoplasmic columns stretching from within the wall of the tube into its cavity, and that the spennatic filaments are produced in connection with the inner ends of the columns as branched lobes, amounting- in general to ten or twelve in number, in which the heads lie outwards imbedded in the protoplasmic stalk, and the filaments or tails are directed inwards towards the central lumen of the tube. Each stalk, or sjjcrmatohlant, as Neumann proposes to name it, possesses a large clear nucleus with nucleolus, and previous to the formation of the heads there are nuclei corresponding in number to them, which do not, however, appear to arise directly from division of the main nucleus of the stalk, but rather to be formed as free nuclei in the protoplasm. Each spermatozoon consists of three parts, which are most easily disting-uished in those which have not reached their stage of full development. These parts are, 1st, the head, or, as it may from its form in some animals be called, the hook ; 2nd, the body or middle pai-t, forming a slight thickening, and frequently of a vesicular appearance ; and 3rd, the filament or tail. The fii-st of these proceeds more immediately from a nucleus, the second is the remains of the protoplasmic covering of a spennatoblastic lobe, the feird is a ciliated production from the last. The bases of the spermatoblasts ai-e attached to the inner sm-face of the fibrous coat of the seminal canals, to which they furnish a complete lining, being set closely upon it like a layer of hexagonal plates. The stalks rise as tapering processes from these plates, and in the intervals between the stalks, necessarily largest towards the periphery, there is a number of opaque gi-anular spherical cells, the exact nature of which is not ascertained, but which it is conjectured may be the source of new spermatoblasts.
An interesting view is presented by Neumann of the analogy of these spennatoblasts of the seminal tubes with the much elongated ciliated cells wliich are found in the canals of the coni vasculosi and tube of the epididymis, in accordance with which it may be held that the spermatic filaments are a peculiar forai of ciliary structm-e, developed from protoplasmic elements of a cellular nature, but v/hich undergo a peculiar modification in connection with the special destination. cf the spermatozoa.
816 DEVELOPMENT OF THE EEPEODUCTIYE ORGANS.
The Ovary. — Considered as a glandular organ the ovary differs from other glands by the absence from it of excretory ducts, and by the
Fig. 610. Fig. 610. — Internal Okgans of a Female
HUJIAN FcETtJS OF 3.| INCHES LONG. MAGNIFIED
(from Waldeyer).
o, the ovai7 full of primordial ova ; e, tubes of the upper part of tlie WoMan body forming the epoophoron (parovarium of Kobelt) ; \V, the lower part of tlie Wolffian body forming the paroophoron of His and Waldeyer ; W', the Wolffian /'- .C"f ' 1 duct; M, the Miillerian duct; m, its upper
/;",':; -V Y' fimbriated opening.
/ .: separation of its conducting passages
/: . from the glandular or productive part
Ijj: -M of its structure. Like the testicle it
|S'; ' begins to manifest its peculiar charac Si:: .::v; ., - ... L.\ \ teristics by the seventh or eighth week,
\^^ -wx-;^^. ;j|^ ^^ when the germ-epithelium has attained
considerable thickness, and forms a decided prominence on the mesial side of the Wolffian body. The farther development of the glandular part of the organ consists mainly in the formation of ovigerms and ova, and the implantation of these in Graafian follicles by a peculiar combination or intermixture of the superficial germinal cells with the deeper blastema which forms the stroma of the organ.
In a former part of this volume, imder Ovary, p. 478, the development of the primordial ova from a certain number cf the cells of the germ-epithelium and their enclosure in Graafian follicles by the growing stroma of the ovary have been described according to the most recent obsen'ations of "Waldeyer, Kolliker and J. Foulis. The publication of the very careful researches of the last observer enables us to add some important details to the previous description.
