Paper - The anatomy of a 7.8 mm pig embryo
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The Anatomy of a 7.8 mm Pig Embryo
Fred W. Thyng
From the Department of Comparative Anatomy of the Harvard Medical School, and The Department of Anatomy, Northwestern University Medical School
Three figures (1911)
Graphic reconstructions of a 12 mm. pig embryo have been published by Dr. F. T. Lewis (03) and used in Minot’s ‘‘ Laboratory Text-book of Embryology.” These reconstructions have been exceedingly helpful to the student in acquiring an understanding of embryology. Therefore it seemed desirable to have similar reconstructions of a still younger stage. At the suggestion of Prof. C. 8S. Minot I have made a series of reconstructions to show the anatomy of a 7.8 mm. pig embryo.
The drawings were made from transverse sections by the modified graphic reconstruction method of His. The surface modeling has been inferred from a study of the sections with the exception of a few instances when wax models were made to aid in its interpretation.
In contour this embryo resembles closely the one represented by Keibel (07) in fig. 15, Taf. II. There are four regions of bending or flexures, the cephalic, the cervical, the dorsal and the sacral. The cephalic is in the region of the mesencephalon and causes the anterior portion of the brain to be bent at an acute angle to the posterior. The cervical occurs in the region of the neck, where the spinal cord joins the brain. It is at an obtuse angle. The dorsal occurs approximately on a level with the middle of the anterior limb bud. It also forms an obtuse angle causing a convexity of the back. The sacral is in the caudal region and is more nearly at a right angle.
- This investigation has been aided by a Bullard Fellowship established at Harvard Medical School in the memory of John Ware.
- The reconstruction represented in fig. 1 of this paper is used in the second edition of Prof. Minot’s ‘Laboratory Text-book of Embryology.’’ Since this plate was made several minor changes have been made in the drawing.
Four branchial arches are observed in fig. 3, separated by ectodermal grooves, the branchial clefts. The fourth is seen at the bottom of a pit (S.cer.) which represents the developing cervical sinus. In size the branchial arches decrease from anteriorly posteriorly. The maxillary processes of the first arch have united on either side with the lateral nasal and globular processes so that the nasal pits are now shut off from the oral cavity. The nasal fossae are, however, separated by an exceptionally broad median nasal process.
The ectodermal invagination on either side to form the lens of the eye has just closed.
The great development of the heart causes a conspicuous swelling of the ventral body wall directly under the branchial arches.
The umbilical cord is comparatively short and bends to the right. The limb buds are conspicuous structures.
The Alimentary Canal and its Appendages
The mouth (M.) shown in figs. 1 and 2 is wide and slitlike. It is bounded posteriorly by the mandibular processes (Mdb.) of the first arch, and anteriorly by the median nasal process united on either side with the maxillary process. In the dorsal wall of the oral cavity there is a distinct longitudinal furrow which leads to the opening of the hypophysis.
The hypophysis (Hyp.) is a flattened diverticulum from the ectoderm of the oral cavity closely applied to the ventral surface of the brain. Its distal end is wider than its proximal part, but does not show any trace of forking. The oral membrane has disappeared except for a small remnant attached immediately posterior to the outlet of the hypophysis. |
The pharynx is dorso-ventrally compressed and has its greatest width in the region of the mouth. It gradually becomes narrower as it extends posteriorly to its bifurcation into the oesophagus (Oe.) and trachea (Tr.) A little above the point of bifurcation it is bent upon itself at an angle corresponding to the neck-bend of the embryo. A little caudal to the hypophysis in the median line occurs the most anterior of the entodermal structures. It is a small conical diverticulum (Seessel’s pocket, S. P. ) resembling somewhat the hypophysis. Still farther caudal the median wall of the pharynx presents another conspicuous projection (x). The development of this structure has been followed by Mrs. Gage (05), who concludes that it represents the bursa pharyngea identified by Killian (’88). In the anterior ventral wall of the pharynx there is a deep conspicuous groove partially separating the ventral ends of the mandibular processes of the first visceral arch. Immediately posterior to this median oral groove is the small rounded elevation, the tuberculum impar (7. 7.), which has been said, (His, ’85) to form the tip and a large portion of the body of the tongue. Some investigators, (Kallius 701, Hammar ’02), however, assert that later in development a more extensive elevation forms from the medial dorsal surface of the mandibular processes, and that it 1s this which forms the greater portion of the anterior part of the tongue. Posteriorly the tuberculum impar is fused to the converging ventral ends of the descending hyoid arches which form the anterior part of a median elevation slightly higher than that of the tuberculum impar. The original separation of the two is now indicated by a slight V-shaped groove. The apex of this groove marks the place of origin of the median thyroid (Th. med.).
The ventral ends of the descending third visceral arches have also converged toward the median line, and have fused with the median elevation of the second arches to form a conspicuous posterior part, the epiglottis (Hpgl.). The remaining anterior portion of the elevation contributes according to His (85) to the development of the posterior part or root of the tongue. Behind the epiglottis and at a much lower level, the internal wall of the pharynx presents a slight median groove bounded laterally by the arytenoid ridges which represent the lateral portions of the furcula described by His (85). This ‘groove leads posteriorly along the ventral wall of the pharynx to the glottis, the slitlike opening of the larynx into the pharynx.
An approximation of the entoderm of the arytenoid folds occurs in the region of the larynx. Thereby two passages for a distance are formed, a broader dorsal channel and a ventral slitlike one, which leads into the trachea. <A little more posteriorly, the dorsal passage forks, one channel passing into the oesophagus and the other into the trachea.
