Book - Text-Book of the Embryology of Man and Mammals 14
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Hertwig O. Text-book of the embryology of man and mammals. (1892)
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- 1 The Organs of the Inner Germ-Layer The Alimentary Tube with its Appended Organs
- 1.1 Introduction to Part 2
- 1.2 The Organs of the Inner Germ-Layer The Alimentary Tube with its Appended Organs
- 1.3 The Formation of the Mouth, the Throat- or Gill-Clefts, and the Anus
- 1.4 Differentiation of the Alimentary Tube into Separate Regions and Formation of the Mesenteries
- 1.5 Development of the Separate Organs of the Alimentary Tube
- 1.6 The Organs arising from the Pharynx : Thymus, Thyroid Gland, Larynx, and Lung
- 1.7 The Glands of the Small Intestine : Liver and Pancreas
- 1.8 Summary
- 1.9 Literature
- 1.10 Glossary Links
The Organs of the Inner Germ-Layer The Alimentary Tube with its Appended Organs
Introduction to Part 2
IN the first part of the text-book, which treated of the fundamental processes of the beginning of development, it was shown how there were formed from the embryonic cells, the descendants of the cleavage-process, several cell-layers : the outer, the middle, and the inner germ-layers, and the intermediate layer which inserts itself into all the interstices between the former. In the further progress of development each of these chief layers, which CARL ERNST v. BAER has called the fundamental organs of the animal body, undergoes a series of manifold changes, and is in consequence gradually converted into the separate organs of the adult body.
The study of the development of the organs constitutes the theme of the second part of this text-book.
A division of the extensive material to be presented here is best undertaken with reference to the separate germ-layers from which the various organs are derived, as was first attempted by REMAK in his pioneer work " Untersuchung iiber die Entwicklung cler Wirbelthiere." But it must be observed at the very outset that the principle of the classification of organs according to the germ-layers can be carried out only with certain limitations. For the completed organs of the adult are ordinarily compound structures, which are not formed out of a single embryonic layer, but out of two or even out of three. Thus, for example, a muscle is developed from the middle germ-layer and the intermediate layer. The teeth arise from the latter and the outer germ-layer ; the alimentary canal with its glands contains elements from three layers, from the inner and the middle germlayers, as well as from the intermediate layer. When, notwithstanding, these organs are cited as descendants of one germ-layer, it is for the reason that the various tissues are of unequal value in the construction and function of an organ, the important components being furnished preeminently by one germ-layer. Thus the structure and the function of the liver or the pancreas are primarily determined by the glandular cells which are derived from the inner germ-layer, whereas connective tissue, blood-vessels, nerves, and serous covering, although they also belong to these glands as a whole, are of less significance, because the characteristic properties of liver or pancreas do not depend upon them. In the anatomy and physiology of a muscle the muscular tissue is the more significant part, in the sensory organs the sensory epithelium.
Guided by such considerations one has a perfect right to designate the intestinal glands as organs of the inner germ-layer, the muscles, the sexual and urinary organs as belonging to the middle germ-layer, and the nervous system together with the sensory organs as products of the outer germ-layer.
Thus the science of the embryology of organs is divisible into four main sections into the science of the morphological products of
- the inner germ-layer
- the middle germ-layer
- the outer germ-layer
- the intermediate layer
The Organs of the Inner Germ-Layer The Alimentary Tube with its Appended Organs
AFTER completion of the formation of the germ-layers and the first processes of differentiation described in the tenth chapter, the body of the vertebrated animal consists of two simple tubes, one within the other (Plate I., figs. 7 and 10), the inner, smaller alimentary tube, and the body-tube separated from the former by the bodycavity (Ih 1 ), each of which is composed of more than one of the primitive cell-layers of the germ.
The alimentary tube, the further development of which will first engage our attention, is composed of two epithelial layers, the entoderm and the visceral portion of the middle layer, which furnishes the epithelial lining of the body-cavity, separated from each other by the intermediate layer, which is at this time little developed. Of the three layers the entoderm is unquestionably the most important, since the further processes of differentiation primarily proceed from it, and since the physiological capabilities of the alimentary canal are determined by the activity of its cells.
The changes which occur in the further course of development are best divided into three groups. First, the alimentary tube comes into communication with the surface of the body by means of a large number of openings, the visceral clefts, the mouth, and the anus. Secondly, it grows enormously in length, and is at the same time differentiated into oesophagus, stomach, small intestine and large intestine, with their peculiarly modified mesenteries and omenta. Thirdly, numerous organs, which are for the most part concerned in the duties of digestion, take their origin from the walls of the alimentary tube.
The Formation of the Mouth, the Throat- or Gill-Clefts, and the Anus
At the beginning of development the alimentary tube opens out to the surface of the germ by means of the primitive mouth (primitive groove), which marks the place at which, during the stage of the blastula, the inner and middle germ-layers have been invaginated (Chapters V. and VI., figs. 44, 47, 54, 55, 78 u). But this opening is only a transitory structure.
Located at the future hind end of the embryonic fundament, it is at first overgrown by the medullary ridges, and establishes a temporary union between the intestinal and neural tubes, the canalis neurentericus (figs. 68 en, 80, 88 ne). Afterwards it becomes entirely closed by the growing together of the edges of the primitive mouth.
Fig. 151. Median section through the head of an embryo Rabbit 6 mm. long, after MIHALKOVICS.
)/(, Membrane between stomodfeum and fore gut, pharyngeal membrane (Rachenhaut) ; lip, place from which the hypophysis is developed ; k, heart ; kd, lumen of fore gut; ch, chorda; v, ventricle of the cerebrum ; v*, third ventricle, that of the between-brain [thalamencephalon] ; v 4 , fourth ventricle, that of the hind-brain and after-brain [epencephalon and metencepnalon,* or medulla oblongata] ; ck, central canal of the spinal cord.
It is affirmed by some that in certain Vertebrates (Petromyzon, several Amphibia) the primitive mouth persists, and becomes the anus of the adult animal.
There arise, however, on the permanent alimentary tube, both at its anterior and posterior ends, new openings, part of ivhich are unpaired, part paired', for the wall of the alimentary tube at several places fuses with the wall of the body, then becomes thinner, and finally breaks through to the outside. The unpaired openings are mouth and anus ; the paired ones are the throat-, yill-, or visceral clefts. The first to be established are the mouth and the gill-clefts, in the regions of head and neck. These are of the greatest importance in the external morphology of the
- [Huxley has employed metencephalon and myelencephalon instead of epencephalon and metencephalon for the fourth and fifth regions of the brain respectively.]
embryo, because with their appearance the head- and neck-regions become distinguishable.
The Development of the Mouth
In all vertebrated animals the epidermis forms on the under side of the rudimentary head, which at first has the appearance of a rounded knob, a small shallow pit (Plate L, fig. 11, and fig. 151), which meets the blind end of the fore gut (kd). In the region of this pit the middle germ-layer is from the beginning absent (KEIBEL, CARIUS). Outer and inner germ-layers meet to form a thin membrane (fig. 151 rh\ which separates oral sinus or oral pit [stomoda?um] and fore gut, and which has been described since the time of REMAK Sisphari/ngeal membrane (Rachenhaut). By its rupture and the degeneration of the shreds of it known as the primitive palatal velum communication with the outside is established (Plate I., figs. 4 and 7 m).
In the case of the Chick the oral pit is observable on the second day of incubation, the front end of the embiyonic fundament having a short time previously elevated itself as a cephalic knob above the extra-embryonic part of the germ-layers. The rupture of the phuryngeal membrane takes place on the fourth day. In the case of an embryo Rabbit of nine days the pharyngeal membrane is not yet ruptured. His has studied in detail this early stage in Man on his embryo " Ly" the age of which he estimates at twelve days.
Fig. 152. Human embryo (Lg of His) 2-15 mm. long, neck measurement.
- Drawing from a reconstruction, after His (" Menschliche Embryonen "). Magnified 40 diameters.
.1/6, Oral pit (ur sinus) ; Ab, aortic bulbus ; Vm, middle part of the ventricle of heart ; Vc, vena cavu superior or ductus Cuvieri ; Sr, sinus reunions ; Vu, vena umbilicalis ; VI, left part of the ventricle ; Ho, auricle of heart ; D, diaphragm ; V.om, vena oruphalomesenterica ; Lb, solid fundament of the liver ; Lby, hepatic duct.
In all anmiotic Vertebrates the entrance to the oral pit (fig. 152 Mb) presents a very uniform condition and appears as a large five sided opening, which is surrounded by five ridges. A knowledge of these is of great importance in studying the history of the formation of the face.
- [It will be seen by an inspection of figure 158 that the longest straight line which can be drawn through the embryo connects the neck- and rump-regions. It is this distance which is designated as the neck, or neck-rump, measurement.]
Of the five ridges one is unpaired, the frontal or naso-frontal process, a broad, rounded projection which bounds the oral pit above. Its origin is connected with the development of the central nervous system, which reaches up to the anterior end of the embryonic fundament, where it is developed into the cerebral vesicles (fig. 153 gh, zh, mJi). Examined by means of a longitudinal section, the frontal process at this stage, therefore, encloses a large cavity belonging to the neural tube, and has the form of a vesicle, which is composed of three layers, the epidermis, a layer of mesenchyma, and the thickened epithelial wall of the neural tube. The primary oral cavity and the fundament of the brain are closely apposed at the beginning of development ; they are separated by only a thin sheet of tissue, within whose territory there is subsequently formed, among other things, the floor of the cranium.
The four remaining ridges are paired structures which surround the oral sinus upon its sides and below. These are produced by growths of the embryonic connective tissue, through which large blood-vessels take their course. They are distinguished according to their positions as upper-jaw (maxillary] and lower-jaw (mandibular) processes. The former are on either side in immediate contact with the frontal process, from which they are separated by a groove only, the naso-optic furrow, which will be discussed in a subsequent chapter, and which runs obliquely upward and outward to that region of the face in which the eye begins its development. The maxillary process is separated from the mandibular process by an incision which corresponds to the place of the future angle of the mouth. The two processes of either side together form the pharyngeal arches, or the membranous jaw-arches.
Before the rupture of the pharyngeal membrane the oral sinus has become still deeper, but only in its upper part, whereas toward the mandibular arch it becomes shallow. This condition is connected with curvatures which in all amniotic Vertebrates as well as Selachians affect that part of the head which encloses the brain-vesicles and lies above the alimentary tube. For the front end of the head is bent down toward the ventral side of the embryo, and finally makes a right angle with the posterior half of the head (fig. 153). Consequently the place at which the so-called anterior wplialic curvature has occurred, and at which the posterior and anterior halves of the head bend into each other, has become an elevation, fospa/rletal [or mid-brain'] elevation (Scheitelhb'cker), SH. The latter encloses the middle brain-vesicle (mJi), the future mid-brain. Furthermore the frontal process, in consequence of the curvature, covers in the oral sinus more and more from above and in front, and thereby contributes to its depth.
As His has shown for the human embryo, the pharyngeal membrane before rupturing extends obliquely backward and upward from the mandibular arch, and becomes firmly attached at the point of curvature hp, where, as a result of the bending, the anterior and posterior halves of the head meet each other at right angles. Even after the rupture of the pharyngeal membrane there is retained, in front of its attachment, a small pit, which constitutes RATHKE'S pocket (fig. 153 lip}.
It is to be noted that the oral sinus, in front of the pharyngeal membrane, and the fore gut, which lies behind it, do not correspond respectively to the cavities designated in the anatomy of the adult as oral cavity and pharynx. But the region of RATHKE'S pocket, which belongs to the embryonic oral sinus, is in the adult referred to the pharynx.
In consequence of the early and complete disappearance of the pharyngeal membrane, it is no longer possible to say at what place in the adult is to be sought the transition from the primitive, epidermis-lined oral sinus to the epithelial layer of the alimentary tube.
The Development of the Visceral Clefts
While the changes described take place in the vicinity of the oral sinus, several visceral clefts make their appearance immediately behind the jaw-arches upon either side of the body. They are developed in the case of Selachians, Teleosts, Ganoids, and Amphibia, as well as Amniota, in a rather uniform manner (figs. 154, 155). From the epithelium of the fore gut there are formed deep outpocketings (sch 1 scA 6 ), which run from above downward on the lateral wall of the throat parallel to the jaw-arches. They crowd aside the middle germ-layers, which extend into this region, and thus grow outward to the surface, where they unite with the epidermis. The latter now become depressed into furrows along the regions of contact (fig. 154), so that one can distinguish inner, deeper throat-pockets, and outer, shallower throat- or gill-furrows. The two are separated from each other for a time by a very thin closin<i membrane, which consists of two epithelial layers, the epidermis and the epithelium lining the fore gut.
Fig. 153. Median sagittal section through the head of a Chick incubated 4.5 days, after MIHALKOVICS.
SH, Parietal [mid-brain] elevation ; sc, lateral ventricle of the brain ; <- ;i , third ventricle ; v*, fourth ventricle ; Sw, aqueductus SYLVII ; gk, cerebral vesicle; zh, between-braiu [thalamencephalon] ; niJi, mid-brain; /./<, cerebellum; zf, pineal process ; /^>, hypophysial (or RATHKE'S) pocket ; ch, chorda ; ba, basilar artery.
The bands of substance which lie between the successive throat-pockets (figs. 154 and 157) are the membranous branchial, throat-, or visceral arches. They consist of an axis, which is derived from the middle germ-layer and the mesenchyma, and of an epithelial covering, which on the side toward the pharynx is furnished by the inner germlayer, on the outside by the outer germ-layer. They are designated according to their sequence as the second, third, fourth, etc., visceral arches, inasmuch as the ridge which surrounds the mouth constitutes the first visceral arch.