Figure 611, copied from some of Foulis's plates (Trans. Roy. Soc, Edin., 1875) will best show what from these observations appears to be the most j^robable view of the mode of development lof ova in the human ovary. At e, fig. 611. B, is seen a portion of the germ-epithelium, and at c', one of the cells undergoing enlargement and conversion into an ovigerm or primordial ovum. Of this the outer jirotoplasm becomes the yolk, and the nucleus the germinal vesicle with its nucleolus or macula. At o, a single o'S'igerm, and at o\ clusters of ovigerms in various stages of development have sunk into the ovarian stroma, and are beingsurrounded collectively and individually by the growth of the connective tissue of the ovarian stroma advancmg from below. Some of the o^^germs in the clusters are more advanced than the rest, and in these, as also in the isolated ovigerm represented in C, a covering of altered connective tissue coi-puscles is seen to be forming round the yolk protoplasm. This is the oi-igin of the cells of the tunica granulosa, which Foulis has shown are not produced, as W^aldeyer believed, from germ-epithelial cells, but from the interstitial connective tissue of the deeper ovarian stroma. In A, o, o, the cell fibres of the stroma («, «,) are seen suiTOunding several individual ova, so as to furnish the first elements of the wall of the Graafian follicles enveloping the ova, and covering immediately the granular cells. In D, representing an ovum somewhat farther advanced, the enlarged yolk-protoplasm and the geiininal vesicle are shown entire, with a fragment of the granular cell covering and fibro-cellular wall of the Graafian follicle ; but the zona pellucida is not yet perceptible.
FARTHER DEVELOPMENT OF THE OVARY.
817
The furtliei* steps in the formation of the ovum, as ascertained by the observations of Foulis, consist mainly in the enlargement of the mass of yolk protoplasm, the formation of a certain quantity of albuminous and fatty granules in combination with it (deutoplasm of Edw. van Beneden) ; and the formation externally of the zona pellucida or yolk-membrane by a consolidation cf the outer
Fig Gil.
T
Fig. 611. — Views of the Formation op Ova and Graafian Follicles in the Ovart
(from Foulis). A, small portion of tlie ovary of a human fcetus of SJ montbs, showing primordial ova imbedded in the stroma ; o, larger primordial ova ; o cluster of earlier ova ; n, fusiform corpuscles of the stroma. B, portion of the ovary near the surface in a human fcetus of 7 A months, showing the manner of inclusion of the germ epithelium corpuscles in groups in the ovarian stroma ; e, germ epithelium ; e', one of the cells enlarging into a primordial ovum before sinking into the stroma ; o, a larger cell imbedded, becoming an ovum ; o', groups of ovigerms or germ cells which have been surrounded by the stroma. C, young ovum from the same ovaiy, isolated ; p, yolk protoplasm. D, ovum more advanced, enclosed in condensed stroma, wliich begins to form a Graafian follicle ; p, yolk protoplasm ; V, germinal vesicle with macula ; g, the fusiform corpuscles now converted into the granular cells ; Gf, condensed stroma forming the wall of the Graafian follicle.
layer of the yolk substance. And here it may be remarked that the recent observations of Oellacher and Balfour on the radiated structure of the yolk protoplasm may explain in some degree, or be connected with the linear radiated marking of the zona pellucida.
Such is the number of ova formed in the manner now described, that in the human foetus of six to seven months the whole substanc-e appears to consist of them and their newly formed Graafian follicles, by which
VOL. II.
3 G
818 DEVEL0P3,IEXT OF THE EErEODUCTlYE ORGANS.
each primordial ovum is closely embraced. A uniform layer of such ova of nearly equal size is especially to be found tovs^ards the surface ; but in the two later months of foetal life some of the ova and follicles advance to a farther stage of development, and increase in size, and this advance is invariably accompanied by a change of position of these ova to a deeper stratum of the ovary. The most advanced of the ova, therefore, are situated deepest in this the earlier stages of the ovarian development. It is different, however, when some "years after Ijirth, and still more towards the age of puberty, a few of the Graafian follicles expand to a great extent, and ultimately when mature reach the diameter of about a quarter of an inch, for then the expanding Graafian follicle gradually approaches the surface of the ovary, or perhaps rather, during the rapid expansion of the follicle, the ovarian stroma gives way by absorption between the follicle and the surface.