The pharyngeal pouches are shown in fig. 2. There are four on either side. As described by Hammar (02) and confirmed by Fox (’08) each possesses a dorsal and a ventral diverticulum. In the 7.8 mm. pig embryo all are in contact with the ectoderm forming closing plates, except the fourth. Anteriorly the pharynx is wide and much compressed dorsoventrally. From the anterior lateral border the first or auditory pouch (Ph. P. 1.) arises. Its dorsal diverticulum is the most conspicuous portion. It is a wing-like projection extending obliquely upward and outward, ending in a pointed process. There is thus formed a prominent ridge extending from the pointed tip of the dorsal process downward, inward and forward to the oral epithelium between the hypophysis and the angle of the mouth. This has been named by Moldenhauer (’77) the sulcus tubo-tympanicus. The contact of this pouch with the ectoderm is made by a lateral ridge extending from the dorsal pointed process posteriorly downward for about three-fourths of its distance. Below the point of contact there 1s a small ventral diverticulum less developed than the corresponding structure of the other pouches, but causing a distinct ventral ridge on the floor of the pharynx extending inward and backward toward the median line.
The pharynx is also quite wide in the region of the second pouch (Ph. P. 2.). Immediately posterior to this it retreats considerably toward the median line. From the lateral and ventral border of this widened portion of the pharynx the second pouch develops. Its dorsal diverticulum somewhat resembles that of the auditory pouch being flat and extending upward and outward. It terminates in two short pointed processes leaving a slight indentation down over which passes the tympanic branch (N. tymp., fig. 3.) of the glossopharyngeal nerve. Between this diverticulum and the one anterior there is a V-shaped notch. The surface of contact to form the closing plate of this pouch is very extensive as shown by Fox (08). Itis formed by a deep plate-like fold of the entoderm jutting out at almost right angles to the axis of the pharynx. The border of contact recedes from the ventral border of the dorsal diverticulum medianly to its extreme lower tip. This fold thus produces @ prominent ventral diverticulum extending down between the second and third visceral arches with its extreme tip pointed medianly. Between the ventral tips of the diverticula of the two sides is the median thyroid (Th. med.) a small branching mass of entoderm.
The third pouch, likewise, arises from the latero-ventral wall of the pharynx. It is plate-like and nearly parallel with thesecond. Its dorsal diverticulum is smaller than the two preceding, but its ventral diverticulum, which forms the thymus gland, is more prominent. Itisa deep fold from the entoderm. Its upper lateral border is nearly parallel to the corresponding border of the third. Medially they tend to converge. A little more than half of its border reaches the ectoderm. Beyond, the ventral part is free. Immediately behind the third pouchthe pharynx recedes abruptly toward the median line.
The fourth pharyngeal pouch (Ph. P. 4.) conspicuously smaller than the preceding joins the pharynx a little anterior to its bifurcation into the oesophagus and trachea. It possesses a rounded dorsal diverticulum, attached to the pharynx, somewhat constricted off from a more ventral tubular portion which extends posteriorly and somewhat dorsally. This posterior ventral portion is comparable with the ventral process of the other pouchesas emphasized by Fox (’08). From the ventral surface of the fourth pouch a prominent ridge extends across the ventral wall of the pharynx to meet the corresponding one from the other side.
Above the cardiac end of the stomach the trachea bifurcates to form two short bronchi, fig. 1. Each entodermal bronchus ends in a symmetrical bilobed enlargement, the lung. The left bronchus has been cut away in the figure. The trachea just above its point of bifurcation shows an indication ot the developing eparterial bronchus (not represented in the drawing).
The liver (hepar) is incompletely divided into four lobes. Two lobes, a dorsal and a ventral, appear on either side. In fig. 1 a median sagittal section of the liver is represented. In fig. 2 it is represented as cut more to the right, and here a better idea is obtained of its comparatively large size.
The stomach (St.) has so revolved on its axis that its dorsal border nearly faces toward the left. Its previous ventral border is now toward the right. Viewed from the left and ventrally it has an oval shape, tapering at either end, while from the left and dorsally it is very much flattened hardly thicker than the oesophagus. Just below the entrance of the oesophagus, the cardiac end presents a small dorsally directed elevation corresponding to the prominent pouch of the stomach of the 12 mm. pig embryo (Lewis ’03, fig. 3).
Gall bladder and pancreas
Ventral to the pyloric end of the stomach is the hepatic diverticulum from which project several cords of cells connecting with the liver trabeculae, one of which will eventually form the hepatic duct. The distinct pouch at the distal ventral end of the hepatic diverticulum together with the more constricted intermediate portion represent the gall bladder and the cystic duct. The hepatic diverticulum opens into the right side of the duodenum. In a younger pig embryo (Thyng ’08, fig. 1), this opening was on the ventral side. The change has occurred by a revolving of this portion of the intestine, thus causing the hepatic diverticulum to be bent at nearly a right angle.
The ventral pancreas (Pane. v.) joins the hepatic diverticulum near its opening into the duodenum. It is an elongated budding mass of entoderm, extending on the right side of the duodenum ventral to the portal vein (V. p.). The dorsal pancreas (Pane. d.) is larger than the ventral. It Joins the duodenum on its dorsal side, posterior to the opening of the hepatic diverticulum. It branches in the mesenchyme of the greater omentum, sending 2 posterior subdivision to the nght and ventrally which overhangs the portal vein. The ventral and dorsal pancreas will later anastomose on the right side of this vein. Neither the dorsal nor the ventral pancreas as yet possesses an open outlet. In the pig the duct of the dorsal pancreas persists, the ventral becomes obliterated (Stoss ’91).
