Paper - The study of mammalian embryology (1900)

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Minot CS. The study of mammalian embryology. (1900) The American Naturalist, 34(408): 913-941.

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This historic 1900 paper by Charles Minot outlines how he intends to organise his later published textbook of embryology.

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The Study of Mammalian Embryology

Charles Sedgwick Minot
Charles Sedgwick Minot (1852–1914)

Charles Sedgwick Minot

This article is intended to give a preliminary account of a Text-Book of Embryology, upon the preparation of which I have been working for a considerable period. The book is intended especially to meet the needs of the student of human anatomy, but it is hoped to render it suitable for use as a general introduction to vertebrate embryology. The distinctive aim of the work will be to direct the beginner in the practical laboratory study of mammalian embryology, guiding him through an intelligent examination of the external form and of the microscopic sections.

The publication of Foster and Balfour’s admirable Elements of Embryology gave a great impetus to the science, but it is, unfortunately, many years since the last edition of that work appeared (1883). Duval’s Atlas is at present the most available guide, but is too elaborate and costly to be Within the reach of most students. It is a work which ought to be in every laboratory, for it is a fine monument of well-directed learned industry. Both of these works deal with the embryology of the chick, — the one chiefly, the other exclusively,—and furnish little aid for mammalian embryology. Marshall’s Embryology is on a more comprehensive plan, but as it scarcely passes beyond the general principles of vertebrate embryology it cannot satisfy the needs of anatomical study. There is in these three works the merit of an adaptation to practical laboratory study, but there are no recent manuals of this type. On the other hand, there is, as all morphologists know, an abundance of recent embryological text-books; but all of them, I think without exception, follow the general pattern established by Kolliker in the first edition of his E7z2‘wz'c/at/mzgsgesc/zicfite (1861), and offer generalized descriptions of the development of the germ-layers, and then of the various organs. A beginner profits more from a presentation of the subject which clings to the actual preparations which he may make for himself. These considerations have led me to the conviction that an introductory work more or less upon the model of Foster and Balfour would be useful. It is such an “Introduction” which I have undertaken.

The plan adopted is briefly as follows :

First, a study of pig embryos of from 9 to 12 mm. Experience has shown that embryos of these sizes can be obtained in considerable numbers from the pork-packing establishments, such as may be found near most large cities. The anatomy of the pig at this stage is readily understood by the student who knows the general anatomy of the adult. Older embryos are more complicated, and yield such long series of sections that the beginner is apt to be discouraged; younger embryos, owing to their spiral twisting, are exceedingly difficult for students to understand when sectioned.
Second, a study of pig embryos of about 17.0 mm. and of the head of an embryo of 20 to 24 mm., which will suffice to render clear the relation of the early embryonic anatomy to the adult structure.
Third, a study of the chick embryo of 20 to 30 hours to illustrate the character of the germ-layers.
Fourth, a study of the foetal envelopes, including the placenta in man and perhaps also in the pig and rabbit.
Fifth, a study of the genital elements, impregnation, segmentation, etc.

This scheme is not logical in its order, but it seems to me to follow that psychological order which is natural forbeginners. in embryology who have already studied anatomy. It is, I believe, often a mistake in introducing a pupil to a new subject to sacrifice the natural, and therefore easy, psychological sequence to the artificialidemands of a logical arrangement. I hope, therefore, that the scheme adopted will prove advantageous for laboratory work.

Most stress will be laid upon the young mammalian embryo. Figures have been prepared to show several stages of the pig, all magnified eight diameters. To represent the first stage to be studied, a pig of 10.0 mm. has been chosen (Fig. I). This figure, as well as the others in this article, are woodcuts after pencil drawings. The drawings were made for me by Mr. E. A. Locke with great care and accuracy. The woodcuts were made by Mr. C. L. Albert Probst, of Braunschweig, Germany, and reproduce the original drawings with remarkable and satisfactory success. The German school of wood engraving uses the black line, while the American school uses the white; so that a German woodcut reproduces microscopical textures better than an American, since in our xmirzcrl sections we rely in our studies chiefly upon the dark or stained parts, and these are indicated by the black-line better than by the white-line engraving. There is the further advantage that the cost in Germany is considerably less. The wood engravings combine softness and delicacy of tone with brilliancy of texture, and thus surpass, it seems to me, very considerably even the best “ process” engravings. As the number of good figures of sections of mammalian embryos is very limited, and as there is no series of illustrations of systematically chosen typical sections of typical mammalian embryos, it is hoped that the present series will be useful, not only to students, but also as a convenience to advanced workers. On the other hand, it cannot be claimed that the new pictures represent definite original research, because they are intended primarily for text-book use. I have pleasure in acknowledging very valuable assistance in the identification of parts rendered by Drs. R. T. Atkinson and F. A. Woods, and by Mr. F. T. Lewis and Mr. J. L. Bremer. Mr. Lewis has made also several dissections of the cephalic nerves, and Mr. Bremer models by Born’s method of the brain and pharynx. I hope the results of these researches will be published separately.

I wish also to express my appreciation of the great excellence of the recent text-books of embryology which have appeared in England, France, and Germany, although I venture to point out that they differ in plan from the proposed new book.