As the Graafian follicle expands with the slightly enlarging ovum, the thickness of the layers of condensed connective tissue or stroma round the ovum increases, and thus there are gradually formed the layers which liave been described as the follicular walls, while blood-vessels penetrate into them so as to form the vascular network of the covering. Within the follicle the granular cells multiply so as to form several layers lining the •whole follicle and closely covering the ovum. As yet there is no space between the ovum and wall of the follicle except that which is occupied by the granular cells, and for a long time the follicle is not larger than to enable it to enclose the ovum ; but in the more advanced stages a proportionally great enlargement of the follicle takes place, in consequence of the separation of two layers of the granular cells, so as to form a space in which fluid accumulates, and thus one or more layers •of cells are left lining the expanded follicle and constituting its tunica granulosa, while those covering the ovum, which is now thrown to one side of the follicle, form the investment known as the discus proligerus, -which appears as a reflected portion of the tunica granulosa (see figs. 335 and 336, p'3. 473 and 475).
As connected with the difference in the seat and mode of development of the ■essential parts of the male and female productive organs, the important question presents itself of the possibility or reality of the simultaneous coexistence in any cases of malformation of ovaries and testes on one or both sides of the body in the same individual. From what has been stated above, the possibility of such coexistence may perhaps be theoretically admitted. On this subject the reader may consult an interesting account by Dr. C. L. Heppner of St. Petersburg (Reichert's and Dubois Reymond's Ax-chiv for 1870, p. 679), of a hermaphroditic child which lived two months after biiiih, in which, along with a considerable amount of the better kno-mi conditions of approximation or mingling of the sexual charapters, it appeared that two organs coexisted, in one of which, agreeing in all respects with the ovary, primordial ova in Graafian folUcles were observed, and in another of a distinctly rounded iorm and compact structm'e, and so far corresponding to the testicle and unlike any of the other known vestigial organs, branched and coiled tubes, filled with cells in a manner exactly the same as those of the seminal canals, were ascertained by microscopic observation to exist. The jiarovarium (epididymis or coni vasculosi) also existed.
The genital passages. — The existence of two sets of tubes between the iuteinal productive organs and the external parts has already been adverted to as a feature common to both sexes. The female organs contrast Avith the male in the large development of one of these tubes, viz..
FARTHER DEVELOPMENT OF THE GEXITAL FASSAGES. 819
the Miallerian ducts into their passages, and in the abortive disappearance of the greater part of the Wolffian ducts ; while in the male the ducts of Miiller suffer in a great measure the abortive retrogradation, and the seminal conducting tubes are produced out of canals formed within special parts of the Wolffian body and the whole of the Wolffian duct. But as in all embryoes of whatever sex both sets of tubes are originally present, while a different one of the original tubes becomes developed into the respective permanent conducting passages, vestiges of the other original tubes are invariably present in various degrees in both sexes.
The Female passages. — In the female, the vagina, uterus, and Fallopian tubes are formed out of the Miillerian ducts. That portion of the ducts in which they become fused together is developed into the vagina, the cervix, and part of the body of the uterus ; and the pecu
Fi-. 612. —DiA- Fig. 612.
GKAJI OF THE FEMALE Type op Sexual Organs.
This and figure
615 represent dia grammatically a
state of the parts
not actually visible at cue time ;
but they are intended to illustrate
the general type in
the two sexes, and
more particularly
the relation of the
two conducting
tubes to the develop inent of one as the
natural passage in
either sex, and to
the natural occurrence of vestiges of
the other tube, as
•well as to the per •sistence of the whole
or i>arts of both
tubes in occasional
instances of hermaphroditic nature.
1, the left kidney ; 2, suprarenal body ; 3, ureter, of which a i)art is removed to show the
parts passing within it ; 4, urinary bladder ; 5, urachus ; o, the left ovary nearly in the place of its original formation ; p o, parovariiun, ejioophoron of Waldeyer ; W, scattered remains of Wolffian tubes near it, parooi^horon of Waldeyer ; d G, remains of the left Wolffian duct, such as give rise to the duct of Gaertner, represented by dotted lines ; that of the right side cut short is marked w ; /, the abdominal opening of the left Fallopian tube ; u, the upper part of the body of the uterus, presenting a slight appearance of division into cornua ; the Fallopian tube of the right side cut short is marked in ; g, round ligament, corresponding to gubernaculum ; i, lower part of the intestine ; V a, vagina ; 7i, situation of the hymen ; C, gland of Bartholin (Cowper's gland), and immediately above it the urethra ; c c, corpus cavernosum clitoridis ; s c, vascular bulb or corpus spongiosum ; n, nympha ; I, labium ; v, vulva.