From the dorsal pancreas the intestine (/nt.) extends in the dorsal mesentery into the umbilical cord (Um. C.) where it receives the yolk stalk (Yk. S.). It then returns to the body and extends posteriorly into the cloaca (Cl.). Beyond the cloaca it is continued out into the tip of the tail as the tail gut (post-anal gut). Posterior to the cloaca it is slender except near its termination where its lumen distinctly enlarges. The. tail gut very early disappears. It has become obliterated in the 12 mm. pig embryo, (Lewis ’03, plate 3).
The cloaca (Cl.) represents a dorso-ventrally expanded portion of the intestinal tract from which the allantois (All.) extends forward and ventrally into the posterior wall of the umbilical cord. The allantois is a slender tube as it leaves the ventral portion of the cloaca. In the umbilical cord it soon becomes considerably dilated. Lateral to the origin of the allantois the cloaca receives the Wolfhian ducts. (D. W.). The anterior portion of the cloaca to which the Wolffian ducts and allantois are connected is partly separated from the dorsal intestinal portion by a fold (uro-rectal) to form the urogenital sinus. In the 12 mm. pig embryo, (Lewis ’03, plate 3), the cloaca has become more extensively separated into its urogenital and rectal divisions. The ventral wall of the cloaca anastomoses with the adjacent ectoderm to form a median raphe, the cloacal membrane. |
The large Wolffian ridges or mesonephroi (Mes.) are shown in part in fig. 2. They consist mainly of the mesonephric tubules and the associated mesial glomeruli. They begin anteriorly near the anterior border of the liver and extend almost the entire length of the body cavity. The posterior portion of each ridge, however, is occupied only by the large Wolffian duct (D. W.) which joins the corresponding antero-lateral portions of the cloaca. The division of the cloaca into which the ducts empty is nearly separated from the dorsal rectal portion as already described. Just before the Wolffian ducts open into the urogenital region of the cloaca, each presents an evagination (Met.) from its dorsal wall, extending into the mesenchyme above. This evagination, the primitive pelvis of the metanephros, shows three lobes developing
from its surface, a posterior bud near its union with the Wolffian duct and two terminal ones.
The longitudinal axis of the brain and spinal cord share the corresponding flexures already described for the embryo. The ventral flexure of the floor of the metencephalon is already quite apparent. The three primary divisions of the brain, prosencephalon, mesencephalon and rhombencephalon, have undergone the following differentiation in the 7.8 mm. pig embryo.
Prosencephalon. The prosencephalon, (figs. 1 and 3) is partially divided into diencephalon (Dien.) and telencephalon (Telen.) by a prominent external groove and a corresponding internal ridge, the velum transversum, (Johnson ’09). This ridge extends from the dorso-median line around the interior of the prosencephalon ending in front of the optic evagination. The cerebral hemispheres are shallow evaginations from the dorso-lateral surface of the telencephalon, the first neuromere of the brain. A small pit in the ventral wall of each hemisphere indicates the developing olfactory area (rhinencephalon). Ventral to the olfactory depression there is a low broad arch which represents the corpus striatum. Caudal to this there is another ridge or arch which overlies the groove leading from the optic cup. In this ridge the optic fibres later develop, and so Johnson has named it the optic ridge. It 1s separated from the developing corpus striatum by a groove, the praeoptic recess which crosses the thin lamina terminalis and extends out into the optic stalks.
Diencephalon. ‘The diencephalon is bounded from the mesencephalon (Mesen.) by an internal ridge extending transversely across the brain toward the tubereulum posterius. This division is narrow, but considerably dilated dorso-ventrally.. A large concavity marks each dorso-lateral wall. In the anterior part of each latero-ventral wall is the optic evagination. In the median line the brain wall is thickened just behind the tip of the oral hypophysis, to form its nervous portion or posterior lobe. Still farther caudal there is a slight depression, the mammillary recess, causing a corresponding external elevation of the floor of the diencephalon. The number of neuromeres entering into the formation of diencephalon is questionable. Johnson (’08) gives two neuromeres to this section of the brain.
Mesencephalon. The posterior boundary of the mesencephalon (Mesen.) is a ridge known as the plica-rhombomesencephalica. Each dorsal zone of this division presents three concavities or neuromeres on its internal surface, and corresponding swellings on its external surface. Schultze (97) found in a pig embryo 2-3 neuromeres in the mesencephalon. Johnson (’09) represents two in this region of the pig embryo. The ventral zone of the mesencephalon is thickened causing a prominent external elevation just anterior to the origin of the oculomotor nerve.
Rhombencephalon. The conventional divisions of the rhombencephalon are not so well differentiated as those previously described. The isthmus (/sth.) is the constricted portion in the region of the plica-rhombomesencephalica. The metencephalon (Meten.) is the portion immediately following the isthmus, already considerably dilated to either side and presenting a developing fold in its ventral wall, the pontine flexure. The myelencephalon (Myelen.) may be arbitrarily bounded from the metencephalon by a line passing transversely across the brain near the posterior extension of the pontal flexure. From here the myelencephalon extends posteriorly arching over the cervical flexure to merge with the spinal cord. The part of the metencephalon from which the cerebellum will later develop does not show any differentiation except a local thickening of the epithelium. The roof plate is thin and considerably expanded. The lateral walls of both metencephalon and myelencephalon are indented by nearly transverse furrows (neuromeres) involving both dorsal and ventral zones. Six appear in the anterior two-thirds of the rhombencephalon. A large probably composite neuromere is found in the posterior third (myelencephalon) over the cervical flexure. Bradley (’04) has described a neuromere occurring in the region of the cerebellum of the pig embryo. In younger pig embryos I have seen neuromeres in the rhombencephalon anterior to those here described. The cranial nerves have been found to be intimately associated with the brain wall underlying these neuromeres.
