Book - A Laboratory Text-Book of Embryology 4 (1903)

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Minot CS. A Laboratory Text-Book Of Embryology. (1903) Philadelphia:P. Blakiston's Son & Co.

A Laboratory Text-Book of Embryology: 1. General Conceptions | 2. Early Development of Mammals | 3. Human Embryo | 4. Pig Embryos | 5. Chick Embryos | 6. Blastodermic Vesicle and Ovum Segmentation | 7. Uterus and the Foetal Appendages in Man | 8. Methods | Figures | Second edition | Category:Charles Minot
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This historic 1903 embryology textbook by Minot describes human development.

This textbook was republished in a second edition 1917: Minot CS. A Laboratory Text-Book Of Embryology. (1917) Philadelphia:P. Blakiston's Son & Co.

See also his earlier 1897 textbook; Minot CS. Human Embryology. (1897) London: The Macmillan Company.

Minot Links: Harvard Collection | 1889 Uterus And Embryo - Rabbit | 1905 Harvard Embryological Collection |1897 Human Embryology | 1903 A Laboratory Text-Book of Embryology | 1905 Normal Plates of Rabbit Embryo Development | Category:Charles Minot

See also: Historic Embryology Textbooks

History Links: Historic Embryology Papers | Historic Embryology Textbooks | Embryologists | Historic Periods | Historic Terminology | Human Embryo Collections | Carnegie - Contributions to Embryology | 17-18th C Anatomies | Embryology Models | Category:Historic Embryology
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Pages where the terms "Historic Textbook" and "Historic Embryology" appear on this site, and sections within pages where this disclaimer appears, indicate that the content and scientific understanding are specific to the time of publication. This means that while some scientific descriptions are still accurate, the terminology and interpretation of the developmental mechanisms reflect the understanding at the time of original publication and those of the preceding periods, these terms and interpretations may not reflect our current scientific understanding.     (More? Embryology History | Historic Embryology Papers)

Chapter IV. Study Of Pig Embryos

Methods of Obtaining Embryos

The pig is recommended for embryological study because specimens of the embryos in sufficiently earl}- stages can be obtained at the larger packing establishments in considerable numbers and with little trouble or expense. When this material is not obtainable, rabbit embryos may be substituted, as these animals are easily kept and breed freely (compare page 305). Owing to the enormous precocious development of the chorionic vesicle in pigs, it produces an enlargement of the uterus which is usually sufficient, by the time the embryo has attained a length of 6 mm., to be observable to the untrained eye. It is, therefore, only necessary to ask the man who removes the viscera from the pigs to lay aside for examination all of the uteri which appear distended. These should not be turned about violently, but handled carefully and should be opened immediately. As soon as the ovum is exposed it will probably-be ruptured, and there will occur a free outflow of opalescent fluid, amniotic and allantoic. With the aid of fine forceps and a horn spoon the embryo may be lifted up — and it should on no account be directly touched — and transferred to a dish containing Muller's fluid, in which the specimen should remain for five or ten minutes. It is then transferred with the help of the horn spoon to Zenker's fluid. Metal instruments cannot be used on account of the corrosive sublimate in the Zenker solution. In one or two hours the embryo should be transferred to fresh Zenker solution and left therein a varyirrg length of time, according to the size of the specimen. In general it may be said that for — Pigs of 6 to 9 mm., 12 hours.

" " 12 " 24 " " " 15 " 36 " " '• 201025 " 4 s " It is undesirable to leave any specimen in the Zenker solution more than forty-eight hours. The Muller's fluid is used for cleaning the specimen. It causes a granular, non-adherent coagulum to form from the foetal fluids. If the specimen is put directly into Zenker's fluid, a fibrous coagulum is formed which often adheres closely to the embryo so as to obscure its shape. Such a fibrous coagulum cannot be removed without injuring the embryo. After having remained a proper length of time in the Zenker solution, specimens are further washed for twenty-four hours in running water, and then treated with alcohol and iodine in the usual manner.

The Making of Serial Sections

Specimens should be colored with alum cochineal in toto, then imbedded in paraffin and cut into serial sections according to the directions given in Chapter VIII.

Selection of the Planes of Section and the Stages for Practical Study.

It is customary to distinguish three fundamental planes — the transverse, the sagittal, and the frontal. It is impossible to so define these planes that the definition shall be exact for all stages. But in general it may be said, the reference being had to the entire embryo, that the transverse plane is one which will be at right angles to the notochord and medullary tube at the level of the heart ; that the frontal plane will be one at right angles to this, passing symmetrically through the limbs of the embryo; and, finally, that the sagittal plane is one parallel to the median plane of the body. As in younger embryos the form is very asymmetrical, both the head and caudal end of the embryo being twisted to one side, the planes which would be true for the body of the embryo in the region of the heart would not be true elsewhere. For the practical use of the student, therefore, in these younger stages it is better to determine the direction of the plane by the floor of the fourth ventricle, so that by " transverse" will be understood a plane of section which cuts the head of the embryo symmetrically, no matter how it may cut the body, and which runs parallel to the floor of the fourth ventricle (medulla oblongata). The frontal plane should be perpendicular to this and also cut the head of the embryo symmetrically. The sagittal plane in these cases is also that of the head and not of the body. Such planes are recommended because in the study of the sections more is gained by having the planes readily understood in the region of the head than in the region'of the body. In later stages, when the body has become straighter, the difference in planes for the head and the body may be practically left out of consideration, except that for the heads of older pigs when they are cut alone, — as on account of the size of the body is often desirable, — the frontal plane is chosen so as to run at right angles to the plane of the palate and symmetrically through the embryo. Sections through the head at right angles to this may be designated as horizontal.* Students will find that it is very much easier to study transverse and frontal sections when they are symmetrical. No pains, therefore, should be spared to orient the embryo properly in the microtome before the sections are cut.

Selection of the Stages. — The most profitable stage to study is that of an embryo of from 9 to 13 mm. in length. Each student should have three specimens of this stage, and it is advantageous that the specimens given each student be approximately of the same size. The embryos should be first studied carefully as to their external form and then cut into serial sections in the transverse, sagittal, and frontal planes. Of these, the transverse series should form the principal basis of study, and the other series should be used principally to clear up the student's conception of the relation of parts. Embryo pigs of the size specified have the typical class characteristics of mammalian embryos, and may readilv be distinguished from the embryos of any other class of vertebrates. The differentiation of the anlages of all the important organs is accomplished, so that these anlages can be identified with certainty and their genetic relations to the adult structures can be clearly grasped by the student. At the same time, although the anatomical differentiation is well advanced, the histological differentiation has made very little progress, so that the stages in question are particularly instructive to beginners. The anatomy of the pig at this stage is, therefore, readily understood by the student who knows the general anatomy of the adult. Older embryos are more complicated and yield such long series of sections that the beginner is apt to be discouraged. Younger embryos, owing to their spiral twisting, are exceedingly difficult for students to understand when sectioned. After having thoroughly mastered the structure of the pig embryo of from 9 to 13 mm., the student may advantageously extend his study of embryos to other sizes. If, as is done in this work, the principal study is made with embryos of 1 2 mm., the student may proceed to make sections of other stages as follows :

Pig embryo of 9 ram., transverse and sagittal series.

" " " 6 " transverse series.

'* " " 17 " transverse series.

" " " 20 " transverse and sagittal series.

(Of the head alone, the frontal series.) Pig embryo of 24 mm., of the head alone, frontal series.

The Study of the External Form

The student should make a careful and thorough study of the external form of every embryo, and make, with the aid of the camera lucida, an exact drawing of every embryo before he cuts it into sections. He will soon learn that such a drawing is almost indispensable for the interpretation of the sections.

  • The system of planes here described is that adopted for the Harvard Embryological Collection, and has been found convenient in practice.

Under the following paragraphs, embryos of 12, 15, and 20 mm. are figured and described from specimens which have been hardened in Zenker's fluid and preserved in alcohol. The description of these stages will be sufficient to enable the student to understand any of the embryos he is required to study.

Fig,. 99. — Pic. Embryo of io mm. A.L, Anterior limb. An, Auditory, or first gill cleft. C.S, Cervical sinus. Mi, Mandibular process. M.L, Milk-line, ,1/t, Maxillary process. N, Nasal pit. Op, Eye. P.L, Posterior limb. Seg, Muscular segment, (/hi, Umbilical cord. Ven, Floor of fourth ventricle (medulla oblongata). > 8 diams.

Pig Embryo of 10 mm. (Fig. 99). — The head, which is very large, in comparison witli the body, forms as a whole nearly a right angle with the back, so that the dorsal outline of the head forms a distinct though rounded angle with that of the back ; this angle marks the position of the neck bend, and corresponds to the junction of the brain with the spinal cord. The neck bend is one of the most marked and distinctive characteristics of the mammalian embryo, being much less developed in birds and reptiles, and being absent in amphibians and fishes. It is probably closely correlated with the cramping of the ventral cervical region, which leads to the formation of the cervical sinus, C.S, and to the disappearance of the second to fourth gill clefts. The neck bend is so great that the mandibular and nasal regions of the embryo are closely appressed to the cardiac region of the body proper. The cephalic region has a second flexure, the head bend proper, which marks the site of the mid-brain, and in the figure appears as a rounded angle obliquely above the eye. From the mid-brain one axis, horizontal in the figure, extends backward through the region of the fourth ventricle, or hindbrain, Ven, to the neck bend, while the other axis extends vertically downward to the region of the fore-brain, which is marked by a rounded protuberance in the outline of the head. The dorsal outline of the body proper forms a long sweeping curve, ending in the tail; this dorsal curvature is another characteristic of the amniote embryo, the back in the embryo of fish-like forms being relatively straight. It is thus brought about that the dorsal side of the embryo is two or three times as long as the ventral. From the ventral side springs the large umbilical cord, the connection of which with the body occupies practically the entire length of the ventral median line of the abdominal region proper. Above the umbilical cord the protuberant outline of the cardiac region passes below the nasal, A, and mandibular, Md, regions toward the cervical sinus, C.S. The long tapering tail extends near the umbilical cord.

The surface modeling of the embryo offers important features. Beginning with the head, we observe first the shallow depression 'constituting the nasal pit, A*. The eye, Op, is entirely without lids; the lens appears in the center and is surrounded by the outlines of the optic vesicle. The small size of the eye is a characteristic of the mammalian embryo ; by which it differs from all sauropsidan forms, but in certain other mammals the embryonic eye is slightly larger than in the pig. Below the eye is the maxillary process, Mx, which is destined to form the greater part of the upper jaw; the anterior boundary of the maxillary process is marked by a shallow depression, the lachrymal groove, which runs from the angle of the eye, Op, to the nasal pit, A. The mandibular process, Md, out of which the lower jaw is developed, is bounded in front by a groove separating it from the maxillary process, and behind by a second groove, Au, the anlage of the future meatus auditorius externus. This groove marks the boundary between the mandibular process and the first or hyoid branchial arch, and is itself the ectodermal member of the first gill cleft. The fourth ventricle, Ven, or cavity of the hind-brain, having very thin walls for its roof, can be readily distinguished. The thickened floor of the fourth ventricle is the anlage of the medulla oblongata. The cervical sinus, C.S, is an area of invagination, presenting at this stage a triangular outline; within the sinus are found the external or ectodermal terminations of the second and third gill clefts. The territory of the mandibular process and cervical sinus corresponds to the pharyngeal region. It is the site of some of the most important, interesting, and complicated developments by which the embryonic is changed into the adult anatomy. The body of the embryo shows the position and number of the segments, Seg, by the external modeling. Both limbs are well advanced, the anterior, A.L, more so than the posterior. From the base of the front to the base of the hind limb extends the milk-line, M.L, curving so as to be nearly parallel to the dorsal outline of the body. Along this line the mammary glands are ultimately developed. Extending across the body are several shadowy lines, shimmering through the translucent body-walls. One marks the position of the embryonic diaphragm ; it extends from the upper edge of the anterior limb obliquely downward toward the edge of the umbilical cord. Another, which extends in a nearly straight line from limb to limb, marks the ventral edge of the large Wolffian body or mesonephros, the dorsal limit of which is approximately indicated by the milk-line, M.L. The outlines of the smaller left dorsal lobe of the liver are distinct, and mark out a pointed area immediately below the forelimb, A.L.

Pig Embryo of 12 mm General Anatomy

Anatomical Reconstruction from the Sections[1] — The four figures herewith presented are based upon the same series of transverse sections (No. 5) from which figures 113 to 121 were drawn. The reconstructions have been made by the method described in Chapter VIII. The actual drawings, especially as regards their shading to indicate the modeling of the surfaces, have been partly made from a wax model of the brain and a wax model of the cavity of the pharynx of the same embryo. The umbilical cord of the embryo used having been damaged, the loop of the intestine in the umbilical cord has been added by a reconstruction from another scries (No. 518) of an embryo of the same size.

Reconstructions are of the greatest assistance in the study of sections, and much facilitate the identification of all the parts. Students using this book should, while examining their sections, constantly refer to the reconstructions. It is unnecessary to give elaborate descriptions of each of them, since the explanations of the lettering of the figures will suffice for the identification of all the parts shown. Certain brief explanations as to each of the figures are, however desirable. They illustrate the general topographical distribution of the organs. The great volume of the central nervous system as compared with the remaining parts is very striking. Of the other organs, the three which are most conspicuous by their size are the heart, liver, and Wolffian bodies. Another striking peculiarity of the embryo is the great diameter of the blood-vessels, and especially of the veins, which are of relatively enormous diameter, being proportionately much larger than in the adult. In marked contrast to this is the small diameter of the cavity of the trachea and lungs and of the entire intestinal canal. Figure 101 represents in the main a median section of the embryo together with the organs of the right side, but with two exceptions, first, the floor of the pharynx is represented as if cut through considerably to the left of the median plane; second, the heart cut to the left of the median plane. The brain and spinal cord are drawn as if opened to show the modeling of the inner surface of the medullary tube. The pharynx is so drawn as to indicate something of the modeling of its floor surface. The opening of the veins into the heart and of the auricle into the ventricle, and the interventricular orifices, are shown. Of the digestive canal only the entoderm is represented, so that the figure displays the entodermal walls of the cesophagus, stomach, and intestine, and shows the two pancreatic anlages. Similarly only the entodermal portion of the trachea and lungs is included, and the same is true of the caudal end of the Wolffian duct and of its outgrowth, which forms the anlage of the kidney. The same is further true of the gall-bladder, of which only the epithelial portion is represented. In this figure the arterial system is fully displayed. The pulmonary artery and the aortic trunk are completely separated. A small artery from the pulmonary arch to the lungs is included, and the figure shows the entire system of branches from the main aorta.

Fig. 100.

Fig. 101. — Pig Embryo of 12.0 mm. Reconstruction from the Transverse Sections, Series 5. For the most part the organs represented are in or near the median plane. The drawing illustrates especially the disposition of the alimentary tract, the arterial system, and the heart. A.bas, Basilar artery, formed by the union of the two vertebral arteries, A. v, and joined, under the mid-brain, by the anterior ends of the internal carotids. A. cm. Caudal artery, the small median prolongation of the dorsal aorta. All, The allantois ; it is joined by the Wolffian duct, and empties into the cloaca,, The cloacal membrane. Ao, Median dorsal aorta. A0.1t, Right descending aorta; a small vessel connecting the dorsal ends of the third (carotid) and fourth aortic arches. Ao.D, Main right descending aorta, passing downward to join its fellow and form the median aorta. A.s, Subclavian artery. An, Left auricle of the heart., The umbilical artery, which runs in the mesodermic wall of the allantois and joins the caudal end of the dorsal aorta. A.v, Vertebral artery., The vitelline artery, which becomes the superior mesenteric artery of the adult, c. Artery known as the cceliac axis in the adult, car.e, External carotid artery, arising from the base of the third aortic arch, car.i, Internal carotid artery. Cbl, Cerebellum. d, Left duct of Cuvier. Dien, Diencephalon. D. V, Ductus venosus Arantii. ep, Small plug of entodermal epithelium in the rectum ; there are several small irregular passages through the plug. Epgl, Anlage of the epiglottis. f, Interventricular foramen, opening into the space a, b, r, of Fig. 105. F.M, Foramen of Monro, f.o, Foramen ovale, between the cardiac auricles, g, Gall-bladder. In, Entodermal wall of the intestine, is, One of the series of intersegmental arteries. A'i, Enlarged blind end of ihe ureter, the anlage of the renal pelvis. Lar, Anlage of the larynx, consisting chiefly of an epithelial plate. Li, Liver. Lu, Entodermal portion of the lung. M.b, Mid-brain. Md.ob, Medulla oblongata. If, A spinal nerve. Neu, Neuromeres of the hind-brain. Oe, CEsophagus. op, Stalk of the optic vesicle ; the stalk is the anlage of the optic nerve. P. A, Pulmonary artery, pan. Dorsal anlage of the pancreas, the narrow part of which becomes the duct ; behind the duct is the anlage of the ventral pancreas, which arises as an outgrowth from the duct of the gall-bladder, p.c. Pericardial cavity. P.V, Pulmonary vein. Sp. c, Spinal cord. St, Stomach ; the letters are placed beside the diverticulum which is characteristic of the pig and is not found in man. /, Posterior portion of the tongue, an eminence on the floor of the pharynx, fcrmed by the union of the ventral ends of the third and fourlh gill arches. T.i, Tuberculum impar, the anlage of the anterior portion of the tongue. Tra, Entodermal trachea., Cloaca. Um.d, Right umbilical vein. Ven, The left ventricle, the letters being placed on the septum inferius. V.mes, Superior mesenteric vein. V.port, Portal vein. V.ri, Vitelline vein. IV. D, Wolffian duct, x, Anastomosis between thvi right and left cardinal venous systems. )'/', Volk-sac. Yk.s, Entodermal yolk-stalk, connecting the yolk-sac with t lie intestine. X '4 diams. — {Drawn by Dr. /•'. 7'. Lewis.)