A mammalian embryo may be conveniently regarded as having assumed its typical class organization at the time when the limb-buds have just become distinct appendages, as found in pig embryos of 9 to 12 mm. At this stage a mammalian embryo is readily distinguished from that of any other class of vertebrates, and the differentiation of the anlagesl of all the important organs is accomplished so that these anlages can be identified with certainty and their genetic relations to the adult structures can be clearly grasped by the student. At the same time, although the anatomical differentiation is well advanced, the histological differentiation has made very little progress, so that the stages in question are particularly instructive to beginners. Fig. I represents a pig embryo of IO mm. The student should make a careful and thorough study of the external form, noting the following features. The head, which is very large in comparison with the body, forms as a whole nearly a right angle with the back, so that dorsal outline of the head forms a distinct though rounded angle with that of the back; this angle marks the position of the neck-bend, and corresponds to the junction of the brain with the spinal cord. The neck-bend is one of the most marked and distinctive characteristics of the mammalian embryo, being much less marked in birds and reptiles, and being absent in amphibians and fishes. It is probably closely correlated with the cramping of the ventral cervical region, which leads to the formation of the cervical sinus (C.S.), and to the disappearance of the second to fourth gill-clefts. The neck-bend is so great that the mandibular and nasal regions of the embryo are closely appressed to the cardiac region of the body proper. The cephalic region has a second flexure, the head-bend proper, which marks the site of the mid-brain, and in the figure appears as a rounded angle obliquely above the eye. From the mid-brain one axis, horizontal in the figure, extends backward through the region of the fourth ventricle, or hind-brain (Vera), to the neck-bend, while the other axis extends vertically downwards to the region of the fore-brain, which is marked by a rounded protuberance in the outline of the head. The dorsal outline of the body proper forms a long sweeping curve, ending in the tail; this dorsal curvature is another characteristic of the amniote embryo, the back in the embryo of fish-like forms being relatively straight. It isvthus brought about that the dorsal side of the embryo is two or three times as long as the ventral. From the ventral side springs the large umbilical cord, the connection of which with the body occupies practically the entire length of the ventral median line of the abdominal region proper. Above the umbilical cord the protuberant outline of the cardiac region passes below the nasal (N.) and mandibular (Md.) regions toward the cervical sinus (C.S.). The long tapering tail extends near the umbilical cord.

  • 1 Anlage is from the German, and has been adopted as a technical term to designate the first accumulation of formative material recognizable as the commencement of an organ, structure, or part in a developing organism.

Fig. 1. Pig embryo of to mm. A .I.., anterior limb; Au., auditory, or first, gill-cleft; C.S., cervical sinus; H.L., hind limb; Md., mandibular process; 1l[.L., milk-line ; Mx., maxillary process; 1\/'., nasal pit; 0/., eye; Sag, muscular segment; I/m., umbilical cord; V¢n., floor of fourth ventricle (medulla oblongata). x 8 diams.

The surface modeling of the embryo offers important features. Beginning with the head, we observe first the shallow depression, constituting the nasal pit (_/V.). The eye (0/J.) is entirely without lids; the lens appears in the center and is surrounded by the outlines of the optic vesicle. The small size of the eye is a characteristic of the mammalian embryo; by which it differs from all sauropsidan forms, but in certain other mammals the embryonic eye is slightly larger than in the pig. Below the eye is the maxillary process (/1/I22), which is destined to form the greater part of the upper jaw; the anterior bounciary of the maxillary process is marked by a shallow depression, the lachrymal groove, which runs from the angle of the eye (0p.) to the nasal pit (./V.). The mandibular process (Z!/Id), out of which the lower jaw is developed, is bounded in front by a groove separating it from the maxillary process, and behind by a second groove (Am), the anlage of the future meatus auditorius externus. This groove marks the boundary between the mandibular process and the first or hyoid branchial arch, and is itself the ectodermal member of the first gill—cleft. The fourth ventricle (Ve7z.) or cavity of the hind—brain, having very thin walls for its roof, can be readily distinguished. The thickened floor of the fourth ventricle is the anlage of the medulla oblongata. The cervical sinus (C5,) is an area of invagination, presenting at this stage a triangular outline; within the sinus are found the external or ectodermal terminations of the second and third gill-clefts. The territory of the mandibular process and cervical sinus corresponds to the pharyngeal region. It is the site of some of the most important, interesting, and complicated developments by which the embryonic is changed into the adult anatomy.

The body of the embryo shows the position and number of the segments (Sag) by the external modeling. Both limbs are well advanced, the anterior (/LL.) more so than the posterior. From the base of the front to the base of the hind limb extends the milk-line (./If./1.), curving so as to be nearly parallel to the dorsal outline of the body. Along this line the mammary glands are ultimately developed. Extending across the body are several shadowy lines, shimmering through the translucent body-walls. One marks the position of the embryonic diaphragm; it extends from the upper edge of the anterior limb obliquely downwards towards the edge of the umbilical cord. Another, which extends in a nearly straight line from limb to limb, marks the ventral edge of the large Wolffiari body or mesonephros, the dorsal limit of which is approximately inclicated by the milk-line (/lf.L.). The outlines of the smaller left dorsal lobe of the liver are distinct, and mark out a pointed area immediately below the fore limb (A.L.).

Of the remaining figures, 2 to 7 represent six transverse sections through a pig embryo of 12.0 mm. But the position of the sections can be easily followed with the aid of Fig. I, which represents a slightly younger embryo. The plane of the sections would be indicated by a nearly horizontal line across Fig. I. The sections are ten microns in thickness, and are 966 in number, not 1200, as the student might expect. The discrepancy is due to the shrinkage of the embryo when imbedded in paraffine. The shrinkage is always very great, and in the case of embryos causes a loss of almost 20 per cent in the length; but as it seems to take place uniformly throughout the embryo, it causes no distortion, so that the embryo in paraffine is an exact though greatly reduced copy—so to speak —of the living embryo. It should be remembered that no correct measurements of the size of organs or cells can be obtained from sections made by the paraffine method. This limitation upon the use of sections is too often forgotten.

The six transverse sections chosen were selected t'o show the most important points in the structure of the embryo, as far as the upper part of the abdomen. The organs of the pelvic end of the abdomen and the character of the muscles can be more advantageously studied in other series and stages.

Fig. 2. Pig embryo of r2.o mm. Series No. 5. Transverse section 185. D.E., ductus enclolymphaticus; F.b., fore-brain; 1Veu., neuromere; 0t., otocyst; Vern, hind-brain (fourth ventricle); 5, trigeminal nerve; 7.8, acustico-facial nerve; 9, glossopharyngeal nerve; 10, vagus nerve; 12, hypoglossal nerve. x 22 diams.

