Paper - The Morphology and Histology of a Human Embryo of 8.5 mm

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Barniville HL. The morphology and histology of a human embryo of 8.5 mm. (1914) J Anat Physiol., 49(1):1-71. PMID 17233012

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This historic 1914 paper by Barniville describes early human embryo Carnegie stage 15 in Week 5.

Modern Notes: Carnegie stage 15

Stage 15 Links: Week 5 | Head | Lecture - Limb | Lecture - Gastrointestinal | Lecture - Head Development | Science Practical - Gastrointestinal | Science Practical - Head | Carnegie Embryos | Category:Carnegie Stage 15 | Next Stage 16
  Historic Papers: 1891 | 1905 Cranial and Spinal Nerves | 1914 | 1926 Precervical Sinus | 1948 stages 15-18
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The Morphology and Histology of a Human Embryo of 8.5 mm

By H. L. .Barniville, B.A., M.B., Demonstrator of Anatomy In University College (N.U.), Dublin.


The embryo which is the subject of the following investigation was obtained in January 1913 in the Frauenklinik at Giessen (director, Professor Opitz) by operation on a woman in whose case an excision of the uterus was rendered necessary on account of tuberculosis, and came into the hands of Professor Keibel through the kindness of Dr Jaschke. It is now included in the Keibel series as No. 1495 at the Anatomical Institute of the University of Freiburg in Breisgau. The embryo came into the fixing fluid in an exceedingly fresh condition; and, when a series had been made, proved to be in such an excellent state of preservation that I undertook, at Professor Keibel's suggestion, a description of the whole anatomy, with the intention of making it as complete as possible as far as the more interesting points were concerned. Accounts of the morphology of individual embryos have been frequent in the literature since the time of His; and although a certain amount of overlapping must obviously occur in the observations of various authors, the value of such work is nonetheless very appreciable. An accurate idea of a given stage in the development of an organ or part cannot be scientifically founded on a single description; and just as our knowledge of adult morphology has been built up by the combination of often very divergent interpretations of a given condition, in the same way must our data, with regard to the course of embryological evolution, be raised to a higher plane of scientific accuracy by the elimination of the personal factor in the equation.

Of recent years two papers have appeared which have a special bearing on the present researchthat of Ingalls on a 4 9-mm. and that of Elze on a 7-mm. embryo; and any subsequent worker along the same lines must necessarily feel himself more especially indebted to these two exhaustive references, providing as they do not only many valuable suggestions but also a definite guide in the treatment of an otherwise somewhat diffuse subject.

The methods employed were those already described in the literature and applicable in a work of the kind. The embryo was fixed in a sublimat-picric acid fluid, and after being photographed (Plate I. fig. 1) was embedded in celloidin-paraffin. The block was provided with guiding lines after the glass-box method of Born and Peter, cut in series with a sectional thickness of 10, u and stained with eosin-azure. The resulting sections are of excellent quality, and provide a well-nigh perfect histological picture, with numerous mitotic figures and a normal condition in the contour of the epithelial linings of the various organs, vessels, and body-spaces. The nerve trunks, although their constituent fibres remain unstained, are sharply defined owing to the characteristic appearance of the sheath-cells, as well as the contrast afforded with respect to the surrounding mesoderm, and can be followed to their ultimate terminations. Figs.7and8onPlateI. will provide an idea of the condition of the cellular histology under high magnification. Small but comparatively unimportant folds occur in some of the sections. The plane of section is not quite horizontal, as will be seen from the text figures, so that the left side of the latter (representing the right side of the embryo) pictures in each case a level somewhat oral in respect to the right side. Graphic profile reconstructions were made of the various systems (figs. A, B, and C), after the methods employed and described by different workers and epitomized by Peter in his Methode in der Rekonstruction. The left side of the embryo was in each case taken as the basis of the reconstruction, but in at least one case a condition occurring in the right side and absent on the left was added in, viz. the connexion between N. IX. and N. X. A few reconstructions from an antero-posterior aspect were also made in the case of some of the organs, viz. the anlage of the lung, in order to obtain a more definite picture of their condition than was possible merely from a study of the sections or from a lateral reconstruction. The guiding-line system employed was twofold, in some cases both being utilised for comparison and to eliminate errors; on other occasions one or other separately was found to be the more accurate. The se were:(a)the guiding lines of the block combined with those which were subsequently drawn on the individual outlines of the sections , and(b)the outline of the whole embryo obtained by an enlargement of the photograph by means of the projection apparatus. Such reconstructions, as has often been pointed out, are not to be regarded as quite free from error even with the most accurate manipulation of the guiding lines, but the results obtained are such as cannot be so conveniently produced by other methods, and indeed, as for instance in the case of the arterial and venous systems, are almost impossible of achievement by the wax-plate method. A comparison of the reconstructions with the text figures will further show that discrepancies are practically absent, and where such are found they will prove to be so minimal as to be negligible for all practical purposes. Wax models after the Born method were prepared of some of the individual organs, and are reproduced in the drawings on Plates I. and II. The text figures were drawn direct from the projection apparatus, and are faithful in outline and detail.

Length of the Embryo

The embryo, measured from the photograph in which it was subjected to a definite known enlargement, has a greatest length of 8.5 mm. When embedded and cut it provided a series of 840 sections of 10y, so that shrinkage, if any, has been very slight. According to Mall's method of reckoning, the embryo would thus have reached the early part of the thirtieth day of its development. In view, however, of the conclusive researches of Bryce, a correction must be made of the Mall formula, viz. the addition of five days to the age, which would place our embryo quite at the end of the fifth week.

External Body-Form

The embryo, as seen from the photograph (Plate I. fig. 1), falls between figs. 9 and 10 of the His Normaltafel and between figs. xi. and xiv. of the Normaltafel of Keibel and Elze, resembling the former more closely than the later. of the embryo (7mm. ca.) which has been the subject of a full description by Elze, and which will be frequently referred to in the following pages. A comparison with that figure, however, will show that the present embryo shows an advancement in many particulars. it is not so curved on itself either cranially or caudally, although the cervical bend is very sharp and approaches a right angle. On the outer surface of the head region the cerebral hemispheres, the optic cup and lens, and the roof of the fourth ventricle show up distinctly. Behind the eye and above the maxilla and mandible is the swelling of the trigeminal ganglion, and still further back above the hyoid arch is the swelling caused by the otic vesicle, with the ductus endolymphaticus continued dorsally from it lateral to the roof of the hind-brain as a very apparent streak. The nasal pit looks laterally as well as towards the heart, and the lateral and medial nasal processes being more prominent, it is deeper than in Elze's embryo. Three visceral arches are to be distinguished. The mandibular arch is subdivided by a horizontal groove into two swellings, while its maxillary process takes a definite part in completing the margin of the nasal pit. It shows further a secondary vertical groove which runs almost parallel with the nasolacrimal groove and ends dorsally caudal to the optic anlage (see Plate I. fig.3). This groove first appears in fig. xv.r.of Keibel and Elze's Normaltafel, and is seen in fig.xvi. from the same source, reproduced as fig.49 of Keibel and Mall's Text-book, vol.i. The hyoid arch is also subdivided by a horizontal groove, and has an opercular prolongation continued caudally from it . The first bronchial arch is sunken in to the sinus cervicalis, which still possesses a wide triangular opening. The placode formations in connexion with N. IX. and N.X., or, more strictly, the openings of the ductus branchialis I.and ductus branchialis IV., can be seen on the surface anterior and posterior to the third arch respectively. In the anterior extremity an oval hand-plate and short forearm, and in the posterior an oval foot-plate are to be recognized. The axis of each limb is set at an oblique angle with respect to the dorsal line. The segmented muscle plates and sclerotomes are very obvious in the dorsal and caudal regions. The tail is spirally twisted on itself and is curved towardsthe left of the umbilical cord, which maintains a more or less medial position. The heart swelling does not quite reach the cord, and ismore definitely marked off from the liver than in fig. xi. above referred to. The re is furthermore a slight surface demarcation between the auricular and ventricular sections of the heart in the form of a short groove or pit about midway along the groove separating the two organs.

Nervous System

The Brain

The morphology of the brain, as seen in outline in the profile reconstruction (fig. A), shows a general similarity to that of the well-known His model of a fourth-week embryo (6'9 mm.), and that of Elze ( A few points may, however, be noted as marking an advancement. The anlage of the cerebralhemispheresismore pronounced, but a partial separation of the two halves is as yet indicated merely by a shallow fossa (fig. 9), at the bottom of which is a low ridge. On the floor of the diencephalon are seen the mammillary and infundibular swellings, and more anteriorly a less well-marked swelling-the torus opticus. Between the two latter is a shallow pit where the hypophyseal pocket lies against the brain-floor. On the roof of the mid-brain, in the middle line and in the position marked by an arrow (fig. A), is a small area, extending over about four sections (10, ), in which the structure of the brain-roof differs from the general type(fig.1). The outer aspect of the brain-wall shows a small, nipple-like projection, corresponding with which, ontheinnersurface, is a shallow pit. The wall of this recess consists of radially arranged cells, of which the nuclei, in marked contrast to the rest of the neural tube, are confined to the outer layers, leaving an inner protoplasmic layer free from nuclei. In the junctional zone between this projecting area and the normal brain-wall are numerous darkly staining nuclei. This finding corresponds exactly in its appearance with the structure described in a similar position by Elze (7 mm.), and in its histology with the description given by Mihalkovics of the histology of the early pineal outgrowth. The recanbelitledoubtthat-itrepresents the earlyanlageofthisstructure. The isthmus is well marked. The very thin roof of the rhombencephalon has become insvaginated into the ventricle duringembedding. The sidewall and floor have well-marked neuromeres, though the most posterior is short and is seen only dorsally at the level of the origin of the vagus(fig.2). The neuromeres are most marked on the inner face of the brain-wall in the form of grooves, but they involve alsothemantlelayer. The first has no nerve-rootin connexion with it, and constitutes the cerebellarneuromere. Itblendsbelowwiththesecond neuromere (or "first" neuromere proper, according to Streeter's reckoning). The relation of the various cranial nerves to these structures is essentially that described by Streeter, etc., for man, except in the case of N. VI., which doesnotariseexclusively from the fourthneuromere(cf.below). Early indications of some of the'future tracts in the rhombencephalicwall are indicated. Inthemarginalzoneo1thefloornumeroustransverselycoursing fibres mark the "Bodenkommisur, " and on either side near the midventral line, is a group of longitudinal fibres-the future median longitudinalbundle. The entering fibresofN.V.sweepinpartforwardsand. upwards, and although, owing to absence of staining, they cannot be traced clear of the -first neuromere, may probably indicate an early tract cerebelaV. Otherfibrestakeabackwarddirection, as is the case with thecranialnerves generaly, the entering fibres assuming in parta longitudinal direction in the reticular marginal zone, without forming definite tracts except perhaps in connexion with N.X. Immediately above the longitudinal fibres the marginal zone diminishes in thickness and almost disappears, .exactly as is seen in the 'Spinal cord dorsal to the posterior funiculus.

Spinal Cord

The lumen of the neural tube can be followed to within four sections (10, A) from the caudal tip, the tube itself blending aborally withtheepitheliumcoveringthetip. The wallinthisregion consists of a single layer of high columnar cells, as Lenhoss6k describes; the only suggestion of layer-differentiation is the frequent appearance of mitotic figuresinthenuclei of the innermostzone. Nearitscaudaltermination the lumen contains two large clumps of degenerating cells attached to the roof of the tube and extending in each case through about ten sections (fig.20). Elze(7mm.)hasdescribedsimilargroupsofcellslyingfreein thelumen. Herecanbemostconvenientlyincludedashortaccountof the condition of differentiation of the wall of the neural tube in general. An examination, forexample, ofa sectionpassingthroughtheupper cervicalregion(fig.3)shows that thewall may be divided fordescriptive purposes into three main layers. This condition is best marked in the region of the anteriorhornanlage. Internallyisabroadlayerofdarkly staining nuclei and radially arranged protoplasmic filaments, the whole affordingtheusual radiallystriatedpictureofearlystages. Itsinnerzone showsnumerousmitoticfigures(germinallayer). Outsidethislayeris the clump of anterior horn nuclei, less darkly staining and having no radial arrangement. Betweenthesetwoisathinlayerofcircumferentially arrangedovalnucleiandfibres-the"zonaarcuata"ofHis. This arcuate zoneiscontinueddorsallyinternal to the posteriorlongitudinalfibre-bundle around which the nuclei become more numerous, constituting an early posterior cell column (His). Thiszoneoffibresandcellsformsamarkedly differentiated mantle layer, but it is not the sole expression of the process ofdifferentiationinthislayer. The outerzone of the radiallystriated layer shows a loosening up of its structure ("aufgelockerter The il " of His) inasmuch as the nuclei are less radially arranged, set farther apart, and stain as a whole less deeply than in the rest of the layer. This process is scarcely noticeable in the region of the sulcus limitans, but is well seen dorso-mesial to the posterior longitudinal fibre bundle, and more particularly opposite the anterior horn anlage, the latter having the appearance of "invading" the inner layer acrossthebarrier of the zonaarcuata. This loosened zone is then discontinuous dorso-ventrally, in contrast to the arrangement figured by His, and the appearances indicate that this further differentiation of the mantle layer occurs earliest in the two above-mentioned situations. The outermost layer is quite free from nuclei, and consists of fibres running in various directions, giving it a reticular structure. it is thickened antero-mnesial to the anterior horn-the funiculus anterior, and opposite the entrance of the posterior root fibres-the funiculusposterior;dorsaltothis it is absent. A well-marked anterior commissure is present, and derives its fibres mainly from the zona arcuata. Some of the fibres, however, do not cross, but sweep around the mesial aspect of the funiculus anterior to remain on the same side of the cord. Traced headwards the anlage of the funiculus posterior fades away a short distance anterior to ganglion C.I. Caudally it can be traced as far as the second sacral segment, beyond which a definite anterior horn also becomes indistinguishable-although ventral root fibres arise down to the level of the first coccygeal segment. The anteriorhornanlagetracedorallycan be followed into the rhombencephalon as a thickening of the differentiated mantlelayer, fromwhichtherootletsofN.XII.arise. The remainderof the wall of this portion of the brain consists of a radially striated layer with its inner germinal zone of mitotic figures and a loosened outer zone, and enveloped by a reticular layer containing the fibre tracts above described. The fore-brain is lined by a radially striated layer quite undifferentiated dorsally and laterally, but ventro-laterally there is a slight loosening-up process apparent in the outer zone, here covered by a thin clear layer free from nuclei.

Cranial Nerves

The general arrangement of the cranial nerves, as seen in the profile reconstruction (fig. A), corresponds with Elze's (7 mm.) description, and in the case of N. IX., X., XI., and XII. with the account given by Streeter for embryos about this age.

N. III. arises by a large number of fasciculi from the floor of the midbrain. In its course it runs internal to the "vena cap it is medialis" (Grosser), and to the ophthalmic division of N. V., with which latter it is in connexion, as will be later referred to under N.V. It ends in two terminal branches in a well-defined mass of condensed mesenchyme in which N.VI. ends more posteriorly. This is the anlage of the orbital pre-muscle mass, and lies postero-dorsal to the optic cup and external to the A. carotidis interna. The nerveshowsnoganglioncellsalongitscourse. Of part could be definitely recognized either in its intra-cerebral or peripheral course. The non-discoveryofthisnerveaftercarefulsearching must be ascribed to its small size, the undifferentiated condition of its nuclei, and more particularly to the fact that it would be cut transversely throughout its whole course. Elze notes only the crossing of the fibresintheroof of the brain, and fails to find any trace of its peripheral fibres.