Fig. 101.

Fig. 102. — Anterior Wall of the Pharynx of a Human Embryo of 3.2 mm. 1—5, Gill arches ; the arches are separated from one another by the entodermal and the corresponding ectodermal gill pouches ; the aortic arches are drawn in dotted lines and arise from the median cardiac aorta. M, Mouth. Of, CEsophagus. Cce, Coelom. X 5° diams. — (A fin- li: His.)

Fig. 103. — Aortic System of His's Embryo, Bl, 4.25 mm.

I— V, Aortic arches. C'i, Mandible. St/, Thyroid gland. A", Main aorta. /', Pulmonary artery. Lg t Lung. Of, CEsophagus. X 3*> diams. — (After W. His.)

Fig. 104.

Fig. 105. — Pig Embryo of 12.0 mm. Reconstruction from the Transverse Sections, Series 5. The figure illustrates chiefly the veins of the right side, and shows the right auricle and ventricle of the heart. A, The right umbilical artery, only a small portion being drawn as it curves around the Wolffian duct, IV. D. a. Tip of aortic septum, which divides the aortic limb of the heart into the pulmonary aorta, P, and main aorta, Ao ; by a growth of the cardiac tissue, a, b, and c of the figure become joined, shutting of) the space around the base of the aorta ; this space communicates by the interventricular foramen with the left ventricle, and serves as the permanent or adult channel of communication between the true aorta, Ao, and the left ventricle. All, Allantois. Ao, Aortic division of the aortic limb of the heart. Au, Right auricle, b, See "a." c, See "a." card', Superior part of the cardinal vein (the anlage of tfie azygos). card", Inferior part of the posterior cardinal vein, cl.i. Opening of the first gill pouch into the pharynx, the pharynx being indicated by dotted shading, cl.2, Opening of the second gill pouch into the pharynx. cl.j, cl.4, Entodermal portions of the third and fourth gill clefts, com, Dotted outline of the omental or lesser peritoneal cavity, d. Left duct of Cuvier. D-C, Right duct of Cuvier, the main venous trunk entering the heart from the right side. D. V, Ductus venosus Arantii. F. VV, Foramen of Winslow, drawn in black. F.pp, Foramen, drawn in black, between the pleural and peritoneal cavities. Gen, Genital tubercle, represented as somewhat displaced from the median line, which it really occupies. G.R, Genital ridge. Jug' , J"S" < Jugu'ar or anterior cardinal vein. Li, Liver. Is, Anlage of the lateral venous sinus. mx. Vein of the inferior maxilla or mandible. P, Pulmonary division of the aortic limb of the heart. p.c, Pericardial cavity. PI, Dotted outline of pleural cavity. Rcc, Rectum. Sc, Subcardial vein, which is derived from the cardinal and on the right side of the body forms part of the vena cava inferior. Scl, Subclavian vein, sis, Anlage of the superior longitudinal venous sinus, which is formed by the union of the veins, /./., from the sides to make a single vessel between the cerebral hemispheres. Ur, Ureter. Um.d, Right umbilical vein, v, Valve of the sinus venosus. Veil, Right ventricle of the heart. V.H.C, Vena hepatica communis, v. op, Ophthalmic vein. V.P, Portal vein. W.b, Wolffian body. W.D, Wolffian duct, x, Anastomosis between the cardinal venous systems of the right and left sides. X '4 diams. — {Drawn by Dr. F. T. Lewis.)

The development of the aortic arches requires a few words of special explanation. The disposition of the vessels is perhaps more clearly shown in figure 1 02. In an earlier stage the heart lies immediately under the pharynx and gives off from its aortic end the short aortic trunk which runs upward toward the floor of the pharynx. From the aortic trunk spring five pairs of vessels, known as the aortic arches (Fig. 102). Ten vessels are symmetrically placed, five on each side of the pharynx. They pass around the pharynx dorsalwards. The five vessels of each side very soon become united by a dorsal longitudinal trunk the descending aorta, which passes down through the cervical region of the embryo and meets its fellow at about the level of the diaphragm, unites with it, and thus forms the single median dorsal aorta. The first arch very soon disappears (Fig. 103). Its prolongations with the vessels on the ventral side run forward to the lower jaw and give rise to the external carotid. The vessel on the dorsal side also persists and gives rise to the internal carotid. Presently the second arch also disappears, and both carotids are, as it were, thereby lengthened. This is the condition which we find in our embryo of 12 mm. (Fig. 101). The third, fourth, and fifth arches are still present. From the base of the third arch runs forward the external carotid, and from the summit of the third arch runs forward the internal carotid. The dorsal ends of the third and fourth arches are still connected, but this connection, instead of being a large aortic vessel, as in earlier stages, has now contracted and almost disappeared, and will soon be lost altogether, so that in the adult there will be no connection between the dorsal ends of the third and fourth arches. The fifth arch is still connected with the dorsal end of the fourth. It gives off the small pulmonary artery to the lungs. On the side toward the heart the relations of the arches are also changed. The main aortic vessel which springs from the heart is, in the 12 mm. pig, divided into two vessels — the pulmonary aorta on the ventral side and the true aorta in a more dorsal position. The division has so taken place that the third and fourth arches are connected only with the true aorta, while the fifth arch is connected only with the pulmonary aorta. The part of the fifth arch on the left side between the origin of the pulmonary artery proper and the main descending aorta offers at this stage an open communication between the pathways of the pulmonary and of the main body circulation. This dorsal half of the fifth aortic arch is known as the ductus arteriosus. It remains throughout the fetal period as an open channel, so that the blood from the right ventricle flows in part to the lungs, in part into the dorsal aorta. The lumen of the ductus arteriosus disappears in man soon after birth. As an anomaly it occasionally persists throughout life, involving serious modifications of the normal circulation. The dorsal part of the fifth aortic arch of the right side has a different history, for it aborts early in embryonic life, and there also occurs an abortion of the entire descending aorta from the end of the fourth arch on the right side downward to the level of the diaphragm. When this abortion has taken place, the entire aortic stream flows from the heart to the left side of the embryo. The aortic branches on the right side appear as follows in the adult : The main stem, from which the five arches originally sprang, is the arteria innominata, which gives off a stem from which spring the two carotids. Next a vessel which represents the persistent third right arch, which no longer has any direct communication with the aorta, but at its end gives off the subclavian and vertebral arteries. The vessel which corresponds to the right third arch is usually described as a portion of the stem of the subclavian artery in the adult.

Fig. 107. — Pig Embryo of 12.0 mm. Reconstruction from the Transverse Sections, Series 5. To show especially the cephalic nerves, c.i, c.2, c.j. Cervical nerves. Cbl, Cerebellum, com, Ganglionic commissure connecting with the jugular ganglion, j. Dien, Diencephalon. ex, External branch of the spinal accessory nerve. F, Froriep's ganglion, which in man completely disappears. G.j, Gasserian ganglion. H, Cerebral hemisphere. ;, Jugular ganglion. Z, Lens. M.b, Mid brain. Mdb, Mandibular process. Md.ob, Medulla oblongata. Mx, Maxillary process. n, Ganglion nodosum. Na, Nasal pit. Op, Optic cup. Ol, Otocyst. reel, Recurrent laryngeal nerve. Ven. I]', Roof of fourth ventricle, j, Oculomotor nerve. 4, Trochlear nerve. 5, Trigeminal nerve. 6, Abducens nerve. 7, Facial nerve. 8, Vestibular ganglion, q, Petrosal ganglion. 10, Vagus nerve. //, Spinal accessory nerve. 12, Hypoglossal nerve. 5 op marks the ophthalmic division of the fifth nerve ; S is on the vestibular ganglion, immediately below which lies the cochlear ganglion ; the seventh nerve has two branches, of which the anterior becomes the chorda tympani ; of the two branches of the ninth nerve, the anterior becomes the tympanic, and the posterior the lingual nerve of the adult. X 2odiams. — (Drawn by Dr. F. T. Lewis.)

Figure 105 is in many respects similar to figure ioi.and is intended to show chiefly the disposition of the veins. There are also included in this figure the Wolffian body and its duct. The pharynx and the heart are supposed to have been cut through, well to the right of the median plane. This makes it possible to indicate in the figure the origin of the pulmonary aorta and of the true aorta. The following are the most important veins: the umbilical, which passes around the umbilical opening and enters the liver; the portal vein, which receives the blood from the abdominal viscera, and also delivers it to the liver. In this specimen there is quite a wide and free connection within the liver between the portal and umbilical veins. In other embryos of this size such a connection does not always exist. The large vena cava inferior is on the right side of the embryo, and also shows the liver, which receives blood directly from the Wolffian bodies and from the cardinal veins. From the upper side of the liver the hepatic vein goes directly to the heart, uniting with the ducts of Cuvier. These ducts receive the jugular veins from the head and the cardinal veins from the body. The cardinal veins are now very much changed. In earlier stages they extended from the duct of Cuvier almost the entire length of the embryo. Of this great vessel there now remains connected with the duct of Cuvier only a comparatively short vessel.

Figure 107 shows the disposition of the cephalic and upper cervical nerves and also the position of the nasal cavity, the eye, and the otocyst.

Fig. 109. — Pig Embryo of 12.0 mm. Reconstruction from the Transverse Sections, Series 5. The embryo has been drawn as if transparent, to show the form of its pharynx and the relations of the pharyngeal gill pouches to the grooves on the outer surface of the embryo. Cbl, Cerebellum. C.S, Cervical sinus. Vint, Diencephalon. H, Cerebral hemisphere. Hy, Hypophysis, which arises as an evagination from the oral cavity. In/, Infundibular gland, which arises as a median evagination from the floor of the fore-brain., Lachrymal groove, m, Maxillary process. M.b, Mid-brain. Mdb, Mandibular process. Md.ob, Medulla oblongata, na.ex. External nasal opening, ni, A thin epithelial plate separating the nasal pit from the oral cavity ; by the rupture of this plate the internal nares is formed later. Oe, CEsophagus. Op, Eye. Ot, Otocyst. par, Part of the thymus, arising from the epithelium of the fourth gill cleft. Sp.c, Spinal cord. 77;, Median thyroid gland, thym, Thymus. Tra, Trachea. /, Entodermal pouch of the first branchial cleft, the anlage of the Eustachian tube and tympanum. 2, Entodermal pouch of the second gill cleft; it actually opens to the exterior, j, Entodermal pouch of the third branchial cleft. 4, Entodermal pouch of the fourth gill cleft, the lower fork of which is the anlage of the lateral thyroid. I, Ectodermal groove of the first branchial cleft ; it is the anlage of the meatus auditorius externus. Y 20 diams. — (Drawn by Dr. F. T. Lewis. )

Figure 109 gives an outline of the head and combines an indication of the extenal modeling of the gill arches with a representation of the shape of the pharynx.

Pig Embryo of 15 mm. — As compared with figure 99, the present embryo (Fig. no) has not only grown in all its dimensions, but has also changed in form. Unlike the embryo proper, the umbilical cord has grown very little. We notice at once that the outline of the back is less curved than before, the ventral side of the body has acquired a convex outline, and at the head it has become considerably larger, both absolutely and relatively, to the body of the embryo, and has, moreover, risen so that the neck bend is diminished. The limbs are beginning to show the differentiation of the feet. Examined more carefully, the embryo offers the following details: the eye, which is characteristically small, has become almond-shaped, and the circular lens can be seen in the midst of it. In the embryos of rodents, carnivores, and primates the eye is larger relatively than in the pig. By the growth of the facial region the development of the snout has been initiated and the opening of the nasal pit now appears as the external nares toward the end of the short snout. The lower jaw is clearly differentiated and the slit of the mouth is distinct. There has been a great growth of the regions of the fore-brain and mid-brain, and it is this growth chiefly which has caused the relative expansion of the head as compared with the rest of the body. The auditory groove now appears distinctly as the anlage of the external meatus of the ear, behind which a protuberance can be seen which is the anlage of the external concha of the ear. The cervical sinus has wholly disappeared. Along the line of the back the primitive segments are scarcely recognizable in the cervical region, but near the upper limb they still show distinctly and from there are indicated with increasing clearness as we pass toward the lower limb. The marks of the segmental divisions do not extend as far on the dorsal side as in the earlier embryos, but are restricted to what may be called the segmental ridge. Along the milk-line a series of small, white, circular spots can be seen. In the specimen figured there were six of these; their number is variable. They are the anlages of the mammary glands, and are at this stage merely the local thickenings of the ectoderm or epidermis. There has been a considerable growth of the dorsal region of the body, and this is perhaps most clearly indicated by the position of the milk-line, which is much further away from the median dorsal line than in the 10 mm. pigThe limbs are both paddle-shaped, and, though still very short, have a broad terminal expansion, which is the anlage of the foot. The front foot has somewhat the outline of a truncated pyramid, while the hind foot is more rounded. In the anterior limb the differentiation into upper and lower divisions is suggested.

1 ic no. — Pig Embryo of 15 mm. X S diams.

Pig Embryo of 20 mm. — As compared with figure no this stage (Fig. in) shows a general progress, but no such striking changes of external form as distinguished the embryo of 15 mm. from that of 10 mm. Embryos of this length vary considerably in their proportions, but the one figured is characteristic of this stage. The enormous transverse diameter of the body as compared with its length is very striking, and the very large size of the head as compared with the body is almost equally remarkable. In the head the .growth of the regions of the fore-brain and mid-brain has continued, and the divisions between the mid-brain and hind-brain are marked by concavities in the outline of the head. The eye is both absolutely and relatively larger. Above it can be distinguished readily the anlages of the great bristles which develop over the eye, corresponding to the human eyebrow. These anlages appear as whitish spots, for they are thickenings of the ectoderm. The snout has increased in length ; the external ear has grown longer and has begun to assume its permanent pointed form. The limbs have increased considerably in length, but not yet enough to pass beyond the line of the abdomen. In both feet the differentiation of five toes is clearly indicated. The milk-line, as a line, has almost completely vanished, but the row of toes, and the anlages of the mammary glands, which develop along the milk-line, persist and will undergo further development in later stages. The number in the specimen figured is five. A row of these anlages marked the position of the milk-line, which demonstrates that there has been a more marked growth of the dorsal region of the body, and comparison of embryos of 15 and 20 mm., during the period comprised between those two stages, indicates that the growth of the dorsal part of the embryo is far greater than of the ventral part. Comparison of figures 1 10 and 1 1 1 shows at once that the area occupied in both figures by the region on the ventral side of the milk-line is about the same. In the pig of 20 mm. there is no indication of the segmental structures recognizable in the surface modeling.

Fig. in.— Pig Embryo of 20 mm. X 8 diams.

Pig Embryo of 12 mm Studied in Sections

A pig embryo of 1 2 mm. has been selected as the center of study in this book because its anatomical relations are such that they may be readily grasped by the student who has already studied the anatomy of an adult mammal, human or other. At the same time the development of the organs is so advanced that their fundamental relations may be observed. From an embryo of this size the transition to the study of younger embryos is, even for the beginner, comparatively easy. It is not necessary that the embryo should be of this exact size; indeed, it may be somewhat advantageous for the student to have an embryo a millimeter larger, or one, two, or even three millimeters smaller, since the figures and explanations referring to the 12 mm. stage will enable him to identify all the structures to be found and yet call upon him for the exercise of care and judgment in identifying, from the data given in the following pages, the various parts in the somewhat different stage he may be studying. The 1 2 mm. size was chosen partly because the author had at his disposal three good series belonging to the Harvard Embryological Collection.

The transverse series is the most important, and should form the basis of the study, and accordingly most of the sections figured are from such a series.

Next in importance comes the sagittal series, but it is desirable that every student should have a series in the three standard planes at his disposal for study. In the practical laboratory study each student should be required to make a series of accurate camera lucida drawings of carefully selected typical sections ; then to name correctly all the parts shown in each section and to identify the distribution of the three germ-layers in every case.* A sufficient number of high-power drawings ought to be added to illustrate the character of the various tissues.

Fig. 112. — Reconstruction of a Pig Emisryo of 12.0 mm. with Indications of the Planes of Sections FlGi'KI D. an, Cloacal membrane. Ao, Aorta. An, Auricle. A.utn, Umbilical artery, a.v, Vertebral artery, bos, Basilar artery, c, Anlage of oecura. ear, Internal carotid, can, Caudal artery. C.Ex, External carotid artery. f.b, Fore-brain. G, Spinal ganglion, g, Gall-bladder, k.i, Hind-brain. In, Intestine. Li, Liver. m.b, Mid-brain, op, Optic vesicle. Ol, Otocyst. pan. Pancreas. Sp, Spinal cord. Si, Stomach, i'm. Umbilical opening. Ven, Ventricle of heart. ///, IV, I", Aortic arches.

Fig. 113

Fig. 114.

Fig. 127.

Fig. 116.

Fig. 128.

Fig. 117.

Fig. 129.

Fig. 11S.

Fig. 119.

Fig. 120.

Fig. 121.