Fig. 2 is at a level about halfway between the eye and the highest point in Fig. I ; it passes, therefore, through the forebrain (F.b.) and the fourth ventricle (Vm.) or cavity of the hind-brain. The section is bounded by the thin layer of epidermis, between which and the brain-walls there is a large amount

Fig. 3. Pig embryo of 12.0 mm. No. 5. .Transverse section 195. EL, ectoderm; 17.5., forebrain; Jug.',_Iug.", jugular vein; In/'., iufundibular gland; Md., medulla oblongata; 01., otocyst; Str., striaa acusticae; Vern, cavity of hind-brain; 5, trigeminal ganglion; 7.8, acustico-facial complex; 9, glossopharyngeus; to, vagus; II, accessarius; :'iz'., third ventricle of the brain. x 22 diams.

of mesenchymal tissue. Alongside the hind-brain lie a series of important structures imbedded in the mesenchyma, which

are identical upon the two sides, although they differ in the section, as the plane of cutting was not symmetrically transverse for the head. These structures are in the following order: 5, the trigeminal ganglion; 7.8, the acustico-facial ganglion complex; the otocyst, 01., with upon one side the separated ductus endolymphaticus, [).E., and upon the other side the opening of the ductus displayed by the section; 9, the glossopharyngeal ganglion; [0, a portion of the vagus ganglion; 12, one of the hypoglossal ganglia. Upon the left side of the figure the connection of the trigeminal ganglion with the angle of the wall of the hind-brain is demonstrated; by this connection and by its large size the trigeminal ganglion may always be identified in the embryo. The brain-wall has a thin outer layer both in the hind-brain and mid-brain; this is the outer neuroglia layer (Randschleier of His) and is conspicuous from the absence of stain; inside of the external layer the wall of the brain is crowded with nuclei, and therefore appears deeply stained in the preparation. Three neuromeres (./Van.) appear in the section, each marked by a concavity on the inner side of the brain-wall.

Fig. 3 is from section 195, and therefore ten sections below Fig. 2. It is given to show three points not shown in the preceding illustration: 1, the spinal accessory nerve root (II) arising from the cervical (in the figure, upper) end of the hindbrain and running forward to join the vagus ganglion; 2, the trigeminal ganglion with the internal jugular vein, which is cut twice (fug.’,]ug.”), abutting close against it, ——a very characteristic relation (this vein takes a sinuous course along the side of the hind-brain, for it passes inside the twelfth, eleventh, tenth, and ninth nerves, then outside the otocyst and the

‘seventh-eighth nerve, and inside the fifth); 3, the infundibular process or gland (/7zf.), springing from the floor of the forebrain Attention may also be called to the structures (Sm), resembling the strize acusticze, but which seem to be rather the fibres of the lateral root of the facial.

The next figure (4) is from a lower level. The section passes through the eyes, the ‘upper end of the auditory cleft (Fig. I, Am), and the upper cervical region of the spinal cord. The spinal cord is clearly divided into a dorsal (D.Z.) and a ventral zone (V.Z.) on each side; the two dorsal zones are connected across the median line by the thin- deck plate, and the ventral zones similarly by the thin floor plate. The lower or ventral limit of the dorsal zone is marked externally by the entrance of the dorsal or ganglionic root, internally by the lateral angle of the central cavity, shown in the section. In the ventral zone the development is more advanced,—the three primary layers in the walls of the neuraxis (or medullary tube) being clearly differentiated, namely: the light outer neuroglia layer; the middle layer, commonly called the mantle layer, but which might be better named the neurone layer; and, finally, the innermost or ependymal layer. These three layers are primary and appear throughout the entire brain and spinal cord. The separation of the dorsal and ventral zones is of fundamental morphological importance, and their characteristics must be clearly understood before the anatomy of the brain is studied. The ventral zone contains all the neurones from which fibres arise forming the ventral nerve root (T/.13.). Following downwards in the figure we come to the section of the jugular vein (jug), just inside of which lies the common trunk (10.11) of the united vagus and accessorius nerves, and also the lower part of the glossopharyngeal ganglion Lower down and nearer the ectoderm lies the facial nerve (7), and again still lower is the large trunk of the inferior maxillary (/I/.r.z'.), or mandibular branch of the trigeminal nerve. Between the two nerves last mentioned is a long oblique slit, which is found at another level to communicate with the pharynx (P/z.); this slit is the inner or entodermal portion of the first or auditory gill-cleft, and is the anlage of both Eustachian tube and of the cavity of the tympanum. Its outer (in Fig. 4, upper) end lies near the bottom of an ectodermal groove; this groove (Fig. 1, Au.) divides the mandibular process from the hyoid arch, and is the anlage of the meatus auditorius externus. The hyoid arch is marked in Fig. 4 by the facial nerve (7) and the arched overlying ectoderm. The internal carotid artery (C.z'.) appears near the optic nerve (Op). External to this artery lies the superior maxillary nerve, which, however, does not show in the figure. The vesicular lens (L) and the cup-shaped retina (Rat) of the eye can be readily identified. The fore-brain has begun to form two lateral expansions, the cavities of which ultimately become the lateral ventricles (L. V.) of the brain, and the walls of which are the anlages of the hemispheres (I-].).

Fig. 4. Pig of 12.0 mm. No. 5. Transverse section 245. C.1'., internal carotid artery; D.R., dorsal root of spinal nerve; D.Z., dorsal zone of spinal cord; Ea, ectoderm; H., cerebral hemisphere; _/ug., internal jugular vein; L., lens; L.V., lateral ventricle; Mx.i., inferior maxillary nerve ; 0).. optic nerve; Plz., pharynx; R:t.,retina; V .R., ventral root of spinal nerve; V;Z., ventral zone of spinal cord; 7. facial nerve; 9, glossopharyngeal ganglion; 10.11, vagus accessorius nerve. x 22 diams.