In all water-inhabiting Vertebrates which breathe by means of gills the thin epithelial closing plates break through between the visceral arches, and indeed in the same sequence as that in which they arose. Currents of water therefore can now pass from the outside through the open clefts into the cavity of the fore gut and be employed for respiration, since they flow over the surface of the mucous membrane. There is now developed in the mucous membrane, upon both sides of the visceral clefts, a superficial, close network of bloodcapillaries, the contents of which effect an exchange of gases with the passing water. Moreover the mucous membrane becomes folded, for the increase of its respiratory surface, into numerous, close-set, parallel branchial leaflets, which are provided with the greatest abundance of capillary blood-vessels. In this manner the most anterior section of the alimentary canal, which lies immediately behind the head, has become converted into an organ of respiration adapted to life in water.
Fig. 154. Frontal (reconstruction) section of the oro-pharyngeal cavity of a human embryo (Bl of His) 4'5 mm. long, neck measurement, from His " Menschliche Embryonen." Magnified 30 diameters.
The figure shows four outer and four inner visceral furrows, with the closing plates at the bottom of them. In the visceral arches separated by furrows one sees the cross sections of the second to the fifth aortic arches. By reason of the greater development of the anterior visceral arches the posterior ones are already somewhat pressed inwards.
The important differentiation of the alimentary canal into an anterior respirator} r chamber and a following nutritive region is possessed by Vertebrates and Amphioxus in common with certain Invertebrates (Tunicates and Balanoglossus).
Likewise in the case of the higher (amniotic) Vertebrates both inner and outer visceral furrows, together with the visceral arches separating them, are, as has already been stated, formed ; but here they are never developed into an actually functioning respiratory apparatus; they belong consequently in the category of rudimentary organs. Upon the mucous membrane there arise no branchial leaflets ; indeed the formation of open cleft's is not always and everywhere achieved, since the thin epithelial closing membranes between the separate visceral arches are preserved at the bottom of the externally visible furrows. Upon this point, however, the opinions of the investigators who have been engaged in the study of the throat-region in late years are very dissimilar. Whereas His, BORN, and KOLLIKER maintain that the closing plate does not as a rule rupture, FOL, DE MEURON, KASTSCHENKO, LIESSNER, and others find that at least the first two or three visceral clefts are temporarily open. The opening takes place to a greater extent in Reptiles than in Birds and Mammals, where it remains limited to a small territory. In the most posterior visceral pockets there can be no breaking through, because they are not as deep, and the closing plate is therefore thicker and contains also a layer of connective tissue. The conditions in Reptiles and Mammals, as well as the differences in the number of visceral arches, to be mentioned directly, express separate stages in the process of regressive metamorphosis, to which the whole visceral apparatus in the vertebrate series has been subjected.
The number of visceral clefts which actually appear in the separate classes of Vertebrates is variable. The greatest number is encountered among the Selachians, where there may be as many as six (fig. 155), in a few species indeed seven or eight. In Teleosts, Amphibia, and Reptiles the number sinks to five. In Birds, Mammals, and Man (figs. 154 and 157) only four arise. We can therefore say in general that from the lower to the higher Vertebrates a reduction has taken place in the number of visceral clefts which make their appearance. In view of theso phenomena, and guided by other comparative-anatomical considerations, many investigators have advanced the hypothesis that in the case of the ancestors of Vertebrates the fore gut has been pierced by a greater number of clefts than is now to be observed even in the Selachians, and further that degraded or metamorjihosed remnants of them are still to be found in the head- and neck -regions.
VAN BEMMELEN has observed in embryos of various Sharks and Skates outpocketings of the lateral wall of the throat behind the last visceral arch, and has interpreted them as rudimentary visceral clefts, which no longer succeed in breaking through (fig-. 155 nsd'). Subsequently there are developed out of them, by growth of the epithelium, glandular organs, the supra-pericardial bodies (BEMMELEN), which are similar in their structure to the thyroid gland. Also in the head-region, which lies in front of the first visceral arch, a reduction and a metamorphosis of clefts has, according to the opinion of various observers, taken place. DOHEN especially has propounded several hypotheses of this kind, for which, however, I do not find valid grounds : (1) that the mouth has arisen by the fusion of a pair of visceral clefts, (2) that the olfactory organs are to be referred to the metamorphosis of another pair of clefts, a view which is also shared by M. MARSHALL and several others, (3) that a disappearance of gill-clefts in the region of the sockets of the eye is to be assumed, and that the eye-muscles are to be interpreted as remnants of gill-muscles.
Fig. 155. Diagram of the development of the thymus, the thyroid gland, and the accessory thyroid glands, and their relations to the visceral pockets in an embryo Shark, after DE MEURON.
sch 1 , self, First and sixth visceral pockets ; th, fundament of the thymus ; sd, thyroid gland ; nsd, accessory thyroid gland.
In the Chick the visceral furrows become visible in the course of the third day of incubation, only three pairs at first, but, at the end of the same day, a fourth pair is added.
In human embryos the visceral furrows are to be seen most distinctly (figs. 157, 154) w r hen the embryo has attained a length of three or four millimetres (His). Outer and inner furrows are in this case deeply excavated and separated from each other by only a thin epithelial closing plate; they diminish in length from before backward. Of the visceral arches which separate them, the first is the largest, the last the smallest ; seen in frontal section they form two rows converging below, so that the oro-pharyngeal cavity tapers funnel-like into the intestinal tube.
From the fourth week of development onward the visceral arches beyin to be displaced in relation to one another, owing to a more rapid growth of the first two than of the following ones (fig. 156). " They glide over one another," as His remarks, " like the tubes of a telescope, in such a way that, viewed from the outside, first the fourth arch is surrounded and covered in by the third, and this in turn by the second, whereas on the inner surface, that which is turned toward the pharynx, the fourth arch lies over the third, the third over the second." As a result the length of the oro-pharyngeal cavity is relatively less in the older than in the younger embryos. In consequence of this unequal growth, which moreover takes place in an entirely similar way in the embryos of Birds and Mammals, there is formed a deep depression of the surface at the posterior margin of the cephalo-cervical region, the neck-sinus, sinus cervicalis (EABL) or sinus prcecervicalis (His) (tigs. 156 and 158 hb}. In the depths of this depression and 011 its front wall lie the third and fourth visceral arches, which are now no longer visible from without. The entrance to the sinus is bounded in front by the second visceral, or the hyoid, arch (zfy. The latter gradually develops a small process backward, which covers over the cervical sinus and has been justly compared by RATHKE with the operculum of Fishes and Amphibia. The opercular process at last fuses with the lateral wall of the body. Thereby the sinus cervicalis, which corresponds to the cavity beneath the operGulum tvMch in Fishes and Amphibia covers in the real gillarches, is closed up.
One easily gets an accurate conception of these important processes of growth by comparing fig. 154 with fig. 156 and iig. 157 with fig. 158.
Fig. 156. Frontal reconstruction of the oro-pharyngeal cavity of a human embryo (## of His) 11 '5 mm. long, neck measurement. From His, " Menschliche Embryonen." Magnified 12 diameters.
The upper jaw is seen in perspective, the lower jaw in section. The last visceral arches are no longer visible externally, since they have moved into the depths of the cervical sinus.
The development of the visceral clefts and the cervical sinus has also a practical interest. Sometimes there occur in the neck-region in Man fistulas, which penetrate variable distances from without inward, and may even open into the pharyngeal cavity. They result from embryonic conditions, the cervical sinus having remained partly open. From this sinus a passage may lead, even in the adult, into the pharyngeal cavity, if abnormally the second visceral cleft has not closed.
Fig. 157. Very young human embryo of the fourth week 4 mm. long, neck-rump measurement; taken from the uterus of a suicide 8 hours after her death, after RABL.
i',, Eye ; ng, nasal pit ; uk, lower jaw ; zb, hyoid arch ; s"', ,s 4 , third and fourth visceral arches ; h, protrusion of the wall of the trunk produced by the growth of the heart ; t's, boundary between two primitive segments ; oe, uc, anterior and posterior limbs.
The Development of the Anus and the Post-anal Gut
The question concerning the fate of the primitive mouth [blastopore] and the development of the anus is not yet settled. Many disclosures are still to be expected from a comparative study of these structures in the different classes of Vertebrates. According to the common representation, which appears to me to correspond on the whole with the real state of affairs, the primitive mouth is a transitory structure without permanent existence. In all Vertebrates it is surrounded, as in Amphioxus, by the growth of the medullary folds, and when these are closed, it no longer leads directly to the outside, but into the posterior end of the neural tube, It has thereby become the familiar canalis neurentericus (fig. 159 ne). Neural tube and intestinal canal together form a U-shaped tube, at the bend of which the rudiment of the primitive mouth, or primitive groove, is to be sought.
Fig. 158. Human embryo of the middle of the fifth week 9 mm. long, neck-rump measurement, after RABL. .s, Mid-brain [parietal] elevation ; an, eye ; ok, upper jaw ; v.Jc, lower jaw ; zb, hyoidarch ; Jib, sinus cervicalis ; ng, nasal pit ; oe, anterior, uc, posterior limb ; m^, muscle-plates (trunk-segments).
The anus is a new structure. It arises on the ventral side of the body (fig. 159 an) at some distance in front of the place where the neural tube bends around into the intestine. Over a small area the entoderm and the epidermis here grow toward each other, and, by crowding aside the middle germ-layer, come into contact and form a thin septum, the anal membrane. Externally this place is characterised in many animals by a depression of the epidermis, the anal pit (fig. 159 an). The opening of the intestine to the outside takes place in most cases at a rather advanced stage of development by the rupture of the thin anal membrane, which consists of only two epithelial layers. The process is therefore similar to that by which the mouth is formed. In one important point, however, there exists a difference between the opening at the anterior and that at the posterior end of the body. Whereas the oral sinus comes in contact with the anterior end of the fore gut, the formation of the anus does not take place at the posterior end of the embryonic intestine, which is occupied by the primitive mouth [blastopore], but at some distance in front of it. (Compare also fig. 126, that of the Chick, in which the region where the anal pit is to be formed is designated by the letters an.) Consequently in the embryos of Vertebrates, when the anus has broken through, the embryonic intestinal tube is still continued for some distance back of the anus to the primitive mouth. This portion is designated as the posi-anal or caudal gut (fag. 126 p.a.y.). The latter designation is appropriate, because the part of the body which lies behind the anus, in which is enclosed the part of the intestine under consideration, becomes the tail-end of the embryo.
The post-anal gut appears to be established as a shorter or longer tract in all Vertebrates ; it has already been observed in the most widely different animals by several investigators : first by KOWALEVSKY in Amphioxus, the Acipenseridse, Selachians, and Teleosts ; then by GOETTE, BOBRETZKY, BALFOUR, HlS, K6LLIKER, GASSER, BRAUN, BONNET, and others in the Amphibia, Selachians, Birds (fig. 126 p.a.y.), and Mammals. In the Selachians (Scyllium) the post-anal section at the time of its greatest development attains about onethird the length of the whole alimentary canal. It exhibits at its end a small vesicular enlargement, which communicates with the neural tube by means of a narrow opening. In an advanced embryo of Bombinator it is also to be seen well developed, as shown in the sagittal section fig. 159. It begins at the place marked by an, at which the epidermis has sunk down to form the anal pit (an) and at which it has united with the intestine, immediately behind the mass of yolk-cells collected in the ventral wall of the latter. From this point it runs backward as a narrow but open tube, and bends around dorsally into the neural tube as the neurenteric canal. The primitive mouth, now closed, formerly lay at the place of bending.
Fig. 159. Sagittal section through an advanced embryo of Bombinator, after GOETTE. m, Mouth ; an, anus ; I, liver ; ne, neuventeric caual ; me, medullary tube ; c/i, chorda ; i>n, pineal glaud.
The post-anal gut, sooner or later, undergoes regressive metamorphosis in all Vertebrates ; it loses its cavity, becomes a solid epithelial cord, afterwards detaches itself from the anal part of the intestine and from the neural tube, and then disappears altogether. Thereby the neurenteric canal, the last remnant of the primitive mouth, has ceased to exist.
A few still more specific statements, in accordance with the representations of STRAHL, KOLLIKER, BONNET, KEIBEL, and GIACOMINI, concerning the formation of the anus in Mammals, may be mentioned in this connection. The first fundament of the anus is demonstrable even in embryos with few primitive segments. At the posterior end of the primitive streak at the anterior end of which the neurenteric canal is situated the anal membrane is formed by the disappearance of the middle germ-layer over a small area and the close contact of entoderm and epidermis. This, however, takes place so that the two latter layers always remain separated from each other by a sharp contour (fig. 160 afm). One might be inclined to regard this position, at the hindermost end of the primitive streak (pr), as deviating from the representation just given, according to which the anus arises on the ventral side of the body somewhat in front of the neurenteric canal. That is not the case, however, as the further course of development teaches ; for in meroblastic eggs, in consequence remaining Vertebrates.
Fig. 160. Sagittal section through the posterior end of an embryo Sheep 16 days old and with 5 pairs of primitive segments, after BONNET.
al, Allantois ; afm, anal membrane ; o.ni, amnion ; ah, amniotic cavity ; al; outer germ-layer, and mk l , middle germ-layer, which share in the formation of the amnion ; np, neural plate as it merges into the primitive streak ; pr, primitive groove in the region of the neurenteric canal ; ik, inner germ-layer ; mk", splanchnic portion of the middle germ-layer ; d, alimentary tube.
of the previously described process of folding, by means of which the body is formed from the flattened-out germ-layers, the region which originally lies behind the primitive groove comes to lie ventral to and in front of the tail-end. At a somewhat later stage than that shown in fig. 160, the primitive streak in front of the anal membrane grows outward as a small ridge and subsequently enlarges into the tail of the Mammal. The neurenteric canal, located in the ridge, is overgrown by the medullary folds, and upon the complete closure of the latter is incorporated in* the neural tube, as in the case of the
In the case of Mammals also there is formed a small caudal gut, which subsequently degenerates. The more the caudal bud protrudes outward (fig. 161 sch}, the more it projects over and beyond the anal membrane (afni), which constantly moves farther toward the ventral side of the body and is now found between the base of the tail (sch) and the fundament of the allantois (/). The rupture of the anal membrane takes place relatively late ; in the case of Ruminants, for example, in embryos that are more than twenty-four days old. Apparently the anus in Birds arises in a manner similar to that in Mammals. According to the statements of GASSER and KOLLIKER its opening, produced by the rupture of the anal membrane, occurs on the fifteenth day.