3 G 2
320
DEVELOPMENT OF THE EErRODUCTIYE ORGANS.
liaritv of the mode of fusion accounts for the occurrence, as a rare anomaly, not only of double uterus, but of duplicity of the vagina, coincident with communication between two lateral halves of the uterus. The next following part of the Miillerian duct, constitutes in animals with horned uteri, the cornu of the uterus ; but in the human subject it remains comparatively short, entering into the formation of the upper part of the organ. The remaining upper portion of the Miillerian duct constitutes the Fallopian tube — becoming at first open and subsequently frino-ed at a short distance from its upper extremity.
The pediculated hydatid of the fimbriated extremity (Hydatid of Morgagni) appears to be the remains of the original upper end of the Miillerian tube. The additional or accessory fimbrise and openings referred to at p. 471, and by Henle in his Handbuch, vol. ii., p. 470, may admit of explanation on the supposition of the duct of Miiller having remained open at these places.
In the human embryo of the third month the uterus is two-homed, and it is by a subsequent median fusion and consolidation that the triangular body of the entire organ is produced. The comua uteri, therefore, of the human uterus con-espond with the separate comua of the divided uterus in animals, and this explains the occasional malformation consisting in the gi-eater or less division of the uterine cavity and vagina into two passages. There is no distinction in the
Fig. 613. A B
Fig. 613.— Female Genital Okgans of the Embryo with the Remains of the
Wolffian Bodies (after J. Miiller).
A, fi'om a foetal sheep ; n, the kidneys ; h, the ureters ; c, the ovaries ; d, remains of ■Wolffian bodies ; c, Fallopian tubes ; /, their abdominal openings ; g, their union in the body of the uterus. B, more advanced from a foetal deer ; a, body of the uterus ; b, comua ; c, tubes ; d, ovaries ; e, remains of Wolffian bodies. C, still more advanced from the hviman fcetus of three months ; a, the body of the uterus ; b, the round ligament ; c, the Fallopian tubes ; d, the ovaries ; e, remains of the Wolffian bodies.
human foetus in the third and fourth month between the vagina and uteras. In the fifth and sixth months the os uteri begins to be formed, and the neck is subsequently gradually distinguished. Thickening succeeds in the walls of the uterine portion ; but this takes place first in the cervix, which up to the time of birth is much larger and thicker than the body of the uterus (KoLLiker).
In the meantime the Wolfiian bodies undergo a partial atrophy, and their ducts
FARTHER DEVELOrMEXT OF THE UTERUS.
821
become more or less obliterated and abortive in different parts. The most constant vestige of the "Wolffian bodies in the female is the now well-known body of Rosenmiiller or Parovarium of Kobelt (Eosenmiiller, Quoidam de Ovariis Embry. Human., Lipsiaj. 1802 ; Kobelt. der Nebeneierstock des Weibes, Heidelberg-, 1847), which has already been described at p. 480 of this volume, the ciHwj)horon of Waldeyer, and which, being produced out of the same elements as the epididymis of the male, presents a remarkable resemblance to that body. The canal uniting the radiating tubes (coni vasculosi) of this organ is also usually persistent, but ceases at a short distance below. In the sow and several ruminants, however, the subdivided upper tubular part or epoophoron has disappeared, and the main tube (middle part of the Wolffian duct) remains in the (hu-t of Gaertner, a strong, slightly undulated tube, which is traceable, first free in the broad ligament
Fig. 614.
Fig. 614. — Adult OvARr, Parovarium anu FalluI'Iaii Tube (from Farre, after
Kobelt).
a, a, Epoophoron (parovarium) formed from the xipper part of the Wolffian body ; b, remains of the uppermost tubes sometimes forming hydatids ; c, middle set of tubes ; d, some lower ati-ophied tubes ; c, atrophied remains of the Wolffian duct ; /, the terminal bulb or hydatid ; h, the Fallopian tube, originally the duct of Midler ; i, hydatid attached to the extremity ; I, the ovary.
of the uterus, and lower down becoming incorporated with the wall of the uterus and vagina, upon which last it is lost.