The accompanying figure 112 represents the outline of the pig embryo which was cut into the series of transverse sections from which figures 1 1 3 to 121 have been made. The student can easily identify the parts in the figure by comparison with that of the pig of 10 mm. (Fig. 99), aided by the accompanying description of the same. The sections of this embryo are ten mikrons in thickness, and are 966 in number, not 1200, as the student might expect. The discrepancy is due to the shrinkage of the embryo when imbedded in paraffin. The shrinkage is always very great, and in the case of embryos causes a loss of almost 20 per cent, in the length ; but as it seems to take place uniformly throughout the embryo, it causes no distortion, so that the embryo in paraffin is an exact though greatly reduced copy — so to speak — of the living embryo. It should be remembered that no correct measurements of the size of organs or cells can be obtained from sections made by the paraffin method. This limitation upon the use of sections is too often forgotten. The horizontal lines indicate approximately the levels at which the sections here figured belong. For convenience the direction and position of the frontal sections represented in figures 1 2 2 to 125 are also indicated approximately on this same outline, although, of course, the frontal series was from another embryo.

The Study of Transverse Sections

The figures and descriptions here presented of eight sections have been selected as illustrating the most important structures, with the exception of the disposition of the umbilical opening and of the kidney, which can be better represented in sections from older or younger stages.

Section through the Upper Part of the Otocyst. — As shown in figure 1 1 2, by the line 185, this section is taken from a level about half-way between the eye and the highest point in figure 112. It passes, therefore, through the fore-brain, F. b, and the fourth ventricle, Ven. IV, or cavity of the hind-brain. The section is bounded by a thin layer of epidermis, between which and the brain-wall there is a large amount of mesenchymal tissue. Alongside the hind-brain lies a series of important structures imbedded in the mesenchyma, which are identical upon the two sides, although they differ somewhat in the section, as the plane of cutting was not symmetrically transverse for the head. These structures are in the following order: N. 5, the trigeminal ganglion; N. 7, 8, the acustico-facial ganglion complex; Ot, the otocyst, an oval vesicle with very distinct epithelial walls.

  • For making camera lucida drawings, a 2-inch objective will be found convenient. An Abbe camera is recommended. Compare the directions for drawing in Chapter VIII.

Fig. 113. — Pig, 12.0 mm. Transverse Series 5, Section 185.

com, Ganglionic commissure between hypoglossal and vagus nerves. mm. II, Ganglionic commissure with fibers of the root of the spinal accessory nerve. D.E, Ductus endolvmphalicus. F.i, Fore-brain. Mi, Medulla oblongata. /V.j, Trigeminal ganglion. .\'.y.S, Aeustico-facial ganglion. .Y./,\ Jugular ganglion of the vagus. Of, Otoeyst. Vtn.IV, Fourth ventricle. X 22 diams.

Xuxt the ninth or glosso-pharyngeal nerve (scarcely appearing in the section on the right side of the embryo) is shown by the upper portion of its ganglion on the left side (the right in the figure); the ganglion is a small, dark mass of triangular outline close to the medullary wall of the hind-brain, and it lies almost even with the posterior edge of the otocyst. A r . 10 is the vagus nerve (the section passes through the upper portion of the jugular ganglion of that nerve) ; it shows better on the right side of the embryo in this section than on the left. Com. 10, and com, refer to the ganglionic commissure which extends above the origins of the hypoglossal nerve. On the left the continuity of this commissure is better shown than on the right, where it offers two parts, one, com, entirely ganglionic, and another, com. 10, which comprises both the ganglionic portion and fibers which share in the formation of the root of the tenth nerve. The trigeminal ganglion is very large, somewhat triangular in shape, the apex of the triangle joining the angle of the hind-brain. This situation is very characteristic, for the trigeminal is always at this angle, and, from its great size and position, is one of the most important landmarks in the study of the topography of the embryonic head. The nerve-cells of the ganglion are grouped, for the most part, on the side toward the ectoderm, where they are closely crowded together, making a deeply staining mass. Nearer the brain-wall the tissue of the ganglion is much less condensed, is somewhat penetrated by small blood-vessels, and contains a considerable number of nerve-fibers, which are gathered into small bundles. Toward the brain-wall the bundles become distinct, and on the right side of the embryo the passage of the nerve-fibers into the brain can be readily seen. The nerve-fibers at this stage are merely neuraxons, that is to say, merely thread-like prolongations of the bodies of the nerve-cells (neurones). The fibers are entirely without sheaths. They stain very lightly, and hence, in the preparation, may be detected by their light appearance. The nerve-fibers may be conveniently rendered conspicuous by counterstaining the sections with Lyons blue. The nervefibers of the trigeminus, which enter the wall of the hind-brain, form in part a bundle of fibers, which extends along past the acustico-facial ganglia within the medullary wall. These fibers represent the commencement of the ascending trigeminal tract of anatomists. The other ganglia associated with the hind-brain are not well shown in this section. The otocyst has a very sharply defined epithelial wall and is imbedded in loose mesenchymal tissue. On the right side of the embryo we have the ductus endolymphaticus, D.E, the opening of which into the main cavity of the otocyst is shown on the left side. The epithelial wall of the ductus is thicker than that of the greater part of the otocyst proper. The wall, Md, of the hind-brain exhibits already characteristic differentiations, for it shows clearly the three primitive layers ; the outer or external neuroglia layer is thin, and appears light in the section because it takes the stain slightly. It is in this outer neuroglia layer (ectoglia) that the entire sensory nerve-fibers are primarily distributed, and, therefore, it is in a portion of this layer that we find the ascending trigeminal tract situated. Next to the ectoglia comes the middle layer, in which the neurones of the medullary wall are situated, and which is, therefore, termed the neurone or gray layer (cinerea), easily recognizable under the microscope by its brighter color, which is due principally to the fact that the nuclei in this layer, though numerous, are much less crowded than in the innermost of the three layers, or primitive ependymal layer, which at this stage is quite thick. Owing to the presence of nuclei, the gray layer is, of course, stained much more than the ectoglia. The nuclei of the brain-wall show as yet very little differentiation. There are numerous mitotic figures which are situated exclusively close to the inner surface of the brain- wall in the fore-brain. The structure of the fore-brain is similar, but the development is less advanced; the differentiation of the neurone layer is only just beginning, and it has acquired little thickness. In the hind-brain we see in the interior, along the region between the otocysts, a series of curved notches which impart a scalloped outline to the wall. A distinct point separates one concavity from the next. Each one of the spaces between two of the projecting points is designated as a neuromere. The neuromeres correspond in number and position to the neighboring primitive segments, and are, therefore, to be designated as segmental structures. They also bear an evident relation to the development of the nerves, and the accepted hypothesis is that from each neuromere springs a single nerve. The attempts which have been made to verify this hypothesis have met with very serious difficulties, for the relations are extremely complicated, and until the matter shall have been much more thoroughly investigated than at present, we must remain in the dark as to the precise morphological value of the neuromeres. But, inasmuch as they appear with the greatest constancy in the embryos of all vertebrates, we cannot help accepting the view that they are really structures of fundamental importance. At the stage we are studying the neuromeres have already begun to lose their distinctness, and in slightly older pigs can be traced only with difficulty. In younger stages their primitive characteristics are better shown (compare page 229). As regards the blood-vessels in the present section : there are small branches of the veins, which show outside of the ganglionic commissure, com; parts of the jugular vein appear in close proximity to the trigeminal ganglion, and again at the side of the head. In the median line between the forebrain and hind-brain, or nearer to the layer, appears a section of the basilar artery. Near the fore-brain on either side is the loop of the carotid artery. There are several important points to be observed in the region between the trigeminal ganglia and the fore-brain. In order to show these more clearly, a separate illustration (Fig. 1 14), on a larger scale, of this portion of the section is given. The trigeminal ganglion, the wall of the fore-brain, and the wall of the hind-brain will be at once identified, so that the correspondence with the general figure is easily followed. Between the trigeminal ganglion and the fore-brain are four veins, two of which, Jug' and Jug'" , are larger and are parts of the main jugular stem passing from the region of the hind-brain to that of the fore-brain, while the two smaller ones, Jug" , are merely branches of the jugular. Close to the section, Jug'" , of the jugular nearest the fore-brain lie the very small sections of the fourth, A../, and third, N.j, cerebral nerves. The fourth nerve is minute in size and lies just behind the jugular. The third nerve, though somewhat larger, is also very small and lies anterior to the jugular somewhat on its medial side. Both of these nerves, owing to their small dimensions, are somewhat difficult to observe with the low power. The detailed figure brings out more clearly other points. It shows very clearly the junction of the trigeminal, N. j, and acusticofacial, N. 7, S, ganglia with the wall of the hind-brain, and also the division of that wall into its three primary layers, the ectoglia, Ec. gl, the gray layer, tin, and the inner or ependymal layer, Epen, and also the median floor-plate or raphe, Raph. Immediately below it is the basilar artery. On either side of the forebrain is the section of the loop of the carotid, A. car, which is passing up to join the anterior end of the basilar artery, which last has been produced by the fusion of the two originally symmetrical vertebral arteries. This portion of the carotid loop probably corresponds to the vessel designated in the adult as the posterior communicating branch, by which the end of the carotid proper anastomoses with the basilar artery. At the side of the fore-brain appears a blood-vessel, Jug. L, which might be called the lateral jugular. It is a branch of the main jugular stem and passes over the side of the fore-brain toward the median dorsal surface thereof, where it meets the corresponding vein of the opposite side, with which it then unites to form a single median vessel. This vessel ultimately acquires great size, and is known as the superior longitudinal sinus. It is shown in figure 105. So much of the vessels as do not unite in the median line persist, to form the lateral sinus of the adult. These sinuses in the embryo are all small branches of the veins when they first appear. Their great enlargement does not occur until comparatively advanced stages. Finally, attention should be paid to the following important modifications in the mesenchyma. Already there has been a rich development of a plexus of fine blood-vessels over the surface of both the foreand hind-brain which has been accompanied by a slight condensation of the mesenchyma between the blood-vessels, thus marking a distinct membrane, in which we can easily recognize the pia mater, Pia. Outside of the pia mater comes a relatively broad zone, Arach, in which the cells are widely separated from one another and are connected by slender and long processes so that the intercellular spaces are very extensive. This broad zone is the anlage of the arachnoid membrane. It is much more differentiated around the ventral portion of the brain than around the dorsal side. Between the arachnoid zone and the external epidermis the mesenchyma is somewhat more condensed and the cells are elongated in form, in part almost spindle-shaped, forming a layer, Cut, which we may consider the anlage of the cutis, and perhaps, also, of the subcutaneous tissue but this is doubtful. Between the arachnoid zone and the cutis zone, so placed that they cannot be quite said to belong to either one or the other, appear numerous blood-vessels. These form a more or less distinct vascular layer, which appears with remarkable constancy in all classes of vertebrates, and over a large part of the body. It may, therefore, be called the panchoroid. It is unquestionably of very great morphological importance, but its history is almost unknown.

Fig. 114. — Portion of Fig. 113 More Highly Magnified. A.bas, Basilar artery. A. car, Internal carotid artery. Arach, Arachnoid zone. Cin, Neurone layer of medulla. Cut, Cutis layer., Ectoglia. Epen, Ependymal layer. G.tri, Trigeminal ganglion. Jug , ,Jug f ', Jug'", Jugular vein. Jug.L, Lateral branch of the jugular. N.3, Oculomotor nerve. X.4, Trochlear nerve. N-S, Sensory root of trigeminus. N.y,8, Acustico-facial ganglion. Pia, Pia mater. Raph, Raphe of the medulla oblongata. r'en./II, Third ventricle of the brain. Vtn.IV, Fourth ventricle of the brain, y 50 diams.

As regards the histological condition of the tissues the student should make careful observations. Attention may be directed especially to the following points : The epidermis at the sides of the section is two-layered and consists of an inner layer of cuboidal cells, the anlage of the Malpighian layer of the adult, and of an outer layer of very thin cells, the epitrichium, the nuclei of which are flattened and appear darkly stained. Toward the median line, above the hindbrain and below the fore-brain, the epidermis becomes gradually one-layered and much thinner. The mesenchyma exhibits three principal forms of cells: First, those which are equally branched in all directions and represent a primitive form of the tissue. Such may be found in the neighborhood of the basilar artery. Second, the elongated cells of the cutis zone; and, third, the cells of the arachnoid zone above described. The blood-vessels have very distinct endothelial walls which are very thin, being thickened only to furnish space for the nuclei, which, unlike those of the adult, project not only into the lumen of the vessel, but also against the surrounding mesenchyma. The blood-corpuscles are rounded cells, sometimes oval, not infrequently somewhat distorted. Their nuclei are nearly spherical and contain a number of fine granules. Mitotic figures are quite frequent. A few of the nuclei are beginning to change by becoming smaller and taking the stain more deeply (compare page 94). In the nervous system the differentiation of the cells in the hind-brain is more advanced than in the fore-brain, but even in the hind-brain the distinction between the young nerve-cells and the young neuroglia cells (neuroblasts and spongioblasts) is not very clear. The nuclei are only just beginning to acquire distinct nucleoli, such as would be characteristic of later stages. The nuclei of the tissues differ markedly from those of the earliest embryonic stages, but can scarcely be said to have assumed in any of the tissues adult characteristics.

Section through the Lower Part 0} the Otocyst. — Figure 1 15 is from section 198, and, therefore, ten sections below figure 113. It is inserted chiefly to bring out three points not shown in the preceding illustration : First, the root of the spinal accessory nerve, N. 11, which arises from the cervical (in the figure upper) end of the hind-brain and runs forward to join the vagus ganglion, N. 10 jug, the jugular ganglion of the adult. Second, the characteristic relations of the jugular vein to the trigeminal ganglion, N. 5. The vein is cut twice, Jug' and Jug" , for it curves around the ganglion, passing on the inside of the ganglion between it and the wall of the brain. At this stage, however, it has begun to expand outward behind, in front of, and below the ganglion. The expansion of the vein outward will continue through later stages until a new venous path is established on the outside of the trigeminal ganglion, which will then appear, so to speak, as an island in the path of the jugular. Still later the vein on the inner side of the trigeminal ganglion will abort, and only the external pathway will be retained. By these changes the vein migrates from its original internal position to its new permanent external position. In the 1 2 mm. pig the jugular vein pursues a very sinuous course along the sides of the hind-brain, for it passes inside of the twelfth, eleventh, tenth, and ninth nerves, then outside of the otocyst, the seventh and eighth nerves, and inside of the fifth. In an earlier stage it lay inside of the otocyst, but is now found to have migrated to the outside thereof, by the process of island formation just described for the trigeminus. The blood-vessel in its original condition is properly termed the anterior cardinal. When it has completely migrated outside, not only of the otocyst and of the trigeminal ganglion, but also outside of the glosso-pharyngeal and vagus nerves, it may properly be termed the jugular. The jugular, therefore, is to be defined as the anterior cardinal vein which, by successive island formations, has migrated to a new position outside of the otocyst and cephalic ganglia. Third, to show the infundibular gland, In}, a small evagination from the ventral floor of the fore-brain, F.b. The evagination is really hollow, but the cavity does not appear in the section figured. It enters into very close relations with another hollow evagination, which springs from the dorsal roof of the oral cavity and is known as the hypophysis. The infundibular gland and the hypophysis become intimately associated with one another in their further development and give rise to the pituitary body of the adult, the gland becoming the posterior lobe, the hypophysis the anterior lobe of that organ . The hypophysis may be best studied in sagittal sections (see page 205). The present section, figure 115, being at a lower level than figure 1 13, passes through the ventral portion of the hind-brain and shows only a narrow part of the cavity of the fourth ventricle, Ven.iv. The three layers in the wall, Md, of the hind-brain are very distinct. At the anterior end of the hind-brain appears a series of light lines, Str, which are caused by nerve-fibers. These lines have been identified as the strice acusticce. They need to be more accurately studied, however, for they seem rather to be fibers of the lateral root of the facial nerve. Close to the anterior section of the jugular vein, Jug' , appear the minute fourth and third nerves, which, however, are not indicated in the figure. Both lie close to the wall of the vein on the side away from the trigeminal ganglion. The fourth nerve lies nearer the outside of the embryo, the third nerve nearer the median plane. At about the same level as this part of the jugular vein, and very close to the wall of the fore-brain, is situated the loop of the internal carotid. Lower down, but not close to the wall of the fore-brain, is the section of the lateral jugular.

Fig. 115. — Pig, 12.0 mm. Transverse Series 5, Section 19S. Ec, Ectoderm. F. b, Fore-brain. In/, Infundibular gland. Jitf, Jttff", Jugular vein. jV.j, Trigeminal ganglion. A'.7,S, Acustico-facial ganglion. N.g, Ganglion nodosum of the glossopharyngeal nerve. A r ./o. Jug, Jugular ganglion of the vagus nerve. .V.//, Root of the spinal accessory nerve. .W, Medulla oblongata. Ot, Otocyst. S/i; Stri;> acusticse. Ven.iii, Third ventricle. W-ii.iv, Fourth ventricle. >;' 22 diams.