Fig. 161. Sagittal section through the tailend of an embryo Sheep 18 days old and with 23 pairs of primitive segments, after BONNET.
sc^, Tail-bud or terminal ridge ; am, auinion ; mk l , its mesodermal (somatic) layer ; afm, anal membrane lying ventral to and in front of the tail-bud ; al, allantois.
It is asserted for many Vertebrates (Petromyzou. Triton, Salamandra, Eana ternporaria, Alytes) that the primitive mouth is converted directly into the anus (GASSER. JOHNSON, SEDOWICK, SPENCER, KUPFFER, GOETTE). But since the development of the posterior part of the body proceeds from the margins of the primitive mouth (formation of the chorda and of the middle germ-layer), it would be difficult to understand how, in these cases, the tail-end of the body and a tail-gut could still be formed. Other investigators (SCHANZ andBoNXET) find that the primitive mouth is divided into two openings an anterior, which is incorporated in the hind end of the neural canal (canalis neurentericus, chorda-blastopore). and a posterior, which becomes the anus (anal blastopore, anal canal). The statements, which are still contradictory, must be cleared up by means of comparative investigations.
Differentiation of the Alimentary Tube into Separate Regions and Formation of the Mesenteries
At first the alimentary tube is broadly in contact (fig. 116) with the dorsal wall of the trunk ; it is united to the chorda (cA), the neural tube, and the primitive segments by means of a broad tract of embryonic connective tissue, in which the fundaments of two large blood-vessels, the primitive aortas (ao\ are enclosed. The right and left portions of the body-cavity are therefore still separated from each other on the dorsal side by a considerable distance. The older the embryo is, the less this distance becomes, until there results a mesentery, a structure which is established along the whole length of the intestinal tube, with exception of the anterior portion, in the following manner (compare, Plate I., figs. 8 and 9 with fig. 10). The alimentary tube recedes from the chorda ; at the same time the broad tract of connective tissue previously mentioned becomes narrower from right to left, but elongated dorso-ventrally (fig. 10, Plate I.) ; the two aortre embraced in it move nearer and nearer together and finally fuse into a single trunk, which lies in the median plane between chorda and intestine. After the further advance of this process the alimentary tube and chorda remain united by means "of only a thin band, which stretches from the front to the hind end of the embryo. This proceeds from the connective tissue enveloping the chorda, encloses along its line of origin the aorta, and is composed of three layers : a connective-tissue lamella, in which blood-vessels run to the intestine, and two epithelial coverings, which are derived from the middle germ-layer and are now composed of greatly flattened cells.
The differentiation of the alimentary tube into separate non-equivalent regions lying one behind the other begins with the development of the stomach. This first becomes distinguishable, at some distance behind the respiratory tract, as a small spindle-shaped enlargement, the long axis of which corresponds with that of the body (figs. 162 and 163 J/<y). Such a condition is attained by the human embryo of the fourth week. Five successive regions may now be distinguished in the whole embryonic alimentary tube : the oral cavity, the throatcavity with its visceral clefts, which is narrowed into the shape of a funnel where it merges into [the third region,] the gullet. This is followed by the spindle-shaped enlargement, the stomach, and the latter by the remaining portion of the alimentary tube, which still is more or less broadly connected (Ds) with the yolk-sac. Excepting the first three regions, the whole alimentary tube possesses a mesentery (mesenterium), the part which is attached to the stomach being designated by the special name mesoyastrium.
In many Fishes and Amphibia this condition is permanent. Even in the adult the alimentary tube takes only a slightly sinuous course through the body-cavity. The stomach appears as a spindle-shaped enlargement of it.
+++++++++++++++++++++++++++++++++++++++++ Fig. 102.
Fig. ir.3. +++++++++++++++++++++++++++++++++++++++++ +++++++++++++++++++++++++++++++++++++++++
Fig. 162. Alimentary tube of a human embryo (R of His) 5 mm. long, neck measurement. From His, " Menschliche Embryonen." Magnified 20 diameters. RT, RATHKE'S pocket; Uk, lower jaw ; SW, thyroid gland ; Ch, Chorda dorsalis ; Kk, entrance to larynx ; Lr/, lung ; My, stomach ; P, pancreas ; Lby, hepatic duct ; Ds, vitelline duct (stalk of the intestine); All, allantoic duct; IV, Wolffian duct, with budding kidney-duct (ureter) ; B, bursa pelvis.
Fig. 163. Alimentary tube of a human embryo (Bl of His) 4 -25 mm. long, neck measurement.
From His, " Menschliche Embryonen." Magnified 30 diameters. The abbreviations mean the same as in fig. 162.
An alteration is brought about in all higher Vertebrates by a more or less considerable increase in the length of the tube, which eventually far exceeds that of the trunk. Consequently the alimentary tube, in order to find room for itself in the body-cavity, is compelled to take a tortuous course. In this way certain parts remain near the vertebral column, whereas others, as a result of the folding, are more distant. The former are attached by means of a narrow mesenter}^ and are consequently less movable, the latter by their change in position have drawn out their suspensorial band into a thin lamella, which sometimes attains a remarkable breadth and allows a correspondingly increased freedom of motion.
The processes of development, which are in part very complicated, are satisfactorily explained by the excellent works of MECKEL, JOHANNES MULLER, TOLDT, and His, even in the case of human embryos, so that these may serve as a foundation for the description.
In human embryos of the fifth and sixth weeks the posterior surface of the stomach, that which is turned toward the vertebral column (fig. 164 gc), is greatly distended; the anterior wall (kc) on the contrary, which upon opening the body-cavity is found to be covered by the already voluminous liver, is somewhat depressed. Consequently a line running along the posterior surface from the entrance of the stomach (cardia) to its outlet (pylorus) is much longer than the corresponding line along the anterior surface. The latter becomes the future lesser curvature (kc) ; the former, along which the mesogastrium is attached, is the greater curvature (gc).
The portion of the tube which follows the stomach has become folded, in consequence of its great increase in length. From the pylorus the intestinal tube (du) at first runs backward [dorsad] for a short distance until it is close to the vertebral column, makes a sharp bend here, and then describes a large loop, the convexity of which is directed forward [ventrad] and downward [caudad] toward the navel. The loop consists of two nearly parallel arms (d l and d 2 ) running near each other, between which is stretched the mesentery (ms), which is likewise drawn out with the loop. One arm (fZ 1 ) lies in front and is directed backward, the other (d 2 ) lies behind it and runs upward, to be again bent near the vertebral column ; thence, supported by a narrow mesentery, it pursues a straight course (r] backward to the anus. The transition from the first to the second arm, or the apex of the loop, is imbedded in an excavation in the fcetal end of the umbilical cord, and it is there in communication with the umbilical vesicle by means of the vitelline duct (d ?> ], now in process of degeneration. At some distance from the origin of the vitelline duct there is to be seen in the second arm of the loop a small enlargement and evagination (cZ 2 ). This is afterwards developed into the coecum, and it therefore indicates the important boundary between the small and large intestine.
Fig. 164. Diagrammatic representation of the alimentary canal of a six-weeks embryo of Man, after TOLDT.
- />, (Esophagus ; kc, lesser curvature ; gc, greater curvature of the stomach ; i/(', duodenum ; d 1 , part of the loop that will become the small intestine ; d z , pait of the loop that will become the large intestine and begins with the coecum ; d 3 , place of connection with the vitelline duct ; 'ni(i, mesogastrium ; //, mesenterium ; m, spleen ; p, pancreas ; r, rectum ; (io, aorta ; d, cceliaca ; mei, mesenterica inferior ; ac, aorta caudalis.
In consequence of these first foldings four regions of the intestine can be distinguished even now ; these are more sharply separated later. The short portion, running from the stomach to the backbone and provided with a small mesentery, becomes the duodenum (du) ; the anterior [ventral], descending arm (d l \ together with the bend in the loop, furnishes the small intestine ; the posterior [dorsal], ascending arm is developed into the colon (<rZ 2 ), and the terminal part, embracing the last bend, into the sigrnoid flexure and the rectum (r).
In embryos of the third and following months there occur, in connection with a further increase in length, important changes in the position of the stomach and the intestinal loops.
The stomach undergoes a double twisting, about two different axes, and thereby early acquires a form and position (figs. 165 A and 7>) which correspond approximately to the permanent condition. First its longitudinal axis, which unites cardia and pylorus and is in the beginning parallel with the vertebral column, takes an oblique and finally an almost transverse position, in consequence of a rotation around the dorso- ventral axis. Thereby the cardia moves to the left half of the body and downwards, but the pylorus more to the right side and somewhat higher. Secondly, at the same time the stomach experiences a torsion around its longitudinal axis, by which the originally left side becomes the front [ventral] and the right the back [dorsal]. Consequently the greater curvature comes to lie below [posterior], the lesser above [anterior]. The terminal part of the esophagus is also affected by the torsion ; it undergoes a spiral twisting, by which its left side becomes the front.
The embryonic processes of growth in the case of the alimentary tube shed light on the asymmetrical position of the two nervi vagi, which pass through the diaphragm, the left on the front side of the oesophagus to be distributed to the front side of the stomach, the right on the back side of the oesophagus to the corresponding surface of the stomach. If we imagine the process of torsion in case of the oesophagus and stomach to be reversed, the symmetry in the course and distribution of the vagi will be completely restored.
Fig. 165. Diagram of the development of the human alimentary canal and its mesentery.
A, earlier, B, later stage.
.'//'. Greater amentum, which is developed from the mesogastrium (fig. 164 mg). The arrow indicates the entrance to the omentum (bursa omentalis). fjc, Greater curvature of the stomach ; gg, ductus choledochus ; <!u, duodenum ; mes, mesenterium ; ;nc, mesocolon ; dd, small intestine ; di, large intestine (colon) ; nul, rectum ; dg, vitelline duct ; bid, ccecum ; iff, appendix vermiformis ; k, place where the loops of the intestine cross each other. The colon with its mesocolon crosses the duodenum.
The torsion of the stomach naturally exercises a great influence on the mesogastrium. ;m<l, as JOH. MULLER was the first to show clearly, initiates the development of the greater omentum (omentum majus). As long as the stomach has a vertical position, its mesentery is a vertical lamella, which stretches from the vertebral column (fig. 164) directly to the greater curvature, that is still directed backward [dorsad]. But in consequence of the torsion it becomes greatly stretched and enlarged, because its attachment to the stomach must follow all the displacements of that organ. From its origin at the vertebral column, it therefore now betakes itself to the left and downward to become attached to the greater curvature of the stomach ; it assumes a shape and position of which the reader will easily form a correct idea if he mentally combines the diagram of fig. 165 with the cross section shown in fig. 166. In this way there is formed a cavity (bursa omentalis, fig. 166 **), separated from the rest of the body-cavity, which has its opening turned toward the right, whose front wall is formed by the stomach and whose back and lower wall is formed by the mesogastrium (gn 1 , gn 2 ). In the diagrammatic figures 165 A and B the entrance to the bursa is indicated by the direction of the arrows.
The bursa omentalis (fig. 166 **) moreover acquires a still greater extension from the fact that the liver (Z) has by this time grown into a large gland, and is united to the lesser curvature of the stomach by means of the lesser omentum (kn), the development of which we shall treat of later. Therefore the bursa does not open, as in the diagram (fig. 165), in which the liver with its ligaments is omitted, at once into the common body-cavity at the lesser curvature of the stomach, but first into an ante-chamber (the atrium bursts omentalis),oi the lesser omental pocket, which lies behind the lesser omentum (kn) and the liver (Z).
Fig. 166. Diagrammatic cross section through the trunk of a human embryo in the region of the stomach and mesogastrium, to show the formation of the omentum, at the beginning of the third month, after TOLDT.
nv, Suprarenal bodies ; o, aorta; I, liver; m, spleen ; p, pancreas ; r/n l , origin of the greater omentum (mesogastrium) at the vertebral column ; gn", the part of the mesogastrium which is attached to the greater curvature (gc) of the stomach ; kn, lesser omentum ; gc, greater curvature of the stomach. * Atrium and cavity of the greater omentum.
The intestinal loop with its mesentery passes through a no less fundamental twisting around its place of attachment in the lumbar region than the stomach does. The descending and the ascending arms at first lie side by side.
Then the latter, which becomes the colon (fig. 165), lays itself obliquely over [ventral to] the former, and crosses the beginning of the small intestine (Jc) transversely. Both parts, but especially the small intestine, continue from the end of the second month to increase rapidly in length and to take 011 a folded condition. Meanwhile the initial part of the colon, or the ccecum (fig. 165 A bid), which exhibits even in the third month a curved, sickle-shaped, vermiform appendage, comes to lie wholly on the right side of the body up under the liver; from here it runs in a transverse direction across [ventral to] the duodenum under [caudad of] the stomach to the region of the spleen, then bends sharply about (flexura coli lienalis) and descends to the left pelvic region, where it is continued into the sigmoid flexure and rectum. Therefore there are distinguishable in the colon, even in the third month, the ccecurn, the transverse and the descending colon. An ascending colon is still wanting. It is formed in the succeeding months (fig. 165 B] by the gradual sinking down of the ccecum, which was at first under the liver, until in the seventh month it is below the right kidney, and from the eighth month onward descends past the crest of the ilium.
Meanwhile the ccecum has increased in length and toward the end of pregnancy is a rather large appendage at the place of transition from the small to the large intestine. It early exhibits a want of uniformity in development (fig. 165 B bid). The terminal part, which often embraces more than half its length, does not keep pace in its growth with the more rapidly enlarging proximal portion ; the former is designated as the appendix vermiformis, the latter as the cwcum. At the time of birth the vermiform appendage is still not so sharply differentiated from the ccecum as it is a few years later, when it has been converted into an appendage of the size of a goosequill and 6 to 8 cm. long.