N. V.- The central fibres of the trigeminal ganglion, on entering the marginal zone of the brain-wall, run in three main directions: one set makes directlyforthe mantle cellsof the first neuromere, [1] and probably also of the second; another set runs longitudinally backwards, constituting the anlage of the spinal root V.; while a third set can be traced some distance forwards. The motor root arises some distance ventrally and becomes closely applied to the inner face of the ganglion, but remaining quite distinct from it. The ganglion in its upper part is dorsal to the vena cap it is ; lower down itcomes to lie lateral to and closely opposed to this structure. It gives off three branches along which ganglion cells are carriedforsomedistance. Especially is this the case with the ophthalmic branch, the root of which constitutes a finger-like process of the ganglion. Thisprocessisnotdistinct(asElze) from the origin of the nerve. The ophthalmic nerve runs forward over the optic cup, crossing external to N. I., and curves downwards, keeping close to the circumference of the cupasindicated in fig. A. The detailofthisbranchanditsrelationto N. I.can be followed only with difficultyowing to the faintness of its loosely arranged fibres, but the following further points can be made out. On therightside, aftercrossingN.I., itexpands in its sectionalarea. This expansion is due to a sprouting from its mesial surface, from which a branch runs through about seven sections (10, u) internal to the main trunk. The point of originofthisbranchisseparated from the anterior branch which N. III. drops down into the orbital pre-muscle mass by the anterior pole of this structure, and from this division of N. III. a small fibre-bundlecanbetracedashortdistanceinthedirection of the ophthalmic nerve. Noganglioncells are to beseen, but the picturesuggestsanearly naso-ciliarybranchwithacommencingciliary system. On theleftside is a similar sprouting of the ophthalmic nerve, without a definite branch, and the arrangement of the mesodermal cells suggests a fibre connexion betweenitandN.III. Nervimandibularisandmaxillariscanbecontinuedfarin to the first arch and its maxillary process respectively. The latter branch on approaching its termination is broken up into bundles by cells which, though mostly of the "sheath" type, are in part of ganglionic appearance. Its relation to the great superficial petrosal nerve will be later referred to. N. mandibularis shows no branching except for a short stump-like branch from its outer aspect twenty-five sections (10, u) from its origin. It receives the entire motor root.

Near its termination the nerve passes through a tunnel of strongly condensed deeply staining mesoderm, which is prolonged distally gutterlike along its outer and posterior aspect-the anlage of Meckel's cartilage. A portion of the nerve, however, splits off and runs a short distance on the outer side of this anlage. The chordtympanibranchisalreadyin connexion with N. mandibularis, as will be described in connexion with N. VII.

N. VI.- The abducens nerve can be well seen on both sides in its peripheral course running forwards from the level of the otic vesicle, internal to the mandibular nerve, to end in the upper and posterior pole of the orbital pre-muscle mass. On both sides the nerve receives numerous radicles from the ventro-lateral border of the hind-brain internal to the otic vesicle and at a level corresponding with the fourth neuromere. At this point and posteriorly the relations become somewhat confused on the right side owing to an unfavourable plane of section. On the left side, although the fibre-bundles are in places exceedingly fine and occasionally disappear in one section to, reappear in the next, a clear picture is nevertheless presented, so that the following description applies mainly to that side. The reconstruction of the nerve in fig. A, though necessarily somewhat schematic, is a faithful representation of the main course of the rootlets. A well-marked radicle, constituting the main posterior root of the nerve, arises from the lower part of the side of the brain internal to N. X. and immediately in front of the most anterior root of N. XII. and in series with it. ' The former is directed forwards, the latter backwards, while the two are separated by a small ascending branch of the vertebral artery. This radicle, at first directed also some what laterally, soon turns dorsally in the mesoderm on the side of the brain, and, reaching a level some twenty sections (10A) higher than its origin, it joins another bundle of fibres running from above and behind downwards and forwards. This bundle, traced upwards, rises to a high level close to the brain-wall until it reaches a point corresponding with the neural crest and about midwaybetweenthedorsalrootsofN.IX.and N.X. Hereitbecomes very small, but before its final disappearance it seems to have a connexion by a finerootletwiththebrain-wall. Thiscannot, however, bestated with certainty owing to the minuteness of the structures and the unstained condition of the fibres. A shortdistancebelowthepointwherethebundle becomes lost it is joined by another fine fibre-bundle coining from behind andbelow. The latterbundlecanbetracedbacktoanorigin from the anterior root of N. XII., and is apparently to be interpreted as a dorsal branch of that nerve. The above dorso-ventrally running nerve, after being joined by the main posterior radicle, runs downwards and forwards, receives a large root from the lower part of the fifth neuromere a short distance caudal to the otic vesicle, and is further joined to the main posterior radicle by a fineloopofconnexion. Internal to the oticvesiclethebundle, now a nerve of relatively considerable size, receives two groups of radicles, one opposite its posterior border and the other a short distance farther forward. The abducens is the reforeinconnexionnotonlywiththe fourth neuromere but directly with the fifth and the brain-wall ventral to the sixth, and indirectly with the posterior unsegmented portion of the rhomnbencephalonthroughtheanteriorrootofN.XII. The questionof the origin of the sixth nerve with regard more especially to its connexion with a given neuromere has been very variously stated by different authors notonlyforlowervertebratesbutalsoforman. An excellentsummary of the wholesubjecthasbeenrecentlygivenbyGraper. Inthecaseof the human embryo most authors (except Broman, who derives' it from the third neuroinere) agree with Streeter in attributing the main origin of the nerve from the fourth neuromere (see footnote on p.10). A posterior origin of the nerve is described by Elze (7 mm.), in whose embryo a radicle arises close to and in series with the roots of N. XII., and indeed a series of radicles extending between the abducens and N. XII. has been more thanoncedescribedintheliterature. Bremer, inamoreextendedstudy of the aberrant roots of the two latter nerves, finds that in young human embryos it is by no means uncommon to find the gap between the two nerves filedin by amore or less segmental seriesof rootlets, some being directed not only backwards or forwards in a ventral direction but also laterallyanddorsally. Ina102-mm.embryoone of the selaterally runningrootsjoinsaseparatebundleofN.IX. Foran 11*5-mm.embryo he figures a condition very similar to that found in our dase (fig. 4 in his paper). Graper, working at the more intimate fibre connexions within the neuromeral area of the rhombencephalon in several lower mammals, chick, etc., findsthatinalcasestheabducensisstrictlyconfined in origin and point of exit to the fourthneuromere. Hedescribes, however, ina 13-mm. sheep embryo, a bundle arising from between the fourth and fifth neuromeres and running ventrally and backwards to join N. IX. It would seem, therefore, from the foregoing more correct to regard this row of ventral roots, which occurs more or less normally in the young human embryo and bridges over the gap between abducens and N. XII., not as belonging morphologically to N. VI. proper, but as constituting transitoryventralbranchesofN.IX.andN.X. The factthattheseroots are primarily connected with one another and with the nerves immediately in front and behind, and the further fact that they may possess dorsal branches corresponding with the dorsal divisions of the spinal nerves (Bremer), afford an explanation of the arrangement so frequently found. This interpretation is in agreement with the view of Graper in that it derives the origin of the abducens solely from the fourth neuromere.

N. VII The ganglion of the facialnerveiswoundspirallyaroundthe acousticganglioncomplex, andgroovesthelater, lyingsomewhat internal above, thenanteriorlyandlowerdownonitsouteraspect. The twocel massesarepartlyfusedbutareinpartseparatedbymesodermal cellsand, especiallybelow, bythemotorrootVII. The centralfibres of the ganglion (N. intermedius), in company with those of N. VIII., enter the marginal zone opposite the third neuromere, some running directly into the mantle layer of this neuromere, but large numbers taking a horizontal direction backwards and constituting the contribution of this nerve to the tractus solitarius. The motorroot, whichissmallrelatively to the size of the ganglion, arises a short distance below the point of entry of the sensory fibres. The trunk of the nerverunsbackwardsandoutwardsunderthe v.cap it is lateralisandcomesintorelationwiththefirstcleft. The connexion here is not in the form of a tubular placode formation, as is the case with the ganglia of IX. and X., but consists of a thick strand of ganglionic cells merging on the one hand with the cells of the ganglion and on the other with the thickened epidermis of the cleft. From the ganglion a short distance distal to this epibranchial organ a short branch is given off which has an independent connexion with that organ and constitutes the nervus epibranchialis described by Futamura in an embryo of27to30days(fig.4). Asthemaintrunksweepsventrallyandcaudally into the hyoid arch it drops off from the lower end of the ganglion an anterior branch, the great superficial petrosal nerve (fig. 4), which carries with it forsomedistanceganglioniccells. Thisnerverunsatfirstlateral to the a. carotidis interna, internal to the dorsal expansion of the first pharyngeal pouch, across the roof of the pouch itself anteriorly, and into the maxillary process lying close to the epitheliumn of the roof of the mouthcavity(fig.5). Afterrunningashortdistancedownwardsin to the maxillary process, it shoots almost horizontally in the sections as a wellmarked bundle across the space separating it from the terminal bundles of n.maxillaris, withoutestablishinganytraceableconnexion. The reisas yetnodefiniteaccumulationofganglioncellsinthisregion. The chord tympani is present as a large branch from the trunk of the nerve (fig. 5), running into the mandibular arch in the angle between the epithelium of the first cleft and the endothelium of the first pouch, which meet above it inthecleftmembrane. Itendsbydividingintotwomainbrancheswhich are directed mesially in the condensed mesoderm of the floor of the mouth, but before this final division it gives off a small branch which curves outwards to connect with n. mandibularis as itruns in the mesodermal Meckelian"gutter"describedabove. Thisconnexionisnotapparenton therightside, althoughasimilarbranchispresent. The nervetrunk finally terminates in a sharply outlined mass of condensed tissue-the hyoid pre-muscle mass, described by Futamura at this stage.

The acoustic complex is markedly differentiated histologically in its upper and lower segments; for while the upper portion resembles the adjacentganglion VII. and the spinal ganglia generally, the cellsin the lower portion are smaller, more darkly staining, and more. widely spaced. This latter "cochlear" segment overlaps the upper "vestibular" segment especially on its outer side, but also internally, so that the vestibular segmenthastheappearanceofbeinginsertedintoitfromabove. The cellsofthislower cochlearportionfurtherfuseinpartwiththeepithelial wall of the otic vesicle, destroying its sharp outline and leaving no definite lineofdemarcationbetweenthetwostructures. Fibresarecontinued upwards from it internal to the vestibular ganglion, and constitute the youngcochlearnerve. The centralfibres from the upperpart of the ganglion join the marginal zone opposite the third neuromere in company with the fibres from ganglion VII.

N. IX.- The ganglion of Ehrenritter is relatively insignificant, and consists of a small collection of cells almost entirely confined to the lateral aspect of the nerve. Itscentralfibresjointhemarginalzone of the hindbrain, some entering the mantle layer of the fifth neuromere, while many turn backwards and upwards, causing a thickening in the " Randschleier" towardsthe point of entry of the centralvagalfibres. The motorrootis distinguishableasasmallbundlearisingashortdistanceventrally. The nerve is continued distally internal to the v. cap it is lateralis (fig. 5, right side)as a fibro-cellular strand in which the fibres predominate, the cells beingapparentlyal of the "sheath"type. Asitapproachesthelevelof the dorsal apex of the second pharyngeal pocket, ganglion cells begin to appear again, and soon the whole cross-section of the nerve consists of a dense mass of darkly staining nuclei forming a large ganglion-the ganglionpetrosum. Inthisstructureadefinitefibrous(motor)elementis indistinguishableassuch, incontrastwithwhathasbeennotedinthecaseof theganglionV.andganglionVII. The ganglion in its lowerpartcomes to be closely applied to the posterior wall of the second pharyngeal pocket (figs. 5 and 6), and here, on the left side, appears a short sprout from its anteriorface. This is the onlyindicationofanervustympanicus, the presence of which has been noted by Futamura at this stage (27 to 30 days). On the right side is a small but very definite fibro-cellular connexion between the ganglion petrosum and the upper part of the ganglion nodosum. Thisconnexionisabsentontheleftside. AccordingtoStreeter, such a connexion between N. IX. and N. X. is absent in embryos up to 7 mm., and usually also in embryos of that stage, while in slightly older embryos the two nerves come into close relation and may be linked together. Elzeina7-mm.embryofindsnosuchconnexion, andbothHis andStreeterfigureitat102mm. Henceintheembryounderdescription, although the two nerves are stil separated by an appreciable interval at the point of connexion, the latter on the evidence must be regarded as of asecondarynature. The nervetrunk, traceddown, in to the thirdarch, runs lateral to the a. carotidis interna, and gradually becomes free from ganglioncellson itsinneraspect, butonitslateralaspectthecellsgradually mergeintoawell-markedplacode. The latter consists of atubularprocess of epithelium, opening below on the lateral face of the third visceral arch inthedeeperpart of the secondcleftor"ductusbranchialisI." The opening is about 140u in diameter from above downward, and from it a finger-like process extends up to the outer side of the distal end of the ganglion (fig. 5), its tip being embedded in the latter structure, so that histologically the two are continuous. A lumen extends through the processforadistanceof90u from the mouth of the placode. Fromthis point the nerve curves forwards and downwards to end just lateral to the (medial)thyroidanlage. Inthispartofitscourse it is situatedsome distanceanterior to the third aortic arch, but drops of, on the leftside, a short internal branch which runs some distance ventrally in close relation to the artery. Ontherightside(fig.6)twosuchbranchesarepresent.[2]

N. X. and N. XI.- The development of the vago-accessory complex has beendescribedindetailbyStreeter. Nevertheless, afewpointscomeup for special mention, especially as our embryo falls in the gap between the 7-mm. and 10-mm. embryos of his series. The general outline of the developmentalstagecanbe seen in thereconstruction(fig.A). A vagus root-ganglion has been definitely differentiated from the neural crest of the hind-brain, itscentralfibresenteringthesixthneuromere (fig.2). The more posterior part of the crest is represented by four or five ganglionic clumps along the well-developed accessory motor portion of the complex, which-clumpsgraduallydiminishinsizefrombeforebackwards. The accessory nerve is first found at the level of the third cervical segment, from which pointitrunscephaladinternal to the secondcervicalganglion. Almost at the level of the I.C. ganglion, but slightly distal to it, it passes external to a small fibre-bundle coming dorsad and somewhat caudad fromthisganglion. Itthendevelopsonitsouterandventralaspecta dense ganglionic clump, corresponding in size with the I.C. ganglion itself, but having no connexion with this latter structure or with the hypoglossal rootlets. Thisaggregationofnerve-cellsmightberegardedeitherasa detachedportion of the I.C.ganglion, an arrangement which, accordingto Streeter, frequently occurs; or, on the other hand, it might with quite as muchprobabilitybelookeduponasaFroriepganglion. Itspositionsome distance caudal to the rootlets of N. XII. is not against this view, for the processwhich, asStreetersuggests, hasbrought about the breaking up of the neural crest into clumps, viz. the growth of the intervening fibretracts, will obviously produce its greatest effect in the case of the most posterior member of the series and thus cause marked caudal displacement. In a 1Q2-mm. and in a 14-mm. embryo Streeter figures the ganglion in a position well caudal to the most posterior rootlet of XII. Its shape furthermore corresponds with the general description given for a Froriep ganglion, in that it tapers off anteriorly in contradistinction to the accessorycelclumps. The reisnowell-definedFroriepformationonthe right side, where the I.C. ganglion is better developed and extends dorsally to the accessorynerve, sendingmostbutnotalofitscentralfibresinternal tothisstructure. The ganglionjugulare, withtheaccessorymotorroot applied to its posterior aspect, is continued down as a fibro-cellular strand, freefromganglioncells, to the ganglionnodosuin(figs.5and6). The v. cap it is lateralis crosses the trunk of the nerve laterally and then runs behind it. The motor portion leaves the upper part of the ganglion nodosum and passes dorsally and laterally, crossing, the vein, N. XII. and N. J.c., to end in three branches in a condensed mnesenchyme, -the anlage of the sterno-cleidomastoidmuscles. The lowerpart of the ganglionlies dorsal to the third and fourth pharyngeal pockets, and opposite the fourth visceral arch it gives off a large branch which curves downwards and inwards in the anterior part of this structure (fig. 6, left side) anterior to the fourth aortic arch, to end dorsal to the ventral aorta close to the hypoglossalnerve. This is the superiorlaryngealnerve, anditgivesof, asis best seen on the right side, an external branch which runs in the posterior part of the archexternal to the artery, andrepresentsinalprobabilitythe externalbranch of the superiorlaryngealnerve of the adult. The placode in connexion with N. X. is a short epithelial cul-de-sac continued from the upperend of the fourthvisceralcleft. The cellsofitstipblendwith those of the ganglion at the level of the origin of the superior laryngeal nerve. The vagustrunkthenpassescaudadlateral to the sixthaortic arch, is crossed by N. XII., and comes tolie in the mesodermal tissue lateral to the esophagusandtrachea(comp.figs.6and7). Itcanbetracedwell beyond the tracheal bifurcation (fig. 10), where it breaks up into branches which run to the developing bronchi, and others which form a plexus around the Esophagus containing much syncytial sympathetic tissue, as willbelaterreferredtoinconnexionwiththatsystem. Aterminal ramification on the fundus of the stomach cannot be made out.