The spinal cord (Med. sp.) shows a differentiation into dorsal and ventral zones, and also indications of neuromeric grooves. McClure (’89) called attention to the neuromeres, ‘‘myelomeres,’ of the spinal cord.
Nn. olfactorius and opticus. Fiber tracts for the olfactory apparatus, and for the eye have not developed.
N. thalamicus. A small elongated clump of ganglion cells (N. th.) were found on either side of the diencephalon dorsal to the corresponding optic vesicle. Between the ganglion and the optic vesicle passes the nasal branch (N. na.) of the ophthalmic nerve. No connection between the nerve and ganglion was found. From the dorsal end of the ganglion, fibres could be traced for a very short distance into the mesenchyma. It probably represents a portion of the neural crest corresponding to the nervus thalamicus of Miss Platt, which was found by growth of the brain wall, to lie directly above the eye stalk (Platt ’91, pp. 97-98). It is possible that this ganglion may contribute to the formation of the ciliary ganglion which has been found by Dixon (’96), to develop in this locality. He, however, is inclined to consider the ciliary as a sympathetic derivative.
N. oculomotorius. The oculomotor nerve (NV. oc.) arises from the thick ventral zone of the mesencephalon by several small rootlets. It extends anteriorly, laterally and ventrally to a developing eye muscle situated lateral to the infundibular region, posterior to the eye vesicle and medial to the anterior cardinal vein.
N. trochlearis. The rootlets of the trochlearis (NV. troch.) are small and are not grouped to form a definite bundle. They leave the brain at the dorsal border of the isthmus and can be traced only a short distance.
N. trigeminus. The trigeminal nerve (N. iri.) consists of a sensory (portio major) and a lateral motor division (portio minor).
It is conspicuous on account of the large semilunar ganglion (G. sl.) connected with it. It gives rise to three branches, the ophthalmic, the maxillary and the mandibular. The ophthalmic leaves the anterior ventral portion of the ganglion and soon gives off a dorso-laterally directed nerve, the frontal (N. fr.) which is related to the ectoderm in the immediate vicinity. A small clump of cells is connected with the nerve near its termination. In front of the right semilunar ganglion, and just above the termination of the frontal there is another small group of cells. A corresponding group on the left did not exist. Beyond the frontal, the nasal division (N. na.) of the ophthalmic extends anteriorly and dorsally above the eye stalk.
The maxillary (NV. mz.) arises from a considerable ventral lobe of the semilunar ganglion, and extends into the maxillary process of the first arch behind the lachrymal groove. As scattered fibres it can be traced as far ventrally as the optic stalk.
The mandibular (N. md.) extends, from the posterior ventral border of the ganglion, laterally and ventrally to the mandibular process (Mdb.) of the first arch. It can be followed some distance below the angle of the mouth. It contains the portio minor or motor part of the trigeminal.
The sensory roots of the trigeminal enter the brain opposite the first and second of the rhombic neuromeres represented for thisembryo. The motor fibres arise from the neuroblasts situated in the basal portion of the same neuromeres.
N. abducens. The abducens (N. abd.) arises from the ventral zone of the fourth rhombic neuromere directly medial to the lower end of the otocyst (V. aud.). That on the right side shortly after it leaves the brain possesses a small aberrant branch coming from the region of the glossopharyngeus or vagus. The nerve extends anteriorly to a dense mass of cells separated from the semilunar ganglion by the anterior cardinal vein.
N. facialis. The motor roots of the facial arise from neuroblasts in the ventral zone of the third rhombic neuromere. At their origin the rootlets are somewhat separated, but as they issue from the metencephalon, they are four or five in number. They soon converge to a single bundle which passes along the posterior border of the geniculate ganglion (G. gn.), and extends posteriorly and laterally along the posterior border of the first entodermal pouch, in intimate association with a bundle of sensory fibres issuing from the neuroblasts of the geniculate ganglion. Just behind the closing plate of the first pouch the associated bundles divide into a posterior and an anterior division, the facial and the chorda tympani (N. ch. tymp.) respectively. The facial can be traced for a short distance into the hyoid arch. The chorda tympani winds around the ventral process of the first entodermal pouch and enters the mandibular process. The geniculate ganglion (G. gn.) is an elongated mass of cells. It extends from the median surface of the dorsal diverticulum of the first pharyngeal pouch, to which it is for a distance closely applied, to a level of the middle of the otocyst. Its fibres, the intermedius, enter the metencephalon in the dorsal zone at the posterior border of the third rhombic neuromere under cover of the entering roots of the acustic ganglion (G. acus). From the ventral tip of the geniculate ganglion a small nerve massed with cells, passes anteriorly and ventrally along the dorsal median border of the auditory pouch. This is the great superficial petrosal.