The Male Passages. — The conversion of the Wolffian duct into the vas deferens of the testicle was first demonstrated in animals by Rathke, in correction of the views of J. Miiller (Meckel's Archiv, 1833), and was further proved and illustrated by H. Meckel and Bidder (H. Meckel, Zur Morphol. der Ham uud Geschlechts-Organe der Wirbelthiere, Halle, 1848 ; Bidder, Male Organs in the Amphibia, Dorpat, 184G). KuUiker showed that a similar process occurs in the human embryo, and that a communication established between the seminal tubes of the testicle (rete testis) and some of the upper tubes of the Wolffian body gave rise to the epididymis.
The observations of Cleland and Banks first pointed out clearly the difference between the structure of the upper nonglomerular, or simpl} tubular part of the Wolffian body, and that of the lov.er and glomerular, or primordial-kidney part.
822
DEVELOPMENT OF THE REPRODUCTIVE ORGANS.
In the male, the Miillerian ducts are destined to undergo little development and are of no physiological importance, while the ducts of the Wolffian Iwdies, and probably also some part of their glandular substance, form the principal part of the excretory apparatus of the testicle. The rmited portion of the Miillerian ducts remauis as the vesicula prostatica, which accordingly not only corresponds with the uterus, as was shown by Weber, but likewise, as pointed out by Leuckai-t. contains as much of the vagina as is represented in the male. In some animals the vesicula prostatica is prolonged into comua and tubes ; but in the human subject the whole of the ununited parts of the Miillerian ducts disappear, excepting, as suggested by Kobelt. their upper extremities, which seem to be the source of the hydatids of Morgagni. The excretory duct of the Wolffian body, from the base of that body to its orifice, is converted into vas deferens and ejaculatory duct, the vesicula seminalis being formed as a diverticulum from its lower part (Waldeyer).
With respect to the fomiation of the epididymis, it appears certain that the larger convoluted seminal tube, which foi-ms the body and globus minor of the epididymis, arises by a change or adaptation of that part of the Wolffian duct which runs along the outer side of the organ. The vas aben-ans or vasa aber
Fig. 61;
Fig. 615. — Diagram of
THE Male Type of
Sexual Organs.
1, 2, 3, 4, and 5, as in iigure 612 ; t, testicle in the place of its original formation ; e, caput eiiididymis ; v d, vas deferens ; W, scattered remains of the Wolffian body, constituting the organ of Girakles, or the paradidymis of Waldeyer ;
V h, vas aberraus ; on, Miillerian duct, the upper part of -which remains as the hydatid of Morgagni, the lower part, represented by a dotted line descending to the prostatic vesicle, constitutes the cornu and tube of the iiterus masculinus ; g, the gubernaciilum ;
V s, the vesicula seminalis ; p r, the prostate gland ; C, CowiJer's gland of one side ; c f, corpora cavernosa penis cut sliort ; s p, corpus spongiosum urethraj ; s, scrotum ; t' , together with the dotted lines above, indicates the - direction in Avhich the testicle and epididymis change place in their descent from the abdomen into the scrotum.
rantia of Haller appear to be the remains also, in a more highly convoluted foi-m, of one or more of the tubes of the Wolffian body still adhering to the excretory duct of the organ, and their communication with the mam tube of the epidi
DEVELO^ME^^T OF THE EPIDIDYMIS AXD YAS DEFEEEXS. S-25
dymis receives an explanation from that circumstance. As to the coni vasculosi in the upper part of the epididymis, it has been customary to regard them as produced by a transformation of the tubes and duct in the upper part of the Wolffian body, according to the views most fully given by Kobelt ; but, according to the more recent observations of Banks, the origin of the coni vasculosi is most jn'obably due to a process of development occm-ring in a new stnictiure or mass of blastema which had been jireviously observed by Cleland, and which is foiTued in connection with the upper end of the "Wolffian body, and close to the Miillerian duct. "Within this blastema Cleland showed that the tubes of the efferent seminal vessels and the coni vasculosi, together with the tube which connects them, are formed anew, while the tubes of the lower primordial -kidney part of the "Wolffian body are undergoing an atroi^hic degeneration. This has been confirmed by the detailed observations of Banks, who has further shown the continuity of their uniting tube with the "\Yolffian excretory duct.
According to this view, the caput epididymis must be regarded, not simply as a conversion of the ujiper part of the "Wolffian body, but rather as a new formation, or superinduced development of tubes in blastema connected with it.