N. XII. arises by a series of ten or eleven rootlets from a longitudinal thickening of the mantle layer already described as a prolongation of the anteriorhornanlagein to the hind-brain. The series, individually separated by ascending branches of the vertebral artery, extends from the level of the vago-accessory trunk to within a short distance of the origin of the ventral nerve-root C.I., the anterior rotlet, as has already been noted, having a fine loop connexion with the origin of N.VI. The serootletsunitetoformthreebundles, whichinturnfuse togethertoformthemainhypoglossaltrunk. The twofan-likeanterior occipitalmyotomes areinlateralrelation to the rotlets, somewhat widely separated above but in contact with them below, where the first rmyotome sendsawedgeofpre-muscletissuebetweenthetwoanteriorroots. The third occipital myotome lies for the most part caudal to the posterior group but overlaps it. From the most anterior root on the left side a large branch detaches itself, running at first laterally across the back of the anterior cardinal vein and then caudally postero-lateral to this vessel (fig. 5), so that dorsal tributaries of the latter separate the nerve from itsparenttrunk. Thisaberrantnerve, whichisaltogetherabsentonthe rightside, rejoinsthemaintrunk of the lypoglossusbelow. The latter lies at first behind the ganglion nodosum, separated from it by the anterior cardinalvein. Itthenwindsspirallyaroundthesestructures, iscrossed by the accessory nerve, and turns sharply upwards and medialwards to end nearthe(median)thyroidanlage(fig.6). Atthelowestpoint of the bend it drops off a branch which can be followed some distance caudally, dorsal to the pericardium and left auricle, to end in the mesodernial anlage of the infra-hyoidmusculature-thedescendenshypoglossi(fig.7, leftside). The hypoglossal trunk is joined posteriorly by the first two and part of the third cervical nerves, the arrangement being essentially similar to that described and figured by Elze and Streeter. In one important point, however, the arrangement indicates an advancement on the Elze (7 mm.) embryo, inasmuch as the cervical contributions join the trunk of N. XII. relatively much lower down; in fact, the C. III. fibres connect with the ramus descendens rather than with the main trunk, so that we are here a step nearer an "ansa" formation typical of a 10-mm. stage. From the proximal end of the nerve trunk and from the distal ends of its constituent roots there extend towards the dorsal aorta cords of typical syncytial sympathetic cells("cellularrami")similartothosetobelaterdescribedin connexionwiththespinalnerves. Dorsal to the aortaineachsectionthere appears at this level a small collection of syncytial tissue constituting a cephalic prolongation of the sympathetic cord. We are here, therefore, dealing with the anlage of the upper end of the first cervical ganglion, the connecting cords probably representing the early stage of the hypoglossosympathetic connexion of the adult.

The Spinal Nerves

Dorsal roots

The neuralcrestextendsunbroken from ganglion C, I. to within a short distance of the tip of the spinal cord. Inthecervicalanddorsalregionithasundergonealmostcomplete segmentation into ganglia, which, however, remain connected by a cellular bridge. The C.I.ganglionhasnoconnexionwiththesecondoneither side. Aswegocaudal to the lowerlumbarregiontheconnectingbridge becomes relativelymore importantinsizethan itsventral ganglionic ofsets, whilestilfurthercaudal the ganglia are represented onlybytheteethof theserratedventralborder of the crest. Inalaboutthirty-fourdorsal spinalgangliaaredefinitelydemarcatedoneitherside. Ingeneralthey areconnecteddorsallywiththethickenedridgeofmarginalzonecontaining the early postero-lateral fibre columns in which the entering dorsal rootfibresassumeforthemostpartalongitudinalarrangement. The distance between ganglion and thickened marginal zone diminishes as we go back, until ultimately, in the lower lumbar region, the dorsal border of the crest comes into direct contact with the postero-lateral ridge through cellular ganglionicprocesses, containingafewfibres. Stillfurthercaudalthe dorsal border of the crest rises towards the dorsal aspect of the spinal cord, higher at some points than at others, so that, although there is here no fibrilation, the dorsal border of the crest shows a serration similar to the ventralborder. Thispicturesuggeststhatintheventralmigration of the neural crest some parts lag behind at regular intervals, this process commencing before the dorsal border has reached the level of the future thickened postero-lateral ridge and giving rise to dorsal processes of the crestitself. Whenthesedorsalserrationsreachthelevel of the ridgethey become here attached to the spinal cord by fibres which are at first very short, subsequently becoming longer owing to the differential growth of the parts. Inotherwords, itwouldscarcelyseem quiteaccuratetoregardthe firstdorsalrootletsasasecondaryconnexionthroughasproutingoffibrous processesbetweentworelativelywidelyseparatedpoints. Ventralrootfibres can be definitely noted as far caudal as the first coccygeal segment, the more posterior groups of the series arising from an undifferentiated neural wall, the more anterior (as far caudal as S. I. segment) from a definiteanteriorhornanlage. The gangliaareconnectedwiththeventral roots in the cervical and dorsal regions by well-marked fibrous processes inwhichneurocytes(Kohn)arescattered. Inthelowerlumbarandmore caudalsegmentstheventralrootfibresrunthroughtheventralpoles of the ganglia(figs.20and21). Thissuggestsaprocesssimilartothatoccurring in the dorsal poles of the ganglia, viz. the pole of the ganglion itself comes in the first instance into immediate relation to the ventral root and short fibre connexions are established, gradually becoming longer as growth of therootsandcordtakesplace. An earlydorsalbranch of the complete spinal nerve is present from C. I. to C. VI. inclusive on the left side (seefig.6). The sebranchesareshort, directedlaterally, andspreadoutfanliketoentertheinnerface of the myotome. The reisnoneinconnexion withC.I., andthatinthecaseofC.VI.isverysmall. Ontherightside they are not so well seen owing to the slight obliquity of the sections, but the reisawell-markeddorsalbranchfromC.I.onthisside. The spinal nervesareconnectedwithoneanother by a nastomosesdown toS.I., those betweentheindividualcervicalnervesandD.I.beingespeciallylarge. The anastomotic loops closely invest the ventral aspect of the root of the membranouscostalprocesses. Inthedorsalregiontheyaremuchfiner, but that between D. XII. and L. I. shows a definite increase in size, while the individual lumbar nerves and the first two sacral nerves form a thick bandofconnexionamongthemselves. The nervesarewelladvancedon their ventral course in the lateral wall of pericardium and coAom, but show nodivisionintoterminallateralandanteriordivisions. The phrenicnerve arises from the anastomoses between C.I., IV., andV. nerves, andruns ventrally and caudally towards the pleuro-pericardial membrane, which it justfailstoreach(figs.9to12). Itliesatfirstdorsal to the anterior cardinal sinus; then, hooking around the termination of the subclavian vein, it passes almost horizontally in a ventral direction lateral to the termination of the posteriorcardinalvein, andatitsterminationliesdorsolateral to the duct of Cuvier (fig. 9). The innervation of the upper extremity is in the form of a plate-like expansion which projects laterally into the root of the limb from the anastomoses between C. V., VI., VII., and VIII., beyond which the individual nerves project ventrally towards the lateralwall of the pericardiumwithoutactuallyreachingit(ramipectorales). This plate is best marked opposite C. VI. and VII. nerves, and divides into two leaves, a dorsal and a ventral, which show a condition of differentiation intermediate between Elze's 7-mm. embryo and his Embryo I. (fig.452, Keibel and Mall). The dorsal plate, besides its main process (radialis), has nearer its oral edge a short process directed towards the pre-axial border of the limb. from the ventralleaf of the platemusculo-cutaneous, median, and ulnar processes are distinguishable. The latter, after a short course as a single trunk, divides on both sides into two branches which can be followed some distance distally in close relation to the marginal (brachioulnar)veininthepostaxialborder of the limb. The nerveanditsbranches lieanteriorandventral(i.e.pre-axial) to the vein. The subelavianartery, as is described in greater detail below, springs from the seventh dorsal segmental artery and pierces the nerve plate between C. VI. and C. VII. nerves. Betweenthedorsalandventralleavesitgivesoffabranchtowards the radial nerve, and soon breaks up into two branches, a larger accompanyingthemedianandasmalleraccompanyingtheulnarnerve. The main vein of the limb videe infra) runs along the postaxial border, then orally dorsal to the dorsal leaf of the nerve plate in its whole extent (fig. 12 et seq.), and crosses anterior and ventral to the phrenic nerve to join thecardinalsinusoppositetheductofCtuvier. The basis of the lumbosacral plexus is represented by the strong anastomoses between the lumbar andsacralnerves. Thisconnectingbandexpandssomewhatoppositethe hind limb into a slightly differentiated plate which sends processes for a shortdistancein to the root of the limb.

Sympathetic Nervous System anid Antage of the Suprarenal Cortex

A definite chain of sympathetic-or sympatho-chromaffin (Zuckerkandl)tissue is present from the level of the ganglion nodosum to the level of the umbilicalarteries. Furtherforwardintheheadregionandinconnexion with the more anterior cerebral nerves, viz. between the distal extremity of the superficial petrosal nerve and the n. mandibularis, a similar type of tissueissuggestedintheformofscatteredcells, whichcannothowever be ascribedwithanycertainty to the sympathetic system. The nervebundle, "almost entirely free from ganglion cells, "which Elze (7 mm.) describes as lying medial to the trigeminal ganglion and which he provisionally regarded as of sympathetic nature, corresponds exactly in position and appearance with N. VI. in our embryo, and is doubtless in the former case also a portion ofthatnerve in its peripheralcourse. The sympatheticcordhasafixed position throughout, lying at first dorso-lateral to the a. carotis interna, then in a similar relation to the aortic roots and later to the aorta itself. The histological characters of its constituent cells are not absolutely constant, but there is always the frequently occurring mitotic figure, the darkly staining nucleus and protoplasmic body provided with short processes, and with a tendency in common with its neighbours to run together into a syncytium as described by Kohn, Zuckerkandl, and others (PI.I.fig.2). In the cervical region there is a commencing ganglion formation, the cord appearing in some sectionsas a few scattered cells and in others as a dense clump with a remarkably definite outline. This appearance is less evident in the dorsal and lumbar regions owing to the factthatthecordheregetsbrokenupintoaloosernetworkbytheinsertion of the brush-likeends of the ramicommunicantes. The first fibrous ramus appears in connexion with N.C. VIII. (fig.15). Moreorallythecordis frequently connected with the cervical nerves, and then anastomoses by loosechainsofsympatheticcellsconstitutingthe"cellularraini"described byKohnfortherabbit(PI.I.fig.2). Suchacellularramusispresent in the case of N. C. I., and, as has already been described, similar chains of cells appear between the ganglion nodosum, the hypoglossal rootlets, and the sympathetic cord at these levels. Fibrous rami are present from C.VIII. to L.I., caudal to which the rami again become cellular. Here, however, in the narrow space between the aorta and the spinal nerves the sympatheticchainbecomes closelyapplied to the later, so that the term "ramus"can scarcely be employed to describe the connexion. Wherea fibrous connexion is present it takes the form of at least two, and in many cases three, distinct bundles from each spinal nerve, the more posterior bundle often fusing with the anterior bundle of the nerve next caudal beforeitreachesthesympatheticchain(fig.21). Especially is this the case with the lower dorsal and upper lumbar nerves. The ramispreadout into an almost continuous brush on reaching the cellular chain, loosening it upintoanetworkofcellsratherthanaclosesyncytium. Inthelumbar region, where the cord is more longitudinally cut in the sections, the protoplasinic processes of the cells can be seen to be directed longitudinally within it, but it is questionable whether distinct nerve fibres are present apart from the enteringfibres of the ramicommunicantes. Anlagen of the celiac, aortic, and hypogastric plexuses are represented by a ventral streamingoffibresandsympatheticcellsfrom the chain aswellasdirectly from the rami. Thisfibro-cellularmigrationismostmarkedaroundthe origins of the cceliac and omphalo-inesenteric arteries (figs. 17, 18, 19), but ispresentinsomedegreedown to the level of the umbilicalarteries. The fibresandcellspassventrallyaroundtheaorta, betweenitandtheWolffian body, and more orally they are closely applied to the inner face of the anlage of the suprarenalcortex. A fibreconnexion of the sympathetic migrations of opposite sides across the anterior aspect of the aorta cannot bemade out, butsyncytialclumpsofsympatheticcellsare present in this position, especiallyaroundtheorigin of the cceliacartery. Similarclumps extend into the root of the mesentery of the stomach and intestine. Surrounding the whole length of the Esophagus, and especially on its dorsalaspect, are numerous collections of darkly stainingsyncytial tisue, undoubtedly sympathetic, in a position similar to that of the " Hals-aorten-geflecht" figured by His for a fourth-day chick, and constituting a mnyenteric plexus in combination with numerous small branches of the vagus(fig.7etseq.) Similarganglioniccollections, butfewerinnumber, are present at the lower end of the trachea around its bifurcation, also in associationwith vagal filaments, and represent the anlage of the cardiac plexus.

The anlage of the suprarenal cortex is present in the first five dorsal segments. it is situatedbetweentheaortamedially, themesonephros laterally, andtheWolffianarteriesdorsally(fig.16etseq.). Itdoesnot reach the dorsal pillar of the pleuro-pericardial membrane anteriorly as it doesinslightlyolderstages(Zuckerkandl) Histologicallyitscomponent cellsare markedly differentiated from the surrounding mesoderm cells, and show an "epithelial" character, the nuclei being larger and more deeply stainingandthecellsrounderandwith a closelyaggregatedarrangement. Caudally the cel condensation is separated from the coelomic epithelium by a relatively wide area of undifferentiated mesoderni, but orally the relation is more intimate, the intervening mesoderm containing numbers of cells of the above type which connect the anlage with a broad ridge of the coelom between the mesonephros and the root of the mesentery. The mesial face of the anlage has closely applied to it the ventrally streaming sympatheticfibresandcells(figs.17, 18, and19). in its substanceare numerous small veins (see fig. 18), which can be frequently seen to open into the posterior cardinal sinus as mesial ventral tributaries of this vessel. The se latter branches more caudally are concerned in the formation of the subcardi-nalveinvideeinfra). Notwithstandingthecloserelation of the Wolffian arteries to its dorsal surface, no branches of these vessels could be seen entering the anlage.

Organs of Special Sense

The Olfactory Organ

The wax model of the nasalorgan(PI.I.fig.3) fals naturally into the series of models by Peter, figured in his Atlas der Entwicklutng der Nase und des Gaurnens, and resembles his model of a9-2-mm.embryo. The nasal area is sunken into a pit, looks laterally as well as ventrally, and is surrounded by a well-developed rampart which is best marked on the lateral aspect and here forms the lateral nasal process. Only anteriorly is the boundary wall indistinct, but even here there is an appreciableelevationdemnarcatingthepit from the generalsurface. The medial nasal process is most prominent posteriorly-processus globularis (Keibel) - and is continued back along the ventral surface of the headsome distancebeyondtheposteriormargin of the pit. Here it is bounded on its lateral aspect by a still shallow oblique groove whichiscontinuedforwards to cut into the posterior margin of the pit and backwards and inwards along the future roof of the mouth-the ventral limiting groove of the maxillaryprocess. The externallimitinggroove of the maxillaryprocess, or naso-lacrimal groove, is continued downwards and forwards from the optic anlage and cuts into the nasal margin a short distance anteriorly and laterally, sothatthetip of the maxillaryprocessformsthepostero-external boundary of the pitforashortdistance, andseparatesthelateralfrom the mesial nasal process.

The nasal pit is continued backwards and somewhat inwards as a blind sac for a distance of about 1 O/A. The floor of the sac is formed by a thick complete raphe of epithelium, the ventral border of which is attached along the groove which we have termed the ventral limiting groove of the maxillary process, and is continuous with the epithelium covering thisprocessandthemedialnasalprocess. The reisnowide groove separating the two processes as in His's figure of a twenty-nine daysembryo, the accuracy of which HochstetterandKeibelhavequestioned. Such a groove is further absent in Peter's series. A comparison of the model with that by Peter for a 105-mm. embryo shows that the ventral limiting groove is much shallower and less marked than in the latter owing to the factthatthetwoprocessesarelesspronounced. The grooveisthus secondary formation, and in neither case can it be regarded as a gutter connecting the hinder end of the nasal pit with the roof of the stomodaeum. In general the appearances confirm the view of an early fusion of the maxillary and medial nasal processes to form the posterior boundary of the nasalpitandthe"primitiveGaumen, "thefusiontakingplacefromwithin outwards(Keibel)astherespectiveprocessesexpand. A bridgingoverby these two processes of a pre-existing groove connecting the nasal pit with the stomod.%um, as described by His, is not compatible with the form of the partsatthisearlystage. The anlage of the organofJacobsonispresent in its earliestform(fig.8). Itdoesnotappearonthemodel, butcanbe observed as a faint grooving of the epithelium of the medial wall of the nasal pit, similar in appearance and position to that figured in the Nortnaltafelof Keibel and Elze (fig. 32) for an embryo of 8.5 mm. The latter is the earliest in which this anlage has been noted.