Section through the First Gill Cleft and Optic Evaginations. — The section shows on the dorsal side the upper cervical region of the spinal cord, on the ventral side the fore-brain giving off the optic nerves. In this and the three sections next following the complicated pharynx appears in various forms. The general shape of the pharynx has been described with the aid of a figure of a wax model of the pharynx made from the same series of sections from which these figures are taken. The shape of the pharynx and of its four pairs of lateral pouches at this stage is remarkably constant, so the student is not likely to encounter any serious difficulty in identifying the parts. The spinal cord is oval in the section. Its cavity has expanded in the middle. The lateral walls are quite thick, the median ventral wall is thinner, and the median dorsal wall (deck-plate) is very thin. The three primitive layers of the medullary tube are very clearly marked out, the ectoglia appearing light, the ependymal layer appearing dark. The differentiation is much more advanced on the ventral side of the spinal cord than on the dorsal side, and, indeed, it is only in the ventral part that the three layers are perfectly differentiated. In the median ventral line we have the floor-plate, in which we can distinguish only two zones, while in the deck-plate there is no differentiation of layers whatever. The spinal cord is clearly divided into a dorsal zone, D. Z, and a ventral zone, V. Z, on each side. The two dorsal zones are connected across the median line by the thin deck-plate, and the ventral zones similarly by the thin floor-plate. The lower or ventral limit of the dorsal zone is marked by the entrance of the dorsal or ganglionic root and by the fibers, which represent the outgoing lateral roots. In the actual section figured, the lateral roots, L. R. n, are those which enter into the formation of the eleventh nerve. The true dorsal root does not appear in the figure . Internally the division between the two zones is marked by the lateral angle of the central cavity shown in the section. In the dorsal zone the differentiation of the three layers has made slight progress. In the ventral zone, however, the development is far more advanced. The most characteristic feature of this movement is the growth of the cinerea or neurone layer, which increases in a twofold manner: first, by encroaching upon the inner or ependymal layer; and, secondly, by the growth of its constituent elements. Examination with a high power shows at once that the cells have grown very much. Their nuclei are larger, granular in appearance, rarely with any indication of a distinct nucleolus. Most of the cells are neuroblasts and have well-marked protoplasmic bodies, finely granular in texture. They have many of them already produced long, slender outgrowths which we can identify as the neuraxons. In order to study the distribution of the neuraxons and the form of the neuroblasts, it is necessary to apply the Golgi rapid method, by which it can be demonstrated that a portion of the neuraxons is distributed entirely within the medullary wall, while another portion passes out to form ventral roots, one of which, N. 12, forming part of the hypoglossal nerve, is shown in the figure. A third portion of the neuraxons, at least in the upper cervical region, as also in the medulla oblongata, passes out to form the lateral roots. The positions of the exits of these two bundles of nerve-fibers are constant and characteristic. The ventral root always passes out from the middle of the ventral zone about half-way between the median floor-plate and the dorsal limit of the zone. The lateral root always passes out at the upper dorsal limit of the ventral zone and immediately below the point of entrance of the true dorsal root. Formerly the lateral roots were not distinguished from the dorsal roots. Following downward in the figure we come to the section of the jugular vein, Jug, just inside of which lies the common trunk, N . 10. n, of the united tenth and eleventh or vagus and accessorius nerves, and also, nearby, the lower part of the petrosal ganglion, N. p. petr, of the glosso-pharyngeal nerve. Lower down and nearer the ectoderm lies the facial nerve, N. 7, situated in what is called the hyoid arch or mass of tissue intervening between the first and second gill cleft. The hyoid arch is further marked by a bulge in the external outline of the section, which leads down into a deep groove beyond which the outline of the section again rises and arches forward to the eye. This groove is the external depression of the first gill cleft and ultimately is transformed into the external auditory meatus. The position of this groove is well shown in figure 99, An, on page 160. Just inside the auditory groove appears the outer end of the first or auditory internal gill pouch, cl.I. It is a long, oblique slit, quite narrow, and is lined by a layer of entoderm. If we follow it along through several sections, we shall find that higher up its outer end comes in contact with the ectoderm at the bottom of the auditory groove, and there the two germ-layers, entoderm and ectoderm, unite to form a single membrane, the closing plate of the gill pouch. Following through the section downward in the series, we can trace the cleft to its connection with the pharynx, Ph. On the posterior side of the cleft we find the internal carotid, Car. in. Only the roof of the pharynx, Ph, is cut, so that it occupies but a small area in the section. On its anterior side it shows a small knob-like projection toward the floor of the forebrain. This is a part of the stalk of the hypophysis. Below the hypophysis appears the very large and conspicuous inferior maxillary nerve, Mx. in, and beneath that the section of the small internal jugular vein, Jug. in. The forebrain is quite complicated in shape, having two lateral expansions, L. V, of its cavity which are destined to form the lateral ventricles. The walls, H, of the lateral ventricles are the anlages of the cerebral hemispheres. From the ventral fin the figure upper) part of the fore-brain spring on either side the optic stalks, Op. These are hollow prolongations of the brain, which expand at their distal ends to form the retina of the eye, Ret, and the pigment layer. The expanded distal ends constitute each a sort of cup, of which the optic stalk is the stem. The cup is two-layered, the space between the two layers being a prolongation of the central cavity of the brain. The inner of the two layers forms the retina proper and is considerably thickened. The outer layer is quite thin and is already quite abundantly laden with pigment granules. At the edge of the cup the pigment layer passes over uninterruptedly into the thick retina layer. In the cavity of the optic cup lies the vesicular lens, L, which arose from an evagination of the overlying ectoderm. The vesicle is, however, now completely separated from the layer which produces it. It has at this stage a very large cavity, and in adjacent sections it can be readily seen that the inner side or that toward the brain is already thickening and changing its character so as to form the main part of the adult lens. The thickening depends chiefly upon the rapid and enormous elongation of the epithelial cells of this part of the vesicle, so that they are transformed into the so-called fibers of the adult lens, each adult fiber being a single epithelial cell.

Fig. 116. — Pig, 12.0 mm. Transverse Series 5, Section 249. Car. in, Internal carotid artery. cl.I, First or auditory gill cleft. D. Z, Dorsal zone of spinal cord. Ec, Ectoderm. H, Anlage of cerebral hemisphere. Jug, Jugular vein. Jug. in, Internal jugular vein. Z, Lens. L.R.11, Lateral root of the eleventh nerve. L. V, Lateral ventricle., Inferior maxillary nerve. JV.y, Facial nerve. N.q.petr, Petrosal ganglion of the ninth nerve. N. 10.11, United vagus and spinal accessory nerve. N.12, Hypoglossal nerve. Op, Stalk of the optic evagination. Ph, Pharynx. Ret, Retina. V.Z, Ventral zone of spinal cord. X 22 diams.

Section through the Second Gill Cleft and Oral Fissure. — The level of this section is such that the head is cut separately and appears in section without connection with the body of the embryo. The space between the head piece and body piece, 0. F, may be designated as the oral fissure, since it is into this space that the mouth opens. In general there is considerable resemblance between this and the section last described, but in the present section the eyes have disappeared and we get the first indications of the nasal pits, 01}. That on the left side of the body shows a trace of the cavity of the pit. The posterior part of the pharynx, Ph, is cut in the section instead of the anterior part, as in figure 1 16. The first gill cleft does not show, but the second cleft, cl. II, does. It lies posterior to the first cleft and, therefore, appears higher up in the figure. "The spinal cord, Sp. c, shows the same general structure as in the previous section. On either side of it may be seen the small and inconspicuous root of the eleventh or accessory nerve. It could not be properly represented in the figure. Some distance below the cord lies the small circular section of the notochord, which differs so slightly in staining from the surrounding mesenchyma that it cannot be well made out without the use of a higher magnifying power. It is enclosed by a distinct membrane which is thick enough to produce a double outline, and contains a considerable number of scattered nuclei, which are, however, nowhere much crowded. The nuclei are round in form, decidedly larger than those of the surrounding mesenchyma, granular, and containing each several more conspicuous, darkly staining granules. There is a very slight gathering of mesenchymal cells about the notochord, as if to form the anlage of a sheath. Just below the notochord there is a broad band of somewhat darker staining, due to a greater condensation of the mesenchyma in that region, and this condensation represents the beginning of the formation of the vertebral structures. On either side we find the transformed myotome, My, or anlage of the striated muscular tissue. This tissue is produced from the secondary segments of earlier stages. The cells have now separated from one another, have lost their distinctly segmental grouping, and have begun to elongate into true muscle-fibers. All that can be distinguished with the low power is the somewhat darker appearance of this part of the section, due to the great crowding of the nuclei. Between the muscular anlage and the notochord the section shows a portion of the first cervical nerve, N. cerv. /, and just within this nerve is a small blood-vessel not represented in the figure. There is a similar blood-vessel symmetrically placed on the opposite side. They are the small vertebral arteries. The jugular or anterior cardinal veins, Jug, are large and conspicuous vessels, but despite their size they have merely endothelial walls and there is no condensation of the mesenchymal cells around them, although such a condensation is to take place later to form the anlages of the muscular and connective-tissue coats (media and adventitia) of the adult. On the dorsal side of the jugular vein and close to it is a light spot in which can be easily distinguished, with the high power, several more or less distinct bundles of nervefibers which are separated from one another by mesenchymal cells. For this reason it is somewhat difficult to recognize this nerve with the low power or to represent it in the figure. On the ventral side of the vein there appears a darkly stained mass, N. 10, the nodosal ganglion of the vagus nerve, and outside of this ganglion is the section of the spinal accessory nerve. Immediately below the nodosal ganglion we have the internal carotid artery, car. in. A little to the inside of the jugular is a small vessel, Ao.D, of great morphological- importance. The corresponding vessel appears on the opposite side. Although very small, this vessel has a distinct coat of condensed mesenchyma around its endothelium. The two vessels are the descending aortae, which have almost completely aborted, and in slightly older specimens will be found to have disappeared altogether. The descending aortae are the longitudinal trunks by which the dorsal ends of the five aortic arches of early stages are connected together. The portion shown in this section is the part of the descending aorta between the tops of the third and fourth aortic arches. The relations are shown in the reconstruction (Fig. 97). The pharynx, Ph., is narrow in its dorsal ventral diameter, but wide transversely, and offers the very characteristic yoke-shaped figure in the section. The distal portions of the second gill clefts are shown, and they appear disconnected with the pharynx, the connection occurring in sections higher up. Each cleft is somewhat slit-like, so that its cavity is an oblique fissure and somewhat parallel in position to the first cleft (Fig. 116). Both the pharynx and the gill clefts are, of course, lined throughout by entoderm, which forms a sharply defined layer crowded everywhere with nuclei, which are of about the same size as those of the surrounding mesenchyma. In the pharynx the entoderm is somewhat thinner on the dorsal than on the ventral side. In the clefts it is thicker than in the pharynx proper, and especially in the clefts it may be observed that the mitotic figures always occupy a superficial position. On the dorsal side of the cleft is a very small blood-vessel, near which, with a higher power, one may see a small nerve, and nearby, but more dorsalwards, a second nerve. Both of these are branches of the glosso-pharyngeus, and lie behind the cleft. They are, therefore, termed the post-trematic branches. Below the cleft and somewhat on its median side is a similar third nerve-branch, the pretrematic of the glossopharyngeus, running in front of the cleft. The outline of the embryo forms a rounded eminence outside of the second cleft ; it represents in part the hyoid arch. In the midst of the mesoderm of this appears a light area with a few nerve-fibers, the end of the facial nerve, N. 7. The mandibular arch or process, Mdb, is very distinct and prominent. It is separated from the hyoid arch by a deep external notch, which corresponds to the external first or auditory cleft. In the interior of the mandibular process there are light spaces differing in their exact distribution on the two sides of the specimen. These spaces contain nerve-fibers and they represent the inferior maxillary nerve. We now come to the oral fissure, 0. F, which separates the body from the head. In the head portion of the section we have the maxillary process, Mx, which is separated in part from the rest of the head by the deep lachrymal groove, L. gr. On either side there shows a shaving from the epithelium of the olfactory chamber, Olf. The fore-brain has expanded laterally, L. V, to form the lateral ventricles, the walls of which, H, are the anlages of the cerebral hemispheres. On the dorsal side, which is the lower side in the figure, the hemispheres project somewhat, leaving a median space between them. This median space is filled with mesenchyma, which may already be regarded as the anlage of the falx. In the tissue of the falx are two very small blood-vessels, the forward prolongations of the lateral jugulars, which are to unite to form the median superior longitudinal sinus. In the previous section these vessels also reappear, but are already united (Fig. 116). In the median dorsal line the wall of the fore-brain is thin and shows a characteristic notch. Close to the surface of the fore-brain there is a very distinctly marked vascular layer, the commencing pia mater, P. M , and with a high power it can be easily seen that the differentiation of the arachnoid zone has already begun.

Fig. 117. — Pig, 12.0 mm. Transverse Series 5. Suction 292.

Ao.D, Descending aorta., Internal carotid artery. ■/.//, Second gill cleft. Ec, Ectoderm. H, Anlage of cerebral hemispheres. Jug, Jugular vein., Lachrymal groove. L.V, Lateral ventricle Mandibular arch. Mx, Maxillary process. My, Myotome, itch, Notochord. N. First cervical nerve. jV. 7, 1'acial nerve. N. q. Glossopharyngeal nerve., Vagus nerve. X.n, Spinal accessory nerve. O.F, Oral fissure or space between the head and mandibles. Olf, Olfactory pit. J'/i, Pharynx. P.M, Pia mater. Sp.c, Spinal cord. X 22 diams.

Section through the Third Gill Cleft and Nasal Pits. — In this section the head is clearly separated by a considerable space from the rest of the section. The transverse diameter of the embryo is here much less than higher or lower, so that the section as a whole seems somewhat narrow. It shows the entire length of the third gill cleft, cl. Ill, exhibiting, on one hand, its connection with the median pharynx, and, on the other hand, its dorsal extremity, where its entoderm joins the ectoderm. The external outline of the embryo makes a deep depression outside the end of the third cleft. This depression is the cervical sinus (compare Fig. 96, C. S; pig of 10 mm.). In the section the cervical sinus displays a narrow downward prolongation. If followed through in the series of sections, this prolongation, which is on the inside of the hyoid arch, Hy, will be found to connect with the second cleft. The spinal cord, Sp. c, presents essentially the same structure as in figures 116 and 117. Our section passes through the roots of the second cervical nerve, TV. cerv. 2, and shows both the dorsal ganglion and the ventral root arising from the ventral zone. These two roots join and form the nerve-trunk, Nv. 2, which almost immediately divides, sending one branch vertically upward into a mass of denser crowded cells (the anlage of the dorsal musculature) and a ventral branch which descends almost vertically toward the pharynx. Just inside of this ventral branch we have the section of the vertebral artery, Art. v. Between the dorsal summit of the ganglion and the spinal cord there is a minute bundle of nerve-fibers not shown in the figure. These fibers constitute the commissural trunk of the eleventh nerve. The third gill cleft is cut almost symmetrically, and extends from the median line to the edge of the section. It is lined throughout by the entoderm, which at the end of the cleft on each side has met and fused with the ectoderm to form the epithelial membrane, the closing plate. The membrane apparently normally remains intact in mammals. In the ichthyopsida the membrane becomes ruptured during embryonic life, and the gill cleft is opened to the exterior. At the end of the cleft the entoderm has undergone a special growth forming a distinct mass, TIivi, on the side of the cleft toward the head. This entodermal structure is the anlage of the thymus gland and is already penetrated by small blood-vessels which are perhaps not capillaries, but sinusoids. The student should clearly understand that the median region of the third gill cleft is really the pharynx proper. From its median ventral line arises the beginning of the trachea, Tr, which should, perhaps, be already designated as the anlage of the larynx. The entoderm extends down in the median line for a considerable distance, making a figure which, in the section, is shaped somewhat like a spear-head. In the center of the section appears a small cavity. Further down toward the lungs we have only an epithelial plate with no cavity observable in it (Fig. 119, Tra), the entoderm of the trachea at this stage forming a solid cord. Ventrad from the trachea, in the median region and between the two aortic arches, is a small, irregular, deeply stained mass of cells, Thyr, the anlage of the thyroid gland. These cells are entodermal, the anlage having been developed by a downgrowth of the epithelium of the floor of the pharynx, although at the present stage the original connection with the pharynx has been lost. The anlage is now isolated from its parent germ-layer and is imbedded in mesenchyma. It is solid, for the cavities of the thyroid follicles are not developed until considerably later. Just above the third gill cleft may be seen a large, darkly stained mass, G. nod, the ganglion nodosum of the vagus nerve. Immediately above it is the section of the jugular or anterior cardinal vein, Jug. Between the ganglion and the vein is a bundle of nerve-fibers representing the twelfth or hypoglossal nerve. Close to the ganglion on its outer side is the section of the spinal accessory nerve, N. 11 , which reappears again in the section below the pharynx, at N. 12. The reason for this double appearance of the hypoglossal nerve may be seen readily by examination of the reconstruction (Fig. 105). A little above the jugular vein is the section of the first cervical nerve, JV. cerv. 1, laterad from which is the external branch, R.ex 9, of the spinal accessory nerve. This branch in the adult innervates the sternocleidomastoid and trapezius muscles.

Fig. 118. — Pig, 12.0 mm. Transverse Series 5, Section 321. ././, Fourth aortic arch. Art.v, Vertebral artery, .lit. External auditory cleft, cl.iii, Third internal gill cleft. C »«/, Ganglion nodosum. //■,•, Hyokl arch. Jug, Jugular vein. L. V, Lateral ventricle. -\',<, Nasal pit. N.ctrv.i, First cervical nerve. N.ceru.z, Second cervical nerve. X. //, Spinal accessory nerve. JV./g, Hypoglossal nerve. Nv.2, Main trunk of second cervical nerve., Olfactory plate. A'.ex.q, Ramus externus of the glossopharyngeal. Sp.c, Spinal cord. Thru, Thymus. Thyr, Thyroid. Tr, Trachea. ■ 22 diams.