Within the region embraced by the bends of the large intestine, the small intestine, which is derived from the descending arm of the loop, is disposed in more and more numerous folds owing to its extensive growth in length (fig. 165 B).
At first all regions of the intestine from the stomach onward are so united to the lumbar region of the vertebral column by means of a common mesentery (mesenterium commune) that they can move freely (fig. 165 A and J3). The mesentery is naturally influenced by the increase in the length of the intestine, inasmuch as its line of insertion on the intestine exceeds in length many times the line of origin at the vertebral column (radix mesenterii), and is thereby laid into folds like a frill. Such an arrangement of the mesentery is found to be the permanent condition in many Mammals, as in the Dog, the Cat, etc.
But in the case of Man, from the fourth month onward, the arrangement of the mesentery is much more complicated. There occur changes which may be briefly characterised as processes of fusion and concrescence of certain portions of the mesenterial lamella with contiguous parts of the peritoneum, either of the posterior wall of the body-cavity, or of neighboring organs. They affect the mesentery of the duodenum and colon, which is always present in the first half of embryonic development.
The duodenum, describing the well-known horseshoe-shaped curve, applies its mesentery, in which the beginning of the pancreas is enclosed, broadly to the posterior wall of the body, and fuses throughout its whole extent with the peritoneum of the latter ; from being a movable it has become an immovable portion of the intestine (fig. 167 du).
The large intestine (figs. 165 and 167 A and B ct) still possesses in the third month a very broad suspensorium arising from the vertebral column, which is nothing else than a part of the common mesentery of the intestine, but which has received the special designation of mesocolon (msc). In consequence of the previously described twisting of the primitive loop of the intestine, not only the colon transversum, but also the considerable mesocolon belonging to it, has been drawn transversely across the end of the duodenum ; for a certain distance it fuses with the latter and with the posterior wall of the body, thereby acquires anew secondary line of attachment (fig. 167 msc) running from right to left, and thus appears as a part that has become detached from the common mesentery. The colon transversum (ct) with its mesocolon (msc) now divides the body-cavity into an upper [anterior] part, which contains the stomach, liver, duodenum, and pancreas, and a lower part, holding the small intestine.
Fig. 167 A B. Two diagrams to illustrate the development of the bursa omentalis.
A, earlier, B, later stage.
zf, Diaphragm ; ?, liver ; p, pancreas ; mg, stomach ; go, its greater curvature ; du, duodenum ; dd, small intestine ; ct, colon trans versum ; *, bursa omentalis ; kn, lesser omentum ; gn l , posterior [dorsal] lamella of the greater omentum, arising from the vertebral column ; gn~, anterior [ventral] lamella of the same, attached to the greater curvature of the stomach (g c ) ! g il *i the part of the omentum which has grown over the small intestine ; gn*, the part of the omentum which encloses the pancreas ; mes, mesentery of the small intestine msc, mesocolon of the transverse colon.
Thus embryology explains the striking condition of the duodenum, which, in order to pass from the upper to the lower space and to become continuous with the small intestine, passes underneath (dorsal to) the transversely outstretched mesocolon (figs. 165 and 167 du).
Also in the case of the suspensorium of the ccecum, and of the ascending and descending arms of the colon, there occurs a more or less extensive concrescence with the peritoneum of the wall of the trunk. Therefore in the adult the parts of the intestine named sometimes lie with their posterior wall broadly in contact with the body-wall ; sometimes they are supported by a broader or narrower mesentery.
There still remain to be described the important changes of the bursa omentalis, the development of which during the first months of embryonic life we have already (p. 299) become acquainted with. The bursa is distinguished, first, by a very considerable growth, and, secondly, by the fact that it fuses with neighboring organs at various places. In the beginning it reaches only to the greater curvature of the stomach (figs. 165, 166), to which it is attached; but even from the third month onward it enlarges and lays itself over [ventral to] the viscera which lie below the stomach, at first over the transverse colon (fig. 167 A <jn l , gn-), then over the whole of the small intestine (fig. 167 A gn 5 ). The bursa consists, as far as it has extended downwards, of two lamellae, which lie close to each other, separated by only a very narrow space, and are continuous at their lower margin. Of these the more superficial, the one which is nearer to the ventral wall of the belly, is attached to the greater curvature of the stomach (gc) ; the posterior [dorsal] lamella, which lies upon the intestines, is originally attached to the vertebral column and here encloses the main part of the pancreas (figs. 167 A p and 166 _p). In the case of many Mammals (Dog) the bursa omentalis remains in this condition. In Man it begins as early as the fourth month to undergo fusions (fig. 167 B}. On the left side of the body the posterior lamella reposes on the posterior wall of the body over a large extent of surface, and fuses with it (gn 4 ), so that its line of attachment to the vertebral column moves laterad up to the origin of the diaphragm (lig. phrenico-lienale). Farther down it glides over the upper [anterior] surface of the mesocolon (msc) and over the transverse colon (ct) ; it becomes fused with both of them, with the former as early as the fourth embryonic month. At the time of birth the two lamellae of the portion of the bursa which has grown over the intestines are, as in many Mammals, separated by a narrow fissure (fig. 167 B gn^} ; during the first and second years after birth they ordinarily fuse, into a, single lamella in which fat is deposited.
Development of the Separate Organs of the Alimentary Tube
The simple growth in length, to which is to be referred the formation of the convolutions just described, is only one and certainly not the chief means by which the inner surface of the intestine is increased. The latter acquires a much greater addition from the fact that the inner, originally smooth epithelial layer, which is derived from the entoblast of the germ, forms evagiiiations and invaginations. By invaginations toward the cavity of the intestine there arise numerous folds, small papillae and villi, which give to the mucous membrane at most places a velvety structure ; by evaginations toward the outer surface of the tube there are developed various kinds of larger and smaller glands.
By this simple device, the formation of folds, the great importance of which in the determination of form in animals was particularly set forth in Chapter IV. of Part I., the mucous membrane acquires to a much greater extent the ability : (1) to secrete digestive fluids, and (2) to absorb the nutritive substances that are mechanically and chemically prepared in the intestine, and to transfer them into the body-fluids.
I discuss the numerous organs which are produced by the process of folding according to the regions into which the intestinal tube is divided, beginning with the organs of the oral cavity.
The Organs of the Oral Cavity : Tongue, Salivary Glands, and Teeth
(1) The Tongue arises, according to the investigations of His upon human embryos, out of an anterior and a posterior fundament (fig. 168).
The anterior fundament appears very early as an unpaired elevation (tuberculum impar, His) on the floor of the oral cavity in the space surrounded by the mandibular ridges. It grows a good deal in width, and its anterior margin projects free over the mandible, thus forming the body and tip of the tongue. Even as early as the beginning of the third month some papillae make their appearance on it (His, KOLLIKER).
The posterior fundament produces the root of the tongue, which, although free from papillae, is richly provided with follicular glands. It is developed out of two ridges in the region where the second and third visceral arches come together in the median plane. The anterior and posterior fundaments unite in a V-shaped furrow, the arms of which diverge in front. The circumvallate papillae are formed on the body of the tongue along this furrow, which persists for a long time. Where the two arms of the V meet there is a deep pit, the foramen ccecum, which His has brought into connection with the origin of the thyroid glands, which will soon be discussed.
(2) The Salivary Glands are demonstrable even in the second month. The fundament of the subm axillary appears first in human embryos at the sixth week (CHIEVITZ), afterwards the parotid in the eighth week, and finally the sublingual.
(3) From a morphological point of view, the Teeth can well be designated as the most interesting structures of the oral cavity. Their development in Man and Mammals is accomplished in a manner Which is neither simple nor Fig ' 168 -- Ton gue f a human embryo about 20 mm. long, neck measurement. After His, easily intelligible ; in the "Menschliche Embryonen." lower Vertebrates, on the contrary, it is simpler, and for that reason I shall make use of the latter as the starting-point of the description.
The teeth, which in Mammals are attached to the edges of the jaws and only bound the entrance to the alimentary tube, possess in the lower Vertebrates a very wide distribution. For in many species they not only cover the roof and the floor of the oral cavity and the inner surface of the branchial arches in immense numbers, as palatal, lingual, and pharyiigeal teeth, but they are also distributed in close-set rows over the whole surface of the skin, and produce, as in the Selachians, a strong and at the same time flexible coat of mail.
The teeth are originally nothing else than ossified papillae of the skin and the mucous membrane, upon the contiguous surfaces of which they are formed. The development of the dermal teeth in Selachians shows this in a very convincing manner.
In young Shark embryos, by a proliferation on the part of the subepithelial cells, there are developed on the otherwise smooth surface of the dermis, which comes from the embryonic mesenchyme, small pnpillre composed of numerous cells (fig. 169 ,?_;;), and these penetrate into the thick overlying epidermis. The latter also undergoes changes on its part, which are directed toward the formation of the tooth ; for those of its cells which immediately cover the papilla grow out into very long cylindrical forms, and produce an organ the function of which is to secrete enamel, the so-called enamel-membrane (fig. 109 sm). By means of further growth the whole fundament next assumes a form which corresponds to the future hard structure (fig. 170).
+++++++++++++++++++++++++++++++++++++++++ Fig. 169. Very young fundament of a dermal tooth (a placoid scale) of a Selachian embryo.
zp, Dental papilla ; sm, enamel-membrane. +++++++++++++++++++++++++++++++++++++++++
Then the process of ossification begins. There is secreted by the most superficial cells of the papilla (o), the odontoblasl-layer (membrana eboris), a thin layer of dentine (zb), which rests upon the papilla like a cap. At the same time the enamel-membrane (sm) begins its secretive activity, and coats the outer surface of the dentinal cap (zb) with a firm, thin layer of enamel (s). The body of the tooth is developed and becomes ever firmer and larger by the subsequent continual deposition of new layers on the first-formed ones, on the dentinal cap new dentine from within through the activity of the odontoblasts ; on the coating of enamel new layers of enamel from without, through the action of the enamel-membrane. Thus the i structure projects more and more above the level of the skin, and the tip of the tooth finally breaks through the epidermal covering. The tooth then acquires a still firmer attachment in the dermis from the fact that, at the surface where the lower margin of the dentine occurs, salts of lime are deposited in the superficial layers of the connective tissue (/A 2 ), and thus a kind of connective-tissue bone, the cementum of the tooth, is produced.
Fig. 170. Longitudinal section through an older fundament of a dermal tooth of a Selachian embryo.
e, Epidermis; e\ the deepest layer of epidermal cells, which are cubical; sch, mucous cells; lh l , the part of the dermis which is composed of connective-tissue lamella? ; lit 2 , superficial layer of the dermis ; :j>, dental papilla ; o, odontoblasts ; r.b, dentine ; s, enamel ; sm, enamelmembrane.
The finished tooth therefore is constructed out of three calcified tissues, which arise from three separate fundaments. The dentine takes its origin from the odontoblast-layer of the dental papilla (mesenchyme), the enamel from the epithelial enamel -membrane (outer germlayer}, and the cementum from connective tissue in the vicinity by means of direct ossification. The finished tooth has, moreover, within it a cavity, which is filled with a vascular connective tissue (pulp), the remnant of the papilla. When the enamel-membrane has fulfilled its office it perishes, for in the process of secretion its cells become shorter and shorter, and are finally reduced to flat scales, which are afterwards thrown off.
In Selachians the formation of the teeth which occupy the edges of the jaws and serve for the comminution of the food differs from this simple process in one important point ; they take their origin, not on the free surface of the mucous membrane, but in its depths (fig. 171). The epithelial tract of the oral mucous membrane which shares in the formation of teeth lias sunk deep down in the form of a ridge (zl) on the inner surface of the jaw-arches, into the underlying loose connective tissue, and now represents a special organ, distinguishable from its surroundings. This important difference is produced by the fact that in the development of the teeth of the jaws more active processes of growth take place, first because these teeth are much larger than the dermal teeth, and, secondly, because they are more rapidly worn out and must consequently be more rapidly replaced by supplementary teeth. As we have often had the opportunity of observing in the study of the production of morphological conditions in animals generally, portions of epithelial membranes that grow more rapidly than their surroundings emerge from the latter and become folded either outward or inward.
Fig. 171. Cross section through the lower jaw of a Selachian embryo with fundaments of teeth. A-, Mandibulav cartilage ; zl, dental ridge ; zp, dental papilla ; zt>, dentine; s, enamel ; siii, enamelmembrane ; b, connective-tissue part of the mucous membrane.
The process of the formation of teeth is the same on the dental ridge itself as upon the free surface of the skin. There are developed on its outer side, which is turned toward the cartilage of the jaw (&), numerous papillae (zp}, lying alongside of and behind one another, which grow into the invaginated epithelium just as the dermal papilla? grow into the epidermis. Thus there arise in the depths of the mucous membrane several rows of teeth, of which the most superficial anticipate in development those which lie deeper ; the former are the first to break through the mucous membrane, to become functional, and, after having been worn out, to be cast off; they are also the first to be supplanted by reserve teeth, which lie behind them, and, developing somewhat later, are consequently younger.
Whereas in the Selachians, as well as in the lower Vertebrates generally, the replacement of teeth by new ones is throughout life an wdimited process, since new papillae are continually being formed in the depths of the dental ridge (polyphyodont), it is in the higher Vertebrates more limited, and in most Mammals occurs only once. There are formed on the ridge two fundaments (diphyodont), one behind the other, one for the milk-teeth and a second for the permanent teeth.
In the case of Man the development of the teeth begins as early as tJie second month of embryonic life. A ridge (zl) (the enamel-germ of older authors) grows from the epithelium of the oral cavity both on the maxillary and niandibular arches as it also does in other mammalian embryos (fig. 290) into the richly cellular embryonic connective tissue. The region from which this growth into the depths takes place (fig. 172 A and B] is marked exteriorly by a groove, which runs parallel to the arch of the jaw, the so-called dental groove (zf). The head of the human embryo represented in figure 289 shows this groove at a little distance behind the fundament of the upper lip.