The Eye

The development of the eyemarksastageinadvanceofthat of Hochstetter's 7-mm. embryo, described in detail by Elze and reproduced asfig.169ofKeibelandMall'sText-book. The opticcupisconnected with the diencephalon by a short widely-open pedicle. The lamina3 of thecuparestilseparated by a wideinterval. The outerlaminaconsists of two or three irregular layers of nuclei and contains traces of pigment. The lamina inversa is greatly thickened and has become differentiated into two layers, an outer radial layer of protoplasmic strands and nuclei resemblingtheradiallayer of the neuralwall, andan innerfibrelayeralso radialbutcell-freeexcept for a few nuclei scattered in each section. The edge of the cup (fig. 5) is closely apposed to the surface epithelium dorsoanteriorly(ixe.relatively to the longaxis of the embryo). Besidesthe mesodermalcellssurounding~theblood-vesselsandthoseflowing, over the edges of the cup, the antrum cupule contains scattered cells which 'may have been derived from these lattersources, or, as pointedout by'Lieberkuhn, from the migrating (2)cells of the otherwise cell-free inner zone of the laminainversa. The chorioidfissureisopenalongitswholelength, but the distal ends of the fissure come very close together on the right side.

The lens vesicle is quite closed, its external contour being complete. Itscavitycontainsdegeneratingcellslyingfreeandpartlyclumpedtogether. Its posterior wall is thickened and differentiated into an outer cellular and an inner radial cell-free layer indicating a commencing fibre formation. it is stilincontactwiththesurfaceepitheliumonbothsides. Ingeneral ithas a circular outline in the sections, but has the appearance of being compressed between the lamina inversa retinas and the surface. This is probably due in part at least to the manipulation of fixing and embedding. At the level of contact with the surface epithelium the lens anlage has a flattened pear-shaped outline, the narrower end of the pear causing aprotrusion of the surface. The surfaceepitheliumnisone-layeredoverthe point of closure in a few sections of the left side, and consists of cubical cells; elsewhere on this side and entirely on the right it is two-layered. The epithelial layers, however, undergo a modification in two respects in the immediate neighbourhood of the point of closure: mitotic figures become more frequent in both layers, and the nuclei of the outer layer lose their typical flattened form and assume the round form seen in the cubical cells of the stratumgerminativum. it is notdefinitelyascertainable whether the single layer above referred to, and which Elze describes for both sides of the 7-mm. embryo, is to be regarded as a continuation of the outerorthe inner layer . The impressionsuggested by a study of the modification of the surrounding epithelium is that it rather represents both layers. Inotherwords, theprocessofclosuremightberegardedasconsisting of a primary growing over of one layer, viz. the superficial layer (Elze), followed later by the other layer; or we might, with greater probability, premise a resolution of the two-layered epithelium into its primary one-layeredformatits"growing-point." Whentheclosurehasbeen effected by the active growing over of this single layer, the latter then differentiates once more into its two layers.

The a. hyaloidea is represented by a large sinus-like blood space lying in the anterior part of the chorioid fissure, projecting into the antrum cupuleandcomingintocontactwiththelensvesicle. it is connected distally with a capillary plexus around the edge of the optic cup, and proximally with a capillary system lying along the pedicle; and through thisapparentlywiththecapillaryvessels of the maxillaryprocess. An arterial branch, to be later referred to, springs from the a. carotis interna over the optic stalk (a. ophthalmica), but there is no connexion present between this vessel and the anlage of the a. hyaloidea except an indirect one through the capillary plexus on the outer face of the optic cup.

The Ear

The anlage of the labyrinthlieslateral to the fourthneuromere of the hind-brain. The ductusendolymphaticus(fig.2)isabout500Muin length on the left side, and on the right 700u, the disparity in length being probably largely apparent and due to the obliquity of the plane of section. Itopensin to the oticvesicleproperonitsmedialaspectabout 130MAbelowthetip of the later. in its upperhalfitshowsaroughlytriangularoutlineoncross-section, theepitheliumatthe anterior and posterior angles being markedly thickened, forming sharp ridges on its external aspect. Thisoccurstoalessextentattheouterangle. in its lowerhalf theanglesbecomeroundedoffandtheoutlineoval. The tipandmostof the outer wall consist of a single layer of epithelium, the inner wall, like therest of the labyrinthineanlage, being2to3cellsthick. Intheoticvesicle itself a lower, more tubular cochlear segment can be distinguished from an upper vestibular portion, in which the anlagen of the anterior and posteriorsemicircularcanalsareindicated by a pouching of the wall. A bulging of the lateralwall somewhat lower down suggests a commencing externalcanal. The lowerend of the vesicleiswidelyseparated from the firstpharyngealpouch, whichliesventrallyandlaterally. Intheintervening mesoderm runs the a.carotis interna with the dorsal root of the incompletesecondaorticarch. Intheprimitivetympaniccavitythereis an anterior and posterior tympanic recess separated by a groove, the groove for the tensor tympani, corresponding with Hamnmar's model of an 8-mm. embryo (fig. p. 274 of Keibel and Mall's Text-book). From the anterior tympanic recess a short groove is continued downwards and forwards on theroof of the pharynx. The relation of the n.chordatympani to the tympanic cavity has been already referred to in connexion with that nerve.

The Alimentary Canal and its Derivatives

On the floor of the pharynx are present the tuberculum impar, with immediately lateral to it on either side a swelling, situated on the ventral end of the firstvisceralarch. The copula is well marked, and is continuous back into high arytenoid folds. The hypophysis has a wide connexion with the roof of the pharynx, and is closely applied to the floor of the diencephalon(fig.5). it is flattened dorso-ventrally and shows a slit-like lumen. Near its distal extremity ends the chorda dorsalis, from which it is separated only by a small interval. Its apex is bifurcated and is continued into two short solid horns.

The first pharyngeal pouch presents nothing for description further thanhasbeenmentionedundertheprimitivetympaniccavity. The second epidermal cleft runs very obliquely forwards as well as in a medial direction, andiscontinuedintoa well-markedductusbranchialisI.(Rabi) into which, as has already been noted, opens the placode in connexion with N.IX. The thirdpharyngealpouchismuchsmallerthanthetwopreceding, and has a short dorsal and a long ventral process, the lumen throughout being reduced to a mere slit (P1. I. fig. 4) by the thickness of its walls. Its closing membrane is situated at the bottom of the sinus cervicalis. The pouch is connected with the pharynx by a stil widely open ductus pharyngo-branchialisIII. The differentiation of the anlagen of the epithelial bodies and thymus is well advanced, and agrees with the descriptionsofGroschuff;Kohn, andMaximow. The ventralprolongation of the pouch-or thymic anlage proper-is continued ventrally and medially, at first dorso-posterior to the third aortic arch, to end close to the thyroid anlage (fig. 6 ) onthelateralaspect of the truncusarteriosus. Itswall consists of several layers of densely packed cells, the posterior wall being thickerthantheanterior. Thisthickeningiscontinuedon to the posterior anddorsalwall of the pouchitself. The anteriorandouterwalls of the latterhavequiteadifferenthistologicalstructure. PI.I.fig.4isa drawingofahorizontalsectionthroughaportion of the pouch. A lumen ispracticallynon-existent. The posteriorwallshowstheclosearrangement of the nuclei in the thymic thickening, while the loose arrangement in the anteriorwalloffersamarkedcontrast. Here the smallerbutmoredarkly staining nucleiarestuddedthroughapale (chromophobe)vacuolatedprotoplasmicsyncytiuminanirregularmanner. Thisportion of the wallof the pouch constitutes the anlage of the "epithelial body III." (Kohn) or parathyroid.

The " caudal pharyngeal pouch complex "-to use the term suggested by Grosser-consists of a relatively involuted fourth pouch with short dorsal andventralpockets. The anteriorwall of the pouchandtheouterwall of the ventral pocket show the same histological differentiation as has been above described for the epithelial body I., and constitute the anlage of theepithelialbodyIV. The reisnodefinite"thymic"thickeningpresent asinthecase of the thirdpouch. The reappearsfurtherinconnexion with this pouch a comparatively long ventro-caudal tubular process at first sight similar to that of the third pouch, but a closer examination of the sectionsshowsthattheresemblanceismerelysuperficial. The process underconsiderationisnotadirectcontinuation of the ventralpocket, which is itself small, but it arises more dorsally and laterally from the body of thepouch. Furthermore, adistinctionisindicatedbyitsrelation to the aortic arches; for whereas the ventral prolongation of the third pouch is in close dorso-posterior relation to the third arch, the structure in question isseparated by a wideinterval from the fourtharchandisincloseanteroventralrelation to the sixtharch(fig.7). Thisarrangementsuggeststhe former presence and later disappearance of such a vessel as Elze (7 mm.) describes as running " close to the ventral wall of the postbranchial body, " namely, the rudimentary fifth aortic arch, which vessel in our embryo is representedonlyatitsextremedorsalend. The structureunderdescription is to be regarded, therefore, as the derivative of a fifth pouch or "ultimobranchial body" of Tandler. Its ventral extremity approaches the pericardium closely (fig. 7), and lies about 300kA caudal and lateral to the thyroidanlage.

The thyroid anlage, as shown in fig. 6, is bilobed, with a dorsal and a lateralsproutfromeachlobe. Alumenispresentintheproximalpartof theleftlobe. The componentcells, ofwhichthenucleiarelarge, round, and darkly staining, are closely packed together and show a commencing columnformation. The distalend of the thyro-glossalductcanbefollowed proximally through nine or ten sections (lOMA) as a solid epithelial cord. The anlageasawholeliesintheapex of the V formedbythethirdaortic arches overlapping these vessels and the bifurcation of the truncus arteriosus.

The Larynx and Lungs

The arytenoid folds are well developed. The lumen of the laryngeal cleft is to a large extent obliterated by a fusion of its walls (fig. 7), but orally a small lumen remains, while posteriorly there isaslit-likecommunicationbetweenCesophagusandtrachea. The lumen of the trachea, at first merely a cleft like that of the larynx, soon broadens intoanoval, andlaterpresentsacircularappearance. Furthercaudally its form varies in cross-section front a circle to an oval, with the long axis eitherlateralorantero-posterior. The bifurcationoccursinthesixth cervicalsegment. The conditionofdifferentiation of the pulmonarysacs, as ascertained from an antero-posterior graphic reconstruction, shows an advance on Narath's figure for a 7-mm. embryo (fig. 342, Keibel and Mall), andonElze'smodel(ca.7mm.)(cf.figs.10-13and1.5). The left stem bronchus has a large lateral bud directed somewhat dorsally (fig. 11) -the "firstventralbud"ofAeby, or"lateral"budofHis-and ends in a swollenbutundifferentiatedflask-shapedextremity. from the rightstem bronchus spring an apical or eparterial bronchus (fig. 12) and a first ventral bronchus bud (fig. 13), the latter of which is somewhat constricted at its origin. Immediatelycaudal to the origin of the ventralbronchusonthis side, the stem bronchus, as can be seen in fig. 13, shows an expansion in a dorsal direction, indicating the formation of a dorsal bronchus bud at this point. The mesodermallunganlageshowsacorrespondinglobingonits outer surface. The relation of the pulmonary arteryto the primary bronchi and their branches is described below (cf. Pulmonary Artery).

The esophagus is at first crescentic in outline, with the concavity directed ventrally; lower down it becomes either circular or eliptical. The wall consists of three or four layers of nuclei grouped for the most part in the middle thickness, leaving an outer basal layer free from nuclei, and an inner layer with few nuclei but with relatively numerous mitotic figures. The epithelial tube is surrounded and closely invested by condensed mesenchyme with large nuclei, many of which are spindle-shaped, arranged concentrically in layers. This mesenchymal condensation is best marked below at the level of the bifurcation of the -trachea, being absent at its upper end, and constitutes the circular esophageal pre-muscle mass. It is covered externally by a continuous nerve-plexus layer, already described as composed of small masses of sympathetic tissue and branches from both vagus nerves(figs.7-10).

The torsion of the stomach is not yet complete (cf. figs. 13, 15, 16, and 17). The widely expanded fundus is blown out to the left, andtheorgan asa whole is arched out well to the left of the plane of the middle line, while the pylorus and beginning of the duodenum are situated in that plane. The concentric "pre-muscle" arrangement of the investing mesoderm, seen in the lower end of the Esophagus, ceases at the cardia. In the root of*the mesentery in this region are clumps of syncytial sympathetic tissue and similar but smaller ganglionic masses are scattered dorsaltoand to the right of the cardiacend of the stomach. No connexions with the sympathetic or vagal fibres are to be observed. The blood-vessels of the stomach are represented by the a. gastrica sinistra, thesplenicplexus(of.below), and the small vein which runs from the fundus through the lesser omentum to join the ductus Arantii.

The duodenum has a patent lumen throughout. The intestinaltube from the pylorus to the apex of the intestinal loop runs somewhat to the right of the plane of the middleline(fig.19). The intestinalloophas undergone a torsion of about 1100, so that the distal limb lies well oral in respect to the proximallimb(figs.A and19). The apex is situated in a funnel-shaped pouch of the ccelominthebase of the umbilicalcord. A short distance proximal to this point the already slender intestinal tube becomes stilfurtherreduced indiameter, lesssharplydefinedagainstthe surrounding mesoderm, and loses its lumen through four sections (in positionmarked + in fig. A). The vitello-intestinal duct has separated, but remains of that structure appear in three distinct places in the sections , in each case through a few sections only, as a slender column of degenerating cells lying free in the ccelom of the cord and surrounded by an aggregation of blood cells representing the extravasated blood of the omphalo-mesenteric artery(cf.fig.17). The distal limb of the loop increases greatly in diameter from the apex to a short distance beyond the cwcum, beyond which it narrows again and is continued caudally in the middle line as large intestine and rectum. The connexion between the latter and the rest of the cloaca-"cloacal duct" of Reichel-is narrowed dorso-ventrally by an extensive development of the urorectal septum (figs. A and 20), and laterally by low broad vertical ridges of mesoderm continued down from the lateral parts of the septum and swelling towards each other from either side. The duct is further constricted below by a transverse ridge from the inner face of the cloacal membrane, and has a dorso-ventral diameter of 130, u. A caudal intestine is present as a small blind finger-like pouch opening into the aboral part of the rectal cloacaand continued back as a solid cord of endothelial cells through nine sections (101). Further caudally and in line with this column a small interrupted epithelial "rest" appears (fig.A). The allantoiscomes of the anterior pole of the cloaca at a sharp angle, as is seen in fig. A, the transition from the latter organ to the former being a sudden one and not agradualmergingofonein to the other(fig.20). At first the allantois consists of a tube of small calibre which enters the root of the body-stalk and gradually insinuates itself between the umbilical arteries (figs. 20, 19, and17). In this position it soon expands into adorso-ventrally flattened vesicle(fig.17), and later, still increasing in calibre, it assumes a roughly circular outline in cross-section. In the distal part of the cord it branches so that in several sections a group of two, three, or more allantoic vesicles istobeseen. The allantois is lined throughout by a single layer of cells: at its origin from the cloaca and in its early tubular portion in the base of the body-stalk the cells are cubical, with pale protoplasmic cel bodies and roundednuclei;moredistally, where the tube expands and becomes vesicular, the cells become flattened and drawn out and the structure as a whole less sharply demarcated against the surrounding mesoderm.