The lower part of the figure represents the section of the head and shows the two nasal fossae, Na, closed toward the mouth side by the olfactory plate, 01. pi, the epithelial membrane somewhat resembling the closing plate of a gill cleft, but it is formed by a fusion of the ectoderm on the two sides of the opening of the nasal pits. When the nasal pits are first formed, they are open throughout their whole extent. The formation of the olfactory plate is the first step toward the separation of the two nasal cavities from the oral cavity. In later stages this plate disappears, and its forward portion is replaced by mesenchyma, so that the separation of the nasal and oral cavities is permanent, but the posterior portion of the membrane becomes first very thin, and finally disappears altogether, thus establishing a secondary connection between the nose and mouth for each nasal chamber, thus leading to the development of the internal choanae. On the dorsal side of the nasal pits (below in Fig. 118), the cerebral hemispheres are cut separately, their darkly stained walls bounding on each side the large lateral ventricle.

Section through the Fourth Gill Cleft. — Of the entodermal gill clefts or pouches the fourth is by far the smallest, and as it appears in sections (Fig. 1 19, cl. IV) is inconspicuous. The section figured differs by two striking features from those of the series above described : first, because the head is no longer included; and, second, because the cardiac structures are beginning to show. On the dorsal side the spinal cord is cut at the level of the ganglion, G.j, of the third cervical cleft. The dorsal root of the ganglion joining the spinal cord, Sp. c, is shown on both sides of the section, and the nerve itself also appears, being best shown on the left side of the embryo, where a short piece, R. D. j, of the ramus dorsalis is included and a much longer piece, R. V. j, of the ramus ventralis. Just inside of the nerve at the level of the notochord, Nch, is the crosssection of the vertebral artery. On the right side of the embryo the section passes through a portion of the second cervical nerve, N. cerv. 2. The jugular vein, Jug, is a verv large vessel. Close to its ventral wall appear a few fibers which represent the first cervical nerve, but they are too indistinct to be represented in the figure. They may easily be found with the higher power. In the median plane is the crescent-shaped section of the pharynx, Ph. Between the jugular vein and the pharynx lies the fourth aortic arch, Ao. 4. The right and left arches are at this stage about equal in size, although the left arch is destined to form the main aortic arch of the body, and only a portion of the right arch will persist to form a portion of the stem of the pulmonary artery. The figure indicates the manner in which these aortic arches pass up from the heart below on either side of the pharynx. A little above the aortic arch on either side may be seen a small, round spot, Sym, which is somewhat conspicuous on account of its deeper staining. It is a section of the cervical sympathetic. Examination with a higher power shows that it consists of somewhat crowded cells, some of which have larger nuclei. These are the neuroblasts. The mesenchymal cells immediately around the anlage are disposed about it in somewhat concentric lines. Between the jugular and the aortic arch is situated the large conspicuous nerve-trunk, N. 10, 11, constituted by the united vagus and spinal accessory nerves. Below this double nerve is a blood-vessel, Ve, a branch of the jugular vein which runs to the lower jaw and tongue. The homologies between this vein and those of the adult have not yet been worked out. Returning now to the pharynx, Ph: on the right side the prolongation of the pharynx to join the fourth cleft can be clearly followed; on the left side of the embryo, the right of the figure, the fourth cleft, el. IV, does not display its connection with the pharynx, but is a separate, small epithelial cavity lined by a cylinder epithelium. Underneath the pharynx appears a vertical plate, Tra, formed by the entoderm of the trachea. This plate is thinnest in the middle, somewhat wider toward the top and bottom of the section. It is quite solid, except for a minute cavity at its dorsal end. This minute cavity may be traced from the opening of the glottis through the series of sections down until it becomes connected with the comparatively large cavities of the developing bronchi of the lung. Below the pharyngeal region descends the thick somatopleure, Som, which encloses the pericardial coelom, Cce, in which the heart is lodged. The inner surface of the somatopleure is covered by the thin mesothelium, msth. Of the cardiac structures we note first the section of the main aorta, Ao, and of the pulmonary aorta, P. A, and finally small sections of the uppermost part of the two auricles, Au. d and Au. s. More of the left auricle is included in the section than of the right.

Fig. 119. — Pig, 12.0 mm. Transverse Series 5, Section 353. Ao, Aorta. A0.4, Fourth aortic arch. An. J, Right auricle. A11.S, Left auricle. Ca, Coelom. </.//', Fourth gill pouch. G.j, Ganglion of third cervical nerve. Jug, Jugular vein, mstli, Mesothelium. N.cerv.2, Second cervical nerve. Neh, Notochord. N. 10,11, United vagus and spinal accessory nerves. P. A, Pulmonary artery. P/i, Pharynx. R.D. j, Dorsal ramus of the third cervical nerve. R. I'.j, Ventral ramus of the third cervical nerve. Som, Somatopleure. S/>.<r, Spinal cord. Sym, Sympathetic nerve chain. Tin, Trachea. Vt, Vein to lower jaw. >( 22 diams.

Section through the Anterior Limbs and Heart. — The section figured is much lower in the series than the last and was selected in order to illustrate the anterior limb-buds, the ducts of Cuvier, and the heart. The position and shape of the limb-buds are sufficiently shown in figure 96. The section demonstrates that the limb-bud is formed chiefly by a dense mass of mesoderm covered by a thin layer of ectoderm. The mesoderm consists of very much crowded cells in which it is very difficult to recognize any distinct differentiations, yet it is probable that here are mingled both true mesenchymal cells and cells which originally belonged to the muscle plates, but which have now broken apart and are developing singly into muscle-fibers. In certain amphibia the cells from the muscle plate can be distinguished from the mesenchymal cells of the limb, and what we know of the development of the muscles in amniota confirms the view that striated muscles and mesenchyma are genetically entirely distinct. No skeletal elements whatever have yet arisen in the limb. We have here a striking illustration of the fact that the skeleton is very late in its development, and, embryologically speaking, is in no sense the framework upon which the body is built up, but rather a late supplementary development. The main morphological features in all parts of the embryo are entirely fixed by the soft tissues before the skeletal structures arise. Both nerves and blood-vessels have grown into the limb. The nerves are the ventral branches of the spinal nerves. Several of these unite together and form the brachial plexus, one part of which, Br. plx, is shown in the section. In the present embryo this nerve-trunk includes fibers derived from both the sixth and seventh cervical nerves. Just above the nervetrunk is the section of the subclavian or axillary vein, which is a branch from the jugular. The dorsal region of the embryo is relatively larger at the level of this section than higher up, owing chiefly to the great development of the mesoderm. The spinal cord, Sp. c, resembles that in figure 1 19, but is both larger and more differentiated. On the left side of the embryo the fundamental morphological characteristics of the spinal nerve are well illustrated in this section. The dorsal root, D. R, bears the ganglion, G, which joins the dorsal zone of the spinal cord. The fibers of this root are produced from the cells of the ganglion and grow from the ganglion into the spinal cord. Other fibers from the same cells grow out in the opposite direction and form the nerve-trunk or root which descends from the ganglion in a nearly vertical direction. The ventral root, V. R, arises from the ventral zone, takes an oblique course, and joins the dorsal root a little below the level of the spinal cord to form a single nerve-trunk, which, however, soon subdivides into its two primary branches. The first or dorsal branch, R.D, bends at an acute angle upward and outward. The second or ventral branch, ramus ventralis, continues downward and curves into the limb. Owing to this curvature, in order to follow its course the nerve must be traced through adjacent sections. If this is done, the ventral ramus will be found to take part in the formation of the brachial plexus. Some distance below the spinal cord is the small notochord. Further down, but also in the median line, appear two small rings of epithelium. Of these, the smaller upper one, (E, is the entodermal lining of the oesophagus, and the larger lower one is the entodermal lining of the trachea. Around each of these rings there has already occurred a slight condensation of the mesenchyma, the first step toward the ultimate differentiation of the submucous and muscular coats of the oesophagus and trachea. The entoderm of both the oesophagus and trachea is a moderately thick layer composed of elongated cells, the nuclei of which are distributed at various levels, but so as to leave the superficial portion of the layer comparatively free. It is in this superficial portion that the mitotic figures always occur. On the ventral side of the trachea and quite close to it appear two small blood-vessels, the pulmonary arteries. By an oversight the two arteries are represented as one in the engraving. To the right and left of the oesophagus appear the circular sections of the two descending aortae, of which the left, Ao. S, is already somewhat larger than the right

Fig. 120. — Pig, 12.0 mm. Transverse Series 5, Section 470. Ao.S, Left descending aorta. Au.d, Right auricle. Br.Plx, Brachial plexus. D.C.S, Ductus Cuvieri sinislra. D.R, Dorsal root of spinal nerve. F, Cardiac fissure. G, Spinal ganglion. L, Anterior limb-bud. Nch, Notochord. Nv, Branch of brachial plexus. (E, CEsophagus. R.D, Ramus dorsalis of spinal nerve. S.a.c, Septum of the auricular canal. Scl.V, Subclavian vein. Som, Somatopleure. Sp. c, Spinal cord. S.s, Septum superius. Tra, Trachea. Val, Auriculo-ventricular valve. Ven.S, Left ventricle of the heart. V.R, Ventral root of spinal nerve. X 22 diams.

Ultimately the greater part of the right aorta will disappear, the aortic arch of the adult being formed from the left aorta. Lower down in the series the two descending aortae unite to form the single median dorsal aorta. The ducts of Cuvier are two enormous venous trunks which deliver the blood to the heart. They lie symmetrically placed to the right and left of the oesophagus and trachea. They extend from the level of the descending aorta? downward and inward to the level of the heart. The duct of the left side, D. C. S, is almost symmetrical with its fellow of the right side, but it has no direct communication with the heart ; but by following down through the series of sections the student can observe that the left duct of Cuvier connects across with the duct of the right side. The right duct opens directly into the right auricle, Au. d, of the heart. All of the venous blood is collected at this stage by the ducts of Cuvier, except that which comes through the liver. The dorsal end of each duct of Cuvier is formed by the union of the jugular or anterior cardinal vein from the head with the posterior cardinal vein from the body. The opening of the right duct into the auricle of the heart is guarded by two small flaps or valves. The lower part of the section is occupied by the large heart lying in the pericardial chamber. The body-wall, Som, or somatopleure, which forms the outer covering of this chamber, is quite thin and without a trace of muscular or skeletal structures. It consists of three distinct layers — an external ectoderm and middle mesenchyma, and the internal mesothelium. The mesothelium is a thin layer of cells which persists throughout life and is known in the adult as the pericardial epithelium. In the present section it is easy to follow this layer from the somatopleure past the ducts of Cuvier on to the heart and completely around the outside of the heart itself. Everywhere it forms the covering or boundary of the coelom of the pericardium. In later stages this mesothelium will have an especial layer of connective tissue close under it. The layer of connective tissue, together with the mesothelium, constitutes the pericardial membrane of descriptive anatomy. The essential fundamental relations of this membrane may, therefore, be easily understood from the present section. From the study of the adult conditions alone it is extremely difficult for the student to grasp these relations. The heart is a very large organ. It consists of two auricles and a ventricle with two limbs. The auricles have thin walls and are separated from one another by a very thin membrane, the septum superius, S.J. The right auricle, Au. d, receives upon its dorsal side the opening of the right vein or duct of Cuvier, the opening being guarded by valves. Of these valves, the one toward the median line disappears, but the other, toward the right of the embryo, persists to form both the Eustachian and Thebesian valves of the adult. As stated above, the left duct of Cuvier delivers its blood to the right duct, and so indirectly to the heart. The ventricles of the heart are much larger than the auricles, and the left ventricular limb or future left ventricle, Ven. S, is already larger than the right limb. The external groove, F, which marks the boundary between the two auricles is clearly shown by the section. The trabecular structure of the ventricles is well developed, and affords a diagnostic path by which the ventricles, if they are cut, may be easily recognized in sections. The development of the trabeculae corresponds to the formation of blood sinusoids of the heart. The trabeculae consist of young muscle cells, and each bundle of cells is closely invested by the endothelium of the heart. The blood thus circulates freely between the trabecular, but remains, as in every blood-channel, separated by the endothelium from the neighboring tissue. The tissues of the heart are thus enabled to get their nourishment from the blood circulating through the organ. The sinusoidal type of circulation which we here encounter appears in the heart of all vertebrate embryos, and is the permanent form of circulation in the frog. In mammals, on the other hand, although the sinusoidal circulation is kept throughout life and the ventricles always have their trabecular structure, yet we find, in addition, the development of a true capillary circulation to supplement the sinusoidal. This capillary circulation is supplied by the coronary arteries, and develops comparatively late. Between the auricles and the ventricles the heart is narrow. This constricted region is known as the auricular canal. A broad partition, S. a. c, divides the cavity of the auricular canal into right and left channels, forming open vessels between the auricles and ventricles. From the lower edges of these channls flaps of tissue project into the ventricles. The flaps are the anlages of the atrio-ventricular valves.

Sections through the Anterior Limbs to Show the Brachial Plexus. — Figure 121 was drawn from a single section, except that the nerves in the limbs represent a reconstruction from several adjacent sections. The limb-bud, A. L, projects freely from the side of the body, is covered by ectoderm, Ec, and filled with a very dense tissue, the cells of which show no very clear histological differentiation. The spinal cord, Sp. c, is fairly well advanced in its development at this level, and shows a darker, inner layer, Epen, a middle, gray layer, cin, and an outer neuroglia, Ec. gl. The cord is completely surrounded by the developing pia mater, which is quite thin, but highly vascular. The ganglia are cut almost symmetrically on the two sides and show their dorsal roots. The descending trunk from each ganglion is joined by the ventral roots, V. R, which arise from the ventral zone of the cord in several bundles which unite about the same time with both one another and the dorsal* root to form the main nerve-trunk, N.S, which enters into the formation of the brachial plexus. Just after the junction of the two roots the nerve gives off a branch which runs obliquely dorsalwards into the anlage of the dorsal muscles, Muse. This branch is, of course, the dorsal ramus. The trunk, N, which runs toward the limb is the ventral ramus. Below the spinal cord is the notochord, Nch, which is completely surrounded by a very dense mass of mesenchymal cells, Vert, the anlage of the body of a vertebraLower down in the section are the two descending aorta? , Ao, which are at this point just uniting to form the single median dorsal aorta. Below the aorta runs a ring of epithelium, CE, representing the entoderm of the oesophagus, and further ventralwards a second layer of epithelium, Tra, the entodermal lining of the trachea. Both of these rings of epithelium are surrounded by somewhat condensed mesenchyma, the differentiation of which about the oesophagus is more advanced than about the trachea. Around the oesophagus next to the epithelium is a thin, looser layer of mesenchyma, the anlage of the mesodermic portion of the future mucous membrane, and perhaps also of the submucosa. Outside of this looser mesenchymal envelope. is a second denser layer in which thecells appear elongated, having begun their differentiation into smooth muscle cells. To the right and the left of the aorta appear the very large posterior cardinal veins, card. From the sides of the trachea project lobes of tissue which represent the anlages of the lungs. These lobes of tissue are each covered by a layer of mesothelium, and protrude, as it were, into the coelom of the pleural cavities, Pleur. Further to one side the coelom, Car, of the abdominal cavity is also in part shown. It is bounded externally by the body- wall, Som, of the embryo. To the right and the left of the oesophagus lie the sections of the vagus nerve, the right nerve a little higher up than the left. Below the trachea in the median line is a small blood-vessel, a section of the pulmonary vein. Above the aorta, on the right and left, lies a small cluster of rather darkly stained cells, intermingled with which one can make out with a higher power the future nerve-fibers. These structures are portions of the sympathetic nervous system. As regards the great nerve of the limb, N.8, it must be remembered that it forms a portion of the brachial plexus and is joined by other cervical nerves. From the voluminous trunk thus developed there arise three principal branches; the first, xx, is at the base of the limb, is small, and runs off dorsally. The other two represent a terminal forking of the nerve-trunk, one, yy, running to the dorsal side of the limb, the other, zz, to the ventral side.