At first the dental ridge is uniformly thin and separated from its surroundings by a smooth surface. There is nothing to be seen as yet of the separate fundaments of the teeth. Then the epithelial cells on the side of the ridge which is directed outwards begin at certain places to grow and to produce at regular intervals from one another as many thickenings as there are to be teeth (fig. 172 A}. In Man, who has twenty milk-teeth, the number of these is ten in each jaw. The thickenings now assume a flask-shaped form (fig. 172 I>), and gradually detach themselves from the outer surface of the epithelial ridge (zl), except at the neck of the flask, which remains in connection with it at a little distance from its deep edge. Because these epithelial growths have relation to the secretion of enamel, they have received the name of enamel-organs.
In the meantime the connective tissue has taken its first steps toward the formation of the tooth (fig. 172 A aiid.#). At the bottom of each flask the connective-tissue ceils exhibit active growth, and give rise to a papilla (cp) corresponding in form to the future tooth. As the papilla? of the dermal teeth grow into the epidermis, so this papilla grows into the enamel- organ, which is thereby made to take the form of a cap.
Then the special layers from which the formation of dentine and enamel proceed are differentiated in both fundaments so far as these are in mutual contact. At the surface of the papilla (fig. 172 E sp] the cells assume spindle-shaped forms and group themselves into a kind of epithelial layer, the layer of the dentine-forming cells (membrana eboris). On the part of the cap-like enamel-organ the cells of the deepest layer, which is in immediate contact with the papilla, are converted into very long cylinders and constitute the enamel-membrane (sm, membrana adamantine). The latter becomes gradually thinner toward the base of the papilla, where it is continued as a layer of more cubical elements (se), which forms the boundary at the surface of the cap separating it from the surrounding connective tissue. Between these two cell-layers (the inner and the outer epithelium of KOLLIKER) the remaining epithelial cells of the enamelorgan undergo a peculiar metamorphosis, and produce a kind of gelatinous tissue, the enamel-pulp (sp) ; they secrete between them a fluid rich in mucus and albumen, and become themselves converted into stellate cells, which are united to one another by their processes, and thus form a fine network. The enamel-pulp is most highly developed in the fifth or sixth month, and then diminishes up to the time of birth in the same ratio as the teeth increase in size. A _>
Fig. 172 A B. Two stages in the development of the teeth of Mammals. Diagrammatic sections.
zj\ Dental groove ; zl, dental ridge ; zl 1 , deepest part of the dental ridge, on which are formed the fundaments of the supplementary teeth ; zp, dental papilla ; am, enamel-membrane ; .sy>, enamel-pulp ; se, outer epithelium of the enamel-organ ; zs, dental sac ; k, bony alveolus. +++++++++++++++++++++++++++++++++++++++++
The connective tissue immediately enveloping the whole fundament acquires numerous blood-vessels, from which branches also make their way into the papilla ; it becomes somewhat differentiated from the surrounding tissue, and is distinguished as dental sac (fig. 172 B zs).
The soft fundaments of the teeth enlarge up to the fifth month of embryonic life, and at the same time acquire the particular forms of the teeth which are to arise from them those of the incisors, the canines, and molars. Then the process of ossification begins (fig. 173) in the same manner as in the dermal teeth. A cap of dentine (zb) is formed by the odontoblasts (o), or dentinal cells ; this cap at the same time acquires a coating of enamel (s) from the enamel-membrane (sm) ; then there are continually deposited on the first layers new ones, until the crown of the tooth is completed. Under pressure of the latter the enamel-pulp (sp) atrophies, and forms only a thin covering to the tooth at birth. The papilla (zp) is converted into a mass of connective tissue containing blood-vessels (g) and nerves, and fills the cavity of the tooth as the socalled pulp. The larger the whole structure becomes, the more it raises up the the gum, covers the edge of tissue of which the and jaw, causes it to become gradually thinner. Finally, it breaks through the gum soon after birth, and at the same time casts oft' from its surface the atrophied remnant of the enamel-organ.
The time has now come in which the third hard substance of the tooth is formed, the cementum that envelops the root. So far as the dentine has received no coating of enamel, the bounding connective tissue of the dental sac (zs) begins, after the eruption of the teeth, to ossify and to produce a genuine bone-tissue with numerous SHARPEY'S fibres ; this bony tissue contributes to the firmer union of the root of the tooth with its connective-tissue surroundings.
The eruption of the teeth ordinarily takes place with a certain degree of uniformity in the second half of the first year after birth. First the inner incisors of the lower jaw break through in the sixth to the eighth months ; then in the course of a few weeks those of the upper jaw follow. The outer [lateral] incisors appear during the period between the seventh and ninth months, those of the lower jaw, again, somewhat earlier than those of the upper jaw. The front molars usually appear at the beginning of the second year, those of the lower jaw first ; then the gap thus left in the two rows of teeth is filled by the eruption of the canine or eye-teeth in the middle of the second year. Finally, the eruption of the back molars, which may be delayed into the third year, takes place.
+++++++++++++++++++++++++++++++++++++++++ Fig. 173. Section through the fundament of the tooth of a young Dog.
k, Bony alveolus of the tooth ; zp, dental papilla ; g, blood-vessel ; o, odontoblast-layer (membrana eboris) ; .16, dentine ; s, enamel ; sm, enainei-nieinbrane ; ;.s, dental sac ; up, enamel-pulp.
The fundaments of the reserve teeth make their appearance at the side of those of the milk-teeth at an extraordinarily early period. They also take their origin from the epithelial ridge. As was previously (fig. 172 A and I>) stated, the ridge extends still deeper (zl 1 ) into the underlying tissue from the place where the enamel-organs of the milk-teeth
have been differentiated from it and where they remain united to it by means of an epithelial cord, the neck. Here in a short time there again appear near the edge of the ridge (fig. 174 s?n 2 , zp 2 ) flaskshaped epithelial growths and dental papillae, which lie on the inner [median] side of the dental sacs of the milk-teeth. In addition there are developed at the ends of the epithelial ridges, in both the right and left halves of the jaw, the enamel-organs of the posterior grinders (the molar teeth of the permanent set), which are not subject to replacement, but are formed once for all. The ossification of the second generation of teeth begins a little time before birth with the first large molars, and is followed in the first and second years after birth by that of the incisors, canines, etc. As a result in the sixth year there are in both jaws forty-eight ossified teeth, twenty milk-teeth and twentyeight permanent crowns, as well as four fundaments of wisdom teeth, which are still cellular.
Fig. 174. Diagrammatic section to show the development of the milk-teeth and permanent teeth in Mammals. Third stage in the series of which figs. 172 A and B are the first and second.
zf, Dental furrow ; zl, dental ridge ; /,-, bony alveolus of the tooth ; h, neck, by means of which the enamel-organ of the milk-tooth is connected with the dental ridge, zl zp, dental papilla ; zp 2 , dental papilla of the permanent tooth ; zb, dentine ; s, enamel ; SHI, enamel-membrane ; sm s , enamel-membrane of the permanent tooth ; sp, enamel-pulp; se, outer epithelium of the enamel-organ ; zs, dental sac.
The shedding of the teeth ordinarily begins in the seventh year. It is initiated by the disorganisation and absorption of the roots of the milk-teeth, under the pressure of the growing new generation. One finds here exactly the same appearances as in the atrophy of osseous tissue, concerning which we have the thorough investigations of KOLLIKER. There arise on the roots of the teeth the well-known pits of HOWSHIP, in which large, multmuclear cells, the osteodasts or bone-destroyers, are imbedded. The crowns are loosened by surrendering their union with the deeper connective -tissue layers. Finally, when the permanent teeth, owing to the growth of their roots, push forth out of the alveoli, the crowns of the milk-teeth are thereby raised up and fall off.
The permanent teeth generally appear in the followiny order : at first, in the seventh year, the first [front] molars ; a year later the middle incisors of the lower jaw, which are followed a little later by those of the upper jaw; in the ninth year the lateral incisors are cut, in the tenth year the first premolars, in the eleventh year the second premolars. Then in the twelfth and thirteenth years the canines and the second molars come through. The eruption of the third molars, or wisdom teeth, is subject to great variation : it may take place in the seventeenth year, but it may be delayed till the thirtieth. Occasionally the wisdom teeth never attain a complete development, so that they are never cut.
The Organs arising from the Pharynx : Thymus, Thyroid Gland, Larynx, and Lung
Whereas in the water-breathing Vertebrates the visceral clefts remain throughout life and subserve respiration, they are completely closed in all Amniota as well as in a part of the Amphibia. The only exception is in the case of the first cleft, lying between the rnandibular and the hyoid arches, which is converted into the drum of the ear (tympanum) and the EUSTACHIAN tube, and thus enters into the service of the organ of hearing, in connection with which it will subsequently engage our attention.
However, the remaining visceral clefts do not disappear without leaving any trace. From certain epithelial tracts of these there arises an organ of the neck-region which functionally is still problematic, the thymus, the morphology of which has been very essentially advanced during the last few years.
has been for several years a favorite object of embryological investigation, since the time when KOLLIKER made the interesting discovery that in mammalian embryos it takes its origin from the epithelium of a visceral cleft. This discovery has since then been corroborated, and at the same time extended ; for also in such animals as persistently breathe by means of gills the thymus is developed out of epithelial tracts of the open and functionally active gill-clefts.
Let us first examine the original condition as exhibited by Fishes. As stated by DOHRN, MAURER, and DE MEURON, the thymus (th) of the Selachians (fig. 175) and the Bony Fishes has a multiple origin and is derived from, separate solid epithelial growths, which take place at the dorsal ends of all the gill-clefts, and, indeed, to a greater extent on the anterior than on the posterior ones.
Fig. 175. Diagram to show the development of the thymus, the thyroid gland, and the accessory thyroid glands, and their relations to the visceral pockets in a Shark embryo, after DE MEURON.
scJt?, First and sixth visceral pockets ; tli , fundaments of the thymus ; sd, "hyroid gland ; nsd, accessory thyroid gland.
Fig. 176. Two diagrams (ventral aspect) of the development of the thymus, the thyroid gland, and the accessory thyroid glands, and their relations to the visceral pockets in a Lizard embryo (A) and a Chick embryo (B), after DE MEURON.
sch 1 , sc/r, First and second visceral pockets ; sd, thyroid gland ; nxd, accessory thyroid gland ; th, fundament of thymus.
In the Bony Fishes the separate fundaments at an early period, even before they have detached themselves from their matrix, fuse together into a spindle-shaped organ lying above the insertion of the gill-arches, which subsequently becomes independent, just as it does in Selachians. The originally epithelial product acquires a peculiar histological character from being penetrated by ingrowths of connective-tissue elements. In the first place lymphcells in great quantities migrate in between the epithelial cells, in a manner .similar to that described by STOHR as of frequent occurrence in the territory of mucous membranes. Secondly, the epithelial growth is traversed in all directions and cut up into small portions by connective tissue, in which lymph-follicles are .formed. The thymus therein acquires the appearance of a lymphoid organ, in which the epithelial remnants are still in part preserved, but only in the form of very small spherical portions, as the corpuscles of HASSALL. At a still later stage of development there arise in the organ irregular cavities filled with molecular granules. These are caused by the disintegration of lymph-cells and the melting down of the reticular connective tissue, which takes place here and there. In the higher, air-breathing Vertebrates the thymus is derived either from the epithelium, of two or three clefts or only from the epithelium of the third visceral cleft, which becomes closed. The former is the case with Reptiles (fig. 176 A th) and Birds (fig. 176 B th), the latter with Mammals. In Reptiles and Birds the two fundaments fuse early upon either side of the trachea into a longish tract of tissue, which in the former is shorter (fig. 177 A), but in the latter very much elongated (fig. 177 J3).
Fig. 177. Semidiagrammatic illustrations to show the ultimate position of thymus, thyroid gland, and accessory thyroid gland on the neck of the Lizard (A),- the Chick (B), and the Calf (C), after DE MEUROX.
sd, Thyroid gland ; nsd, accessory thyroid gland ; th, thymus ; th 1 , accessory thymus ; Lr, trachea ; li, heart ; cj vena jugularis ; c, carotid vein.
In Mammals it is principally the third visceral cleft which contributes to the formation of the thymus. According to KOLLIKER. BORN, and KABL this is the only one which comes into consideration, whereas DE MEURON, KASTSCHENKO, and His give an account which differs from this, but only in minor details.
The further changes of the fundament of the thymus in Mammals and in Man may be briefly summarised as follows. The thymus-sac, which probably takes its origin from the third visceral pocket, encloses only a very narrow cavity, but possesses a thick wall composed of many elongated epithelial cells (fig. 178). It then grows downward toward the pericardium, and at the posterior end begins to form, like a botryoidal gland, numerous rounded lateral branches (c). (KOLLIKER.) These are from the beginning of their formation solid, whereas the sac-like part (), which occupies the neck-region, always continues to exhibit a narrow cavity.
The budding continues for a long time, and meanwhile extends to the opposite end of the originally simple glandular sac, until the whole organ has assumed the lobed structure peculiar to it. At the same time an histological metamorphosis is also taking place. Lymphoid connective tissue and blood-vessels grow into the thick epithelial walls and gradually destroy the appearance which so resembles a botryoidal gland. With the increase in the size of the organ the lymphoid elements coming from the surrounding tissue predominate more and more ; the epithelial remnants are finally to be found only in the concentric bodies of HASSALL, as MAURER has shown for Bony Fishes and as His has undoubtedly rightly inferred for Man and Mammals. The cavity originally present and resulting from, the invagination disappears, and instead of it there arise new irregular cavities, probably the result of a breaking down of the tissue.
Fig. 178. Thymus of an embryo Rabbit of 16 days, after KOLLIKER. Magnified.
a, Canal of the thymus ; 6, upper, c, lower end of the organ.