Liver, Pancreas, Spleen, and Caecum

The entodermal liver tissue extends on the left side as the left coronary appendage dorsally and orally as far as the ventral pillar of the pleuroperitoneal membrane, in the substance of the Apericardio-peritoneal membrane"(fig.12). from the latterstructureitbulgeslaterallyin to the antero-lateral recess and appears anteriorly in a few sections in a fold which is attached to the lateral aspect of the pleuro-peritoneal membrane (fig.11). Caudo-ventrallyitliesembeddedinthesubstance of the mesoderm of the dorsal pericardial wall (septum transversum) and bulges into theperitonealcavity, coming almost into contact with the lateral abdominal wall. Further ventrally, havingjoinedtherightlobe, itreachesandfuses with the anterior abdominal wall caudal to the pericardium (compare figs.12, 13, 15, and16). Posteriorlytheleftlobeappearsasawing-like expansion of the right lobe (fig. 16), demarcated from the latter ventrally by the groove for the left umbilical vein, which here runs in the middle linetojointheductusArantii. The rightlobe, thoughexceedingtheleft inbulk, doesnotextendsofardorsallyandorally;itscoronaryappendageis lessmarkedandfailstoreachthepleuro-peritonealmembrane(fig.13). It extends, however, much further caudally, [3] frees itself from the dorsal mesentery, and bounds orally and ventrally the peritoneal cleft which connectsthebursaomentaliswiththegreaterperitonealcavity-thehiatus communis recessuurn or future foramen epiploicum (cf. fig. 17 and below). The gall-bladder(PlateI.fig.5)liesina deepwidegroove of the liver, filedwith dense mesenchyme which separatesit from the liversubstance. Proximally the cystic duct becomes free from the liver, the mesoderm surrounding it causing a slight swelling under the peritoneum, and just beforeitsjunctionwiththehepaticductalumenappearswithinit. At this point the hepatic duct itself also acquires a lumen which is partially double. The common ductcontainsalumen in its wholeextent, and near its junction with the duodenum gives off the ventral pancreas anlage (PlateI.fig.5, andfig.18). The commonductatitsorigin from the duodenumisatfirstdirecteddorsally, to the rightandorally. After givingofftheventralpancreasittwistsonitselfand runs orallyventrally and to the right, the cystic duct continuing this direction, while the hepatic ducttakesanalmostdirectlyoralcourse(PlateI.fig.5). The latter continues orally ventral to the omphalo-mesenteric vein (fig. 17) and soon becomes embedded in the liver, where its outline against the liver tissue proper becomes in many sections irregular and indistinct, the appearances suggestingthegivingoffoflivertrabeculse. Itcanbetraced, however, somedistanceinthesubstance of the liverasanirregularcolumn of cells, smaller, darker, and more denselypackedthanthesurroundinglivercells, until finally it loses its identity by merging gradually into normal liver tissue. The cells of the livertrabeculeeconsistoflargepolyhedralcells with dense granular protoplasm fitting close together and containing a large round, more lightly staining nucleus with usually two or three dark nucleoli. The endothelium of the numerousbloodspacescloselyinvests thetrabeculae, sothataperivascularspaceisseldomapparent. Typical "blood-forming" cellsareabsent, althoughacertainnumberofdarkly stainingcells, withlitleprotoplasm andmuchsmallerthanthenormal liver cel, occur scattered through the trabeculae and show a general resemblancetoordinarymesodermcells. Afewoccursinglyimmediately beneath the endotheliumn.


A wax model has been prepared of the dorsal and ventral pancreas anlagen, the hepatic ducts and a portion of the left lobe of the liver and omphalo-mesenteric vein. A drawing of the model is reproduced in Plate I. fig. 5. The ventral pancreas anlage is in the form of a lobulated sessile knob attached to the common bile duct close to its origin from the duodenum. The lobules contain wide-open longitudinally running lumina (fig. 18), with a general arrangement of one central lumen with a lateral lumen on either side. The central lumen opens in to the bile duct, and its major portion is situated caudal to this communication. In the case of the lateral lumina no direct connexion either with the central lumen or with the bile duct could be made out, but the histological picture presented by the endothelial walls suggests an opening in both cases in to the centrallumenatitsjunctionwiththebileduct. Owingto the torsion of the proximal portion of the bile duct around the duodenum the ventral pancreas is directed dorsally and somewhat to the right and abuts against the vena omphalo-mesenterica, the wall of which it indents (fig.18). it is separated from the proximalportion of the dorsalanlage only by a smallinterval. The dorsal pancreassprings from the duodenum dorsally and to the left side immediately proximal to the entrance of the bileduct. it is lobulatedliketheventralpancreasandismuchlarger, and lies in the dorsal mesogastriumn with its long axis almost dorsoventral. The vena omphalo-mesenterica, in its spiral course dorsally and orally around the left aspect of the intestinal tube, is in close relation to the anlage grooving it on its right side and caudal face (PlateI.fig.5, and fig.18). Inthelatterpositionthegrooveisespeciallydeep, owingmainly to the fact that a process of pancreatic tissue is continued caudally in the interval between the vein and the duodenum, lying close against the former (fig. 18). This process is separated from the duodenum by a deepnotch. The anlagecontainsirregularluminathroughout, including the above-mentioned caudal process, in which one of the lumina is relativelylarge. The proximalportion of the lumen, orcommonduct, enters the duodenum in an obliquely oral direction, so that the latter appears in this position to have a double lumen through a few sections.


The anlage of the spleen is represented by a thickening of the celomic epithelium over a relatively large area of the dorsal and lateral aspects of the left leaf of the dorsal mnesogastrium, with proliferationofitselements(figs.17and18). The thickeningisbestmarkedat the level of the origin of the dorsal pancreas, and results in the production of an epithelium several layers thick and consisting of a somewhat chromophobe protoplasm in which numbers of small round nuclei are set. It is demarcated from the underlying mesoderm by a layer of this lightly stainingprotoplasmwithfewnuclei. The area of the mesogastriumcorresponding with the epithelial thickening projects from the surface and, where the proliferation is best marked, is notched on its ceelomic aspect. The underlying mnesoderin, which shows no special differentiation except for the presence of numerous mitotic figures, contains a plexus of bloodvessels which is connected on the one hand with the omphalo-mesenteric vein caudal to the dorsal pancreas, and on the other with the left gastric branch of the coeliacarteryvideeinfra).

The Caecal Region

Fig. 6 on Plate a drawing of a wax model of the apex and major portion of the intestinal loop, with a part of the superiormesentericartery. Onthedistallimb of the loopashortdistance from the apexisanampullaryswelling. from the lateralaspect of the ampulla springs a hollow diverticulum in the form of a short blunt cone directedorallyandlateraly. The diverticulumshowsnodistinctioninto basal and apical portions except in so far as its lumen may be divided intoanampullarysectionandadiverticularsectionproper. The ileum and colon are not quite in the same straight line, the latter being displaced somewhat laterally at its commencement relative to the former. This relation of these two portions of the tube can be observed from the outer aspect, but is better marked as regards the lumen, so that the arrangement is best described thus:-the lumen of the colon followed orally divides as into the two limbs of a Y, one limb being continued into the ileum, the otherendingblindlyattheapex of the diverticulum. Atthejunctionof thelimbs of the Ythelumenundergoesenlargement. The ileumisthus inserted obliquely into the tube formed by the appendix and colon taken together. Immediatelyproximal to the appendixisasecondsmallerand solidepithelialappendage of the intestinaltube(PlateI.fig.6). Ithas a similar direction as the former structure, i.e.orally along the ileum and laterally, and, though possessingnolumen, itscells, especially in its basal portion, arearrangedregularlyaroundapotentialcentralcleft. Proximally it is partially embedded in the wall of the ileum, while its base winds round to the oralanddorsalaspect of the intestinaltube. The exactnature ofthisdecidedlyabnormalstructureisnotapparent. Itspositionwith respect to the apex of the intestinal loop on the one hand, and the appendix on the other, would seem to rule out the possibility of its being a Meckel diverticulum, but it must be remembered that our knowledge of thegrowthrelations of the variousorgansisveryincomplete. The lack of accurate observations in this direction, and especially in the case of the alimentary canal, has been recently emphasised by the work of von Berenberg-Gossler, who proves conclusively that the whole of the rectum andcolonasfarasthecoacummustarise from the cloaca. Inthepresent case the only safe criterion-namely, the relation of the structure to associated blood-vessels-is impossible of application, so that the question isincapableofdefinitesolution. Anotherpossibleinterpretation of the abnormality is that it is of the nature of those intestinal diverticula which have been described in the literature as occurring not only in the neighbourhood of the hepatic outgrowth but along the whole length of the ileum, and which have been generally regarded as pancreatic in nature. In this connexion it is interesting to note the observation of Wright, who finds an accessory pancreas in Meckel's diverticulum near the umbilicus.

Heart and Blood-Vessels

The sinus venosus receives the two ducts of Cuvier and the v. hepatica revehenscommunis. Itsslit-likeopeningin to the atriumisboundedby twowell-developedvenousvalves. The leftvalvepassesbelowtowards the left to end near the distal extremity of the dorsal prolongation of septumI. Orallyitfuseswiththerightvenousvalvetoformthevenous septum (septum spurium). A few sections cephalad from the point of fusion of the two valves a septum detaches itself from the left aspect of the venous septum near its base, and, rapidly increasing in height, passes obliquely over the posterior wall of the spatiumn septo-valvulare to end nearthesmallseptumI. Thispartofitscourseismarkedontheouter surface of the heart by a distinct groove (fig. 9). This septum would seem to be a continuation of the left valve, as though the latter had again parted company with the venous septum, but that it is in reality of such a nature cannot be definitely stated owing to the complete fusion of the twovalvesimmediatelybelow. Septum I. is welldeveloped(fig.9), its free border being thickened and covered by a thickened endocardium. Its dorsal and ventral prolongations can scarcely be said to actually reach the endocardial cushions of the auricular canal; the dorsal one approaches its associated cushion very closely, but the ventral horn fades away into the ventral wall of the atrium before reaching the ventral cushion. The attachedbase of the septumshowstwosmallperforationsanearlyforamenovaleI. A commencingseptumI.ispresentasashort low ridge of myocardium to the right of the upper part of septum I. (fig.9). The interventricularseptumisstillow. Inthebulbusarteriosus proximal and distal bulbar swellings are well marked, and by their union with one another have brought about a partial separation of the bulbus into truncus aorticus and truncus pulmonalis. in fig. 14arereproduced tracings of six sections at various levels through the bulbus which exhibit themainfeatures of the processatthisstage. The proximalbulbar swellings A and B (Tandler) have the classical arrangement, the former beginning on the anterior wall of the bulbar portion of the ventricle and running distally on to the posterior wall of the bulbus, the latter passing from the posteriorwallproximally to the anteriorwalldistally. of the distal bulbar swellings described by Tandler in his model of the bulbus of the embryo H6, and numbered 1 to 4, two, viz. 1 and 3, can be identified in the sections, and have produced the distal bulbar septum by uniting across the bulbar lumen, dividing it at this point into pulmonaryandaorticconstituents. The bulbarswellingAreachesthedistal bulbar swelling 1 (fig. 14a), while swelling B fuses with swelling 3-that is, withtheanteriorpillar of the distalbulbarseptum. Further, ablunt wedge of mesoderm juts into the bulbar lumen from an oral and dorsal aspect, so that both sixth aortic arches arise by a short common stem (figs.7and14). Thisstructure, theseptumaortico-pulmonale, reaches the distal bulbar septum only through its dorsal and ventral prolongations, leaving between the edges of the two septa a foramen of communication betweentheaorticandpulmonarytrunci.

Three complete aortic arches are present (fig. B). A pulmonary artery arises on each side from the sixtharchnearitsventralorigin. Eachlies at first ventro-lateral to the trachea, passing dorsally later, the left before theright. The lattervesselrunscaudallyventral to the apical(eparterial) bronchus bud (vide Lung, and fig. 12) and then dorsal to the first ventral bud, as His has shown for a'105-mm. human embryo, and Flint for the pig. The leftarterypassesbackdorsal to the firstventralbud of the right side, and both arteries end by breaking up into a capillary plexus on the dorsal aspect of the flask-shaped extremity of the stem bronchus.

Into the lateral aspect of the dorsal extremity of the sixth arch on the left side opens a very small vessel, which can, however, be followed through afewsections This stump is in the position of the originaldorsaltermination of the fifth arch, and corresponds with the findings of various observers, viz.Elzeina7-mm.embryo. There can be little doubtthat this structure represents the extreme phase of the involution of this vessel.

The truncus arteriosus bifurcates anteriorly into the third aortic arches. From the commencement of the third arch on either side a small vessel runs forwards beneath the pharynx-the external carotid. It becomes lost at the level of the first cleft and cannot be followed into the first visceralarch. The onlydefinitebranchwhichitgivesoffisonepassing outwardsanddownwardsasthesecondarch-thea.lingualis. The reis, however, also in the arch a short upwardly directed vessel which might perhapsberegardedastheventralend of the secondaorticarch. From the dorsal extremity of the third arch the dorsal aorta passes forward as theinternalcarotidartery. Immediatelybehindthelevel of the otic vesicle it gives off a comparatively large branch which is directed backwardsanddownwardsin to the secondvisceralarch. Ontherightside thisvesselterminatesbydividingintotwobranches, bothdirectedventrally. This vessel must be identified as the dorsal part of the second arch, or, afterTandler, theroot of the stapedialartery. The a.carotisarchespast the hypophysis, and as it passes over the optic cup it gives of a small ophthalmicbranch. Thisvesselontheleftsidesoondividesintotwo vbranches, one of which can be traced to a capillary plexus on the upper pole of the retinalanlage. Ontherightside, wherethearteryisina less contracted condition and the plane of section at the same time more favourable, the ophthalmic artery can be seen to pass out laterally over the optic cup and to connect with a plexus surrounding the edge of the cup andespeciallyitstemporalrim. Thisplexusisconnectedbelowwith the anlage of the a. hyaloidea through a relatively large branch which passesoutthroughthedistalend of the chorioidfissure(videEye). The a. hyaloidea itself is represented by a large sinus-like and plexiform vessel lying in the anterior part of the chorioid fissure and projecting into the antrumcupulhesoastoabutagainstthelensvesicle. Tracedproximally, the artery, besides connecting with a capillary plexus lying along the caudo-temporal aspect of the pedicle, is found to give off a short, stout branch which passes dorsally around the caudo-temporal aspect of the pedicleatthe"insertion" of the latterin to the cup. it is questionable whether this vessel represents the true proximal continuation of the a. hyaloidea or, as would seem more probable from the observations of Dedekind, Versari, andFuchsonlaterstages, thea.ciliarisnasalis. The connexion of the a. hyaloidea with the ophthalmic branch of the a. carotis isthusaveryindirectone. A similarindependenceofthisarteryisnoted by Dedekind (6 mm.) and by Elze (7 mm.), and the condition receives an explanation in the findings of Fuchs in the rabbit, in which mammal he derives the a. hyaloidea from a marginal vessel encircling the edge of theopticcup.

Aftergivingofftheophthalmicartery, thecarotidtofollowthe interpretation of de Vriese-divides into an anterior and a posterior branch. The former sweeps round the optic anlage, giving ofto the side of the fore-brain the anterior chorioidal artery, as described by Mall for 9 mm., and continuing ventrally internal to the nasal pit it breaks up into a capillaryplexusovertheventralsurface of the fore-brain. Throughthe plexus there is probably an anastomosis between the arteries of either side. Asthearterypassesinternal to the olfactoryepitheliumitgives itabranch(fig.8)seenonbothsides. The posteriorterminalbranchof the carotid curves dorsally and backwards beneath the mid-brain and uniteswithitsfellow(inpositionmarked+ in fig. B)toformthea.basilaris, butbeforedoingsoitgivesoftwo strongbranches to the floorand sides of the mid-brain. The latter together constitute the posterior cerebral group (Mall).

The arteria vertebralis cervicalis arises as the first cervical dorsal segmental artery opposite the dorsal extremity of the sixth aortic arch. It gives branches to the medullary tube identical with those of the segmental arteries generally (vide infra), but continues forward ventrolateral to the hind-brain to unite with its fellow (in position x in fig. B) to formthebasilarartery. from the vertebralandbasilararteriesnumerous strong twigs ascend on the side wall of the hind-brain, many passing between the individual rootlets of N. XII. The most marked member of the series supplies the area immediately behind the isthmus, i.e. the cerebellarregion. The sevesselsareunitedwithoneanother by a n irregular longitudinal anastomosis.

The roots of the dorsal aorta unite at the level of the 8th cervical segment(fig.B, +). Atthelevel of the 5thlumbarsegment, aftergiving offtheaa.umbilicales, it is continuedasthecaudalaorta to the caudal tip. In several parts of its course the latter appears as two vessels of varying relative calibre, and at more than one level assumes a plexiform arrangement with " Inselbildung, " so that the aorta appears at these points in the sectionsasthreeormoresmallvesselsgroupedtogether. Evenwherea single vessel is present it often shows strands of mesoderm traversing its lumen, so that we have here a well-marked instance of "several paths being used by the arterial blood before the reduction to a single vessel" (Evans).