Section through the Stomach and Liver. — We now pass to a section well below the heart in order to study the characteristics of the Wolffian body, stomach, and liver. At this level, as comparison with figures 118 and 120 will show, the body of the embryo has its greatest dimensions. The upper edge, Urn, of the umbilical cord appears in this section. The spinal cord with its ganglia and nerves presents essentially the same features as in figure 120. The notochord, Nch, forms a small circle in section and is surrounded by the anlage of the body of a vertebra, which appears as an area relatively large, over which the mesenchymal cells are more crowded or condensed than elsewhere. At its periphery the anlage merges without divisional boundary into the surrounding mesenchyma. It is more expanded laterally than ventrally. In the median line below the notochord is the large dorsal aorta, Ao, which is formed by the union of the two descending aortse shown in figure 120, and which extends through the abdominal region of the embryo to the pelvic region, where it forks to form the two allantoic arteries, which, passing on either side of the intestine, continue their course along the side of the internal allantois or future bladder, until they reach the umbilicus, where they enter the umbilical cord to supply the extra-embryonic or placental circulation. The aorta is surrounded by mesenchyma, and to this are, so to speak, appended the large Wolffian bodies, W . B, one on each side. From the dorsal region of the embryo to the umbilical cord extends the somatopleure or body-wall, Som, which, like that around the pericardial chamber, consists of an external ectoderm, Ec, a middle mesenchyma, mes, and an internal mesothelium, tnsth. It is important for the student to understand the arrangement of the germ-layers in the somatopleure. The mesothelium is commonly known in the descriptive anatomy of the adult as the peritoneal epithelium. The peritoneal membrane consists of this epithelium and of all underlying connective tissue. In sections like that figured it can be readily followed not only over the inner surface of the body-wall, but over the surface of the Wolffian body and liver, and upon the left side of the body also over the surfaces of the greater omentum, stomach, and lesser omentum. The relations of the abdominal viscera to the peritoneum which are so perplexing to the student of adult anatomy are here shown diagrammatically, as it were, by the section of the actual embryo. It is evident from such a section that the abdominal cavity (splanchnoeele) is completely bounded by mesothelium, and that all the abdominal viscera are outside of the cavity. This conception, which is so important, yet so difficult to the student of anatomy, is easily mastered by the study of embryonic relations. The Wolffian body, W. B, is the foetal or embryonic kidney, and is also termed the mesonephros (compare page 101). It is much larger relatively to other parts in the pig than in man or the rabbit. It consists of numerous epithelial tubules very much contorted with blood spaces between them, of glomeruli which always lie toward the median and inferior side of the organ, and, finally, of a single longitudinal canal, the Wolffian duct, into which all of the tubules open. The tubules are formed by the cuboidal epithelium. The glomeruli resemble in their structure those of the kidney. Each is a bunch of blood-vessels covered in by a layer of epithelium which forms one boundary of the space into which the glomerulus projects. The opposite side of the space is also bounded by epithelium, which at the stalk of the glomerulus becomes continuous with the covering of the glomerulus itself, the whole structure resembling closely that of a Malpighian corpuscle of the true kidnev. The space around each glomerulus is really the beginning of a Wolffian

Fig. 122. — Pig, 12.0 mm. Transverse Series 5, Section 633. Ao, Dorsal aorta. Ec, Ectoderm. G, Spinal ganglion., Gall-bladder. Gen, Anlage of genital gland. Li, Liver, mes, Somatic mesenchyma. msth, Somatic mesothelium. X, Spinal nerve. Nch, Notochord. Om.maj, Omentum majus. Om.min, Omentum minus. Som, Somatopleure. Sp.c, Spinal cord. St, Stomach. Urn, Umbilical cord. V.card, Posterior cardinal vein. V.C.I, Vena cava inferior. Vert, Anlage of vertebra. V.U.D, Right umbilical vein. V.U.S, Left umbilical vein. //'. B, Wolffian body. X 22 diams.

tubule. The spaces between the tubules are almost entirely blood-channels, and are lined by endothelium, which, for the most part, is closely fitted against the epithelium of the tubules. Occasionally a small amount of mesenchyma can be found between the tubules, or between the tubules and the nearest endothelium. We have, accordingly, in these organs a typical sinusoidal circulation. The blood spaces of the Wolffian body really belong to the posterior cardinal veins into which the Wolffian tubules in the course of their development have, as it were, penetrated, although without destroying the continuity of the vascular endothelium. It is by the intercrescence of the tubules and of the endothelium that the sinusoidal condition is established. A portion of the original channel remains on the dorsal side of the Wolffian body, more or less free, V. card. We thus learn that, owing to the development of the Wolffian body, the posterior cardinal veins as such disappear. The Wolffian duct is always on the ventral side of the organ, and can easily be traced through as a continuous tube from section to section. In the figure it may be easily found in the left mesonephros, it being there the lowermost of the cavities drawn in the organ. On the median lower surface of the Wolffian body, underneath the glomeruli, is an accumulation of tissue, Gen, the anlage of the genital gland, which is yet very slightly advanced. Below the aorta on the right side of the embryo is a large trunk of the vena cava inferior, V. C. I, on its way past the right dorsal lobe of the liver. Near the aorta on the left is the mesogastrium, Om. ma], or future great omentum, by which the stomach is suspended from the median dorsal wall or the abdomen. The stomach, St, is entirely upon the left side of the body and is directly connected, by means of the anlage of the lesser omentum, Om. mm, with the liver. The walls of the stomach are constituted by the splanchnopleure, and, therefore, comprise a layer of thickened entoderm, which bounds the cavity of the organ, and a relatively thick layer of mesoderm which forms the greater part of the wall, and the very thin superficial mesothelium. The entoderm is a smooth layer of moderate thickness composed of elongated epithelial cells. It forms no folds and shows no trace of differentiation into gastric glands. In the mesenchyma there are some capillary blood-vessels. The mesothelium is thicker than over the liver and somatopleure, and contains crowded, more or less nearly spherical nuclei. The liver is by far the largest organ of the body. It takes up nearly half the section. It is divided into four main lobes, the two dorsal and two ventral; two on the right and two on the left. The reference line, Li, runs to the left dorsal lobe. The liver consists of a complicated network of relatively large blood sinusoids, the spaces between which are entirely occupied by the embryonic liver cells. Near the median plane, between the ventral lobes, appears the gall-bladder, G. bl, which is cut three times. The liver is attached in the median ventral line to the body-wall and to the base of the umbilical cord. The two umbilical veins (compare Fig. 132) enter the liver directly from the cord. The veins are originally of equal size, but in this embryo the right vein, V. U. D, has already become smaller than its fellow, V. U. S, of the left side, and in later stages the right vessel is found to have disappeared altogether. The two veins are here connected respectively with the right and left ventral lobes of the liver. It will be noticed that the right and left sides of the abdominal cavity are completely separated from one another, and that there is a special part of this cavity shown in the section between the stomach and the right dorsal lobe of the liver, and which is known as the lesser peritoneal space or cavity of the omentum. In another section the lesser peritoneal space is found to connect, by means of a very small and narrow foramen of Winslow, with the general cavity of the abdomen.

The Study of Sagittal Sections, Embryo of 12 mm

From a sagittal series of this stage many significant pictures are obtainable. Two sections only have been selected for illustration and description: first, the median section of the head; the second one passing through the principal cephalic ganglia. To the student of anatomy these sections are highly instructive, for they exhibit each, or in a single picture, many important fundamental relations of the brain, cephalic nerves, and other structures of the head.

Median Sections of the Head. — The plane of the section (Fig. 123) is almost exactly median for the region of the hypophysis and infundibular gland, but in the region of the spinal cord it is a little to one side ; hence the actual plane of the section is slightly oblique to the true median plane of the embryo. The present section is especially instructive as regards the shape of the brain and the relations of its various parts to one another. The hind-brain begins at the spinal cord, Sp. c, and has a very large cavity, the fourth ventricle, Ven. IV. It is separated from the region of the mid-brain by a constriction which is very marked on the dorsal side, Isth. The constriction is known as the isthmus. It is always from the dorsal side of the isthmus that the fourth nerve takes its origin. It is one of the fixed landmarks of the brain. The mid-brain, M . B, also has a large cavity, and, as a whole, forms the great arch which corresponds to the head-bend of the embryo. It passes forward and downward, without any very definite line of demarcation at this stage, into the fore-brain, the cavity of which is larger in diameter than that of the mid-brain. The fore-brain is partially subdivided into two regions; the anterior, Pros, is the prosencephalon and gives rise to the lateral outgrowths, which form the cerebral hemispheres. Already the deep depression separates this part of the fore-brain on its dorsal side from the posterior part, which is termed the diencephalon. The limits of the diencephalon at this stage are very indistinct; later its boundary against the mid-brain becomes clearly marked by the differentiation of the epiphysis and posterior commissure.

The spinal cord, Sp.c, forms almost a right angle with the axis of the hind-brain. This angle marks and corresponds to the neck-bend of the embryo. On its dorsal side the hind-brain has a thin ependymal roof, open, which, however, toward the isthmus thickens considerably to produce the anlage, Cbl, of the median portion of the cerebellum. On the ventral side the wall of the fore-brain varies in appearance. Where the section is exactly median, it displays the raphe or floorplate of the region. Where it is off the median plane, it shows instead the thicker, lateral wall of the medulla oblongata. The walls of the mid-brain on the dorsal side, Q, are almost uniform in thickness and texture. They are, however, later to be differentiated into the corpora quadrigemina. The ventral side of the mid-brain, Ped, is considerably thicker than the dorsal, and forms a strongly marked arch. It is represented in the adult essentially by a part of the peduncle of the cerebrum. The floor, Dien. fl, of the diencephalon is a thin membrane of which the part nearest to the mid-brain will produce the mammary bodies, and the part further from the mid-brain the tuber cinereum. It has already formed a special outgrowth, In}, the anlage of the infundibular gland, which extends out from the brain and arches over the end of the hypophysis, Hyp. The hypophysis is an outgrowth from the ectodermal lining of the mouth, Or. Its method of development can be clearly made out at this stage. The infundibular gland in older embryos extends further on the posterior side of the hypophvsis. Meanwhile the hypophysis loses all connection with the epithelium of the oral cavity, somewhat as does the otocyst with the overlying epidermis which produces it. The hypophysis proper and the infundibular gland undergo their further development in intimate association. The result of their differentiation is the pituitary body, which is really a duplex organ. Below the infundibular gland the wall of the brain shows a thickening, Chi. op, which can be followed through in the series laterally until it connects with the optic stalk. This thickening of the brain-wall in later stages furnishes the passage for the fibers of the optic nerve, and is, therefore, the anlage of the optic chiasma. Between the infundibular gland and the optic chiasma extends the post-optic laminae, L.p.o. On the opposite side of the chiasma follows the lamina terminalis, which leads us forward to the wall of the hemispheres, H. Underneath the hind-brain extends the large basilar artery, .4. bos; at its posterior end, A. bas. p, the basilar artery is joined by the two vertebral arteries from the fusion of which it is really produced. Underneath the fore-brain we have the opening of the mouth, Or, from the dorsal side of which springs the elongated evagination of the hypophysis. The oral cavity runs into the pharynx, the floor of which is formed in part by the anlage of the tongue, Ton, and of the epiglottis, Epgl, a rounded eminence very different in shape at this stage from the adult epiglottis. The pharynx can be followed along until it passes over into the oesophagus, Oe, which, however, is not well shown, as the section passes through it away from the true median plane. Between the oesophagus and the anlage of the epiglottis is a mound of tissue, La, which represents the lateral wall of the developing larynx. The mound is separated from the anlage of the epiglottis by a deep notch. In the median plane the mound is filled with entoderm which forms a wide plate through which there is only a narrow opening leading down into the trachea. Finally, we see from the base of the mandible the somatopleure, Sow., extending off to form the boundary of the pericardial chamber. The figure also includes a presentation of the inferior maxillary vein, V. m.x. i, and of the thyroid gland, Thyr, which immediately overlies the main trunk of the ventral aorta. This aorta gives off on either side of the pharynx three principal branches, of whch the smallest is the base of the carotid and corresponds to the third aortic arch. The second and third branches are much larger and correspond to the third and fourth aortic arches. The pulmonary aorta, P. Ao, is already separated from the main aorta of the bod)'.

Sagittal Sections of the Head through the Principal Ganglia. — The section (Fig. 124) is to one side of the median plane. It exhibits the optic nerve, the trigeminal, acustico-facial, petrosal, jugular, and nodosal ganglia; but, on the other hand, exhibits little of the brain, there being only a shaving from the lateral wall of the fore-brain, H, and a section of the widest part of the hind-brain which shows the cavity or lateral recess, R. L, of the fourth ventricle. The auditory vesicle is cut, Ot. It is formed by a layer of epithelium derived from the ectoderm, although now not connected with the overlying part of the epidermis by the invagination of which the otocyst is developed. It shows a narrow, upward prolongation, the anlage of the ductus endolymphaticus (compare Fig. 127). The epithelial otocyst lies in a line with the great cephalic ganglia and occupies its invariable and permanent position behind the acustico-facial ganglion, Ac. F, and in front of the glosso-pharyngeal, G. petr. The position of the otocyst makes it an invaluable landmark in the study of sections of the head. Only the lateral portion of the pharynx, Ph, appears. It forms a wide, almost slit-like diverticulum, from which extend further laterally the first and second entodermal gill pouches. In the figure can be seen a small depression extending downward from the oesophageal or posterior end of the pharynx. This depression marks the beginning of the second cleft. Nothing is seen of the third and fourth clefts in this section, as they both lie nearer the median plane. The pocket or diverticulum of the cervical sinus, Cerv. S, lies near the ganglion nodosum, G. noil. From its appearance it might easily be mistaken for the section of a gill cleft, but it is in reality lined not by entoderm, but by ectoderm, and its cavity can be easily traced through the series of sections of the exterior of the embryo where the epithelium lining the sinus becomes continuous with the epidermis. Cephalad from the sinus, but close to it, lies a small dark rounded mass, the anlage of the thymus gland (compare Fig. 118, Thm). The thymus anlage is produced by proliferation of the entodermal cells on the anterior side of the third cleft, and is penetrated by blood-vessels which seem to be sinusoids, although their history has not been worked out. The great vein of the head, which for convenience we may term the jugular, — although the application of this name to the vein in its present condition is somewhat inexact, — is cut several times, owing to its irregular course. Its main stem, Jug"" , arises nearly vertically through the cervical region and is, relatively to the size of the embryo, of huge diameter. It continues upward, Jug'" , along the dorsal side of the vagus to about half-way between the ganglion nodosum and ganglion jugulare. At that point the vessel curves inward and forward, and, therefore, is not encountered again in this section until, having bent upward again, it shows, Jug' , on its way past the trigeminal ganglion. A branch of the jugular is cut just above the ganglion, Jug" , and another small and probably not very important branch is shown at Ve.

Fig. 124. — Pig .F, Acustico-facial ganglion complex, sinus, just in front of which shows the anlage of the thymus, which is deeply stained. Cerv.6, Sixth cervical nerve. Cce, coelom around the heart or pericardial cavity. G.jug, Ganglion jugulare of the vagus nerve. G.nod, Ganglion nodosum of the vagus nerve. G.petr, Ganglion petrosum of the glossopharyngeal nerve. G.lri, Ganglion of the trigeminus nerve. H, Lateral wall of the cerebral hemisphere. Jul;, Jugular vein. {Jug', Behind the trigeminus. Jug" , Branch in front of the trigeminus. Jug'", Main stem behind the vagus. Jug"", Main stem descending to join the duct of Cuvier. ) m, An undetermined structure, probably the anlage of a lingual muscle. A/,/, Mandible. jV.j, Root of the fifth or trigeminal nerve. N.op, Optic nerve. N.12, Twelfth or hypoglossal nerve. Oi, Otocyst. PA, Pharynx. R.L, Recessus lateralis of the fourth ventricle. Ve, Small branch of the jugular vein. Vent, Ventricle of the heart. V 22 diams.

The nerves are shown as follows : The optic nerve, N. op, still has its central cavity, which, nearer the median plane, opens into the third ventricle of the brain, and in the section resembles in shape an inverted U. On the side of the nerve toward the mouth there is a deep notch, — the section of the choroid fissure. The trigeminal ganglion, G. tri, is very large, and its trilobate form is clearly indicated by the figure. The lobe to which the reference line, G. tri, runs gives off the ramus ophthalmicus ; the lobe nearest the jugular gives off the ramus maxillaris inferior, while the middle lobe gives off the ramus maxillaris superior. From the ganglion the fibers and nerve-cells extend upward to form the root, N. 5, which joins the hind-brain at a characteristic point, — namely, at the summit of the Varolian bend and where the hind-brain is widest (compare Figs. 113 and 1 25). By its great size and by its topographical association with the lateral apex of the recessus lateralis of the fourth ventricle, the trigeminal ganglion may always be readily identified in sections of embryos. The acustico-facial ganglia, Ac. F, may also be readily determined by their typical position immediately in front of the otocyst, Ot. But it is quite difficult to identify the four components of this complex structure, — namely, i°, the motor root of the facial nerve; 2°, the facial or geniculate ganglion ; 3 , the vestibular ganglion ; 4 , the cochlear ganglion. In figure 124 three divisions are shown. The large, darkly stained division, to which the reference line, Ac. F, runs, and which lies nearest to the otocyst, is the vestibular portion of the acoustic ganglion; the small, light area occupying a middle position in the inferior part of the complex is the motor division of the seventh nerve, or lateral root of the facial ; it can be followed to the brain, which it enters as four bundles of fibers; its path of entrance is shown better in frontal sections (Fig. 1 26, t.m). Just in front of the facial motor root lies a second smaller dark mass, the geniculate ganglion of the facial, with an upward prolongation, the sensory root. The ninth or glossopharyngeal nerve is represented by the ganglion petrosum, G. petr, and its ascending sensory root. This nerve may be quickly identified because it is the first behind the otocyst. The upper ganglion of this nerve, the so-called Ehrenritter's ganglion, is represented by an accumulation of cells in the upper part of this root. As regards the tenth nerve, or vagus, both its ganglia and the fibrous trunk connecting them are shown. The upper or jugular ganglion, G. jug, is nearly on a level with the otocyst, while the lower or nodosal, G. nod, lies near the cervical sinus. To the nerve-trunk between the two ganglia are adjoined the fibers of the eleventh or spinal accessory nerve, which does not otherwise appear in this section. A small piece only of the hypoglossal nerve can be seen, N. 12. The space occupied by this nerve is blank in the engraving; in the specimen it shows horizontal fibers.

The Study of Frontal Sections, Embryo of 12 mm

The frontal series has special value for the study of the hind-brain and associated structures, as the plane of the section is approximately at right angles to the axis of the hind-brain. It also furnishes instructive pictures of the relations of developing vertebrae and nerves.