N. acusticus. The ganglion of the auditory nerve (G. acus.) is an elongated mass of cells closely applied to the middle anterior median border of the otocyst. In the middle portion where an upper group of fibres to the ear will later appear, the ganglion is fused with the epithelium of the otocyst. From the ventral end of the ganglion a small nerve extends posteriorly to the epithelium of the lower median portion of the otocyst. According to Streeter (’08), this later will innervate the posterior ampulla and the saccule. The entering fibres from the extreme dorsal part of the acustic ganglion pass into the dorsal zone of the metencephalon opposite the third rhombic neuromere as two rootlets just above the entering intermedius.
N. glossopharyngeus. The glossopharyngeal (N. glos.) like the vagus to be described presently is characterized by possessing two ganglia, the ganglion superius (G. sup.), and the ganglion petrosum (G. petros.). The latter is more sharply outlined than the former-and intimately connected at its lower end with the posterior border of the dorsal diverticulum of the second pharyngeal pouch, and the adjacent ectoderm. This nerve possesses both sensory and motor fibers. The more numerous sensory rootlets are covered with ganglion cells continuous with those of the superior ganglion. They enter the dorsal zone of the myelencephalon opposite the fifth rhombic neuromere. A ganglion crest or bridge still connects the superior ganglion with the jugular ganglion (G. jug.) of the vagus. The motor fibres arise from neuroblasts in the mantle layer of the ventral zone of the fifth rhombic neuromere. They extend laterally through the marginal velum of the brain and emerge as a number of small rootlets ventral to the sensory components. The glossopharyngeal divides distally sending a branch, the tympanic (N. tymp.), down over the dorsal diverticulum of the second pharyngeal pouch into the hyoid arch. Posterior to the dorsal diverticulum it divides into two small branches, an anterior, the lingual ramus and a posterior, the pharyngeal ramus.
N. vagus. The vagus (N. vag.) possesses a dorsal ganglion the jugular(G. jug.) and a ventral one, the ganglion nodosum (G. nodos.) The jugular ganglion is large and 1s connected with an extensive posterior portion, a part of the neural crest, which ends in an S-shaped group of cells. The cells in the posterior limb of the § are more compact than those in the anterior limb, and fibres from its dorsal apex enter the brain as a distinct root. The ganglionic crest connecting with the jugular, is the undifferentiated representative of several ganglia, companions of groups of the more ventral roots composing the hypoglossal nerve (NV. hyp.). The jugular ganglion is also continuous with cells which overlie the sensory roots quite close to their entrance to the brain. These roots are here represented diagrammatically. The ganglion nodosum is also quite elongated, and is covered externally to a considerable extent by vagus fibres. It 1s connected intimately with the ectoderm of the posterior wall of the fourth ectodermal groove.
The vagus contains both sensory and motor fibres. The sensory fibres arise from cells of the jugular ganglion, including the ganglionic crest. The motor components arise in part from neuroblasts in the dorso-lateral part of the ventral zone of the sixth rhombic neuromere. These fibres leave the brain under the entering sensory roots. Another portion of its motor fibres are derived from the spinal accessory to be described presently. Posterior to the third pharyngeal pouch the vagus gives off an anterior branch the superior laryngeal (NV. lary’g. sup.). Beyond this it passes behind the pulmonary arch and can be traced for a distance where it is ventral and lateral to the oesophagus. The auricular and recurrent branches of the vagus are not formed in this embryo.
N. accessorius. The spinal accessory (N. acc.) is derived from small rootlets issuing from the dorso-lateral part of the ventral zone of the myelencephalon and the spinal cord as far posteriorly as the sixth cervical ganglion. Caudally the rootlets issue from the spinal cord below the main trunk of the accessory and much nearer the ventral roots than they do more anteriorly, showing somewhat of a gradual transition into ventral roots. Lubosch (99) who has made a comparative study of the accessory nerve, concludes that as a general rule its more posterior rootlets are nearer the dorsal roots of the spinal ganglia. Exceptions, however, to this rule were noted (pp. 530 and 545). The spinal accessory composed of only a few fibres in its posterior part, gradually increases in caliber as it extends forward under the spinal ganglia. It finally turns ventrally along the posterior border of the vagus. Below the jugular ganglion it becomes closely connected with the vagus, contributing to its (vagus) motor part. A portion of the accessory, however, bends dorsally, away from the vagus for the later innervation of the trapezius and sternomastoid muscles.
N. hypoglossus. The hypoglossal (NV. hyp.) 1s a composite motor nerve arising by several rootlets from the ventral part of the ventral zone of the myelencephalon. These rootlets soon converge to form the main trunk. The corresponding dorsal ganglia are represented by an undifferentiated portion of the neural crest which, however, shows indications of ganglion formation in its posterior part.
Portions of the anterior five cervical nerves (Nn. sp. 1-8.) and their ganglia are shown in the reconstruction. The ganglia are still joined together by portions of the neural crest. Ganglion cells continuous with those of the spinal ganglia cover the sensory rootlets close to their entrance into the spinal cord.
The extent of the nasal invagination is represented in fig. 2. It is a shallow laterally compressed pit (Ff. na.). Its thickened epithelium does not reach the wall of the oral cavity.
The right optic vesicle (V. op.) with its connecting stalk 1s represented in fig. 3. It is cup-shaped on its lateral surface except for a groove which leaves the cup, and extends along the border of the stalk toward the brain. The lens is still connected with the ectoderm by a slender stalk cut across in the drawing.
The otocyst (V. aud.) is also represented in fig. 3. It is an oval ectodermal vesicle lying adjacent to the lateral surface of the rhombencephalon. Above it projects the ductus endolymphaticus which arises from its median dorsal surface.