Thirty-two dorsal segmental arteries are present, viz.twenty-five from the aortaproperandseven from the caudalaorta. Distally to the last of the caudal segmentals a few fine capillary vessels are directed dorsally towards theneuraltube. Inthemoreanteriormembers of the seriesaltheprincipalbranchesofsupply to the spinalcordhavebeenlaiddown. Each artery runs through the mesoderinal intervertebral foramen in company with the vein and anterior to the corresponding nerve, and at the level of the junction of anterior and posterior spinal nerve roots divides into three main branches: (a) towards the mid-ventral line of the cord-a. radicularis ventralis (figs. 5 and 15); (b) a branch passing between the dorsal ganglia and the wall of the neural tube and extending as far dorsally as the point of entry of the dorsal rootsthe a. radicularis dorsalis (fig. 15); and (c) a branchdirectedlaterallytowardsthemyotome. The a.radicularisventralis when it reaches the lateral edge of the Bodenplatte penetrates the substance of the tube, as the a. centralis, and passes dorsally between the differentiating anterior horn anlage and the undifferentiated radial layer (videSpinalCord), helpingtodemarcatethetwoareas(fig.3). Between the individual aa. rad. vent. as they turn into the spinal cord a longitudinal anastomosisformstheprimitiveanteriorspinalartery. The a.radicularis dorsalis can in many cases be seen to give a branch into the neural tube atthelevel of the dorsalextremity of the anteriorhorn. The branch(c) is present only in the case of the most anterior members of the series, viz the first segmental on the right side, which sends a large vessel laterally againstthemyotome. from the lattersmallbranchesarecontinued through the myotome accompanied by venous vessels, while to the outer side of the inyotoinenumerousfinecapillariesappear. Wemaythusspeak of a penetrating "ramus cutaneus dorsalis inedialis" (Evans).[4] The re is no traceable longitudinal anastomosis between the cervical dorsal segmentalstoindicateacervicalvertebralsystem. The firstthreearteries, however, show a remarkable diminution in their lumina as they approach theaorta, sothatattheiroriginstheyarelitlemore than capillaryinsize. As we go caudally the proximal portions of these vessels increase in importance, and this fact, taken in conjunction with the well-developed condition of the spinal branches of the most anterior members, must be regarded as indicating an involution of the aortic roots of these latter vessels.

Ventral Branches of the Aorta

The celiac artery springs from the aortabytworootsoppositethe5ththoracicdorsalsegmental. from the loop between the two roots a small vessel passes ventrally and orally into the left leaf of the dorsal mesogastrium-a gastrica sinistra-and connects at the level of the pylorus with a relatively large plexiform vessel lying in themesodermbeneaththesplenicanlage-a.lienalis(fig.17). Thisplexus, as has already been mentioned (vide Spleen), drains into the v. omphalomesenterica caudal to the dorsal pancreas (fig. 18).

The superior mesenteric artery arises opposite the 7th and 8th thoracic dorsalsegmentals by a smalleranteriorandalargerposteriorroot. It runs into the mesentery to the left of the intestinal tube and between the two limbs of the intestinal loop towards the caudally directed (originally right)face of the mesentery(PlateI.fig.6, andfigs.19and20). Itthen passes towards the right across the caudal aspect of the proximal limb of theloop. Asitreachesthesurface of the mesenteryitstilpossessesa relatively large calibre, but at this point the wall of the artery has been torn in the narrow interval between the mesentery and the lateral coelomic epithelium of the cord so that it cannot be followed distal to this point as adistinctvessel. The reis, however, acontinuous"clot"ofbloodcells traceable through about sixty sections and contained in a gutter of parietal coelom without any definite wall (fig. 17). That this collection of cells can be identified as the extra-embryonic portion of the artery is more than doubtful, and it is more probable that it is the result of a tearing of the vessel and an outpouring of its blood as the result of interference with thecordbeforefixing. it is , however, significantthattheremains of the vitello-intestinalductisfoundamongthesecells. The inferiorinesenteric artery is small, and arises opposite the 2nd lumbar dorsal segmental by two mainrootssomewhatconfusedby"Inselbildung." Itrunsthroughabout fifteensections(10Mx)beforebecominglostinacapillaryplexus. Itgives aWolffianbranch to the mesonephrosvideeinfra).

The umbilical arteries arise in the 5th lumbar segment and encircle the posterior end of the celom lateral to the Wolffian duct as the latter runs inthecaudalend of the Wolffianmesentery. A shortdistance from it s origin(fig.B)eacharterygivesoff from it sposterioraspectthea.ischiadica, which possesses a smaller and a larger root and runs distally and laterally between the teeth of the serrated edge of the hind-limb nerve-plate to break up into a large indefinite plexus. The two aa. umbilicales then approach each other at the root of the body stalk, the allantois inserting itself gradually between them (figs. 20, 19, and 17), a position which it maintains in its coursethroughthecord. Nearthedistalend of the latter the two arteries anastomose and after a short course as a single vessel separateagainintotwo. The lumenofeacharteryislargerinthedistal than the proximal part of its course.

From theventralaspect of the caudalaortabetweenthelasttwodorsal segmentals a comparatively large branch runs on either side of the cloaca, just proximal to the cloacal membrane, and into the area oral and ventral to the urogenital section of the cloaca, the angle between it and the allantois(figs.AandB). Thisareaisdrained by a venousplexusforming ananastomosisbetweenthetwoumbilicalveins(cf.). Elzedescribesthis arterial branch in an embryo of 7 mum. (circa) as running " zur lateralen Seite des Enddarmes an dessen Vbergang in die Kloake, " but fails to find itinembryosof9-5mm.and11mm. The vesselinquestionwouldtherefore appear to undergo rapid development and equally rapid involution.

Wolifian Arteries

On the left side eighteen Wolffian arteries spring from the lateral aspect of the aorta; on the right side nineteen arteries come from the (samesourceandone from the inferiormesentericartery. The y arise irregularly and show no segmental arrangement, the 6th and 7th on the left side for instance arising by a short common stem, while many, particularly in the case of the more caudal members of the series, divide in theirdistalcourseintotwoandsupplyadjacentglomeruli(fig.21). The most anterior artery on the right side shows no apparent lumen, appearing merelyasacordofcells, whilethesecond hasadefinitelumenonlyat its entrancein to the glomerulus. The appearancesprovidedbythesearteries areprobablyassociatedwiththedegeneration of the correspondingWolffian tubules, whichcannot, however, bedefinitelydiagnosedinthetubulesthemselves. AsitenterstheglomeruluseachWolffianarteryshowsalocalized increaseincalibre. The moreanteriorvesselsareinverycloserelationto thedorsalaspect of the anlage of the suprarenalcortex.

The subclavian artery arches dorsally and laterally from its origin from the 7th dorsal segmental artery.[5] It perforates the base of the nerve plate between C. VI. and C. VII. nerves without previously giving off any branches. Appearingbetweenthedorsalandventralleaves of the plate, it continues for some distance as a definitely single large vessel on the dorso-lateral aspect of the ventral leaf-the "a. brachialis profunda" of Erik Muller (figs.12 and 15). Itgivesoffabranchwhichisdirectedalong the radial nerve andr apidly becomes plexiform. The profundatrunk itself then shows island formation without the production of a definite plexus, the lumen of the vessel being crossed by strands of mesoderm, while numerousshortsproutsaregivenoff from it . The vesselthendivides into two, one branch continuing along the median nerve, the other along the ulnar, and each remaining as a single trunk for a short distancebeforeappearingintheformofaplexus. The former is the larger of the two and constitutes the main continuation of the arterial trunk. Beforepenetratingthebrachialplexusthesubclavianarterygives offa branch which remains on the medial aspect of the ventral nerve-plate andshowsaplexiformarrangementatitsorigin. from the latterpoint this vessel sends a small but definite branch medially and ventrally towards the pleura (internal mammary ?), and continues ventro-caudally as a single trunk-" a.brachialis superficialis" of Muller-medial to the ventral nerveplate and rami pectorales (fig. 15, right side), to end by turning laterally around the caudal edge of the plate to join the artery accompanying the ulnarnerve. Ontheleftsideacompleteanastomosisbetweenthesetwo vessels cannot be traced owing to the fact that the artery accompanying the ulnar nerve is smaller on this side than on the right and breaks up earlierintoacapillaryplexus. Furthermore, thepictureissomewhat obscuredatthispointbysmallfoldsintheroot of the limb. The a.brachialissuperficialisgivesoffinthesame positiononbothsides, i.e.opposite C. VII. nerve, a small branch which pierces the base of the nerve-plate in a dorsaldirection. Ontheleftsidethisvesselendsbesidethesubclavian vein on the dorsal aspect of the plate.

The Veins

(fig. C).- The anterior cardinal sinus begins anterior to the Gasserian ganglion as the v. cap it is medialis, and is formed by the flowing together of two venous plexuses, one draining the side of the fore-brain and lying dorsal to the optic anlage, the other gathering its radicles from themid-brainandisthmusregion. AccordingtoMall, thesetwoplexuses together constitute the anterior cerebral vein. Both plexusesreachthe mid-dorsal line, and form here a more or less definite longitudinal system. Especially is this the case dorsal to the cerebral vesicles, where a fine venous vessel is present on either side of the middle line-the sinus sagittalis(fig.8). More posteriorly the arrangement becomes less regular. Receivingseveralsmallertributaries, the v.cap it is passes back internal to ganglion N. V. and here receives a large branch from the maxillary arch beneaththeopticcup. The latter before its junction with the main trunk isjoined by a veincoining from the hypophysis. Closely applied to the lateralsurface of the ganglionisa capillaryvenous plexusinconnexion anteriorly and posteriorly with the v. cap it is , providing a basis for the later-formedexternalvenousring. Immediately caudal to the ganglion the vein-now the v. cap it is lateralis-gets a double tributary from the anteriorpart of the hind-brain(middlecerebralvein, Mall), from which point it runs caudally external to N. VII. and N. VIII., the otic vesicle and N. IX. getting small branches from the first and second visceral arches on itscourse. BetweenN.IXandN.X.theposteriorcerebralveinentersit in the form of a large vessel from the posterior part of the hind-brain, and immediatelyposteriorlyisasmallerbranchbelonging to the same system. The v. cardinalis anterior then sweeps round the vago-accessory trunk altogether lateral to it, but quickly getting on to its dorsal aspect and separatingitfromN.XII. it is crossedlaterallybytheaccessorynerve, and lower down by the hypoglossus trunk and its c6, vical contributions.

Heretheveinbecomesmuch increasedincalibreandcomestolieoncemore lateral to the vagus. Alargevenousplexuslyinginthefloor of the mouth drains back across the ventral ends of the visceral arches towards the lateral aspect of the vagus trunk-the linguo-facial vein (Lewis). No definite connexion with the v. cardinalis anterior could be made out on eitherside. Asthelatterliesdorsal to the auricleitgivesoffinthe5th cervical segment the duct of Cuvier, relatively longer with a smaller diameter on the left side, shorter and wider on the right. This vessel immediately enters the pleuro-pericardial membrane and sweeps ventrolaterallytoenterthelateralhorn of the sinusvenosus(figs.9-12). Inthe latter part of its course the v. cardinalis anterior receives several dorsal segmentaltributaries. OppositetheductofCuvier, i.e.inthe5thcervical segment, it isjoined by a large vein which drains the post-axial border of the limb, its tip, and the distal part of its pre-axial border-the brachioulnartrunk. Astheprimitiveulnarveinentersthebody-wall from the post-axial part of the root of the limb it isjoined by a vein which drains, through superficial tributaries, the lateral wall of the thorax caudal to the arm-bud-thethoraco-epigastricveinofLewis. The trunkthusformed runs oralwards as the subclavian vein dorsal to the nerve-plate of the limb plexus (figs. 9, 12, 13, 15), hooks around the anterior end of the latter and the phrenic nerve, and isjoined before itstermination by a small vein from the proximal part of the pre-axial border of the limb.

The posterior cardinal sinus is formed at the level of the 1st sacral segmentbythejunction of the ischiadicvein from the post-axialborderof the hind-limb bud and the relatively small caudal vein from the tip of the tail. The caudalveinsremainseparate(figs.20and21), butareconnected witheachotherthroughanastomosesinfront of the caudalaorta. From this level the posterior cardinal vein runs oralwards, passing lateral to the origin of the umbilical artery and dorsal to the mesonephros, and receives thedorsalsegmentalveins. The subcardinalsystem, theextentofwhich is indicated in the reconstruction reproduced as fig. C, is present in a welldevelopedcondition. The v.cardinalisposteriorasitrunsdorsal to the Wolffianbodyreceivestwosetsoftributaries, amesialandalateral. The former are the larger and are best marked in the lower dorsal and lumbar segments, where they anastomose to form a definite longitudinal vessel, lying internal to the Wolffian body and on a plane well ventral to the posteriorcardinal-the(medial)subeardinalvein(fig.19). Atthehinder end of the mesonephros this vein is in connexion with its parent posterior cardinal trunk by a large anastomotic vessel (fig. 21), present on both sides, similar to that modelled by Felix in a 9-5-mm. embryo (fig. 574, Keibel and Mall), and forming in his description the posterior termination of the medialsubeardinal. The latterveinis, however, intheembryounder description, continuedcaudallytothisanastomosisinternal to the umbilical artery at its origin and dorsal to the mesenchyme of the metanephros to join the posterior cardinal vein just oral to the junction of ischiadic and caudalveins. The rearethustwodefinitechannelsbywhichtheblood coming from the posterior end of the body can enter the subeardinal vein. In the upper dorsal segments the mesial Wolffian tributaries do not anastomosetoformalongitudinalvessel, thoughmany of the mareT-shaped as described by Felix. The more anterior members of the series run through the anlage of the suprarenal cortex and take over the drainage of thisstructure(fig.18). Thisanteriordisconnectedportion of the subcardinal system is further related on the right side to the posterior extremity of the developing anlage of the inferior vena cava, to be later described. The lateral Wolffian tributaries drain dorsally between the individualcollectingtubules of the mesonephros. They also unite to form a lateral anastomosing vessel, which, however, fadesawaybehind. The medial and lateral vessels are united together by anastomoses, which can be seen passing between every two Bowman capsules, an arrangement whichsuggestsafunctioningmesonephricportalsystematthisstage. The subeardinal veins of either side have no connexion with each other across thefront of the aorta. of the veneeumbilicalestheleftisverylarge, and entering the body-wall from the umbilical cord, it receives, besides superficial tributaries from the body-wall, a larger vein coming from a caudal direction. This vein is presental soon the rightside, and both commence in a plexus which forms an anastomosis between the two and lies in the angle between the antero-ventral surface of the cloaca and the allantois. This vein therefore drains the area supplied by the cloacal branch of the caudal artery, which has already been described in connexion with the lattervessel. Strikingtheanteriorsurface of the liver, itgroovesthe ventral surface of that organ for some distance (fig. 16) before joining with thev.omphalo-mesentericaventro-lateral to the stomach. Beforeits termination it gives off a large vena advehens to the left lobe of the liver ramusangularis(Mall)-andasmallerone to the rightlobe. The vena umnbilicalis dextra has reached an advanced stage of involution and has no longer an independent existence, opening at both ends into the left vein, viz. in the umbilical cord and again immediately before the left vein strikes the liver (fig. 17). Like the latter vein it receives numerous tributaries from the body-wall.

The peripheral portion of the degenerated vitelline vein is present as a blind stump projecting freely into the coelom ventral to the mesentery, and provided with a coating of mesoderm (fig.19). Entering themesentery it encircles the intestinal tube on the left side, and as it winds round to get on to the dorsal surface of the latter it is joined at this point (i.e. in the position of the original dorsal or middle vitelline anastomosis) by the superiormesentericvein(fig.5, PlateI.). The latter beginsinafewsmall radiclesnearthetip of the intestinal loop and runs in the mesentery, keeping on a plane to the left of the small intestine and duodenum.

The omphalo-mesenteric vein formed by the junction of the vitelline remnant and the superior mesenteric vein winds on to the dorsal aspect of the duodenum, grooves the under surface and mesial aspect of the dorsal pancreas anlage, and applies itself so closely to the now dorsally directed pancreas that its wall is indented by that structure(fig.18). Continuing its spiral course, the vein plunges obliquely into the left face of the right lobe of the liver to the right of the stomach(PlateI.fig.5), and joins the v.umbilicalissinistra. Beforeitsjunctionwiththelatterveinitgivesoff into the right lobe of the liver a large vena advehens which immediately divides into two, one branch being directed ventrally-ramus arcuatus venmsport (figs.17, 16)- and the other dorsally and constituting the continuation of the main omphalo-mesenteric trunk or right vitelline vein (Mall). from the junction of the v.umbilical is sinistraandv.omphalomesenterica the wide ductus Arantii runs orally and dorsally through the liver (fig. 15), receiving at least one small tributary from the fundus 6f the stomach through the lesser omentum, as Broman described, to open into the v. hepatica revehens communis or caudal prolongation of the sinus venosus from the leftside. Thislatterreceivesalsothecommon opening of the rami dextra and sinistra vene hepaticae on its anterior aspect and to the right of the opening of the ductus Arantii, and on its right side the persistingterminalportion of the omphalo-mesentericvein(fig.15). The vena hepatica revehens communis is continued caudally into liver sinusoids lying in the dorsal margin of the liver, and through these is in connexion with adefinitevenousvesselwhichrunscaudalwardsintheborderlinebetween the liver tissue and the mesoderm bounding it dorsally, to the right of the recessus mesentericus and in the right leaf of the dorsal mesogastrium, i.e. inthecavalmnesentery. More posteriorly it leaves the dorsal surface of the liver, passes dorsal to the hiatus communis recessuum and comes to lie ventro-mesial to the right inesonephros, and comes into relation with the anterior extension of the subeardinalis system, with which it is connected by a nastomosis. Wehavethereforeherepresentadefiniteanlage of the inferiorvenacava(comparefigs.15-18). A singlepulmonaryveinopens into the caudal part of the atrium to the left of the lower end of septum I. It gets a large branch from the mesoderm in front of the left lung anlage andsmallertributaries from the right.