The further history of the thymus in Man permits the recognition of two periods, one of progressive and one of regressive development.
The first period extends into the second year after birth. The thymus of the right side and that of the left move in their growth close together into the median plane and here fuse into an unpaired, lobed organ, whose double origin is to be recognised only by the fact that the organ is ordinarily composed of lateral halves separated by connective tissue. It lies in front of [ventral to] the pericardium and the large blood-vessels beneath the breastbone, and is often elongated into two horns which extend upwards to the thyroid gland.
The second period exhibits the organ undergoing regressive metamorphosis, which usually leads to its total disappearance, the particulars of which can be learned from the text-books of Histology.
The Thyroid Gland
is found on the anterior surface of the neck, and appears to be developed in almost all classes of Vertebrates in a tolerably uniform, typical manner from an unpaired and a paired evagiiiation of the pharyngeal epithelium. We must therefore distinguish unpaired and paired fundaments of the thyroid gland.
The unpaired fundament has been longest known. There is not a single class of, Vertebrates in which it is wanting, as has been established especially by the investigations of W. MULLER. It appears to be an organ of very ancient origin, which shows relationship to the hypobranchial furrow of Arnphioxus and the Tunicates.
DOHRN has opposed this hypothesis and has expressed the view, which is also shared by others, but which lacks proof, that the thyroid gland is the remnant of a lost gill-cleft of the Vertebrates.
The unpaired thyroid gland arises as a small evagination of the epithelium of the front wall of the throat in the median plane and in the vicinity of the second visceral arch. Then it detaches itself completely from its place of origin, and is converted either into a solid spheroidal body (Selachians, Teleosts, Amphibia, etc.) or into an epithelial vesicle having a small cavity (Birds, Mammals, Man, etc.). The vesicle subsequently loses its cavity.
In Man the development of the unpaired part of the thyroid giand is related to the formation of the root of the tongue, as His states in his investigations of human embryos. The previously described ridges lying on the floor of the throat-cavity in the vicinity of the second and third visceral arches, which unite in the median plane to form the root of the tongue, surround a deep depression, which is the equivalent, of the evagination of the pharyngcal epithelium in the remaining Vertebrates. By the further approximation of the ridges the depression becomes an epithelial sac, which remains for a long time in communication with the surface of the tongue by means of a narrow passage, the cluctus thyreoglossus.
The paired fundaments of the thyroid gland were discovered a few years ago by STIEDA in mammalian embryos, but they have been more fully investigated by BORN, His, KASTSCHENKO, DE MEUROX, and others in Mammals and other Vertebrates (excepting Cyclostomes). In the Amphibia, as well as in Birds and Mammals (fig. 176 7>), there are formed, a little while after the appearance of the unpaired fundament, two hollow evaginations of the ventral epithelium of the throat behind the last visceral arch and in connection with the last visceral cleft. They come to lie immediately on either side of the entrance to the larynx. In many Reptiles (fig. 176 A nsd] there is an interesting deviation due to the fact that an evagination is developed only on the left side of the body, while on the right it has become rudimentary. Even in the Selachians (fig. 175), as DE MEURON appears rightly to maintain, paired fundaments of thyroid glands are present. They are the previously mentioned supra-pericardial bodies discovered by v. BEMMELEN. These arise as evaginations of the epithelium of the throat behind the last pair of gill-clefts near the anterior end of the heart. In all cases the evaginated portions of the epithelium become detached from their parent tissue and enclosed on all sides by connective tissue ; they then undergo a metamorphosis similar to that of the unpaired fundament of the thyroid gland.
In regard to their ultimate position there exist considerable differences between the separate classes of Vertebrates. In the Selachians the supra-pericardial bodies remain far away from the unpaired thyroid gland, being located in the vicinity of the heart ; but in the other Vertebrates they move more or less close to the gland, and have here acquired the name of accessory thyroid glands (fig. 177 A and B nsd}. Finally, in Mammals and Man the approximation has led to a complete fusion of the unpaired and the lateral, paired fundaments (fig. 177 C). Together they constitute a horseshoe-shaped body that embraces the larynx. It is, however, to be observed, that at the time of their fusion the lateral fundaments, in comparison with the median one, are only very small nodules. Consequently KASTSCHENKO, who is probably in the right, ascribes to the former an inconsiderable importance for the development of the whole mass of the thyroid gland, whereas His maintains that they become in Man the voluminous lateral lobes, and that the unpaired fundament becomes the small middle part of the organ.
The further development of the thyroid gland is accomplished in a very similar manner in all Vertebrates. Two stages are distinguishable.
During the first stage the whole fundament grows out into numerous cylindrical cords, which in turn push out lateral buds (fig. 179). By the union of these with one another there is formed a network, into the interstices of which are distributed branches of the blood-vesse1s together with embryonic connective tissue. In the case of the Chick it is found that the thyroid gland has reached this stage of development on the ninth day of incubation, in the Rabbit embryo when it is about sixteen clays old, in Man in the second month.
During the second stage the network of epithelial cords is resolved into the characteristic follicles of the thyroid gland. The cords acquire a narrow lumen, around which the cylindrical cells are regularly arranged. Then there are formed on the cords at short intervals enlargements, which are separated by slight constrictions (fig. 180). By the deepening of the constrictions the whole network is finally subdivided into numerous, small, hollow epithelial vesicles or follicles, which are separated from one another
- [The elevation caused by the mid-brain may be called the apex or crown (Scheitel). In later stages the distance between crown and rump is greater than that between neck and rump, hence the measurement is made from the crown. Compare foot-note, p. 283.]
Fig. 179. Right half of the thyroid gland of an embryo Pig 21 '5 mm. long, crown-rump measurement,* after Boiix. .Magnified 5O diameters. The lateral (LS) and median (MS) thyroid glands are in pi-oce;: of fusion, g, Blood-vessels ; tr, trachea.
by highly vascular embryonic tissue. Subsequently the follicles increase in size, especially in the case of Man ; this results from the epithelial cells secreting a considerable quantity of colloid substance into the cavity.
A few further details concerning the thyroid gland of Man, for which we are indebted to His, may be of interest. First, it is to be noted that the lateral fundaments are considerably more voluminous than the middle part, and that the future fundamental form of the organ is thus from the beginning predetermined. Secondly, some rare anatomical conditions (His) are explained by the development, such as the ductus lingualis, the ductus thyroideus, and the glandula suprahyoidea and praehyoidea. As was previously stated, the unpaired fundament of the thyroid gland is connected with the root of the tongue by means of the ductus thyreoglossus. When the thyroid gland moves from its place of origin farther down, this duct becomes elongated into a narrow epithelial passage, whose external orifice remains permanently visible as the foramen coecum at the base of the tongue. The remaining part usually undergoes degeneration, but occasionally some parts of it also persist. Thus the foramen coecum is sometimes elongated into a canal (ductus lingualis) 2 cm. long, that leads to the body of the hyoid bone. In other instances the middle part of the thyroid gland is prolonged upward in the form of a horn, which is continued as a tube (ductus thyroideus) to the hyoid bone. Finally, according to His, the glandular vesicles now and then to be observed in the vicinity of the hyoid bone the accessory thyroid glands, as well as the glandula supra- and prse-hyoidea are to be interpreted as remnants of the ductus thyreoglossus.
Fig. 180. Section through the thyroid gland of an embryo Sheep 6 cm. long, after W. MULLER.
sch, Sac-like fundaments of the gland ; /, glandular follicles in process of formation ; b, interstitial connective tissue with blood-vessels (g).
Lung and Larynx
The lung with its outlet (larynx and trachea) is developed, like a lobed gland, out of the oesophagus in a tolerably uniform manner, as it appears, for all amniotic Vertebrates. Immediately behind the unpaired fundament of the thyroid gland (fig. 181 Sd) there arises on the ventral side of the oesophagus a groove (k), which is slightly enlarged at its proximal end. It is to be seen in the Chick at the beginning of the third day, in the Rabbit on the tenth day after fertilisation, and in the human embryo when it is 3'2 mm. long.
Soon the groove-like evagination becomes separated from the overlying portion of the alimentary tube by two lateral ridges ; this furnishes the first indication of a differentiation into cesophagus and trachea (fig. 181). Then there grow out from the enlarged posterior ends of the groove (figs. 181, 163) two small sacs (Lg) toward the two sides of the body (in the Chick in the middle of the third day), the fundaments of the right and left lung. Enveloped in a thick layer of embryonic connective tissue, they are in immediate contact behind with the fundament of the heart; laterally they project into the anterior fissure-like prolongation of the body - cavity. With this the essential parts of the respiratory apparatus are established; at this stage in amniotic Vertebrates they resemble the simple sac-like structures which the lungs of Amphibia present permanently. In the further course of development the fundaments of trachea and oesophagus, which communicate by means of a fissure, become separated by a constriction which begins behind, where the pulmonary sacs have budded out, and gradually moves forward. The constricting off is here interrupted at the place which becomes the entrance to the larynx. The latter is distinguishable in the case of Man at the end of the fifth week as an enlargement at the beginning of the fundament of the trachea. It acquires its cartilages in the eighth or ninth week. Of these the thyroid cartilage arises, according to the comparative-anatomical investigations of DUBOIS, from a fusion of the fourth and fifth visceral arches, whereas the cricoid and arytenoid cartilages, as well as the half-rings of the trachea, are independent chondrifications in the mucous membrane.
Fig. 181. Alimentary tube of a human embryo (R of His)
5 mm. long, neck measurement. From His, " Menschliche Embryonen." Magnified 20 diameters. R T, RATHKE'S pouch ; Uk, lower jaw ; Sd, thyroid gland ; Ch, chorda dorsalis ; Kk, entrance to the larynx ; Lg, lung ; Mg,, stomach ; P, pancreas ; Lbg, primitive hepatic duct ; Ds, vitelline duct (stalk of the intestine) ; All, allantoic duct; W, Wolffian duct, with kidneyduct (ureter) budding out of it ; B, bursa pelvis.
Two stages are recognisable in the metamorphosis of the primitive lung-sacs of Man and Mammals.
The first stage begins with the elongation of the sac, which is attenuated at its origin from the trachea, but is enlarged at its opposite or free end. At the same time in Man from the end of the first month (His) it pushes out, in the manner of an alveolar eland, hollow evao-inations, which crrow out into the thick connective tissue envelope and enlarge at their ends into little sacs. The first bud-like outgrowths on the two sides of the ~body are not symmetrical (fig. 182), because the left lung-sac produces two, the right three bud-like enlargements. An important feature of the architecture of the lungs is thus established from the beginning, namely, the differentiation of the right lung into three chief lobes, and of the left into two.
The further budding is distinctly dichotomous (fig. 183). It takes place in the following way : each terminal vesicle (primitive lung- vesicle), which is at first spheroidal, becomes flattened and indented on the wall (Ib) which lies opposite its attachment. Thus it becomes divided, as it were, into two new pulmonary vesicles, each of which is then differentiated into a long stalk (lateral bronchus) and a spherical enlargement. Inasmuch as such a process of budding is kept up for a long time, in Man until the sixth month, there arises a complicated system of canals, the bronchial tree, which opens into the trachea by means of a single main bronchial tube from either side of the body, and the ultimate branches of which, becoming finer and finer, terminate in flask-shaped enlargements, the primitive lung-vesicles. The latter are at first confined to the surface of the lung, while the system of canals occupies its interior.
+++++++++++++++++++++++++++++++++++++++++ Fig. 182. View of a reconstruction of the fundament of the lungs of a human embryo (Pr of His) 10 mm. long, neck measurement, after His.
Trachea ; br, right bronchus ; sp, oesophagus ; bf, connective-tissue envelope and serous membrane (pleura) into which the epithelial fundament of the lung grows ; 0, M, U, fundaments of the upper, middle, and lower lobes of the right lung; 0\ U\ fundaments of the upper and lower lobes of the left lung.
During this budding the lungs as they increase in volume continue to grow downwards into the thoracic cavities, and thereby come to lie more and more at the right and left of the heart. With their ingrowth into the cavities of the chest (fig. 314 brh), they push before them the serous lining of the latter, and thus acquire their pleural covering (the pleura pulmonalis, or the visceral layer of the pleura).
During the second stage the organ, which up to this time has the typical structure of a botryoidal gland, assumes the characteristic pulmonary structure. The metamorphosis begins in Man, as KOLLIKER states, in the sixth month, and comes to a close in the last month of pregnancy. There now arise close together on the fine terminal tubules of the bronchial tree, on the alveolar passages, and on their terminal vesicular enlargem ents, very numerous small e v a g i n ations. But in distinction from the earlier ones, these are not constricted off from their source of origin, but communicate with the latter by means of wide orifices, and thus constitute the air-cells or pulmonary alveoli. Their size is only a third or fourth as great in the embryo as in the adult ; from this KOLLIKER concludes that the increase in the volume of the lung from birth up to complete development of the body is to be attributed exclusively to the enlargement of the vesicular elements which exist in the embryo.
Fig. 183. View of a reconstruction of the fundament of the lungs of a human embryo (N of His) older than that of fig. 182. After His. Magnified 50 diameters.
Ap, Arteria pulmonalis ; lr, trachea ; sp, oesophagus ; l/j, pulmonary vesicle in process of division ; 0, upper lobe of the right lung with an eparterial bronchus leading to it ; M, U, middle and lower lobes of the right lung ; O 1 , upper lobe of the left lung with hyparterial bronchus leading to it ; U 1 , lower lobe of the left lung.
The epithelial lining of the lung is variously modified in different regions during development. In the whole bronchial tree the epithelial cells increase in height, acquire in part a cylindrical, in part a cubical form, and from the fourth month onward (KOLLIKER) have their free surfaces covered with cilia. In the air-sacs, on the contrary, the cells, which are arranged in a single layer, become more and more flattened, and in the adult become so thin that formerly the presence of an epithelial covering was wholly denied. Then they assume a condition similar to that of endothelial cells; as in the case of the latter, their boundaries are demonstrable only after treatment with a weak solution of silver nitrate.