In fig. 1 the heart is represented as sectioned on the left side of the median line. The trabeculae of the ventricle here as also in fig. 2, are represented diagrammatically. The left atrium is now largely separated from the right by the septum primum (atrial septum) united ventrally with two thickenings of the atrial canal. Between these thickenings a small ventral passage still persists which unites the two atria. It is the interatrial foramen (Ff. 7a.). A secondary opening in the septum primum the foramen ovale (F.0.) occurs more anteriorly and dorsally.
In the posterior median part of the left atrium there is a small opening marking the outlet of the single pulmonary vein (V. pul.). Ventral to this vessel the left common cardinal vein is cut obliquely. Between the left atrium and the corresponding ventricle a median antero-posterior ridge formed by the union of the thickenings of the artial canal, represent the anlage of the right portion of the bicuspid valve. Immediately ventral to this ridge there is a large opening, the interventricular foramen (fF. iv.) bounded ventrally by the free edge of the ventricular septum. Through this opening the two ventricles communicate. Later in development it comes to open into the ventral aorta (Ao.v.) by the union of the aortic and ventricular septa. The aortic septum is already indicated in the aortic bulb, fig. 2, by two folds or ridges, a and b. The ventricular septum connects with the left fold (6) while the one on the right (a) connects with the right ventricular wall just anterior to the atrio-ventricular foramen. The left ventricle already has a much thicker wall of muscular trabeculae than the right.
In fig. 2. the heart is sectioned on the right of the median line. Into the right atrium (Al. d.) opens the sinus venosus (S. 2.) between its two valves which unite anteriorly and extend along the atrial wall as aridge, the septumspurium. The atrium opens into the corresponding ventricle by the right atrio-ventricular foramen. On the right side of this foramen is figured the anlage which contributes to the formation of the tricuspid valve. The right ventricle opens into the bulbus arteriosus. This outlet is being subdivided by the formation of the aortic septum as already described. The permanent outlet will be through the pulmonary trunk which as indicated in the figure, leaves the antero-ventral part of the ventricle and passes dorsally and medially across the first portion of the aortic channel.
The arteries are figured in figs. 1 and 2. In fig. 2 the ventral aorta (Ao. v.) after leaving the heart is seen to give off on the left five aortic arches one to each visceral arch. Their relation to the pharyngeal pouches is clear. The slender first and second arches on either side arise from a common trunk as the ventral aorta bifurcates just behind the median thyroid | (Th. med.). In fig. 1 the corresponding aortic arches on the right are represented as showing through the pharyngeal wall. The intermediate portion of the second arch on the right has become obliterated. The most posterior or pulmonary arches on account of the developing aortic septum leave the aorta by a short common trunk. A little above the bifurcation of this trunk each pulmonary arch gives off posteriorly a small pulmonary artery (A. pul.). The right pulmonary artery arises from the corresponding arch by two small branches. The aortic arches of each side extend around the lateral border of the pharynx and join the descending aorta (Ao. desc.) of the corresponding side. The two descending aortae extend caudally and unite near the cardiac end of thestomach to form the median dorsal aorta (Ao. med. dor.). Each descending aorta from where it. is joined by the first aortic arch is continued anteriorly into the maxillary arch posterior to the optic stalk. This represents a portion of the internal carotid artery (A. car. int.). It gives off a branch above the optic stalk which represents the anterior cerebral artery. It is here joined by the posterior communicating branch (A. com. post.) of the basilar (A. bas.) which gives off in front of the oculomotor nerve the posterior cerebral. Caudal to the posterior communicating branches the anterior part of the basilar is a single vessel and gives off several lateral branches, but along the greater part of the pontine bend it is represented by two vessels intimately connected by cross branches. From each of these vessels numerous branches extend laterally. Posteriorly these two longitudinal vessels merge with a complicated system of capillaries spreading along the ventral and lateral surfaces of the spinal cord.
Six intersegmental arteries arise from each descending aorta (Ao. desc.). From the median dorsal aorta corresponding pairs are given off at regular intervals along its course to the tail. Those on the left are represented as cut away. Each intersegmental artery extends from the aorta dorsally and laterally to the lateral surface of the spinal cord alongside the corresponding spinal nerve. From the seventh intersegmental artery the most anterior of those arising from the median aorta, a lateral branch, the subelavian extends into the upper limb-bud. A series of ventrolateral branches from either side of the aorta extend to the glomeruli of the corresponding mesonephros.
The most anterior of the median ventral branches of the aorta is the coeliac axis (A. coel.) which arises opposite the pyloric end of the stomach. The omphalo-mesenteric (Aa. omp. mes.) is represented by three median branches which anastomose in several places. In their course to the yolk sac, they cross the intestine on the right and give off branches to the mesentery. The two anterior roots of the omphalo-mesenteric willlater atrophy the posterior persisting to form the superior mesenteric artery. In a human embryo of 5 mm. Tandler (’03), found that the omphalo-mesenteric artery was represented by four or five median branches from the aorta. These united to form a ventral longitudinal anastomosis extending downward in front of the aorta. Later in development all these roots disappeared with the exception of the posterior one which formed the main stem of the superior mesenteric artery.