Note on the Blood

On Plate I. fig. 7, a, b, and c, are pictured the main characters of three nucleated red blood cells from the spatium septovalvulare of the heart. The nucleusshowsagradationinthedensityof itsnetwork-"ichthyoid"and"sauroid"typesofMinot. The size of the nucleus in relation to the size of the cell-body diminishes from a to c, while thethreecellsshowavaryingdepthofcolourintheirprotoplasm. In general it may be stated that practically al the types of red cel are present which have been described by Maximow for mammalian embryos. His "primitive blood-cells" or the "primitive mesameeboids" of Minot couldnotbedefinitelyidentified. Inawehaveatypical"normoblast" of Maximow, and c, according to his interpretation, is a "primitive erythroblast, " while b, in as far as the deep red coloration of its protoplasmisconcerned, must be classedasa"megaloblast." Mitoticfigures occurfrequentlyinthenucleatedredcells of the blood-stream. Nonnucleated plastids occur but sparingly in the embryonic vessels, but beneath the amnion in the distal part of the cord and in the "chorionic cavity, "artificialyproduced by a strippingoff of the amnion from the surface of the cord, numerous non-nucleated red cells are found in company with nucleated normoand megaloblasts. The se are much smaller than the embryonic blood cells proper and very irregular in shape, and have resulted probably from an extravasation of maternal blood from the placenta. PlateI.fig.7, d, representsthreeleucocytes("lymphocytes" of Minot) with typical reniform nuclei which contain numerous relatively largechromatinaggregations. The nucleiarebasophileandstainblue in the sections . The protoplasmisclear, withavacuolatedappearance, andisnon-granular. Suchcellsarerelativelyfewinnumberandalways occur in groups, in which however the cel boundaries are distinct. Schridde, in his paper on the development of the blood in early human embryos, asserts that up to a stage of 12 mm. no leucocytes are present in the blood-stream, being first formed in the liver when that organ enters on its blood-forming function. Maximow strongly opposes this view, and in the face of the present findings the position of Schridde would appear quite untenable.

The Urogenital System

The Mesonephros is situated between the 8th cervical and 3rd lumbar segments inclusive. On the left side thirty-seven tubules are present, all of which are S-shaped and provided with Bowman capsules and have collecting tubules opening in to the primary excretory duct. On the right side occur thirty-eight complete tubules with capsules and collecting tubules opening into the duct, and posterior to these is a 39th and rudimentary member of the series. The latter is in the form of as mall, more or less olive-shaped epithelial body, containing a lumen and with a solid pole abutting very obliquely against the mesial aspect of the Wolffian duct without opening into it. AccordingtoFelix, nonewtubuleanlagen are added at the caudal end after the 7-mn. stage; and since this tubule scarcely corresponds in its relations or appearance either with a normally developingor with a normally degenerating tubule, the structure in question is probably to be regarded rather as of an abortive nature. The more caudal tubules on both sides cannot be said to have completed their development, inasmuch as the capsules here consist of widely open saucerlike structures, in contrast to the double-sphere form with only a small aperture for the glomerular vessels seen in the more anterior members of theseries. On the left side there are no definite signs of degeneration either cranially or caudally, except in the somewhat shrunken appearance of the more anterior glomeruli and their smaller size in relation to the comparativelyl arge capsule. Fortherightsidethesamestatementholds good with regard to the anterior tubules, but here there is further to be observed an involution of the two anterior Wolffian arteries. The first possesses no apparent lumen, consistingmerelyofa cord ofcellsconnecting the capsule with the aorta, while in the case of the second no opening in to the aortacouldbemadeout. This negative finding may be due to a slight folding in this region, but the artery is nevertheless far from normal and its glomerular plexus less evident than those of its caudal neighbours. The primary excretory duct is anteriorly and on either side continuous at an angle with the first collecting tubule, and runs caudally close beneath thecoelomicepitheliumlateral to the Wolffiantubules(figs.15-21). At its caudal end it curves towards the middle line and runs internal to the origin of the umbilical arteries to end by opening into the dorso-lateral aspect of the urogenital cloaca (fig. A). At the junction of each collecting tubule the duct undergoes a localized enlargement (compare the two sides offig.18). Towardsthecaudalend of the mesonephrostheenlargements run together, and the duct appears in consequence as a tube of much largercalibre. Caudal to the last mesonephric tubule it suddenly narrows, and remains relatively small down to the point of origin of the ureteric outgrowth. Posteriortothispointitexpandsintoafunnelwhichis still definitely marked off from the cloacal lumen. No pronephric rudiments are to be seen on eitherside. The vessels of the mesonephros have been fully described in the sections on the circulatory system.

The Metanephros

The ureter arises from a somewhat lateral as well as dorsal aspect of the primary excretory duct of the mesonephros a short distance from the opening of the latterin to the cloaca(fig.A). The renal pelvis is differentiated from the ureter proper and has the form of a flask-shapedterminalswellingslightlyflattenedlaterally, thewholeanlage being curved with its apex directed orally as well as dorsally (see fig. A). The pelvis is capped by a layer of very dense mesodermal tissue in which thenucleiarearrangedinamarkedlyradiatingmanner. Fromthiscap a thick cord of condensed mesoderm stretches forward to a point caudomesial to the tailof the mesonephros and ends close to the later, being separated from it only by the large anastomotic vessel which here connects thesubcardinalwiththeposteriorcardinalvein(fig.21). The subcardinal is continued caudally dorsal to this condensation, as has already been mentioned in connexion with that vein.

The Genital Epithelium and " Genital Cells"

A germinal epithelium exists only in a very early and indefinite condition. The reissomethickening of the ccelomicepitheliumbetween the Wolffian ridge and the root of the mesentery in the lower dorsal region on either side, with some proliferation of its elements in a dorsal directionagainstthemesoderin. Thisthickeninghasnodefiniteanterioror posterior boundaries, and passes gradually laterally and inesially into the unthickenedareawithoutanydistinctlineofdemarcation. Inconnexion with the reproductive epithelium, however, mention may be made of a very characteristic type of cel found in the dorsal part of the mesentery and betweenthispointandthegerminalregionatthelowerdorsallevels. In PlateI.fig.8, a, one of the se cellsisfiguredlyinginthemesenteryabout midway between its base and the intestinal tube. Its large size and definite outline at once differentiate it from the surrounding mesoderm cells. The nucleusisregularlyroundedandeccentricandcontainsa large irregular chromosome resulting from a clumping of the nuclear chromatin network (von Berenberg-Gossler), and divided into two portions, which, however, remain connected by a fine bridge. The cell-body contains a clear protoplasm with a generally vacuolated appearance, but in close contactwiththenucleusandinthelongaxis of the celisaclose network -a more or less typical "Golgi-net"containing a single dark spot, probably of the natureofacentrosome. in fig. 8, b, PlateI., asomewhat similar cel is shown, in which, however, the Golgi-net is absent and the chromosomeinthreeportions. ThiscelwasfoundbetweentheWolffian body and the root of the mesentery some distance dorsal to the ccelomic epithelium. Fig.8, c, isofacel of the sametype, butfromamongstthe cells of the proliferatinggenitalepithelium. The cellsherepictured, which occur only sparingly in the situations mentioned, closely resemble those which have for many years been described in the literature as "genital cells" forthevariousvertebrateclases, andtheirappearanceatonce suggeststhepossibility of the irbeingofthisnature. The originof primitive "sex cells".from other sources than the coelomic germinal epithelium has long been an established fact, although observations in this direction on the human embryo have been very limited, and the question whethersuchcellsaregeneticallycontinuouswiththedefinitivegenitalcells hasremainedunsolved. Felix, recording the occurrence of "primary extra regional genital cells" inhuman embryosof25, 26, and49mm.-inthe earlier stage in the neighbourhood of the cloaca, in the later stage in or under the coelomic epithelium near the root of the mesentery-states that in amniotes al the primary cells either disappear entirely or enter on alatentstagewithapossiblelaterappearanceasdefinitivesecondarycells. Fuss(1912), ontheotherhand, inhisworkonthegenitalcellsofman and mammals, traces the path of these cells from the entoderm of the yolk-sac to the genital region in later stages, and holds that inclusive of the human embryo the so-called " germinal epithelium " plays only a very unimportant roleinthegenesis of the sex-cells, andbeginstoproliferateonlywhen the majority of the genitalcells, coming originallyfrom theyolk-sacentoderm, havearrivedattheirdestinationinthegenitalregion. VonBerenbergGossler (1912) has further worked out and pictured the histogenesis of the primitive genital cells in the chick and embryo duck, and it is an interesting factthat the celfiguredinourPlateI.fig.8, a, isalmost identicalwith those in his figures of threeto four-day embryos after staining with ironhoematoxylin. Thisauthorfindsthatcytologicalanalysisprovidesno definite proof that these cells are in realitythe ancestors of themale or female reproductive cells, and, with Fick, regards the peculiar clumping together of the nuclear chromatin as a " resting-network, " associated with diminished functional activity. The suggestion that the appearance of suchcellsasthoseheredescribedforthepresent embryo isdue merelyto an enlargement and modificationofordinary mesoderm cellsinpreparation for mitosis is at once disposed of by the striking analogy which exists between these cells, and those of the chick, in which mitotic division is absentorexceedinglyrare. Furthermore, wheremitoticdivisionoccursin the surrounding mesoderin, as it frequently does, the cel in this case, although assuming a rounded-off appearance and increase in bulk, as describedby Maximow, neversimulatesthecellsinquestion either in size orhistologicalcharacter. That, finaly, theyarenottobeidentifiedasof the nature of "Wanderzellen" of Maximow is shown by the regular rounded nucleus which they in every case possess.

The Connective Tissues

The chorda dorsalis extends from a short distance behind the hypophysis (fig.5) to the tip of the tail. it is incontactwiththe-epithelium of the roof of the pharynx. On cross-section it shows a thick homogeneous membrane enclosing a clear, apparently homogeneous protoplasm in which thenucleiarearrangedinasinglecircleinthemiddlethickness. No cartilageor"pre-cartilage"ispresentinthemembranous skeleton. Parts of the base of the skull are indicated in membrane, viz. the pars petrosa, aswellasthevertebralbodieswithcostalprocessesandafewribs. The anlage of Meckel's cartilage has been mentioned above in connexion with themandibularnerve. Threeheadsegmentsandthirty-sixorthirty-seven bodysegmentsarepresent. Inalthree of the formerdistinctdifferentiated muscle fibres are to be observed, as is the case with the body segments as farcaudalasthefourthsacral.

The Body-Cavities

The lateral halves of the pericardium communicate with each other in frontanteriorandventral to the truncusarteriosus. Soon, however(fig.7), the two become separated by the inesoderm surrounding the truncus and bulbus, whichextends from the dorsal to the ventralwall of the pericardium. Itsdorsalattachment, oranteriorsection of the dorsalmesocardium, islitle more than the mesoderm surrounding the truncus and sixth aortic arch as theseleavethepericardialcavity. Ventrallythebulbusiscloselyapplied to the anterior pericardial wall through about thirty sections (10M), and partialfusionoccurs(fig.7). The reisineverysection, however, abroken line of epicardium demarcating the two, so that the fusion is very incomplete, and one cannot speak of a true ventral mesocardium in this region. Behind the point where the anterior section of the dorsal mesocardium ceases the broad tracheal ridge juts dorsally into the pericardium, and graduallyincreasingindorso-ventralheightitfinallyfuses, from the tracheal bifurcation caudally, with the dorsal surface of the heart to form the main orcaudalsection of the dorsalmesocardium(figs.9and10). Fromthis pointthedorsalwall of the pericardium, withwhichthedorsalmesocardium iscontinuousoneitherside, slopesrapidlyventrallyandcaudally, andishere formed by the comparatively smooth antero-ventral surface of the septum transversum. On either side of the trachealridge("bourreletmesent6rique" ofBrachet)anteriorlyisabroadgroovewithoutany definitelateralboundary (onleftsideoffig.8). Tracedbackwardsthisgroovegraduallydeepensand narrows ("gouttiere pleuro-pericardique" of Brachet) and a well-marked ridgeappearstodemarcateitlaterally(rightsideoffig.8). Morecaudally stilthe groove becomes bounded on its lateral aspect and later centrally also by the duct of Cuvier as it sweeps caudally and mesially, and by the mesodermal membrane which connects that vessel with the body-wall (fig.9). The groove("gouttierepleurale")is, however, stilincomplete below, inasmuch as its appearance in the above figure as a closed " cavity " is due to the presence of a portion of the atrial wall, and it is only when the mesoderm on the dorsal and mesial aspect of the duct of Cuvier fuses with thedorsalmesocardiumandthuscompletesthepleuro-pericardialmembrane that the pleural cavity is fully shut off from the pericardium proper (fig.10). Betweenthe anterior and posterior sections of the dorsalmesocardium the pleuro-pericardial grooves communicate with each other, at first freely but more caudally through a slit-like space between heart and trachealswelling. The partplayedbytheductofCuvierinboundingthe pleuro-pericardialopeningcanbewell seen in figs.8to11. The ductwith itsinesodermal connexion with the body-wall bounds the groove at first laterally and then later below, but the ridge thus produced, as Brachet and others have pointed out, does not constitute the anlage of the pleuropericardialmembraneproper. from the mesialaspect of the ductof Cuvier at its oral and dorsal end a prominent ridge, already mentioned, consistingofcondensedmesodermwiththickenedepitheliumandanirregular surface, extends oralwards and gradually moves on to the dorsal aspect of the pericardium, reaching in front the lateral part of the "bourrelet inesent4rique." This is the anterior extension'of the pulmonary ridge (Mal'). Behind, itspreadsout, firston the mesial, later on the dorsal aspect of the Cuvierian duct, and adds itself to the mesodermal membrane, connecting the latter with the body-wall, while, as Brachet has shown, it joins the mesoderm around the duct, by its active growth, with the dorsal mesocardium, and through this with the mesodermal lung anlage (fig. 10). It thus converts the pleuro-pericardial groove into a "cavity, " and is connected at both ends with the mesoderm surrounding the lung, and can later, by active growth, complete the closure of the pleuro-pericardial opening. The pulmonaryridgebeingthusthetrueanlage of the pleuropericardial membrane, the'duct of Cuvier lies at first in its lateral edge and latercomestohang from it sventralsurface(fig.10). Stilllateritleaves the pleuro-pericardial membrane altogether (fig. 11) to run along the lateral aspect of the dorsal mesocardium before entering this structure on itsway to the sinusvenosus. Inotherwords, the"lateralmesocardium" and the pleuro-pericardial membrane are two distinct structures, the former bringing about a constriction of the pleuro-pericardial communication withouttakinganyactivepartintheactualclosingprocess. Infigs.10 and 11 the antero-lateral recess of the peritoneal coelomx has appeared (on the right side of both figures), and here the pleuro-pericardial membrane forms a T-shaped junction with the pleuro-peritoneal membrane extending dorsally and the " pericardio-peritoneal membrane " ventrally and laterally. The latter is at no point a true membrane like the other two, being very much thicker, and constitutes in reality the anterior or dorsal edge of the septum transversum, in which, a few sections posteriorly (fig. 12), liver trabeculaeappear. it is thusseenthatthepleuro-pericardialmembraneis itselfaportion of the anterioredge of the septum. in fig. 11thedorsal attachment of the pleuro-peritoneal membrane is the dorsal body-wall under the cardinal sinus; ventrally and mesially it is continuous with the dorsal surface of the septum transversum, through the pleuro-pericardial and " pericardio-peritoneal"membranes. Ithasspreadoutonitsmesialsurfacethe epithelium of the pulmonary ridge, and is partly made up of that structure. Followed forward it is seen to be attached to the posterior aspect of the dorsal part of the duct of Cuvier (Brachet), as well as being continued orallyfromthatlevelandmesiallyasthepulmonaryridge. Posteriorlyit ends in a caudally directed free edge with dorsal and ventral prolongations whichdiffersomewhatonthetwosides. Ontheleftsidethedorsalprolongation, or dorsal pillar of Uskow, is the better marked, is continued into a ridge mesial to the anterior extremity of the Wolffian mesentery (fig. 12), andgraduallyfadesawayinthisposition. The ventralpillaristhicker andflattensoutrapidlyin to the septum transversum, or, more correctly, is invaded and spread out by liver tissue so that it soon loses its ridge-like form. Ontherightside, wheretherightlobe of the liverhasnotreached the ventral pilar, the latter is high and sharp (fig. 13) and is continued backalongtheuppersurface of the septumtransversum. Ingeneralthe pleuro-peritoneal membrane is situated in a sagittal plane, but is bulged laterally by the mesodermal lung anlage.