FIG. 5.—Pig of 12.0 mm. No. 5. Transverse section 321. 110.4, fourth aortic arch; Au., ectodermal depression of the first or auditory gill-cleft; C¢rv.I, first cervical nerve; Car:/.2, ganglion of the second cervical nerve; G.mza'., ganglion nodosum (lower vagus ganglion); Hy., hyoid arch; ]ug., jugular vein; L. V., lateral ventricle; 111.111., median thyroid duct; 1Va., nasal pit; No.2, second cervical nerve; 1V.xz'i., hypoglossal nerve; N.xi., spinal accessory nerve;, nasopharyngeal plate; R.ex.xi., ramus externus accessorii; Sf.C., spinal cord; 7‘lxyr., median thyroid; 7'Ima., thymus; Ve., vein; z'z'i., third gill-cleft of the right side. x 22 diams.

Fig. 5 is from section 321, and is therefore considerably lower down in the series than the section represented in Fig. 4. It passes through the cervical sinus and olfactory pits (compare Fig. I), and therefore also through the spinal cord of the neck. The section comprises two distinct unequal parts: the first and larger is the section of the cervical, pharyngeal and mandibular regions; the second and smaller is the section from the tip of the low bent head. The spinal cord (.S‘p.c.) in the upper part shows clearly both the three primary layers of its walls and the division into dorsal and ventral zones. The outermost layer is very lightly stained, and may be, perhaps, best defined as the anlage of thewhite matter. The inner layer is deeply stained, while the middle layer (mantle or gray stratum) is intermediate in color. The nerve cells develop most abundantly and rapidly in the ventral part of the mantle layer, which is consequently more enlarged than the dorsal part and has caused a bulging sidewards and downwards of the ventral zone. The ganglion (C£’7’”u.2) is the second cervical; it sends a root upward to enter the dorsal zone of the spinal cord, and a tract downward to join the ventral root, thus constituting the second spinal nerve (N7/.2), which subdivides almost at once into a dorsal and a ventral ramus. Between the dorsal summit of the ganglion and the spinal cord there is a small bundle of nerve fibres, not shown in the figure. These fibres constitute the commissural trunk of the eleventh nerve. The third gill-cleft (z'z’z'.) is cut almost symmetrically, and extends from the median line to the edge of the section; it is lined throughout by the entoderm, which at the end of the cleft on each side has met and fused with the ectoderm to form an epithelial membrane (Verschlussplatte), which closes the cleft laterally. All vertebrate embryos probably have their gill-clefts at an early stage all closed by a similar membrane ; but, whereas in fishes and amphibians the membrane is soon broken through, in mammals, on the contrary, it remains intact, and the clefts are, it is thought, normally always imperforate. At the end of the cleft the entoderm has undergone a special growth, forming a distinct mass (T/zm.) on the side of the cleft towards the head. This entodermal structure is the anlage of the thymus, and is already penetrated by small blood vessels, which are perhaps not capillaries but sinusoids. The third gill-cleft extends to the bottom of the cervical sinus, a large and deep depression of the outer surface, clearly shown both in Figs. I and 5. The sinus is bounded cephalad by the large and protuberant first or hyoid branchial arch (H;/.), and this arch is separated from the mandibular region by a groove (Am) or ectodermal depression, which is later transformed into the meatus auditorius externus. In mammals there are four pairs of gillpouches, or so-called clefts, all of which are lateral diverticula of the pharynx. Owing to the curvature of the pharynx these diverticula are brought to different levels, and into different planes in the pig embryo at the stage we are considering; hence in a series of sections the various gill—pouches are encountered in different portions of the series, and thus it occurs that only one pouch, the third, is shown in the section. The student should clearly understand that the median region of the two-thirds gill-pouches in Fig. 5 is the pharynx proper, and that it gives rise to the anlage of the median thyroid, of which both the stalk (./W1/z.) and the glandular portion (T/zyr.) appear. Just above the third gill-cleft may be seen the large, darkly stained lower ganglion (ganglion nodosum) of the vagus nerve (G.72oa’.), and just above the ganglion is situated the section of the jugular vein (jug). Close to the ganglion on its ectal side appear two fibrous nerve trunks, of which the one nearest the pharynx is the accessorius or eleventh nerve (./V.:r.z'.), while the other nearest the jugular vein is the hypoglossus, which reappears (1V.:rz'z'.) below the aortic arch. A little above the jugular vein is the section of the first cervical nerve (C227'72.I), laterad from which is the external branch (]€.ex.xz'.) of the spinal accessory nerve. This branch in the adult innervates the sternocleido-mastoid and trapezius muscles.

The lower part of Fig. 5 represents part of the head and shows the two nasal fossae (Na) closed towards the mouth side by the olfactory plate (0Z.pZ.), an epithelial membrane somewhat similar to the closing plate of a gill-cleft. On the dorsal side of the olfactory foss2e—below in Fig. 5—the cerebral hemispheresare cut, their darkly stained wall bounding on each side the large lateral ventricle (L. V.).

Fig. 6. Pig of no mm. No. 5. Transverse section 470. Aa.S., left dorsal aortic trunk; Au.d., right auricle of the heart; D.R., dorsal root of spinal nerve; D. C .S ., left dnctus Cuvieri; _/., external groove between right and left auricles of the heart; 0., ganglion of spinal nerve; L., anlage of fore-limb; Na/:., notochord; Nu, nerve of limb; 0e., cesophagus; R.D., ramus dorsalis of spinal nerve; R.V., ramus ventralis of spinal nerve; S .a.c. , septum of auricular canal; Sci. V ., subclavian vein; .S‘am., somatopleure; S_fi.c., spinal cord; S.:., septum superius; Tra., trachea; Val., atrio-ventricular valve; Ven.S., left ventricle; V.R., ventral root of spinal nerve. x :2 diams.