Each common iliac (A. JI. com. d. and s.) arises from the aorta by two short rootlets (cut off on the left) and extend laterally and ventrally to the region of the posterior limb-bud to which it gives off a branch, theexternal ihac. It then continues forward and passes into the umbilical cord as the umbilical branch. The original vessels which connected the umbilical arteries with the latero-ventral wall of the aorta are now represented by a single trunk (A. mes. inf.) just anterior to the origin of the iliacs. This soon divides after leaving the aorta into right and left branches each of which can be traced ventrally through a strand of tissue passing across the posterior part of the coelom to the corresponding umbilical branch. This vessel which earlier represented a part of the umbilical arteries apparently becomes the inferior mesenteric artery of the adult.
The vitelline vein (V. w., fig. 1.) arising on the surface of the yolk sac passes into the mesentery and extends parallel with the first limb of the intestinal loop, finally crossing it on the left a little posterior to the pancreas. Here it is joined by another vein of about the same caliber, the superior mesenteric (V. mes. sup.) which arises in the mesenchyma of the mesentery by numerous tributaries. The common trunk known as the portal vein (V. p-) divides into two branches corresponding to the original right and left vitelline vessels. The left branch is small. It passes on the left of the dorsal pancreas where it is resolved into a plexus of small vessels extending in the mesenchyma of the mesogastriumm. It originally extended on to the liver. The right branch extends anteriorly between the ventral pancreas below and a subdivision of the dorsal pancreas above. It then bends to the right and enters the liver (V. p., fig. 2). In the hver it opens into the sinusoids which represent subdivisions of the vitelline veins, (Minot,’ 00).
The umbilical veins secondarily have acquired an opening into the hepatic sinusoids. The right (V. um. d.) represented in fig. 2, anastomoses with the larger one on the left within the cord. It then passes anteriorly through the somatopleure of the ventral body wall, receiving tributaries in its course from the posterior limb-bud and body wall. It finally enters the liver and breaks up into the hepatic sinusoids. The left umbilical upon entering the liver passes, as a large channel, the ductus venosus (D. v.) through the sinusoids toward the heart.
From a plexus of veins overlying each dorso-lateral surface of the prosencephalon from which the superior longitudinal sinus will later develop a lateral branch extends on either side across the diencephalon. The one on the right is here represented. Dorsally this 1s joined by several veins coming from the region of the mid-brain, isthmus and anterior part of metencephalon. Ventrally it is Joined by the ophthalmic vein (V. oph.) which arises from a plexus in the mesenchyma near the nasal epitheluum, and coursing along under the lachrymal groove receives branches from the region of the eye. The part of the anterior cardinal (V. card. ant.) formed by these branches is internal to the semilunar ganglion and has been called the cavernous sinus. The anterior cardinals then extend posteriorly, lateral to the ventral wall of the rhombencephalon and outside the otocyst and cranial nerves with the exception of the hypoglossal which as it passes forward crosses the vein on its outer side. Behind the otocyst each anterior cardinal receives a large branch proceeding from a plexus of capillaries overlying the lateral surface of the myelencephalon. Posteriorly it passes to the common cardinal vein (V. card. com. d.). Just anterior to the common cardinal its dorsal wall is connected with an anastomosing plexus of vessels with which the most anterior intersegmental veins unite. While ventrally it receives the linguo-facial vein (V. ling.-fac.) (Grosser 01, Lewis ’03 and ’09) arising on the corresponding side of the branchial arches. Posterior to the opening of the linguo-facial vein, from the median side of each anterior cardinal, small branches extend in front of the trachea. These eventually will form the vena anonyma sinistra.
The anterior and posterior portions of the right posterior cardinal (V. card. post.) are well defined. In the region of the mesonephric tubules it has been divided into Wolffian sinusoids (Minot ’00), and its course along the dorsal border of the Wolffian body has been interrupted in places. Into the ventral wall of the persisting portions of the vein enter numerous sinusoids which extend around the lateral side of the metanephros. Four of these are represented in the drawing. In a similar manner a series of sinusoids earlier passed around the mesial side of the metanephros and connected with the posterior cardinal. Between these median sinusoids there now has formed on either side a longitudinal anastomosis, the subcardinal vein (V. scard. d.), (Lewis 702). The dorsal portions of some of the median sinusoids on the right are seen, either connecting the subcardinal and posterior cardinal or upon the interruption of the latter, taking over to the sub-cardinal the thus isolated parts with the entering intersegmental veins. The ventral portions of the median sinusoids have in part atrophied. The subcardinal on the left connects anteriorly with the posterior cardinal. The subcardinal on the right which forms a part of the inferior vena cava has made a secondary connection with the ductus venosus by a ‘‘tapping of the hepatic sinusoids”? (Lewis, ’02). Its connection with the corresponding posterior cardinal has been lost, and its anterior extremity is now a small branch arising from its dorsal wall near where it bends toward the liver. At the lower border of the liver a small sinusoidal connection with the subcardinal is seen. Between the right and left subcardinals twelve places of anastomosis were found. The four posterior, situated a little below the posterior or permanent root of the omphalomesenteric artery, probably coalesce to form the large renal anastomosis represented for the 12 mm. pig embryo by Lewis.
It also seems probable that in the pig between the subcardinal and posterior cardinal divisions of the inferior vena cava there is formed an intermediate portion also from Wolffian sinusoids, but dorsal to the mesonephric arteries and medial to the posterior cardinal. An indication of this vessel is shown united anteriorly with the subeardinal by a broad connection but with slender cross connections posteriorly. The posterior cardinal in this region is slender and its continuity in places is lost. The right subclavian vein (V. scl. d.) arises from the posterior cardinal between the seventh and eighth intersegmental vessels.