Portions of three sections illustrating the structure of the hind-brain and associated parts are given below. The following remarks on the hind-brain are intended to make the significance of these sections clearer. The wall of the hindbrain is, of course, produced by the development of the wall of the medullary tube. Its most striking peculiarity is the enormous expansion of the deckplate, which forms the very wide epithelial layer, epen, the so-called ependymal roof of the fourth ventricle. It starts from the upper edge of the dorsal zone, Fig. 125, D. Z, and forms a wide arch which is covered in externally by a rather thin layer of mesoderm, mes, and the nearby epidermis, Ec, of the embryo. The covering is so slight in development at this stage that in the fresh specimen the roof of the fourth ventricle, including its coverings, appears as a translucent membrane through which we can readily distinguish the great cavity of the fourth ventricle itself. The expanse of the ependymal arch is greatest at the region of the trigeminal root. From there backward toward the spinal cord its expanse gradually diminishes. In correspondence with the growth of the deck-plate the lateral walls of the medullary tube become bent outward and downward, so that, though they remain near together on their ventral side, where they are united by the floorplate or median raphe (Fig. 127, raph), yet their upper dorsal edges are far apart. In consequence of this change of their position the original lateral walls appear as the floor of the hind-brain, and we recognize in them the anlages of the medulla oblongata. We distinguish here, as everywhere in the medullary wall, the dorsal and ventral zones. The ventral zone is intimately united with its fellow by the short median raphe. Between them is a deep fissure (Fig. 126, /), which is never wholly obliterated. The floor-plate undergoes a great development in later stages and is transformed into the median raphe of the adult medulla. The lateral or morphologically dorsal limit of the ventral zone is marked by the exit of the lateral roots (Fig. 125, L. R). The ventral limit of the dorsal zone is marked by the entrance of the sensory or ganglionic fibers (Fig. 125, G. tri; Fig. 126, Fac). Toward the dorsal side the dorsal zone gradually thins out and passes over into the ependymal, epen. The great development of the lateral roots is perhaps the most important single 6haracteristic of the medulla oblongata. They furnish the principal motor or efferent nerve-tracts of the brain and form an important constituent part of four nerves: first, the trigeminal or fifth; second, the facial or seventh; third, the glosso-pharyngeal or ninth; and fourth, the vagus or tenth. There are no lateral roots known to occur anterior to the medulla oblongata, unless possibly the fourth nerve, the relations of which in many respects are peculiar, should turn out to be a lateral root. In the spinal cord we find lateral roots in the upper cervical region, and it is not improbable that they may yet be found associated with the dorsal roots of spinal nerves lower down. But even in the cervical cord the lateral roots attain but a slight development. The contrast with other portions of the central nervous system makes the great development of the lateral roots in the medulla oblongata all the more striking. The dorsal zone of the hind-brain lags considerably behind the ventral zone in its development, and at all stages the ventral zone forms a larger proportion of the medulla than does the dorsal zone. Section through the Trigeminal Roots. — The section passes through the widest part of the hind-brain, the cavity of which is enormously distended. It is bounded on the dorsal side only by the very thin ependymal roof, epen, which does not form any part of the true nervous structure, although it passes into and is directly continuous witli the dorsal zone, D. Z, which is thus seen to be only a thickened portion of the wall of the neural tube, just as the ependyma is the attenuated deck-plate. The trigeminal ganglion, G. tri, is very large and sends its sensory fibers upward into the dorsal zone to form there a distinct bundle of nerve-fibers which persists throughout life and is known in the adult as the tractus solitarius, T. S. The other root of the nerve, L. R, is lateral. It lies below the ganglion near the median plane. Its fibers arise from neuroblasts in the ventral zone and gather together as a distinct bundle which starts near the median line, takes a curving course through the ventral zone, and makes its exit from the medullary wall at the dorsal limit of the zone. It has a striking resemblance to the root of the facial nerve. We do not yet know whether such a course of the fibers is characteristic of all lateral roots or only of the trigeminal and facial roots. On the medial side of the trigeminal ganglion is a large vein, Card, the anterior cardinal vein, which by island formation is to migrate to the outside of the ganglion to form a portion of the permanent jugular trunk. In the median line in the mesenchyma immediately below the raphe is the section of the basilar artery, and considerably below that is the small section of the notochord which it is very difficult to distinguish with a low power. Between the notochord and the cardinal vein is the section of the carotid artery.

Fig. 125. — Pig, 12.0 mm. Frontal Series 6, Section 284.

Card, Anterior cardinal vein. D.Z, Upper portion of the dorsal zone of His. Ec, Ectoderm. epen, Ependymal roof of the fourth ventricle. G.tri, Ganglion trigemini. I..R, Lateral root of the trigeminal nerve, mes, Mesenchyma. T.S, Tractus solitarius of W. His. X 22 diams.

Section through the A custico- facial Ganglion. — In this section (Fig. 126) the thickened ventral wall of the hindbrain (i. e., the anlage of the medulla oblongata) is not spread out nearly horizontally, as in the trigeminal region, but rises obliquely on either side from the median line. The right and left sides of the medulla are divided from one another by a deep median fissure, /. In the median line we see also the basilar artery, A. bas, and still lower the wide, slit-like pharynx, Ph, the outer portion of which ascends obliquely toward the jugular vein, Jug. The ascending lateral part of the pharynx is a portion of the first gill pouch or future Eustachian tube, and is quite clearly marked off from the pharynx proper by its oblique direction. Of the acustico-facial ganglion complex the section shows four parts: the ganglion vestibuli, G. vest; the geniculate ganglion, G. gen; the sensory root, Fac, of the facial nerve arising from the geniculate ganglion and entering the brain to form there a distinct fiber-tract which is oval in the section and lies just below the entering vestibular fibers, and is clearly indicated in the drawing; and, finally, the motor tract, t. m, of the facial nerve. This tract is a very distinctly marked bundle of nerve-fibers which arise from neuroblasts of the ventral zone, traverse that zone almost horizontallv, then bend downward and pass out from the brain-wall, appearing as the lateral root of the facial nerve. The root runs first toward and then past the geniculate ganglion. The jugular vein, Jug, lies outside of the ganglia; not, as does the earlier cardinal vein, inside. In the mandible below the pharynx appear two nerves. Of these, the upper is the hypoglossal, N. 12, which lies near the

angle formed by the jugular of the first Fig. 127—Pig, 12.0 mm. Frontal Series 6, ;jj deft wkh the pharynx The lower

Fig. 126. — Pig, 12.0 mm. Frontal Series 6, Section 340.

A.bas, Arteria basilaris. D.Z, Dorsal zone of the medulla oblongata. Ec, Ectoderm, epen, Ependymal roof of the fourth ventricle. /, Median fissure of the medulla oblongata. Fac, Sensory root of the facial nerve. G.gen, Geniculate ganglion of the facial nerve. G.vtst, Ganglion vestibuli of the acoustic nerve, /ng, Jugular vein, mes, Mesenchyma. Mx.i, Inferior maxillary branch of the trigeminal nerve. N.12, Hypoglossal nerve. Ph, Pharynx, t.m, Motor tract of facial nerve. X 22 diams.

Section 3S0. of the two nerves, Mx. 1, is the inferior A.oas, basilar artery. Loch, Cochlea. D.e, Ductus endolymphaticus. cptti, Ependyma. Facta, maxillary.

Motor division of the facial nerve. Jug, Jngu- Section through the Otocyst. — The lar. Md.obl, Medulla oblongata. Pk, Pharynx. fio;ure ig fron] a sec tion . not far from raph, Median raphe of the medulla oblongata. , „ ... , .V. Anlage of the semicircular canals. Vc, Vein. the last. Ihe hind-brain has Harrowed X 22 diams. considerably; its thickened floor, Md.

obi, the anlage of the medulla oblongata, rises steeply from the median line. Its ependymal roof, epen, is less expanded than in figures 126 and 127. It forms a sharp angle in the dorsal median line. The median ventral fissure between the two sides of the medulla is deeper than further forward. The pharynx, Ph, is wide and has expanded laterally into the common beginning of the first and second gill pouches. Between the pharynx and the raphe the basilar artery, A. has, has been cut transversely. Below it and near the pharynx is the small notoehord, which, however, can only be clearly recognized with the higher power, and is, therefore, not represented in this or the preceding figure. The otocyst is a large epithelial vesicle with three well-marked divisions: first, the common chamber, S. c, out of which the three semicircular canals are to be differentiated. Second, a slender canal, D.e, which one easily identifies as the anlage of the ductus endolymphaticus. It lies between the semicircular canal and the wall of the medulla oblongata. Third, the long, curving, but not spiral cochlea. The common chamber formed by the union of these divisions is later subdivided to form the upper utriculus and lower sacculus. Outside the cochlea lies the crosssection of the jugular vein, just below which is the section of the motor portion, Fac. m, of the facial nerve. The sensory portion of the facial nerve at this stage is much smaller, and runs only a short distance downward from the geniculate ganglion and is entirely separate from the motor portion. The morphological constitution of the facial nerve is still very obscure, and a satisfactory account of its development is, for the present, impossible.

Section through the Dorsal Vertebra (Fig. 1 28). — Owing to the curvature of the embryo the spinal cord is cut twice ; once, Sp. c', toward the head end of the embryo, and again, Sp. c", lower down toward the tail end. Alongside the sections of the spinal cord appear the large, darkly stained masses of the ganglia, G. The section also passes through the bases of the anterior limbs, A. L, in one of which we can see one of the branches, N. br, of the brachial plexus. Between the two pieces of the spinal cord of the section the plane passes on the ventral side of the spinal cord and shows the series of vertebral formations, together with the nerveroots, N", the intersegmental arteries, A. i. s, and the segmental veins, small vessels which lie close to the intersegmental arteries. The nerves are sections of the dorsal root below the ganglia. Each nerve has a distinct outline and is partly penetrated by ingrowing mesenchymal cells which subdivide the nerve into rounded fiber bundles. In each bundle the nerve-fibers appear as fine dots, which, however, by the use of the fine adjustment can be followed up and down through the section, and thus identified as fibers. The single fibers are more or less isolated from one another, and between them are delicate threads, the nature of which is not known. Between the adjacent rounded bundles of fibers there is often a distinct space. The vertebral anlages, Vert, are formed entirely from condensed mesenchyma, and therefore stand out somewhat conspicuously in the section, owing to their darker staining. Each anlage is bow-shaped, the concavity of the bow facing toward the tail of the embryo. The ends of the bow pass behind the nerve-trunk of the segment to which the vertebral anlage belongs. The anlages extend completely across the median line, and by following through in the series of sections, it may be found that the condensed mesenchyma surrounds the notoehord. which, therefore, passes through the central portion of each vertebral anlage. The vertebrae at this stage are entirely without any distinct limitation and merge into the surrounding loose mesenchyma. Near the anterior border of each nerve-trunk, and usually somewhat toward the median side of it, lie the intersegmental vessels, which are of small size and vary greatly in their exact position and number, according as they are more or less branched. Between the ends of the vertebral bows outside of the nerve-trunks can be seen with higher power clusters of elongated cells with developing musclefibers which are here still segmentally arranged between the processes of the developing vertebrae.

Fig 12S— Pig 12.0 mm. Frontal Series 6, Section 572.

of the body of a vertebra. X 22 diams.

Pig Embryo of 9 mm

Pig embryos of this stage supplement very instructively those of 1 2 mm. It will, of course, be advantageous for the student to prepare serial sections himself. When that is not possible, there should at least be sections prepared for the laboratory which the student may examine. Five sections are illustrated and described below. They have been chosen to supplement the descriptions of the sections of the pig of 12 mm., and they will be found to illustrate certain fundamental morphological relations in the embryo more clearly than older stages.

Transverse Section through the Region of the Branchial Arches. — The branchial arches are much more conspicuous at this stage than in later ones, being separated from one another by deep ectodermal depressions, figure 1 29, I, II, III, IV; and, although 777 and IV are already being turned in, preparatory to the formation of the cervical sinus, they are still distinct and their order in the series is evident. The section (Fig. 129) shows on the dorsal side the spinal cord, in which we can already recognize the subdivision into dorsal zone, D. Z, and ventral zone, V. Z. To the dorsal zone is appended the dorsal root ; from the middle of the ventral zone comes off the ventral root of a cervical nerve, N. Just between the dorsal root and the wall of the spinal cord can be seen the section of the accessory nerve. The secondary segment, My, is sharply defined and has a distinct growing edge showing at its upper limit in the section. The inner leaf of the secondary segment is stained more lightly than the neighboring tissue corresponding to the modifications which the cells are undergoing preparatory to their change into young muscle-fibers. In the 12 mm. pig in this region the cells of the muscle plate have already broken apart and no distinct plate can any longer be recognized. Below the muscle plate follows the section of the jugular vein, Jug. Lower down and in the median line we have the section of the pharynx, Ph, lined by the epithelial entoderm. The pharynx is surrounded by the very large aortic vessels, which start from the ventral side of the pharynx and pass upward along its sides to join the descending aorta, Ao. d. 4, at about the level of the jugular veins. The vessels shown are the fourth aortic arches.

Fig. 129. — Pig, 9.0 mm. Transverse Series 9, Section 171. Ao.d.4, Descending aorta receiving the right fourth aortic arch. <-/.///, Third entodermal gill cleft. D.Z, Dorsal zone of spinal cord. F.B, Fore-brain. Hy, Hyoid branchial arch. Jug, Jugular vein. Mdb, Mandibular branchial arch. My, Muscle plate. N, Nerve. Nch, Notochord. 01/, Olfactory plate. Ph, Pharynx. V.Z, Ventral zone of spinal cord. I, II, III, IV, First to fourth ectodermal gill clefts. X 35 diams.

Their symmetry and their relations to the pharynx are beautifully demonstrated in this section. Below the aorta we find a section of the third internal gill cleft, cl. Ill, a narrow, slit-like cavity lined by entoderm. On the left-hand side of the embryo the junction of the entoderm of the internal pouch with the ectoderm is shown. The two germ-layers have united to form a typical closing plate. Above the third gill cleft the outline of the embryo shows a deep depression which is due to the commencing formation of the cervical sinus. From the upper end of this depression runs upward the ectodermal fourth cleft, and from its lower part extends downward the ectodermal third cleft. Between the third and fourth clefts the external surface of the embryo protrudes somewhat. This protuberance corresponds to the so-called third branchial arch. Between the third external cleft and the second, II, is a still greater protuberance on the outside of the embryo. This marks the third branchial arch. The third aortic arches are somewhat imperfectly shown, but the connection of the left third arch with the central aorta appears. Between the second and first external clefts we have the second or hyoid branchial arch, Hy; and, similarly, between the first or auditory cleft, /, and the oral fissure, which separates the head from the body of the embryo, we have the very large and protuberant mandibular arch, Mdb. The head of the embryo is completely separated in this section from the body. It shows the cavity of the fore-brain, F. B, bounded by the ectoderm of the medullary wall, and on one side also shows the thickening of the epidermis, 01} , which forms the olfactory plate or plakode, which is to become the lining of the nasal pit.