The next section figured is No. 470, and is therefore much lower in the series. It was selected in order to show the anterior limb—buds, the ducts of Cuvier, and the heart. The position and shape of the limb anlages are shown in Fig. I, and the section demonstrates that they are formed by mesoderm, with a thin covering layer of ectoderm. The mesoderm is very little differentiated, none of the skeletal elements being yet formed. The nerves and blood vessels are growing into the limb; the nerves are the rami ventrales (R. V) of spinal nerves, and form branches within the limb; one of these branches, as yet unidentified, is shown at A72). The vein Sol. 17. is the so-called subclavian or axillary vein, a branch of the jugular, as explained below. The lower part of the section is occupied by the large heart lying in the pericardial chamber. The body-wall (Sam), or somatopleure, which forms the outer covering of this chamber, is quite thin, and without a trace of muscular or skeletal structures. The heart consists of two auricles and two ventricles. The auricles have thin walls and are separated from one another by a very thin membrane, the septum superius (S5,). The right auricle (Azz..zz’.) receives upon its dorsal side the opening of the vein or duct of Cuvier, this opening being guarded by valves; of these valves, the one towards the median line disappears, but the other, towards the right of the embryo, persists to form both the Eustachian and Thebesian valves of the adult. The corresponding opposite vein, or left duct of Cuvier (D.C.S.), is almost symmetrically placed, but does not have any communication with the left auricle, being instead connected at a lower level by a transverse venous trunk with the ductus Cuvieri of the right side. The upper portion of the left duct (D.C.S.) is seen in the section to be somewhat constricted off from the lower portion, and in fact it shows the jugular vein of the head at its actual junction with the ductus. The ventricles of the heart are much larger than the auricles, and the left ventricle (Vm.S.) is already larger than the right; the external groove (f.), which marks the boundary between the two ventricles, is clearly shown by the section. The trabecular structure of the ventricles is well developed and affords a diagnostic mark by which the ventricles, however cut, may be easily recognized in sections. The development of the trabeculze corresponds to the formation of the blood sinusoids of the heart, to whichl have recently directed attention. The constricted region of the heart, which connects the auricles with the ventricles, is known as the auricular canal. A broad partition (S.zz.c.) divides the canal into right and left channels, and at the ventricular ends of these channels the formation of atrio-ventricular valves (Va/.) is well advanced. The dorsal or trunk region of the section is formed chiefly by the more or less dense mesenchyma. It includes important structures. The spinal cord (Spa) resembles that in Fig. 5, but is both larger and more differentiated. The fundamental morphological characteristics of a spinal nerve are well illustrated by the left nerve of Fig. 6. The dorsal root (D.R.) bears the ganglion (G.) and is joined a little below the level of the cord by the ventral root (VI?) to form a single nerve trunk, which, however, soon subdivides into its two primary branches; the first or dorsal branch, ramus dorsalis (R.D.), bends at an acute angle upwards and outwards; the second or ventral continues downward and curves into the limb; owing to this curvature it must be traced through adjacent sections, but in Fig. 6 it can be seen again (R. V) at the base of the limb, where it crosses the subclavian vein (Sc[.V). Some distance below the spinal cord is the small notochord (IV?/z.). Much lower, and between the two ducts of Cuvier, appear the oesophagus <06.) and trachea (Trzz), each a ring of entodermal epithelium with commencing condensation of surrounding mesenchyma. This condensed tissue is the anlage of the future submucosa and muscularis. Above the oesophagus, to the right and left, appears the dorsal aortic trunk, of which the left only (Ao.S.) is completely retained throughout life. It should be noted that the blood vessels, including the largest, have at this stage only endothelial walls, the adventitial and muscular coats being added later. All blood vessels are endothelial chambers, and this fundamental conception ought to be made clear to the student.

We now pass to a section (No. 633) well below the heart, in order to study the characteristics of the Wolffian body, stomach, and liver. At this level, as comparison of the figures will show, the body of the embryo has its greatest dimensions. The upper edge (Um) of the umbilical cord also appears in this section. The spinal cord, with the ganglia and nerves, presents essentially the same features as in Figs. 5 and 6. The notochord (No/2.) forms a small circle in section and is surrounded by an area over which the mesenchymal cells are more crowded or condensed than elsewhere. The condensed mesenchyma is the anlage of a vertebra (Vert.). It is more expanded laterally than dorso-ventrally. In the median line below the notochord is the large dorsal aorta (A0,), which is formed by the union of the two dorsal aortic trunks of Fig. 6, and which extends through the abdominal region of the embryo to the pelvis, where it forks to form the two allantoic arteries, which run to the umbilicus, and entering the umbilical cord supply the extra-embryonic or placental circulation. The aorta is surrounded by mesenchyma, and to this are, so to speak, appended the large Wolffian bodies (I/I/.19.), one on each side. They are much larger, relatively to other parts, in the pig than in man or the rabbit. The Wolffian body is the foetal or embryonic kidney, and is also termed the mesonephros. It consists of numerous epithelial tubules (I/I/.z‘.), very much contorted, with blood spaces (sinusoids) between them, and of glomeruli, which always lie towards the median and inferior side of the organ. All of the tubules open into the single longitudinal canal, the Wolffian duct. This duct is always situated close to thepsurface on the ventral side of the organ, and is very easily distinguished in dissected embryos after the removal of the intestines and liver. In the figure it may be easily found in the left mesonephros (DI/.b’.), it being there the lowermost of the cavities drawn. On the median lower surface of the Wolffian body, underneath the glomeruli, is an accumulation of tissue (Gem), the anlage of the genital gland. Below the aorta, on the right of the embryo, is the large ductus venosus, or upper end of the vena cava inferior, on its way past the right dorsal lobe of the liver towards the heart. Below the aorta on the left is the mesogastrium (077z.mzzj.), or future great omentum, by which the stomach is suspended from the median dorsal wall of the abdomen. The stomach (St.) is entirely upon the left side of the body, and is directly connected by means of the anlage of the lesser omentum ('7z.) with the liver. The liver is by far the largest organ of the body; it takes up nearly half of the section. It is divided into four main lobes, two dorsal and two ventral, two on the right and two on the left. The reference line (LL) runs to the left dorsal lobe. The liver consists of a complicated network of relatively large blood sinusoids, the spaces between which are occupied by the embryonic liver cells. Near the median line between the ventral lobes appears the gall—bladder (G.é[.), which is cut three times. The liver is attached in the median ventral line to the body-wall and the base of the umbilical cord. The two umbilical veins enter the liver directly from the cord. The right vein (V.U.D.) is already smaller than its left fellow (V.(Z5.). They are connected respectively with the right and left ventral lobes of the liver. The right vein in a little later stage is no longer recognizable as an open channel. It will be noticed that the right and left sides of the abdominal cavity (splanchnocoele) are completely separated from one another, and that there is a special part of this cavity shown in the section between the stomach and the right dorsal lobe of the liver, and which is known as the lesser peritoneal space or cavity of the omentum. The body-wall ($0772.), or somatopleure,1 consists of three layers,—the ectoderm (E6), the mesenchyma (me.v.), and the mesothelium (ms!/1.). ‘It is of the greatest importance for the student to understand the arrangement of the germ-layers in the somatopleure. The mesothelium (ms!/z.) is commonly known in the descriptive anatomy of the adult as the peritoneal epithelium. In sections like that of Fig. 7 it can be followed not only over the inner surface of the body-wall but over the surface of the Wolffian body and liver, and upon the left side of the body also over the surfaces of the greater omentum, stomach, and lesser omentum. We see, in fact, that the body cavity is completely bounded by mesothelium, and that all the abdominal viscera are therefore outside of it. This conception, which is so important yet so difficult to the student of anatomy, is easily mastered by the study of the embryonic relations.