The Glands of the Small Intestine : Liver and Pancreas
In the section which treats of the liver we must enter upon a discussion not only of the development of the parenchyma of the gland, but also of the various hepatic ligaments the lesser omentum, the ligamentum suspensorium, etc. ; in fact, we must begin with the latter because they are developed out of a structure a ventral mesentery which is ontogenetically older than the liver itself. In view of the manner in which the body-cavity arises, as a pair of cavities, such a structure ought to be found along the whole length of the ventral side of the alimentary canal in the same manner as on its dorsal side. Instead of that, it is found only at the anterior region of the alimentary canal, along a tract which extends from the throat to the end of the duodenum.
This ventral mesentery acquires a special significance, because several important organs take their origin in it ; in front, the heart, together with the vessels that bring the blood back to it the terminal parts of the veme omphalomesentericre and of the vena umbili calis ; immediately behind the latter, the liver with its outlet and its blood-vessels.
The part which, during an early stage of development, encloses the heart is called mesocardium anterius and posterius ; we shall return to it later in considering the development of that organ. The portion (fig. 184) which joins this behind [caudad] has been hitherto less regarded by embryologists. Since it stretches from the lesser curvature of the stomach and the duodenum (du) to the anterior [ventral] wall of the trunk, it may be especially designated as the ventral gastric and duodenal mesentery, or, under a more comprehensive title, as ventral alimentary mesentery (Ihd + Is). It has been described by KOLLIKER on sections of Rabbit embryos as liverridye (Leberwulst), and by His in his " Anatomie menschlicher Embryonen r> as preliepaticus (Vorleber) ; it has the form of a mass of tissue rich in cells, which inserts itself between the wall of the belly and the regions of the intestine previously mentioned. In cross sections through human and mammalian embryos there are encountered in it the capacious vense omphalomesentericse. As far as a mesocardium and a mesogastrium anterius are developed in Vertebrates, the body-cavity appears even subsequently as a paired structure.
Fig. 184. Diagram (view of a cross section) to show the original relations of duodenum, pancreas, and liver, and of the ligamentous structures belonging to them.
HR, Posterior wall of the trunk ; du, duodenum ; p, pancreas ; I, liver ; dins, dorsal mesentery ; Hid, ligamentum hepato-duodenale ; Is, ligamentum suspensorium hepatis.
Fig. 185. Cross section through the anterior part of the trunk of an embryo of Scyllium, after BALFOUB.
Between the dorsal and ventral walls of the body, where the attachment of the stalk of the yolksau is out, there is stretched a broad mesentery which contains numerous cells and completely divides the body-cavity into a right and a left half. The duodenum ('.In), lying in the mesentery, is twice cut through ; dorsally it gives rise to the fundament of the pancreas (pan), ventrally to that of the liver (Jip.<(). Further, the place where the vitelline duct (umc) emerges from the duodenum is to be seen. s/>.c, Neural tube (spinal cord) ; sp.g, ganglion of posterior root ; a,-, anterior root ; dn, dorsally directed nerve springing from the posterior root ; mp, muscle-plate ; >,ip\ part of muscle-pJate already converted into muscles ; 'iiiji.l, part of muscle-plate which gives rise to the muscles of the limbs ; nl, nervus lateralis ; ao, aorta ; ck, chorda ; sy.y, sympathetic ganglion ; ca.v, cardinal vein ; sp.n, spinal nerve ; sd, segmenta] duct (duct of primitive kidney) ; st, segmontal tube (pronephric tubule).
The cross section through a Selachian embryo (fig. 185) shows this distinctly. The duodenum (du) is enclosed in the connective -tissue mesentery, which reaches from the aorta (ao) to the front [ventral] wall of the trunk ; dorsally the pancreas (pan) is budded forth from its wall, ventrally the liver (hp.d).
The liver begins to be developed very early in the ventral mesentery (liver-ridge or prehepaticus), and in this exhibits, as will appear later, two modifications, which are, however, unessential ; for sometimes it appears in the form of a single, sometimes as a paired evagination of the epithelial lining of the ventral wall of the duodenum.
The first is the case, for example, in the Amphibia and Selachii. In Bombinator (fig. 159), as GOETTE has shown, the liver arises as a broad ventrally directed evagination of the intestine, which lies immediately in front of the accumulation of yolk-material. The liver remains permanently in this simplest form in the case of Amphioxus lanceolatus, in which it is located immediately behind the gill-region as an appendage of the intestinal canal.
In the case of Birds and Mammals, on the contrary, the fundament of the liver is from the beginning double. As has been known since the investigations of REMAK, in the case of the Chick (fig. 186) on the third day of incubation, two sacs (I) grow out of the ventral wall of the duodenum immediately behind the spindle-shaped stomach (<St). They grow into the broad cell-mass of the ventral mesogastrium (the Leberwulst), one passing forward to the left, the other backward to the right, and thereby embrace from above the vena omphalomesenterica on its way to the heart. The process in. Mammals is somewhat different. According to the observations of KOLLIKER in the case of the Rabbit, the primitive hepatic tube of the left side is formed in the embryo of ten days, to which a right duct is added in the course of another day. Also in the case of human embryos 4 mm. long His demonstrated that at first there is only a single hepatic duct, and that some time afterwards a second appears (fig. 163 Lbg).
In 'the further course of development both the unpaired and the paired hepatic fundaments are metamorphosed quite rapidly into a tubular gland with numerous branches ; this acquires a special character, differing from that of simple tubular glands, owing to the fact that the tubes early become joined together to form a fine network, since the primitive hepatic tubes send out numerous lateral buds, which in some Vertebrates
(Amphibia, Selachii) are from the beginning hollow, in others (Birds, Mammals, Man) solid. Imbedded in the embryonic connective substance of the ventral mesogastriuin, they grow out in the former case into hollow tubes, in the latter into solid cylinders. These in turn are soon covered with corresponding lateral processes, and so on. Inasmuch as these grow toward one another, and where they meet (fig. 1 87 Ic) fuse, there arises a close network of hollow glandular canals or solid hepatic cylinders in the common connectivetissue matrix.
Simultaneously with the epithelial network there is formed in its meshes a network of blood-vessels (y). From the vena omphalomesenterica, which, as previously stated, is embraced by the two hepatic tubes, there grow out numerous shoots, and these by forming lateral branches unite with one another in a manner corresponding to that of the hepatic cylinders.
The liver of the Chick is found to be in this condition on the sixth day. It has become even now a rather voluminous organ, and is composed, as in the case of Mammals and Man, of two equally large lobes, each of which has arisen from one of the two primitive hepatic ducts by budding. The two lobes produce on the ventral mesentery two ridges, one of which projects into the left body-cavity and one into the right (fig. 184).
Fig. 186. Diagrammatic view of the alimentary canal of a Chick on the fourth day, after GOKTTK.
The heavy line indicates the inner germ-layer, the shaded portion surrounding it the splanchnic portion of the mesoblast. ly, Lung ; St, stomach ; p, pancreas ; I. liver.
A further increase in the size of the liver is due to the fact that from the hepatic cylinders united into a network new lateral branches grow forth and undergo anastomosis, whereby new meshes are being continually formed.
Herewith the essential parts of the liver are present in the fundament : (1) the secretory liver-cells and the bile-ducts, (2) the peritoneal covering and the suspensory apparatus, both of which are derived from the ventral mesentery. The changes in these parts which lead to the permanent condition are now to be considered.
Fig. 187. Section through the fundament of the liver of a Chick on the sixth day of incubation.
Slightly enlarged. Ic, Network of hepatic cylinders ; lc l , hepatic cylinder cut crosswise ; rt, blood-vessels ; gw, wall of the blood-vessel (cndothelium) ; t>l, blood-corpuscles ; 6/, peritoneal covering of the liver.
The epithelium of the ducts and the secretory liver-parenchyma are derived from the two hepatic tubes and from the network of hepatic cylinders, products of the entoblast.
The parts of the two primitive liver-tubes first formed become the right and left ductus hepatici. In Birds and Mammals these open at first, as we have seen, into the duodenum close together ; then at their place of entrance there is formed a small evaginatioii of the duodenum, which receives the two ductus hepatici. The evagination gradually increases to a long single canal, the bile-duct or ductus choledochus, the result of which process is that the whole liver is farther removed from its source of origin.
By an evagination either of the ductus choledochus or of one of the two ductus hepatici, the gall-bladder with its ductus cysticus is established. In Man it arises from the ductus choledochus, and is present as early as the second month.
The network of hepatic cylinders, which are sometimes hollow, sometimes solid, is metamorphosed in two ways.
One part becomes the excretory ducts (the ductus biliferi). In the cases in which the hepatic cylinders are at first solid, they begin to become hollow and to arrange their cells into a cubical or cylindrical epithelium around the lumen. In this process some of the branches of the network must degenerate. For, whereas all hepatic cylinders at first communicate with one another by means of anastomoses, this is, as KOLLIKER remarks, no longer the case in the adult, except at the outlet of the liver (Leberpforte), where the well-known network of bile-ducts exists.
The remaining part of the network furnishes the secretory parenchyma of liver-cells. The character of a netlike tubular gland, which becomes so evident during development, is to be recognised even in the fully developed .organ in the case of the lower Vertebrates, the Amphibia and Reptiles. The tubules of the gland, which were from the beginning hollow, subsequently exhibit an exceedingly narrow lumen, which is demonstrable only by means of artificial injection, and which in cross section is surrounded by three to five liver-cells. Through their manifold anastomoses they produce an extraordinarily fine network, the small meshes of which are filled up by a network of capillary blood-vessels, together with a very small amount of connective substance.
In the higher Vertebrates (Birds, Mammals, Man) the tubular structure of the gland subsequently becomes very inconspicuous and the liver acquires a complicated structure, information concerning the details of which is given in the text-books of histology.
There are three tilings which, from a developmental point of view, are not to be lost sight of: first, the capillaries of the bile-duct have arisen by canali.-;ition of the primitive hepatic cylinders ; secondly, they are bounded by only two liver-cells, which are very large and flake-like ; thirdly, they send out evaginations between and even into the liver-cells themselves. In this way a greater complication is brought about in the arrangement of the tine biliary capillaries and the hepatic cells, to which there also corresponds a greater complication in the distribution of the capillaries of the blood-vessels. By means of all this the original tubular structure of the gland becomes almost entirely obliterated in the fully developed organ. In the adult, as is well known, the parenchyma of the liver is divided by means of connective-tissue partitions into small lobes (acini or lobuli). At the beginning of development nothing is seen of the lobulated structure, because all the hepatic cylinders are united into a network. Detailed information concerning the development of the lobules is wanting.
Now a few words concerning the ligaments and the conditions of form and size which the liver presents up to the time of birth.
The ligamentotis apparatus, as was remarked in the beginning, is preformed in a ventral mesentery (the Vorleber). Owing to the fact that the two hepatic sacs grow out from the duodenum into this ventral mesentery, and by continual branching produce the right and the left lobes of the liver (figs. 184, 185, and 188), the ventral mesentery becomes divided into three portions : first, a middle part, which furnishes the peritoneal covering for both lobes of the liver; secondly, a ligament which proceeds from the front convex surface of the liver in a sagittal direction to the ventral wall of the body, extending as far as the navel and embracing in its free margin the subsequently disappearing umbilical vein (ligamentum suspensorium and teres hepatis, figs. 184, 188 Is) ; and thirdly, a ligament which proceeds from the opposite, concave or portal surface of the liver to the duodenum and the lesser curvature of the stomach, and which contains the ductus choledochus and the afferent hepatic blood-vessels (omentum minus, which is divided into the ligamentum hepato-gastricum and hepato-duodenale). (Fig*- 184 lhda,nd 188 kn.) The lesser omentum or omentum minus soon loses its original sagittal position and is stretched out into a thin membrane running from right to left (fig. 166 kn) ; this is due to the fact that the stomach undergoes the previously described displacement, and moves into the left half of the peritoneal cavity, whereas the liver grows out into the right half more than into the left. In consequence of the formation of the liver and the lesser omentum, the greater omentum, produced by the torsion of the stomach, receives an addition, which is designated as its antechamber (atrium bursse omentalis). For there comes to be associated with the greater omentum that part of the body-cavity which lies behind the liver and lesser omentum, and which in the adult possesses, as is well known, only a narrow entrance (the foramen of WINSLOW) lying below the ligamentum hepato-duodenale.
Fig. 188. Diagram to show the original positions of the liver, stomach, duodenum, pancreas, and spleen, and the ligamentous apparatus pertaining to them. The organs are seen in longitudinal section.
/, Liver ; m, spleen ; p, pancreas ; dd, small intestine ; dy, vitelliue duct ; bid, ccecum ; mil, rectum ; kc, lesser curvature, gc, greater curvature of the stomach ; mes, mesentery ; kn, lesser omentum (lig. hepato-gastricum and hepatoduodenale) ; Is, ligamentum suspensorium hepatis.
Concerning the development of the coronary ligament, see a subsequent part which treats of the diaphragm.
As far as regards the conditions of form and size which the liver presents up to the time of birth, there are two points which are worthy of attention : first, the liver early acquires a very extraordinary size ; secondly, its two lobes are developed at first quite symmetrically. In the third month it nearly fills the whole bodycavity ; its free sharp margin on which a deep incision between the two lobes is observable reaches down almost to the inguinal region, leaving here only a small space free, in which, upon opening the bodycavity, loops of the small intestine are to be seen. It is a very vascular organ, for a great part of the blood returning from the placenta to the heart passes through it. At this time the secretion of bile begins, although only to a slight extent. This increases in the second half of pregnancy. In consequence of this the intestine gradually becomes filled with a brownish-black mass, the meconium. This is a mixture of bile with mucus and detached epithelial cells of the intestine, to which is added amniotic water with flakes of epidermis and hairs that have been swallowed. After birth the meconium is accumulated in the large intestine, from which it is soon afterwards eliminated.