The coni vasculosi, so fonned, become connected with the body of the testicle by means of a short straight cord, which is afterwards subdivided into the vasa efferentia. The peritoneal elevation descending from the testis towards the lower extremity of the "Wolffian body, is the upper part of the plica gubernatrix, and becomes shortened as the testicle descends to meet the lower end of the epididjTiiis ; the peritoneal elevation which passes do'OTi into the scrotum, and is continuous with the other, is the more important part of the plica gubernatrix, connected with the gnbernactilum testis. The spennatic arteiy is originally a branch of one of those which go to the "Wolffian body, and ascend from the surface of the Wolffian body to the upper part of the testis, along the ligaments connecting them ; but, as the testis descends, the artery lies entnely above it, and the secreting substance of the Wolffian body remains adherent to it ; and hence it is that the organ of Gii-aldes, which consists of persistent "\\'olffian tubules, is found in a position superior to the ei^ididymis. (For a fuller account of this subject the reader is referred to Banks "-On the Wolffian Bodies," Edin 18G-i.)
Fig. 616. YiEW FROM BEFORE OF Fig. 616.
THE Adttlt Testis and EpididtMis (from Farre, after Kobelt).
a, a, convoluted tubes in ■ the head of the epididymis developed from the upper part of the Wolffian body ; b and /, hydatids in the head of the epididymis ; c, coni vasctdosi ; (/, va.sa aberrantia ; /(, remains of the duct of ]\Udler ■with i, the hydatid of IMorgagni at its upper end ; /, body of the testis.
The Descent of the Testicles.
— The testicles, which are originally sitixated in the abdominal cavity, pass down into the scrotum before birth. The testicle
enters the internal inguinal ring in the seventh month of foetal life : by the end of the eighth month it has usually descended into the scrotum, and, a little time before birth, the naiTow neck of the peritoneal pouch, by which it previously communicated with the general pei-itoneal cavity, becomes closed, and the process of peritoneum, now entu-ely shut off from the abdominal cavity, remains as an independent serous sac. The peritoneal pouch, or processus vaginaliB, which, passes do-mi into the scrotum, precedes the testis by some time in its descent, and into its posterior part there projects a considerable columnar elevation
82i DEYELOPMEXT OF THE EEPRODUCTIYE ORGANS.
akeady alluded to, which is filled with soft tissue, and is termed 2>lica (juhcrnatrix . There is likewise a fibrous structui'e attached inferiorly to the lower part of the scrotum, and surrounding the peritoneal pouch above, which may be distinguished as the (jul}t'rnacular cord, both this and the plica gubernatrix being included in the general tei-m g uhernaciiluvi testis (J. Hunter). The gubernacular cord consists of fibres which pass downwards from the sub-peritoneal fascia, others which pass upwards from the superficial fascia and integmnent, and others again which pass both upwards and downwards from the internal oblique muscle and the aponeurosis of the external oblique ; it exhibits, therefore, a fusion of the layers of the abdominal wall. Superiorly, it surrounds the processus vaginalis, without penetrating the plica gubernatrix ; and the processus vaginalis, as it grows, pushes its way down through the gubernacular cord and disperses its fibres. By the time that the testis enters the internal abdominal ring, the processus vaginalis has reached a considerable way into the scrotum ; and. as the testis follows, the plica gubernatrix becomes shoiler, till it at last disappears ; but it cannot be said that the shortening of the plica is the cause of the descent of the testicle, and much less that (as has been held by some) the muscular fibres of the gubernacular cord are the agents which effect this change of position. The arched fibres of the cremaster muscle make their appearance on the surface of the processus vaginalis as it descends, while its other fibres are those which descend in the gubernacular cord. (See, for a further account of this process, and the various views which have been held with regard to the descent of the testicles, Cleland, " Mechanism of the Gubemaculum Testis." Edinburgh, IS.JG.)