Fig. 7. Pig of 12.0 mm. No.5. Transverse section 633. .40., aorta; D. V., ductus venosus; Ea, ectoderm of the somatopleure; G., ganglion on dorsal root of spinal nerve; 6.51., gall bhddefi GM-1 lnlage of genital gland; L12, liver; men, mesenchyma of somatopleure; mstln, mesothelium of the somatopleure; Ncln, notochord; 1V., spinal nerve; 0m.maj., omentum major;’n., omentum minus; Svm., somatopleure; $1., stomach; I/m., umbilical cord; VerI., anlage of the body of a vertebra; VJ/.I)., right umbilical vein; V.U.S., left umbilical vein; W./3., Wolflian body or mesonephros; W .t., Wolfiian tubule. X 22 diams.

From the sagittal series of the same stage many instructive pictures are obtainable. I have selected a median section of the head, and one passing through the principal cephalic ganglia, for engraving, and give here the latter. The magnification is 22 diameters, the same as for the transverse sections Figs. 2 to 7. To the student of anatomy such a section as is shown in Fig. 8 is highly instructive, for it exhibits in a single picture many important fundamental relations of the cephalic nerves, particularly of the second, fifth, seventh, eighth, ninth, and tenth. As the section is remote from the median plane, little of the brain appears, there being only a shaving from the lateral wall of the fore-brain and a section of the widest part of the hind-brain, which shows the cavity or lateral recess (./3.1,.) of the fourth ventricle. The auditory vesicle or otoc'yst (0!) is cut; its narrow upward prolongation is the anlage of the ductus endolymphaticus. The otocyst lies in a line with the great cephalic ganglion, occupying its invariable and permanent position behind the acustico-facial ganglion (Ac.F.) and in front of the glosso— pharyngeal. Only the lateral portion of the pharynx (P/z.) appears; this portion forms a wide diverticulum, almost slitlike (compare Fig. 4), from which extend the first and second entodermal gill—pouches. In the figure the pharynx has a small depression extending downward at the oesophageal end of the part shown in the section; this depression marks the beginning of the second cleft. Nothing is seen of the third and fourth clefts in the section, as these both lie nearer the median plane. The pocket or diverticulum of the cervical sinus (compare Fig. 1) appears (C677/.5.) ‘near the ganglion nodosum (G.7zod.). It might be mistaken for a gill-cleft, but it is lined by ectoderm and its cavity can be easily traced through the series to the exterior. Cephalad from the sinus but close to it lies a small dark mass, the anlage of the thymus gland. The mass is produced by proliferation of the entodermal cells on the anterior side of the third cleft (compare Fig. 5, '1‘/z-222.), and is penetrated by blood vessels which seem to be sinusoids, although their history has not been worked out. The jugular vein (]ztg.), owing to its irregular course, is out several times ; its main stem (]zzg.’”’) rises nearly vertically through the cervical region, and is, relatively to the size of the embryo, of huge diameter, and continues (fzzg/”) upward along the dorsal side of the vagus nerve to a level about halfway between the ganglion nodosum and ganglion jugulare, and as the vessel there curves inwards and forwards it is not encountered again until it bends outwards (fag/) on its way past the trigeminal ganglion. A branch of the jugular vein (fag/’) is cut just above the ganglion.

  • 1 It is much to be regretted that German embryologists use the term “ somatopleure ” erroneously.