In the second half of pregnancy the growth of the two lobes of the liver becomes unequal, and the left is surpassed more and more in size by the right. Before birth the lower margin of the liver projects downward for some distance beyond the costal cartilages, almost to the umbilicus. After birth it diminishes rapidly in size and weight, in consequence of the change in the circulation produced by the process of respiration. For the stream of blood which during embryonic life has branched off from the umbilical vein into the liver now ceases. During the growth of the body the liver also increases in size still further, but less than the body taken as a whole, so that its relative weight is constantly undergoing reduction.
The pancreas is developed in all Vertebrates with the exception of a few in which it is wanting (Bony Fishes) as an evagination on llu i dorsal side of the duodenum, usually opposite to the origin of the liver (figs. 162, 163, 186 p). In the Chick (fig. 186) the first fundament is distinguishable as early as the fourth day ; in Man it appears somewhat later than the primitive hepatic tube, and has been demonstrated by His in embryos 8 mm. long as a small evagination (figs. 162 and 163). The sac, usually hollow, grows into the dorsal mesentery (ligs. 184, 188 _p) by giving off' hollow, branching, lateral outgrowths.
In the case of Man the pancreas is present as early as the sixth week in the form of an elongated gland (fig. 164p), the free end of which has penetrated upward [cephalad] into the mesogastrium, and thus, midway between the greater curvature of the stomach and the vertebral column, it can move freely. It is therefore compelled to share in the alteration of position which the stomach together with its mesentery undergoes. In embryos of the sixth week its long axis still corresponds approximately with the longitudinal axis of the body. The free end then moves into the left half of the body-cavity, the whole organ being turned (fig. 166) until finally its long axis comes to lie in the transverse axis of the body, as in the adult. In this position its head is imbedded in the horseshoe-shaped curvature of the duodenum, whereas its tail reaches to the spleen and left kidney.
Inasmuch as the pancreas in its development has grown into the mesogastrium (figs. 164, 166, 188), it possesses in the first half of embryonic life, as TOLDT has shown, a mesentery, on which it accomplishes the turning previously described. But at the fifth month this disappears. (Compare the diagrams fig. 167 A and B p.) For as soon as the gland has taken its transverse position, it attaches itself firmly to the posterior wall of the trunk and soon loses its freedom of motion, because its peritoneal covering and its mesentery become fused with the adjacent parts of the peritoneum (fig. 167 B gn 4 ). In this manner the pancreas of Man, which was developed, like the liver, as an intraperitoneal organ, has become a so-called cxtraprritoneal organ, owing to a process of fusion between the serous surfaces that come in contact with each other. By means of this also the attachment of the mesogastrium is displaced from the vertebral column farther to the left.
It still remains to be mentioned, in regard to the outlet of the pancreas, that during development it is continually moving nearer to the ductus choledochus, and that finally it opens in common with the latter into the duodenum at the diverticulum of VATER.
Orifices of the Alimentary Canal
- The original orifice of the alimentary canal (resulting from the invagination of the inner germ-layer), the primitive mouth (blastopore), becomes closed later, owing to the circumcrescence of the medullary ridges, and furnishes temporarily an open communication with the neural tube, the canalis neurentericus.
- The neurenteric canal likewise disappears subsequently by the fusion of its walls.
- The alimentary tube acquires new openings to the outside (visceral clefts, mouth, anus) by the fusion of its walls with the body-wall at certain places, and by the regions of fusion then becoming thinner and rupturing.
- The visceral clefts nrise on both sides of the future neck-region of the body, usually five or six pairs in the lower Vertebrates, four pairs in Birds, Mammals, and Man. (Formation of outer and inner throat-furrows ; breaking through of the closing plate.)
- In water-inhabiting Vertebrates the visceral clefts serve for branchial respiration (development of branchial lamellae by the formation of folds of the mucous membrane) ; in Reptiles, Birds, and Mammals they become closed and disappear, with the exception of the upper part of the first fissure, which is employed in the development of the organ of hearing (external ear, tympanum, Eustachinn tube).
- The mouth is developed at the head-end of the embryo by an unpaired invagination of the epidermis, which, as oral sinus, grows toward the blindly eneling fore gut, and by the breaking through of the primitive pharyngeal membrane. (Primitive palatal velum.)
- The anus arises, in a manner similar to that of the mouth, on the ventral side at some distance in front of the posterior end of the body, so that the intestinal tube is continued for a certain distance beyond the anus toward the tail.
- The post- anal or caudal intestine, which at first stretches from the anus to the posterior end of the body (tail-part of the body), becomes rudimentary afterwards and wholly disappears, so that the anus then marks the termination, as the mouth does the beginning, of the alimentary canal.
Separation of the Alimentary Tube and its Mesentery into Distinct Regions
- The alimentary canal is originally a tube running straight from mouth to anus, near the middle of which the yolk-sac (umbilical vesicle) is attached by means of the vitelline duct (stalk of the intestine).
- The alimentary tube is attached throughout its whole length to the vertebral column by means of a narrow dorsal mesentery ; it is also connected with the anterior wall of the trunk, as far back as the umbilicus, by means of a ventral mesentery (mesocardium anterius and posterius, anterior [ventral] gastric and duodenal mesentery). (Vorleber.)
- At some distance behind the visceral clefts, the stomach arises as a spindle-shaped enlargement of the alimentary tube ; its dorsal mesentery is designated as mesogastrium.
- The portion which follows the stomach grows more rapidly in length than the trunk, and therefore forms in the body-cavity a loop with an upper [anterior], descending narrower arm, which becomes the small intestine, and a lower [posterior], ascending more capacious arm, which produces the large intestine.
- The stomach takes on the form of a sac, and becomes so turned that its long axis coincides with the transverse axis of the body, and that the line of attachment of the mesogastrium, or its greater curvature, which was at first dorsal, comes to lie below, cr caudad.
- The intestinal loop undergoes such a twisting that its lower, ascending arm (large intestine) is laid over [ventral to] the upper, descending arm (small intestine) from right to left, and crosses it near its origin from the stomach.
- The twisting of the intestinal loop explains why in the adult the duodenum, as it merges into the jejunum, passes under the transverse colon and through its mesocolon. (Crossing and crossed parts of the intestine.)
- The lower arm of the loop, during and after its twisting and crossing of the upper arm, assumes the form of a horseshoe and permits one to distinguish the coecum, the colon ascendens, c. transversum, and c. descendens.
- Within the space bounded by the horseshoe, the upper arm of the loop becomes folded to form the convolutions of the small intestine.
- The mesentery, which is at first uniform and common to the whole alimentary tube, becomes differentiated into separate regions, for it adapts itself to the folds and to the elongations of the alimentary tube. It is elongated and here and there undergoes fusion with the peritoneum of the body-cavity, by means of which it either acquires new points of attachment or in certain tracts wholly disappears ; some portions of the intestine are thus deprived of their mesentery.
- The mesentery of the duodenum, and in part also that of the colon ascendens and c. descendens, fuses with the wall of the body (extraperitoneal parts of the intestine).
- The mesentery of the colon transversum acquires a new line of attachment running from right to left, and becomes differentiated from the common mesentery as mesocolon.
- The mesogastrium of the stomach follows the torsions of the latter and is converted into the greater omentum, which grows out from the greater curvature of the stomach to cover over all the viscera lying below.
- Fusions of the walls of the omentum with adjacent serous membranes take place : (1) on the posterior wall of the body, in consequence of which the line of origin from the vertebral column is displaced to the left side of the body ; (2) with the mesocolon and colon transversum ; (3) on the part of the sac which has overgrown the intestines, where its anterior and posterior walls come into close contact and fuse into an omental plate.
Development of Special Organs out of the Walls of the Alimentary Tube
- The surface of the alimentary tube increases in extent inward by means of folds and villi, and by glandular evaginations outward.
- There are developed, as organs of the oral cavity, the tongue, the salivary glands, and the teeth.
- The teeth, which in the higher Vertebrates are found only at the entrance of the mouth, are distributed in the lower Vertebrates (Selachians, etc.) over the whole of the cavity of the mouth and throat, and indeed as dermal teeth over the whole surface of the body.
- The dermal teeth are dermal papillae ossified in a peculiar manner, in the development of which both the superficial layer of tin- curium and also the deepest cell-layer of the epidermis investing the latter are concerned.
- The corium [dermis] produces the abundantly cellular dental papilla, which secretes the dentine at its surface, where a layer of odontoblasts is formed.
- The epidermis furnishes a layer of tall cylindrical cells, the enamel-membrane, which covers the dentine-cap with a thin layer of enamel.
- The base of the dentine-cap acquires a better attachment in the dermis from the fact that the latter becomes ossified in its vicinity and furnishes the cementum.
- At the margins of the jaws the tooth-forming tract of the mucous membrane sinks down into the underlying tissue ; there is first developed by a proliferation on the part of the epithelium a dental ridge, on which the teeth of the jaws arise in the same way that the dermal teeth do on the surface of the body.
- The development of a tooth takes place on the ridge in the following way : the epithelium grows more rapidly at one point, and a papilla of the connective-tissue part of the mucous membrane grows into this proliferated part or enamel-organ. The dental papilla forms the dentine, but the enamel-organ, developing an enamel-membrane, secretes the enamel ; finally, the connective- tissue dental sac becomes ossified and furnishes the cementum.
- Beneath the milk-teeth there are early formed in Mammals and Man, at the deep edge of the dental ridge, the fundaments of supplementary teeth.
- From the throat-region of the intestine there are developed thymus, thyroid gland, accessory thyroid gland, and lungs.
- The thymus arises by the thickening and peculiar metamorphosis of the epithelium of several pairs (Selachii, Teleostei, Amphibia, Reptilia), or of only one pair, of visceral clefts.
- In Selachians and Teleosts there is a proliferation of epithelium at the dorsal ends of all the visceral clefts, which are penetrated by growths of connective tissue and blood-vessels.
- In Mammals and Man there is formed from the third pair of visceral clefts a pair of epithelial thymus-sacs, which send out lateral buds and become peculiarly altered histologically.
- In Man the two thymus-sacs are joined in the median plane to an unpaired body, which begins to degenerate in the first years after birth.
- The thyroid gland is an unpaired organ, which arises in the region of the body of the hyoid bone from either a hollow or a solid outgrowth of the epithelium in the floor of the pharyngeal cavity.
- The epithelial rod detaches itself from its parental tissue and forms lateral rods.
- At a later stage these epithelial cords become separated into small epithelial spheres, which secrete in their interiors colloid substance and are converted into wholly closed glandular sacs enveloped in highly vascular capsules of connective tissue.
- The accessory thyroid glands are paired and arise from evaginations of the epithelium of the last pair of visceral clefts, which undergo metamorphoses similar to those of the unpaired thyroid gland.
- The accessory thyroid glands in most Vertebrates remain separated from the unpaired thyroid gland by a greater (Reptiles) or less (Birds) space, whereas in Mammals they appear to fuse with it to form a single body.
- The lung is developed out of the floor of the alimentary canagf in the throat-region, behind the fundament of the unpaired thyroid gland.
- A groove-like evagination, which is constricted off from the alimentary canal as far forward as its anterior end, the entrance to the larynx, becomes larynx and windpipe.
- From the posterior end of the groove there grow out two sacs, which acquire at their ends vesicular enlargements and constitute the fundaments of the right and left bronchus, together with the corresponding lung.
- The want of symmetry between the right and left lung is early exhibited, since the right sac provides itself with three vesicular lateral buds, the fundaments of the three lobes, whereas the left sac forms only two buds.
- The further development of the lungs allows one to distinguish two stages, of which the first exhibits a great similarity to the development of an acinous gland. In the first stages the primitive pulmonary sacs increase in number by constrictions and at the same time become differentiated into a narrower conducting part, the bronchial tubes, and a broader vesicular terminal part. In the second stage the air-cells or pulmonary alveoli are formed.
- From the intestinal canal proper there are formed only two glands, which are large and developed from the duodenum the liver and the pancreas.
- The liver is developed as a branched tubular gland which becomes a network.
- There grow out i'roiu the duodenum into the ventral mesentery or prehepaticus (Vorleber) two liver-tubes, the fundaments of the left and right lobes of the liver.
- The tubes form hollow or solid lateral branches, the hepatic cylinders, which are united into a network and become in part bile-ducts, in part the secretory parenchyma of the liver and biliary capillaries.
- The ductus choledochus arises as an evagination of the wall of the duodenum which receives the two hepatic tubes, and it forms at one place an evagination which becomes the gall-bladder and the cystic duct.
- From the ventral mesentery, into which the hepatic tubes grow, are derived the serous investment and a part of the ligamentous apparatus of the liver, namely, the lesser omentum (ligamentum hepato-gastricum and hepato-duodenale) and the ligamentum suspensorium hepatis.
- The panci as grows from the duodenum into the dorsal mesentery and into the mesogastrium.
- The mesentery which the pancreas originally possesses subsequently disappears by becoming fused with the posterior wall of the trunk ; at the same time, in consequence of the twisting of the stomach, the long axis of the pancreatic gland comes to lie in the transverse axis of the body.
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Text-Book of the Embryology of Man and Mammals: Description of the Sexual Products | The Phenomena of the Maturation of the Egg and the Process of Fertilisation | The Process of Cleavage | General Discussion of the Principles of Development | The Development of the Two Primary Germ-Layers | The Development of the Two Middle Germ-Layers | History of the Germ-Layer Theory | Development of the Primitive Segments | Development of Connective Substance and Blood | Establishment of the External Form of the Body | The Foetal Membranes of Reptiles and Birds | The Foetal Membranes of Mammals | The Foetal Membranes of Man | The Organs of the Inner Germ-Layer - The Alimentary Tube with its Appended Organs | The Organs of the Outer Germ-Layer | The Development of the Nervous System | The Development of the Sensory Organs | The Development of the Skin and its Accessory Organs | The Organs of the Intermediate Layer or Mesenchyme | The Development of the Blood-vessel System | The Development of the Skeleton
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