The External Organs. — In the human embryo, as in that of animals, the external organs are up to a certain time entirely of the same form in both sexes ; and the several organs which aftenvards distinguish the male and female externally take their origin respectively from common masses of blastema of precisely similar structure and connections. The common cloaca exists till after the fifth week, and the genital emmence from which the clitoris or penis is formed makes its appearance in the course of the fifth and sixth weeks in front of and within the common orifice. In the course of the seventh and eighth weeks the common orifice is seen to become divided into two parts, viz., the longer slit of the genito-urinary apeiture anteriorly, and the naiTower and more rounded anal aperture posteriorly : but the exact manner in which the separation of these two apertures takes place has not yet been acciu'ately traced. It -is intimately connected with the formation of the urogenital cord as an independent stmcture, and is probably mainly effected by the advance from the sides and posteriorly of septal bands which separate the lower pai"t of the intestine. Somewhat later, or in the ninth and tenth weeks, a transverse integumental band completes the division between the anal and the urogenital orifices, which band forms the whole of the so-called perineum of the female, and the part of the perineal integument in the male which is situated behind the scrotum ; the raphe being most obvious in the male sex.
The cutaneous folds, or cii-cular genital ridge, which are afterwards converted into mons Veneris, labia majora, and scrotum, as well as the lips of the urogenital furrow, which are converted into the nymphte of the female and unite as integument below the penis in the male, are both of early formation and at first precisely the same in all embryoes. In this condition, which continues until the eleventh or twelfth week, the parts aj^pear alike in both sexes, and resemble very much the more advanced female organs. The rudiments of JJartholui's or Ceirpcrs glands are, it is said, seen at an early period, near the root of the rudimentary clitoris or penis, on each side of the genito-ui-inary passage.
In the female, the tvro lateral cutaneous folds enlarge, so as to cover the clitoris and form the labia majora. The clitoris itself remains relatively smaller, and the gToove on its under surface less and less marked, owing to the opening out, and subsequent extension backwards, of its margins to form the ntjinplKP. The vascular bulbs remain distinct and separate, except at one point where they run togetlier in the glans clitoridis. The hijmcn begins to appear about the fifth month as a fold of the lining membrane at the opening of the genital passage
FARTHER DEVELOPMENT OF THE EXTERNAL ORGANS. 825
into the urogenital sinus. Within the vestibule, which is the shortened but ■widened remains of the urogenital sinus, the urethral orifice is seen, the lu'ethra itself undergoing considerable elongation.
In tlie male, on the contrary, the j;c'«w continues to enlarge, and the margins of the groove along its under surface gradually unite from the primitive urethral orifice behind, as far forwards as the glans, so as to comiilete the long canal of the male itrcthra, which is therefore a prolongation of the urogenital sinus. This is accomplished about the fifteenth week. "When this union remains incomplete, the abnormal condition named iLUpo.^padlas is produced. In the meantime the prcjniec is formed, and, moreover, tlie lateral cutaneous folds also unite from behind forwards, along the middle line or raphe, and thus complete the scrotum, into which the testicles descend in the course of the eighth month of fojtal life.
The corpora cavernosa, which are at first separate, become united in their distal portions in botli sexes ; but the corpus spongiosum lu'ethraj which is also originally divided in all embryoes, and in the female remains so in the greater part of its extent, becomes enlarged in the male in the glans penis, and its two parts united mesially both above and below the urethra, so as to enclose the whole of that tube from the bulb forwards to the glans.
TYPE OF DEVELOPMENT AND ABNORMAL FORMS OF THE GENITAL
ORGANS.
The type of development of the genital organs may be stated to differ in the several parts of the system in the two sexes as follows, viz. : —
1st. It is single and homological in the external organs.
2nd. It is double and heterological in the middle organs or passages.
3rd. It is partially double and heterological in the productive organs.
Accordingly the congenital malformations of the reproductive organs admit of being distributed under the following divisions : —
1st. Abnormal forms attributable to deficient, redundant, or abnormal modes of development of one or more of the external organs in either sex, producing an approach to tlie fonn of the otlier sex.
2nd. Forms referrible to deficient, redundant, or abnonnal modes of development of one or other of the two sets of sexual passages, viz., of the Wolffian or Miillerian ducts, so as to lead to the greater or less predominance of sexual characters in a part or the whole of these passages inconsistent with tliose prevailing in other parts of the system, or to the coexistence of both sets of passages in whole or in i:)art.
8rd. Extremely rare forms referrible to the possible coexistence of the productive parts of testicles and ovaries in the same individual, usually combined with more or less of the foregomg kinds of malformation.
Upon the subject of these malformations the reader may consult the learned and able article Hermaphroditism by Sir James Y. Simpson in tlie Cyclop, of An at. and Physiol.