Fig. 8. Pig of 12.0 mm. No. 7. Sagittal section 25. Ama, auricle of the heart; Ac.F., acusticofacial gan lion complex ; Can, coelom around the heart or pericardial cavity ; Car:/..S‘., diverticultnn of tie cervical sinus, just in front of which shows the anlage of the thymus, which is deeply stained; G._/7tg., ganglion jugulare of the vagus nerve; G.7zoa’., ganglion nodosum of the vagus nerve; G./ztr., ganglion petrosum of the glossopharyngeal nerve; G.trz'., ganglion of the trigerninus nerve; I-1., lateral wall of the cerebral hemisphere -,]ng., jugular vein ;Jug.’, behind the trigeminus: jug/', branch in front of the trigeminus ; _/z/g.’”, main stem behind the vagus ; j7¢e."", main stem descending to join the duct of Cuvier: m., an undetermined structure. probably the anlage of a lingual muscle; Mai, mandible; 1V.V., root of the fifth or trigeminal nerve; N.o;$., optic nerve; 1V..1-/1',, twelfth or hvpoglossal nerve: 01., otocyst: P/1., pharynx; R.L., recessus lateralis of the fourth ventricle; V2,, small branch of the jugular vein; Vcmfl, ventricle of the heart. X 22 (liams.

The nerves are shown as follows : The optic nerve (/V.o/J.) still has its central cavity, which, nearer the median plane, opens into the third ventricle of the brain, and in the section resembles in shape an inverted U. On the side of the nerve towards the mouth there is a deep notch, - the section of the choroid fissure. The trigeminal ganglion (G.trz'.) is very large, and its trilobate form is clearly indicated by the figure. The lobe to which the reference line (G.z7'z'.) runs gives off the ramus ophthalmicus; the lobe nearest the jugular gives off the ramus maxillaris inferior, while the middle lobe gives off the ramus maxillaris superior. From the ganglion the fibres and nerve cells extend upwards to form the root (A/(V), which joins the hind-brain at a characteristic point,— namely, at the summit of the Varolian bend and where the hind-brain is widest (compare Figs. 2 and 3). By its great size and by its topographical association with the lateral apex of the recessus lateralis of the fourth ventricle, the trigeminal ganglion may always’ be readily identified in sections of embryos. The acustico-facial ganglia (Ac.F.) may also be readily determined by their typical position immediately in front of the otocyst (Oat). But it is quite difficult to identify the four components of this complex structure, namely: 1°, the motor root of the facial nerve; 2°, the facial or geniculate ganglion; 3°, the vestibular ganglion ; 4°, the cochlear ganglion. In Fig. 8 three divisions are shown. The large, darkly stained division, to which the reference line (/1c.F.) runs, and which lies nearest to the otocyst, is the vestibular portion of the acoustic ganglion; the small, light area occupying a middle position in the inferior part of the complex is the motor division of the seventh nerve, or lateral root of the facial; it can be followed to the brain, which it enters as four bundles of fibres ; its path of entrance is shown better in frontal sections (Fig. 10). just in front of the facial motor root lies a second smaller dark mass, the geniculate ganglion of the facial, with an upward prolongation, the sensory root. The ninth or glossopharyngeal nerve is represented by the ganglion petrosum (G.pctr.) and its ascending sensory root. This nerve may be quickly identified because it is the first behind the otocyst. The upper ganglion of this nerve, the so-called Ehrenritter’s ganglion, is represented by an accumulation of cells in the upper part of this root. As regards the tenth nerve, or vagus, both its ganglia and the fibrous trunk connecting them are shown. The upper or jugular ganglion (Gjztg) is nearly on a level with the otocyst, while the lower or nodosal (G.7zod.) lies near the cervical sinus. To the nerve trunk between the two ganglia are adjoined the fibres of the eleventh or spinal accessory nerve, which does not otherwise appear in this section. A small piece only of the hypoglossal nerve can be seen (1V..t'z'z'.). The space occupied by this nerve is blank in the engraving ; in the specimen it shows horizontal fibres.

The frontal series has special value for the study of the hindbrain, its nerves, and of the otocyst. From this series I present here three figures of the head region. The first of these sections enormously distended; it is bounded on the dorsal side only by a very thin layer of cells, the epithelial roof or ependyma (Ep), which does not form part of the true nervous structures, although it passes into and is directly continuous with the dorsal zone (D.Z.) of the medulla oblongata, which is thus seen (also in Figs. 10, II) to be only the thickened ventral wall of the neural tube, just as the ependyma is the attenuated dorsal wall. The trigeminal ganglion (G.irz'.) is very large and sends its sensory fibres upwards into the dorsal zone to form there the tractus solitarius (T.S.). Its motor or lateral root (L.R.) is below the ganglion andnearer the median line. As it enters the brain it curves and as a still distinct bundle runs towards the median plane. Its similarity to the motor root of the facial is very striking, but so far as I am aware has not been recorded hitherto (compare Figs. 9, 10). Inside of the trigeminal ganglion lies the jugular vein.

FIG. 9.—Pig of 12.0 mm. Series No. 6. t Ons passes through Frontal section 284. Ea, ectodernig 1 . .

the widest part of the hind— E_fl., ependymal roof of the fourth ven . tricle; 17.Z., upper portion of the dorsal braln and through the roots zone of His; G.z‘r1'., ganglion trigemini; of the trigeminus. The Ca‘/_ _]ug., jugularlvein; L.R., lateral root _ _ _ _ of the trigeminal nerve; //zc:., mesenlty Of t h e h 1 ll T311’) 1 S chyma; T.S., tractus solitarius of W.His‘

The next figure (Fig. 10), through the acustico-facial ganglion, shows the thickened ventral wall of the hindbrain (z'.e'., the anlage of P/z. the medulla oblongata)

no longer spread out nearly horizontally, but rising obliquely from the

FIG. 10. —Pig of 12.0 mm. No.6. Frontal section 340. rnedlan hne' T116 nght art.,arteria basilaris; D.Z., dorsal zone of the me- and Sides Of the clulla oblongata; Ea, ectoderm; E/2., ependylnal

roof of the fourth ventricle; f., median fissure of tl\e 1-neduna Oblong-ata are medulla oblongata; Frzr:., sensory root of the facial divided from one another nerve; G.ge1z., geniculate ganglion of the facial

nerve; Gs:/z.vt., ganglion vestibuli of the acoustic 21 (16613 l’1’lCdl3.I1 fiSSuI'C nerve; ]ng., jugular vein ; 1/M5,, mesenchyma; /|[,r.z'.,

inferior maxillary branch of the trigeminal nerve;

N..rz'z'.,l1ypoglossal nerve; P/L., pharynx; z‘.m., motor just below appgars thg tract of facial nerve. X 22 cliams.