When theconditiondescribedaboveiscompared, forinstance, withthat found in a 7-mm. embryo, in which, as Mall says, the pulmonary ridge "connects the dorsal end of the septum transversumn with the Wolffian body, " as is seen in the large Piper-Ziegler model of a 6 8-mm. embryo (1902); and when it is further considered that this ridge is the anlage of both pleuro-pericardial and pleuro-peritoneal membranes, it is possible to appreciate the ful significance of Brachet's description (1897) in which he regards both anlagen as " spreading so to speak from one point to develop in different directions, one caudal and the other cranial."

The liver bulges from the dorsal surface of the septum transversum, reaching farther forward on the left side, where it ends in a wing-like expansionextendingin to the antero-lateralrecess(fig.1). On theright side, thoughnotextendingsofarforward, itfilsup most of the anterior portion of the greaterperitonealsac. Wherethelargevenaumbilicalis sinistra enters the liver from the body-wall the ventral mesentery may be said to commence, and it is continued caudally from the posterior aspect of the vein as a thin membrane (fig. 17) connecting mesentery and bodywall for about twenty sections (101A). Behind this it is produced as dorsal andventralpilarsonthenmesentericedgeandventralbody-wallrespectively as far back as the umbilical ccelomic diverticulum.

Fig. 17 shows the incomplete foramen of Winslow, or hiatus communis recessuum, through which the lesser peritoneal cavity has a wide commnunicationwiththegreater. The hiatusisboundedabovebythepoint of separation of the caval inesentery from the dorsal surface of the liver, behind by the caval mnesentery with the early interior vena cava, in front by the posterior surface of the right lobe of the liver, and is widely open to the rightandbelow. Fromthisopeningthelesserperitonealcavity extends orally and expands in three main directions: (a) ventrally between the stomach and right lobe of the liver, recessus hepato-entericus; (b)to the left over the dorsal aspect of the pylorus and distal part of fundus, r. mesenterico-entericus; and (c) orally, partially separating the lower end of the Esophagus and the stomach from the right lung anlage, r.pneumatoentericus(cf.figs.13, 15, 16, and 17).

Caudal to the origin of the pancreas the dorsal mnesentery follows the main intestinal torsion, and merits no special description (figs. 19 and 20).

My thanks are due to Geheimrat Professor Wiedersheim for permitting me to work in his laboratory over an extended period, and for placing the resources of the Institute at my disposal; to Professor Keibel for the loan of his embryo and continuous supervision and helpful criticism during the course of the work; and to Dr von Berenberg-Gossler for many valuable suggestions.

Freiburg In Breisgau, April 1914.

List Of Principal Abbreviations brachial artery. p.= profunda. s.=superficialis. A.c. arteria centralis (of spinal cord). A.c.e. carotisexterna. A.c.i. carotisinterna. A.cdel. c(eliacartery. cloacalbranchofcaudalaorta. A.D.S. dorsalsegmentalartery. A.f.b. arteryoffore-brain(a.cerebri ant.etLied.). A.m.s. arteriamesentericasuperior. A.P. pulmonary artery. 1.=left. r.=right. A.r.d. a.radicularisdorsalis. A.r.v. a.radicularisventralis. A.S. subclavianartery. A.Spl. splenicartery A.U. umbilicalartery. A.V. vertebralartery. A.W. Wolffianartery. Ad. adrenalcortex. Al. allantois.

Ao.= aorta. A.= aortic arch. C. =caudal. d.=dorsal.

Ap. appendix.

At.c. atrial canal.

B.A. bulbus arteriosus.

B.cap. Bowman's capsule.

B.D. common bileduct.

B.E. eparterialbronchusbud.

B.L. lateralbronchusbud.

B.S. bulbarswelling. A and B = proximal. 1 and 3=distal.

B.St. stem bronchus.

Ctec. csecum.

C.D. cysticduct.

Ch.d. chordadorsalis.

Cl. cloaca.

Co. colon.

D. duodenum.

D.A. ductusarteriosus. ductusbranchialis.

D.E. ductinseudolymphaticus.

Ep.B.mI. epithelial body of third pharyngeal pouch.

F.B. = fore-brain. G.B. gall-bladder. G.F. Froriepganglion. Gl. Wolffianglomerulus. G.S. spinalganglion. G.c. cervicalganglion. d.= dorsal. 1.= lumbar. V.= Trigeminal. H.B.r. roofofhind-brain. H.C.R. hiatuscommunes recessuum. H.D. hepaticduct. Hem. cerebralhemisphere. Hy. hypophysis. I1. ileum. J.organofJacobson. K. kidney. L. liver. La. larynx. M.B. mid-brain. M.p.p. pleuro-pericardialmembrane. pleuro-peritonealmembrane. Mx. maxilla. My.0.3. thirdoccipitalmyotome. N.acc. accessory nerve. N.B.P. brachialplexus. N.c. cervicalnerve. N.Ch.T. chordatympani. N.d. dorsalnerve. N.E. nervus epibranchialis. N.La. laryngealbranchofN.X. N.L.P. lumbarplexus. N.Md. mandibularnerve. Mx. maxillaryn. N.O. ophthalmicn. phrenicI1. superficialpetrosaln. N.rad. radialn. N.S. sacraln. N.X. vagus n. N.XII.d. descendenshypoglossi. Neu. neuromere. N.L. naso-lacrimalgroove. 0. cesophagus. OP. opticanlage. OT. oticvesicle. P. pericardium. P.D.=dorsalpancreas. Ph.p. pharyngealpouch. P1. placode. P.R. pulmonaryri(lge. P.U. pillarofUskow. d.=dorsal. v.= ventral. P.V. ventralpancreas. Py. pylorus. R. rectum. R.C. ramuscommnunicans. R.H.-E. recessus hepato-entericus. R.M.E. , , mesenterico-entericus. R.P.E. , , pneumato-entericus. S. spleen. S.A.P. septumaortico-pulmonale. S.B.D. distalbulbarseptum. Sp.s.v. spatiumsepto-valvulare. S.s. sinus sagitalis. S.v. sinusvenosus. Sy. sympatheticcordandventral branches. T. trachea. T.A. truncusarteriosus. Tb. thyroidanlage. t.s. secretingWolffiantubule. Ty. thymusanfage. V.c. caudalvein. V.Cu. Cuvierianvein. V.C.I. vena cava inferior. V.Hep. hepatic vein. c.= common. 1.= left. r.=right. V.Is. ischiadicvein. V.m.s. superiormesentericvein. omphalo-mesentericv. V.p. pulmonaryv. V.r.a. ramus arcuatus. V.r.ang. ramus angularis. V.S. subclavianvein. V.sub. subcardinalv. V.T.E. thoraco-epigastricv. V.U. umbilicalvein. V.Ul. ulnar vein. V.V. venous valve. W.D. Wolffianduct. W.R. Wolffianridge.


The original monographs of the principal authorities cited will be found indexed in the exhaustive bibliographies of Keibel and Mall's Text-book and the Normaltafel of the Development of Man of Keibel and Elze. The following is a list of the authors whose work has been more especially consulted:

(1) v. BERENBERG-GOSSLER, H., "Contributions to the Subject of the Development of the Caudal Part of the Intestine and Urogenital System, based on Terato logical Material, " Anatomisehe Hefte, 149, Bd. xlix. (1913).

(2) v. BERENBERG-GOSSLER, H., " The Primitive Genital Cells of the Chick of the 3rd-4th Day of Incubation, etc., " Archivfiir mik. Anat., Bd. lxxxi., Abteilung i.(1912).

(3) BORN, " Contributions to the Development of the Mammalian Heart, " Archiv fur mik. Anat., Bd. xxxiii. (1889).

(4) BRACHHT, " The Development of the Larger Body-cavities and their Separation from one another, etc., " Ergebnisse der Anat. undt Entwicklunysgeschichle, Bd. vi. (1897).

(5) BREMER, J. L., "Description of a 4-mm. Embryo, " Amer. Jour. of Anat., vol. v. (1906).

(6) BREMER, J. L., " Aberrant Roots and Branches of the Abducens and HypoglossalNerves, " Jour. ofCompar. Neurol. and Psychol., vol.xvi., No. 6 (1908).

(7) BROMAN, J., "Description of a Human Embryo, " Morphologische Arbeiten, Bd. v. (1896).

(7a) BRYCE, T. H., and TEACHER, J. H., "An Early Ovum embedded in the Decidua, " Glasgow (1908).

(8) DEDEKIND, F., "Contribution to the Development of the Eye-vessels of Man, " Anat. Ilefte, Bd. xxxviii. (1908).

(9) ELZE, C., "Description of a Human Embryo of 7-mm. ca. greatest length, etc., " Anat. Hefte, Bd. xxxv. (1907).

(10) EVANS, H. M., " On the Developmuent of the Aorta, Cardinal and Umbilical Veins, etc., "Anat. Rec., vol.i.(1909).

(11) FELIX , W., " The Development of the Urinary Organs, " Hertwig's llandbuch dervergleichendenund experimentellenEntwicklun~kisgesclhichte, vol.i.(1905).

(12) FUCHS, H., "On the Development of the Vertebrate Eye, " Anat. Hefte, Bd. xxviii. (1905).

(13) Fuss, A., " The GenitalCellsofMan andMammals, "Archivfirmik.Anat., Bd. lxxxi., Abt. i.(1912).

(14) FUTAMURA, R., " On the Development of the Facial Musculature in Man, " Anat. Hefte, Bd. xxx. (1906).

(15) GRAPER, L., "'['he Rhombomeres and their Nerve Relations, " Archivfur mik. Anat., Bd. lxi., Abt. i.(1913).

(16) GROSCHUFF, K., "On the Occurrence ofa Thymus Segment in the Fourth PharyngealPouch in Man, " Anat. Anzeiger, Bd. xvi.(1900).

(17) GROSSER, O., "On the Ultimobranchial Body in Man, " Anat. Anzeiger, Bd. xxxvii. (1910).

(18) HERTWIG, Text-book of Embryology (1906).

(19) His, W., AnatomyofHuman Embryos, Leipzig(1880-1885).

(20) His, W., jun., " The Development of the Sympathetic in Vertebrates, etc., " Verhandlnqen der Anat. Geselechaft, Vienna (1892), Anat. Anzeiger, Bd. vi., Erg.Heft.

(21) HocHSTETTER, " Development of the Vascular System, " Ergebrisse der Anat. und Entwvicklung, Bd. vi.(1892).

(22) HOCHSTETTER, '"Development of the A. vertebralis in the Rabbit, etc., " Morphologisches Jahrbuch. Bd. xvi. (1890).

(23) INGALLS, N.W., "DescriptionofaHuman Embryoof49mui., "Archiv ftirmni/. Anat., Bd. 1x. (1907).

(24) KEIBELJ, F., Normaltafel of the Developmnent of the Pig, Jena (1897).

(25) KEIBEL, F., "Development of the Vertebrate Eye, " Klin. Monatsblatt fur Augenheilkunde, Bd.xliv., New Series(1906).

(26) KEIBEL, F., and ELZE, C., Normaltajel of the Embryology of Man, Jena (1908).

(27) KOHN, A., "On the Development of the Peripheral Nervous System, " Anat. Anzeiqer, Erg. Heft, Bd. xxvii. (1905).

(28) LEWIS, F. T., and THYNG, F. W., " The Regular Occurrence of Iitestinal Diverticula in Embryos of Pig, Rabbit, and M1an, " Amer. Jour. (ofAnat., vol. vi. (1908).

(29) MALL, F. P., "A Human Embryo 26 days old, "Jourmi. of Morph, vol. v. (1891).

(30) MALL, F. P., " A Study of the Structural Unit of the Liver, " Amer. Jour. ofAnalt., vol.v.(1906).

(31) MALL, F. P., "On the Development of the Blood-vessels of the Brain in theHuman Embryo, "Amer.Jour.ofAnat., vol.iv.(1904).

(32) MAXIMOW, A., " Studies in Blood and Connective Tissue, " Archiv ftir mik. Anat., lPd. lxi. (1909).

(33) MULLUR, E., "Contributions to the Morphology of the Vascular System: I. The Arm Arteries of Man, " Anat. Hefte (1903); "II. The Arm Arteries of Mammals, " Anat. Hqfte (1904).

(34) PETER, K., Methods of Reconstruction.

(35) PETER, K., An Atlas of the Develolpment of the NoseandPrimitivePalate (1913).

(36) PIPER, "On a Ziegler Model of a Human Embryo of 6-8 mim., " Anat. Anzeiger, Bd. xxi. (1902).

(37) SCHRIDDE, H., " The Origin of the First Embryonic Blood-cells in Man, " Transactions of the German Pathological Society (1907).

(38) TANDLER, J, " The Development of the Human Iintestinal Arteries, " Anat. Hefte, Bd. xi.(1903).

(39) TANDLER, J., "On the Development of the Fifth Aortic Arch and Fifth Pharyngeal Pouch in Man, " Anat. Hefte, Rd. xxxviii. (1909).

(40) WRIGHT, J, H., "Aberrant Pancreas in the Region of the Umbilicus, " Jour. of the Boston Soc. of Med. Sci., vol. V. (1901).

Explanation Of Plates

Plate I

Barniville1914 plate01.jpg

Fig.1. Photograph of the Embryo. x4ca.

Fig.2. Drawing of a portion of a section containing a "cellular ramus communicans." Ao., aorta; N.c.3, third cervical nerve. x1120ca.

Fig. 3. Drawing of a model of the right olfactory region, seen from below and somewhat from the rightside. Mid.l., middle line; Mx., maxilla;N.L., nasolacrimal groove; OP., optic anlage. x40ca.

Fig. 4. Drawing of a portion of a section passing almost horizontally through the upper part of the third pharyngeal pouch. Ep.B.iii., anterior wall or anlage of the epithelial body of the pouch;Ty., posterior wall or thymic anlage. x1120ca.

Plate II

Barniville1914 plate02.jpg

Fig. 5. Drawing of a model of the pancreatic outgrowths with the biliary ducts and a portion of the right lobe of the liver. D, , duodenum; G.B., gall-bladder; H.D., hepatic duct; L., liver; P.D., dorsal pancreas; PV., ventral pancreas; St., stomach; V.owi., oinphalo-nesenteric vein; V.v., vitelline vein; V.m.s., superior mesenteric vein. x75.

Fig. 6. Drawing of a model of a part of the intestinal loop, with a portion of the superior mesenteric artery. A.m.s., superior mesenteric artery; Ap., appendix; Co., colon;IL., ileumn. x75.

Fig. 7. Drawing of some blood-cells from the spatium septo-valvulare of the heart. a, b, c, erythrocytes; d, leucocytes. x1500ca.

Fig. 8. Drawing of three genital cells a, cell lying in the mesentery, from the lower dorsal region; Cce., chorlonic epithelium; b, cel from the mesoderm dorso-mesial to the coelomic epithelium, with a neighbouring mesoderm cell; c, cel from the left genital region. x1120ca.

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  1. In the following account the neuromeres are reckoned exclusive of the first or "cerebellar" neuromere. Thus neuromere 1 is "neuromere a."of Streeter and" neurornere 2" of v. Kupffer, Bradley, Graper, etc. Seefig.2.
  2. IntheprofilereconstructionN.IX.semstoruninthesecondarch;thisisdueto the fact that while the nerves are projected on the mid-sagittat plane, the visceral arches, etc., arerepresentedinsurfaceoutline, sothatthedeeperrelations, viz.theobliquity of the secondcleft, arenotshown.
  3. Makingeveryallowanceand corectionfortheobliquity of the sectional plane, which hasbeendonethroughout.
  4. The a. rad. dorsalis from the 1st right dorsal segmental passes dorsally external to theganglion, anarrangementwhichappearsquiteexceptional.
  5. The system of reckoning the dorsal segmental arteries here employed is that of Mall, Tandler, Broman, andEvans. Hochstetter, forinstance, describesthearteriasubclaviaas arising from the sixthdorsalsegmental. See footnote, p.605 of Keibel and Mall's Text-book.