Upon the subject of malformations in general the following works are recommended, viz. : —
Isid. Geoff. St. Hilaire, Hist. Gen. et Partic. des Anomalies de I'Organisation, &c., 3 torn. Paris, 1832 — 6 ; Cruveilhier, Anat. Pathol., &c., Paris, 1S;>() — 42. Otto, Sexcentorum Monstrorum desc Anat. Vratisl., 1841 ; Th. L. W. Bischoff. Uber Missbildungen, &c., in R. Wagner's Handworterbuch der Physiol., 1843 ; Wm. Vrolik, Tab. ad illustr. Embryol. Hom. et Mammal, tam Natiu-. quam Abnormem, Amstel., 1849, and tlie article " Teratology"' by the same author in Todd's Cyclop, of Anat. and Physiol. ; Aug. Forster, Die Missbildungen des Menschen. &c., Jena, 1861 ; as also the systematic works of Rokitanski and others on Pathological Anatomy.
The following tabular scheme of the Coreesponding Parts of the ffenito-urinary organs in the two sexes, and of their relation to the Formative Rudiments of the common embryonic type, may be useful in
826 DEVELOPMENT OF THE EEPRODUCTIYE ORGANS.
fixing attention on the more important points of tlie foregoing description, and indicating more clearly the homologies of the parts : —
Fejule Perjiaxext. Common Embryonal. Male Permanent.
I. -COMMON BLASTEMA OF REPRODUCTIVE GLANDS.
Ovary. Body of Testicle.
Fnniislies the ovigerms and re- 1. Germ-epithelium covering . . Uisuppear.s, and is replaced by mains on the .surface. serous covering of tunica
vaginalis.
Forms stroma of the ovary and 2. Deeper blastema Forms glandular seminal tubes
the Graalian follicles. of the testis.
II.- WOLFFIAN BODIES.
Transverse tubes of epoophoron 1. Upper tubular non-glonieru- Vasa efferentia and coni vas or organ of Rosennuilkr lar part. culosi of the epididymis. (Parovarium).
Paroophoron (Wald.) 2. Lower glomerular part (pri- Paradidymis (Wald.), organ of
mordial kidneys). Giraldes, and ^'asa aberrantia.
Koimd ligament of the uterus . . 3. Ligament of the "Wolffian Gubemaculum testis.
body.
Ill- WOLFFIAN DUCTS.
Tube of the Epoophoron 1. Upper and middle parts .... Convoluted tube of the epididymis.
Ducts of Gaertner, in cow and pig 2. Lower part Vas deferens and vesiculse
seminales.
IV— MULLERIAN DUCTS.
Pimbriated abdominal opening 1. Upiwr extremity Hydatid of Morgagni.
and terminal and oecasiunal hydatids
Paliopian tubes 2. Middle part Occasional tubular pi'olonga ticns (it uterus masculinus.
Vagina and uterus 3. Lower single or meiiian part Uterus masculinus (vesicuU
prostatica).
v.— GENITAL CORD AND SINUS UROGENITALIS.
Tissue uniting female urethra and 1. Substance surrounding geni- Prostate gland. Muscular and vagina. tal cord. glandular tissue.
Female urethra 2. Upper part of cavity or Upper part of prostatic portion
urinai-j' pedicle. of tlie urethra.
Ostium vaginae . Hymen 3. Confluence of urinary and Verumontanum.
genital ]iarts. Vestibule 4. Lower part Lower ]iart of prostatic portion and membranous part of urethra. Glands of Bartholin 5. Common blastema Cowper's Glands.
VL— EXTERNAL ORGANS.
1. — Vascular parts.
Crura and corpus clitoridis a. Corpora cavern<jsa Crura and corpus penis.
Glans clitoridis and vascular 0. Corpora spongiosa Glans penis and spongy body
bulbs separate) of urethra (united). 2. Integumental parts.
Preputium clitoridis «. On genital eminence Preputium penis.
Integumental folds of nymphee b. Lijjs of genital furrow Integument and raphe below
(separate). '• . ]ieiiis.
Labia majora (se]iarate) c. Genital ridges (lateraP Scrotum and raphe (united).
Perineum of female, with raphe . U. Transverse interauogcnital Perineum of male beliind
baud. scrotum, with raphe.