A large vein from the liver, the vena hepatica communis (V. hep. com.) extends anteriorly, and unites with the right and left common cardinal veins to form the sinus venosus which opens into the right atrium as already described.
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Explanation of Figures
ABBREVIATIONS FOR Fig. l
All, allantois Isth. isthmus
Ao. cau. aorta caudalis M, mouth
Ao. desc. Ao. descendens Mab. processus mandibula Ao. med. dor. Ao. mediana dorsalis ris
Arc. aor. 4 arcus aorticusquartus Med. sp. medulla spinalis
Arc. pul. arcus pulmonalis Mesen. mesencephalon
A. bas. arteria basilaris Met. metanephros
A. car, int. A. carotis interna N. sp. sec. nervus spinalis secun A. coel. A. coeliaca dus
A. com. post. A. communicans pos- Neu. a neuromere terior Oe. oesophagus
A. wl. com, d. A. iliaca communis Pane. d. pancreas dorsale dextra Pane. v. pancreas ventrale
A. il. com. 8. A. iliaca communis S.P. Seesel’s pocket sinistra St. stomach
A, mes. inf. A. mesenterica in- Telen telencephalon ferior Th. med. thyreoidea mediana
Aa.omp.-mes. Aa. omphalo-mesen- 7’. 1. tuberculum impar tericae Ir. trachea
Ch. d. chorda dorsalis Um. C. umbilical cord
Cl. cloaca V. hep. vena hepatica
Dien. diencephalon V. mes, sup. VY. mesenterica
D.W. ductus Wolffii superior
Engl. epiglottis V. p. V. portae
F ia. interatrial foramen V. pul. V. pulmonalis
F, tv, foramen interventri- V. v1. V. vitellina culare ‘ x, pharyngeal diverti Fo. foramen ovale culum
Hyp. hypophysis Yk. 8. yolk stalk
A. car. A. bas. S.P. int. Neu. Mdb, T, i, Th. med. Epgl. x
Ao. dese. Ao. ¥,
A. car. int, Aa. mes. A. pul. At. d,
Ch. d. D. wv.
D. W. F. na. Med. sp. Mes. Mesen. Met. Meten. Myelen. Ph. P. 1, 2, 8, 4
Fig. 1 Reconstruction from a pig embryo of 7.8 mm. (Harvard Embryological Collection 1358). The drawing illustrates especially the arterial system and the epithelial portion of the digestive tract and its appendages. It also shows the left atrium and ventricle of the heart, and the interior of the brain and spinal cord. x .15 diams.
ABBREVIATIONS FOR FIG. 2
right fold or ridge of aortic septum aorta descendens aorta ventralis arteria carotis interna Aa. mesonephricae A. pulmonalis atrium dextrum left fold or ridge of aortic septum chorda dorsalis ductus venosus ductus Wolffii . fossa nasalis medulla spinalis mesonephros mesencephalon metanephros metencephalon
S. v. Th. med. VY. card, ant.
. card, com. a. . card. com. s.
card. post. hep. com. ws. ling.-fac. oph.
. scard. d.
. sel. d. .umb, d. Ven. a.
SN NNNSNNXN OS OS
vena cardinalis anterior
V. cardinalis communis dextra
V. cardinalis communis sinistra
V. cardinalis posterior
V. hepatica communis
V. subcardinalis dextra
V. subclavia dextra
V. umbilicalis dextra
car. Vv. Meten. int. " Ph. P2 -.? nica Pe ; Th, med. Myelen. M tou ’ a mest esen. fi - aattne LF _ ff More if ae i - “4 ey Z es 5 f * 4 2 7 a f ey a. +a V. oph..j | ee = ¥ N j b. 2 x PPP ey = - r a. Fo na s in, At. d aa * ? Z Ven. d. Z ue 2 be V. card. DA he com. &. By Uy iia we H tj, “tts, ( eras ae rm ‘ i sae ni ¥ \ Met! > ¢ D. W. _ § 4 > V. card.post. Aa. mes.
i Ao. desc.
ae Ph. P. 4
A. pul. V. ling-fac.
V. card. com. d,
V. hep. — com.
V. card. post,
V. scl. d.
V. scard. d.
Fig. 2 Reconstruction from a pig embryo of 7.8 mm. (H. E. C. 1358) It represents chiefly the venous system of the right side, and the right atrium and ventricle of the heart, and shows the relation of the aortic arches and pharyngeal pouches of the left side. XX .15 diams. 44
D. acus. G. gn.
G. jug. G. nodos. G. petros. G. sl.
G. sup. Mab.
Med. sp. Mesen. Meten. Myelen. N. abd.
N. ch, tymp.
ABBREVIATIONS FOR FIG. 3
N. chorda tympani
md. me. na.
. oc. Nn. sp. N., th.
N. troch. N. tymp. N. vag. S. cer. T'elen. V. aud. V. op.
. lary’ g. sup.
N. laryngeus superius
Nn. spinales 1-5
G. G. VY. G. jug. sup. aud. acus. G. gn. N. abd. Meten.
Myelen. N. glos.
N. lary’ g-Sup.
Nn. sp. 1, 2, 3, 4,5, G. Ss. G. N. WN. ch. Mdb. N. md. Telen. nodos. cer. petros. tymp. tymp.
Fig. 3 Reconstruction to show the branchial arches, the exterior of the brain, and the cranial nerves of a pig embryo of 7.8 mm. (H. E. C. 1358) X .15 diams.
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