In the median line basilar artery (zzrzfi), and still lower the wide, slit-like pharynx (P/1.), the outer portion of which ascends obliquely towards the jugular vein. The ascending part is a portion of the first gill-pouch (future Eustachian tube) and is quite clearly marked off from the pharynx proper. Of the acustico-facial complex the section shows four parts: the ganglion vestibuli (‘.); the geniculate ganglion (; the sensory root (Fac.) of the facial nerve arising from the geniculate ganglion and entering the brain to form there a distinct fibre tract, which is oval in section, lies just below the entering vestibular fibres, and is indicated in the engraving; finally, the motor tract (z‘.m.) of the facial ; this tract is a very distinctly marked bundle of nerve fibres, which traverse the Ventral zone of the medulla almost horizontally, then come downwards and, passing out from the brain—wall, form the lzzteml root of the facial; this root runs first towards and then E/’-\ past the geniculate ganglion. Thejugularvein (_/zzg.) lies outside the V5‘ ganglia, not inside, as in Fig. 9. In the man- D-S dible appear two nerves ; 5,,_ of these the upper is the hypoglossal (./V.xz'z‘.); it lies near the angle /"3" formed by the junc- FIE’?!tion of the first gillcleft with the pharynx proper; the lower is the inferior maxillary. P/A

The next figure is from a section not far from the last and chosen

FIG. n.—Pig embryo of 12.0 mm. No. 6. Frontal section art, artery; Cacfl., cochlea; DAEIJ ductus endo The hind_brain has nat-_ lymphaticus; E/5., ependyma ; Fain:/1., motor division of _ ~ the facial nerve; f1¢g., jugular‘, 11[zz’.o&l., medulla oblon considerably 1tS gata; P&., pliarynx; R/)/z., median raphe of the medulla

thickened floor < Z, [ (1,. :),l:ill(])i1ga>:aq;2 :‘i.acl.],ma11lage of the semicircular canals; Va, 061.), the anlage of the

medulla oblongata, rises steeply from the median line; its ependymal roof (E1?) is less expanded than in Figs. 9 and I0 and forms a sharp angle in the dorsal median line. The two sides of the medulla are separated by a deep fissure (cf. Fig. 1o,f.), underneath which the wall of the brain is correspondingly thinned, constituting the floor plate (Rf/1.), which is transformed in the adult into the median raphe. The pharynx (P/z.) is wide and is expanded laterally into the common beginning of the first and second gill-pouches. Between the pharynx and the raphe the basilar artery (arm) has been cut transversely. The otocyst is a large epithelial vesicle with three well-marked divisions : first, the common chamber (5.c.), out of which the three semicircular canals are to be differentiated; second, the slender canal (D.e.), the anlage of the ductus endolymphaticus, which lies between the semicircular canals and the medulla; third, the long, curving, but not spiral, cochlea (Cot/z.). The common chamber formed by the union of these three divisions is later subdivided to form the upper utriculus and lower sacculus. Outside the cochlea lies the cross-section of the jugular vein (jug), just below which is the section of the motor portion (Fzzc.m.) of the facial nerve. The sensory portion of the facial is much smaller, runs only a short distance downwards from the geniculate ganglion, and is entirely separate from the motor portion. It seems to me, therefore, that the anatomical description of this nerve, in view of the embryonic conditions, needs revision.

The absence of advanced stages of histological differentiation at this period of development is a general and characteristic state of the tissues. Especially noteworthy is the simple character of the mesenchyma, for it offers no marked differentiation except that of the merely endothelial blood vessels and their contained blood. The blood corpuscles are all nucleated red cells, which are for the most part in embryos of twelve millimeters, in a transitional stage. In younger pigs the corpuscles are in the ichthyopsidan stage and have a full-sized nucleus with‘ quite distinct chromatin granules. In older pigs the corpuscles have much—contracted nuclei, which stain darkly, as in the blood cells of Sauropsida. In our embryo the nuclei are partly contracted and modified. The mesenchyma proper is very voluminous and constitutes a large proportion of'the total bulk of the body. Its cells vary much in the degree in which they are crowded in the different territories of the tissue, and they vary also in form, but they can hardly be said to offer any positive specialization, and there is probably at this stage no differentiation of the fibrils and elastic networks, which are so conspicuous in many of the adult tissues which are derived from the embryonic mesenchyma. The simplicity of mesenchymal structure is further emphasized by the absence of distinct layers. Thus, between the brain and the ectoderm there is a continuous layer of tissue nearly uniform in appearance; later, out of this layer there will have arisen the cutis, subcutaneous tissue, periosteum, bone, dura mater, arachnoid, and pia mater, while the complexity of organization will have been increased by the ingrowth of nerves and striated muscles. The entoderm and ectodermal epidermis are both quite simple epithelial structures, although the entoderm, it must be added, already presents some very important modifications, such as the notochord, the thymus and thyroid glands, and the liver. On the other hand, the neuro-ectoderm shows advanced differentiation of the neurones with their accompanying axons, from the neuroglia cells. The mesothelium also offers more complexity, owing to its share in the development of the striated muscles, of the Wolffian body, and of the genital glands.

The preceding illustrations and descriptions are published, partly in the hope that in spite of their fragmentary character they will prove useful to some students of mammalian embryology, partly in the hope that the author will receive from other teachers criticism and suggestions in regard to the proposed text-book.

Harvard Medical School, Boston, Mass.

October 23, 1900.

Cite this page: Hill, M.A. (2024, June 15) Embryology Paper - The study of mammalian embryology (1900). Retrieved from

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