Sagittal Section to the Right of the Median Plane. — In the accompanying figure 130 the cephalic end of the embryo is omitted; a portion of the heart, the entire length of the Wolffian body, and the tail are included. The dorsal outline of the embryo forms a characteristic curve. A long series of spinal ganglia, G, are shown arranged in regular succession and following the curvature of the back. The ganglia are easily recognizable by their dark staining ; each of them is so large as to occupy at least four-fifths of the length of the segment to which it belongs. The boundaries between the adjacent primitive segments are indicated by the positions of the intersegmental arteries, A. is. Even when their cavities do not show, the position of these vessels is marked by the darker line of tissue. The origin of one of these intersegmental vessels from the dorsal aorta, Ao, is indicated in the lower part of the figure. The Wolffian body, W. b' , W. b", extends from the level of the lungs and liver well down toward the pelvic end of the embryo. Its ventral limit is marked by the body-cavity, Cos, and it is, of course, covered by a layer of mesothelium, msth, which here, as everywhere and at all stages, forms the boundary of the coelom. In the Wolffian body we distinguish readily numerous sections of the epithelial Wolffian tubules, and toward the ventral side of the organ the characteristic glomeruli, Glo. Between the glomeruli and the mesothelium there is a layer of mesenchyma, but between the tubules there is little tissue, the intertubular spaces being almost entirely occupied by sinusoids developed from the cardinal vein. The larger sinusoid or venous space, V. msn, is due to the section of the venous trunk which joins the lower end of the vena cava inferior, and is known as the mesonephric vein. In the upper part of the figure we encounter a section of the descending aorta, Ao. d, and of the lungs, Pul, or pulmonary anlage. It consists of a ring of entoderm bounding the central cavity and enclosed by a thicker layer or mesenchyma, which, again, is bounded by a layer of mesothelium. The space or coelom about the lung is shown in the figure to be continuous with the coelom of the abdominal region. On the ventral side we have the heart partly shown, the ventricle, Ven, being so cut as to exhibit the trabecular structure of the network of the sinusoidal spaces. The auricle, Au, is without sinusoids. The great venous trunk or ductus venosus, D. ven, opens into the auricle, the opening being guarded by two valves, that on the dorsal side of the opening in the figure, V. s, being the left valve. The ductus venosus receives its blood-supply from the liver, Li, which consists of liver cells or hepatic cylinders and numerous sinusoids of many diameters. On the lower side of the liver there is a considerable accumulation of mesenchyma by which the liver is united on the one end to the body- wall, Som, to the umbilical wall, Urn. w' , and to the mesentery by which the intestine is suspended from the liver. In this mesenchyma are lodged three spaces bounded by entoderm, the uppermost of which is indicated by the reference line, A. It is just at this point that the development of the gall-bladder and the intestine takes place, and the exact identity of the three entodermal structures just referred to has not yet been satisfactorily worked out. Underneath the liver in the section of the mesentery is situated the portal vein, P. v. From the mesentery extends out the intestine (duodenum). It is a somewhat cylindrical tube which curves over ventralwards and passes out through the opening of the umbilicus. It consists of a very small tube of entoderm, Ent, with only a small internal cavity (compare Fig. 132, Red.). The thickness of the intestinal wall is due chiefly to the considerable development of the mesenchyma. The external covering of the intestine is a layer of mesothelium which becomes the peritoneal epithelium of the adult. In the tissue of the organ we distinguish the narrow vitelline artery, Art. v. The umbilical opening is quite wide and is bounded both above and below by a prolongation, Um. w', Urn. w", of the somatopleure of the embryo. The wall on the upper side is much thicker than on the lower. The opening of the umbilicus is very wide. It is partly occupied by the duodenum. Appended to the inferior wall of the umbilicus is the allantois, All, which arises from the enlarged caudal end (cloaca), Clo, of the intestine. It passes out first forward, then makes an acute but rounded angle, and extends outward through the umbilical opening. It may, therefore, be said to consist of two limbs, one within the body of the embryo joining the cloaca, and the other passing out through the umbilical opening. The limb arising from the cloaca is completely united with the body-wall, and is, of course, upon the side toward the coelom covered in by mesothelium. The lining of the allantoic cavity is an epithelium, and is a portion of the entoderm. Along the second limb of the allantois the mesothelium on the side toward the cavity of the umbilicus forms a series of clumsy projections, Vil, the mesothelial villi of the allantois. They are smallest toward the embryo and increase in size distally. With higher power one can see that the mesothelium of the villi is very thin and the mesenchyma in their interior of quite loose texture. In later stages the mesothelium grows, the mesenchyma in large part disappears, and the villi then seem hardly more than small bags of mesothelium with but little contents, save some coagulum. They continue to enlarge until the embryo is 17 or 1 8 mm. long, after which they begin to abort. In these older stages the villi extend far into the abdomen and are packed in between the abdominal viscera, presenting curious appearances in section. As the tail of the embryo is bent to one side, it offers us a section of a portion of the spinal cord, Sp. c, and at its tip a glimpse of three primitive segments, Seg.

Fig. 130.— Pig, 9.0 mm. Sagittal Series 53, Section 213. A, Entodermal cavity (probably gall-bladder). AM, Intersegmental artery. All, Allantois. Ao, Median aorta Ao.D, Descending aorta. Art.v, Arteria vitellina. Au, Auricle. Clo, Cloaca. Ca, Crelom D vm Ductus venosus. Ent, Entoderm. G, Ganglion. Glo, Glomerulus. Li, Liver, msth, Mesothelium' Pi,/. Lung. Seg, Segment. So,,,, Somatopleure. Sp.c, Spinal cord. Um.w', Upper wall of umbilicus Um.w ', Lower wall of umbilicus. I'; Vein in liver. Ven, Ventricle of heart. Vil, Villus V.msn Vena mesonephrica. /:/, Portal vein. V.s, Valvula sinistra. II'J.', IS'.!,", Wolffian body ^22 diams.

Frontal Section through the Mid-brain and Fore-brain. — Comparison with figure 99 (pig, 10 mm.) will make it clear that in a frontal series we shall obtain a few sections of the head which include only mid-brain and fore-brain and show no other special cephalic structures. The mid-brain, M. B, is somewhat rounded in form and passes over into the fore-brain, which is quite long and which already shows traces of its subdivision into two parts, the diencephalon, Dien, which lies nearest to the mid-brain, and the prosencephalon, Pros, which constitutes the terminal portion of the brain and which produces the lateral expansions which are to form the cerebral hemispheres. The expanding prosencephalon is separated by a constriction from the diencephalon, which in its turn is similarly separated from the mid-brain. The diencephalon and prosencephalon together represent the fore-brain. They are subdivisions of the primary first cerebral vesicles. It is important to note that they do not correspond to complete subdivisions, and have not the same morphological value as the three primary vesicles. The histological development is much less advanced than in the pig of 12 mm. The ectoderm is very thin, consisting for the most part of a single layer of cells, but here and there the formation of a second layer is seen to be beginning. The mesoderm is very simple in character and almost uniform in appearance, but there is a distinct difference between the mesenchyma around the brain and that underneath the epidermis, the former having cells further apart. This is almost the first stage in the differentiation of the arachnoid zone around the brain. The pia mater, however, though quite thin, is well defined, by the condensation of the mesenchymal cells and by the somewhat numerous small blood-vessels in it. The medullary wall is everywhere quite thick and crowded with nuclei. In the region of the diencephalon the ectoglia is distinctly formed, but elsewhere has hardly begun its differentiation. On the inside of the medullary wall, close to the surface, there are everywhere very numerous mitotic figures.

Frontal Section through the Umbilical Opening. — The illustration (Fig. 132) is part of the same section in the series from which figure 131 is taken. For convenience of comparison the position has been reversed so as to bring the dorsal side of the embryo uppermost in figure 132. It results from this that the right and left sides of the embryo are reversed in the engraving as compared with the other figures of transverse and frontal sections. By examining figure 99 (pig, 10 mm.) the student will see that sections in the frontal plane, owing to the curvature of the posterior end of the bodywall, furnish transverse sections of the spinal cord of the pelvic region. Therefore, the section here figured, although part of a frontal series, is directly comparable to a transverse section of the body. In the upper part of the figure we have the spinal cord, Sp. c, and on one side of that the ganglion, G. Owing to the spiral twist of the embryo the section is not symmetrical, so that the posterior limb, P. L, appears only on one side of the section. Laterad from the nerve shown in the figure is the large muscle plate, My, the cells of which are already beginning to change into muscle-fibers. On the dorsal side of the plate we find its growing edge, in. pi, where the tissue of the muscle plate proper bends over and passes continuously into the external wall of the segment. From this growing edge the cells are added to the muscle plate by which it extends upward. The similar edge on the ventral side provides for the extension of the muscle plate downward. In the median line, below the spinal cord, we have the small notochord, Nch, and the large median dorsal aorta, Ao. In the ventral portion of the embryo appears the large body-eavity into which protrude the Wolffian bodies and the intestine. The coelom also has a downward prolongation into the beginning of the umbilical cord, and in this prolongation lies the loop of the intestine, In. The coelom is bounded everywhere by the layer of mesothelium represented in the engraving as a continuous line. With a higher power the mesothelium is seen to consist of a single layer of cells, but varying somewhat in thickness in different regions. By following the contour of the mesothelium the student will recognize at once that all of the viscera are, in the anatomical sense, outside of the coelom. The Wolffian bodies, W. b, are voluminous organs projecting from below the aorta on either side of the large intestine, Rect, and extending far into the abdominal cavity. At the lower ventral edge of the Wolffian body appears the Wolffian duct, IV. D, a wide, longitudinal canal into which the Wolffian tubules open. The large size of the duct is characteristic of this stage. In later stages it is smaller. The tubules are very large, contorted in their course, and appear, therefore, variously cut. They are formed by a cuboidal epithelium and are provided with a sinusoidal circulation. The endothelium of the blood spaces can generally be seen fitting closely against the epithelium of the tubules. Here and there, however, there is some mesenchyma between the blood spaces and the walls of the tubules. On the median side of the Wolffian body are the glomeruli, which are of large size, and similar in structure to the glomerulus of the permanent kidney, though differing from the renal glomeruli in their proportions and in the details of their structure. It is not difficult to make a reconstruction of the course of a single tubule by following it through a few neighboring sections. The general course of a tubule is in the transverse plane, but it is much contorted. Each tubule begins at one of the glomeruli, with which it is in open communication. It then bends so as to make a somewhat irregular S-shaped figure, and finally opens into the Wolffian duct. After leaving the glomerulus it widens somewhat, but before it joins the Wolffian duct it again diminishes in diameter. The changes in diameter are gradual. The blood spaces or sinusoids of the Wolffian body are derived from the posterior cardinal veins. The veins and tubules, when the latter first become distinct, lie near together. As development continues both enlarge and encroach upon one another's territory; hence there is an intimate intercrescence of the blood-vessels and of the tubules, resulting in the formation of sinusoids. The whole of the Wolffian body might from one point of view, therefore, be regarded as a modification of the cardinal vein, and morphologically all of the blood spaces between the tubules belong to that vein. There remain typically two portions of the cardinal vein which are more or less open and distinct. The one on the dorsal side of the Wolffian body, card, may be conveniently regarded as representing the original cardinal vein. The other, on the ventral side of the Wolffian body, is at first not a very distinct channel, but gradually becomes more and more so, and is known by the distinctive name of subcardinal rein. It is a vessel of great morphological importance, since on the right side of the embryo it acquires a connection with the liver which renders it possible for the blood of the right subeardinal vein to pass through the blood spaces of the liver directly to the heart. This makes a very direct channel, a more direct one than existed previously, when the blood from the subeardinal came to join that of the cardinal, passing up to the duct of Cuvier and then back to the heart. The new channel through the liver rapidly enlarges and becomes recognizable as the vena cava inferior. This important venous trunk is a combined vessel, comprising, first, a part of the sinus venosus of the heart; second, the ductus venosus of the liver; third, a large channel developed from the sinusoids of the liver; fourth, the upper part of the right subeardinal vein; and, fifth, the lower part of the right cardinal. The vena cava inferior has already been developed in the pig embryo of 9 mm. Between the two Wolffian bodies hangs down the large intestine, Red, suspended by its mesentery in the median line. The entodermal portion is a very small circle of epithelium with an extremely minute lumen, which in the section is scarcely larger than a single nucleus. The mesentery and intestine are covered by a well-defined mesothelium and have a considerable amount of mesenchyma, in which there is no distinct histological differentiation beyond the presence of a number of small blood-vessels. At this stage the large intestine runs nearly in the median plane to the pelvic end of the body. In the opposite direction, toward the head, it bends to the left of the embryo, making a loop which passes over into the end of the ileum. The ileum forms the continuation of the loop and extends into the coelom at the base of the umbilical cord. There it bends back and returns toward the dorsal side of the embryo to pass over into the duodenum and join the stomach. Owing to the fact that the small intestine extends into the extra-embryonic coelom of the umbilical cord, there makes a loop, and returns to the embryonic region, we get typically a double section of the intestine as shown in the figure, one of each limb of the loop. The entoderm, In, in these loops forms a small ring, which, however, is much larger than the entodermal ring of the large intestine at this stage. Each loop contains a large amount of mesenchyma, mes, the cells of which are somewhat crowded, so that the tissue appears dark in the stained section. The boundary between the body of the embryo and the tissue of the umbilical cord is marked by the position of the two umbilical veins, that of the left side, V. U.S, being very much larger than that of the right side, V. U. D. By following down the somatopleure, Som, of the embryo, it will be seen that these veins are lodged therein, and that the continuation of the somatopleure beyond these veins forms the substance of the umbilical cord. The limb-bud, P. L, is a large mass of rather dense mesenchyma, entirely without muscles or nerves and covered by ectoderm. At the edge of the limb-bud the ectoderm shows a special thickening, F. The theory has been advanced that this thickening is homologous with the ectodermal fold which produces the fin of fishes, or at least that portion of the fin in which the fin-rays are developed.

Fig. 131. — Pig, 9.0 mm. Frontal Series 54, Section 194. Dim, Diencephalon. M.B, Mid-brain. Md, Medullary wall of brain, mes, Mesenchyma. Pros, Prosencephalon. Ve, Vein. X 22 diams.

Fig. 132. — Pig, 9.0 mm. Frontal Series 54, Section 194. Ao, Aorta, card, Cardinal vein. F, Ectodermal fold at the border of the limb-bud. G, Ganglion, gen. Genital ridge. Glo, Glomerulus. In, Small intestine (jejunum), mes. Splanchnic mesoderm (of the intestinal wall),, Dorsal growing edge of the muscle plate. My, Muscle plate of secondary segment. Nch, Notochord. P.L, Posterior limb. Rid, Large intestine. Sow, Somatopleure. Sp.c, Spinal cord. V.U.D, Right umbilical vein. V.U.S, Left umbilical vein. U'.b, Wolffian body. IV. D, Wolffian duct. X 35 diams.

Frontal Section through the Second and Third Gill Clefts. — In this preparation (Fig. 133) the section hits the posterior wall, Ot, of the otocyst and is just anterior to the origin of the glossopharyngeal nerve. The appearance of the section of the hind-brain is characteristic for this region of young embryos. The deck-plate has grown gradually in size and forms a wide membrane, epen, the ependymal roof of the fourth ventricle. Owing to this growth of the deck-plate, the upper or dorsal limits of the dorsal zones, D. Z, are brought far apart and the cavity of the hind-brain is thus enlarged. The dorsal zone is marked by an angle in the interior and by the point of entrance of the nerveroots on the exterior from the ventral zone, V. Z. On their dorsal side the dorsal zones thin out and pass over gradually into the ependyma. The ependyma consists of a single layer of cells. In the dorsal zone the differentiation of the three primary layers of the medullary wall has scarcely begun, but in the ventral zones the three layers are already distinguishable, though not far advanced in their differentiation. In the floor-plate there are two layers. Below the medullary tube lies the basilar artery, A. bas, and below that, not far from the upper wall of the pharynx, lies the small round notochord in the midst of loose mesenchvmal cells, which have not yet begun to condense themselves about the notochord. The pharynx is a wide space of rather small dorso-ventral diameter, and having a much thinner layer of entoderm on its dorsal than on its ventral side. Above the pharynx on either side lies the section of the descending aorta, Ao. d. The reference line to this vessel crosses a dark mass of cells which belong to the ganglion nodosum of the ninth nerve. Below the pharynx the section shows the third aortic arch, Ao. j, and the fourth aortic arch, Ao. ./, just springing off from the median aortic trunk above the heart, so that the two fourth arches are connected across the median line. Between the third and fourth aortic arches on either side is a small cavity lined by entoderm, cl. Ill, a diverticulum from the third gill cleft. Immediately below the otocyst is the jugular vein, Jug. From a point below the jugular there extends a prolongation, Hy, which may be taken as a portion of the hyoid or second branchial arch. It extends downward and consists of a mass of mesenchyma covered by ectoderm. It encloses a space, cl. II. ex, which may be regarded as the external portion of the second gill cleft. In a neighboring section (455) the prolongation of the pharynx shown in figure 133 can be traced still further until it opens into this space, cl. II. ex. The second cleft is open upon both sides of the embryo, the first and third have closing membranes, the fourth cleft is not yet so far developed that its entoderm has come in contact with the epidermis of the embryo. The second cleft probably always becomes open, differing in this respect from all the others. Why it has this peculiarity we do not know. The opening does not persist, but the exact history of its closure is at present unknown. The process, Hy, described as shutting in the external portion of the second gill cleft has sometimes been termed the operculum, because it covers a gill cleft opening, as does the operculum of a bony fish.

Fig 133. — Pig, 9.0 mm. Frontal Series 54, Section 459. A. ins, Basilar artery. Ao.d, Descending aorta of the left side. A0.3, Third aortic arch. A0.4, Fourth aortic arch arising from the median ventral aorta. cl.II.ex, External portion of the second gill cleft. el. Ill, Third gill cleft. D.Z, Dorsal zone of the medulla oblongata. Ec, Ectoderm. Eptu, Ependymal roof of the hind-brain. Hy, Ilyoid arch. Jug, Jugular vein, mes, Mesenchyma. Or, Posterior wall of the otocyst. P. Ac, Pulmonary aorta. P/i, Pharnyx. Som, Somatopleure. V.Z, Ventral zone. X 22 diams.

In the lower part of our figure a portion of the somatopleure, Som, is shown where it extends ventralwards to form the wall of the pericardial cavity. There is also included in the drawing a part of the pulmonary aorta, P. Ao.

Pig Embryo of 6 mm

Of this stage two transverse sections are figured in order to give more exact notions as to the structure of neuromeres and the secondary segments. The first section is taken through the level of the head , and may be directly compared with figure 113. The relations are so closely similar that it is unnecessary to describe the present section (Fig. 134) in detail. The explanation of the figure is sufficient for the identification of the parts. The otocyst is large and conspicuous, and the arrangement of the nerves is essentially similar to what we find in the older embryos. The neuromeres, however, are very distinct, especially those upon the left side of the embryo, N. 1, 2,3, 4. Of these, the third is perhaps the most characteristic. Each neuromere is separated from its fellow by an internal sharp ridge, so that the inner boundary of each neuromere toward the cavity of the fourth ventricle is a small arc of a circle. The cells are elongated and are placed radially to the inner curved surface of the neuromere. A thin but distinct layer of ectoglia is present.

Fig. 134.— Pig, 6.0 mm. Transverse Series 9, Section 90., Carotid artery. Jug, Jug', Jugular vein. Md, Medullary wall of the fore-brain. N.i, N.2, N.3, N.4, Neuromeres of the hind-brain. .V.j, Trigeminal ganglion. N. 7 ,8, Acustico-facial ganglion. N.g, Root of the glossopharyngeal nerve. Of, Otocyst. Vcn.III, Third ventricle or cavity of the fore-brain. Ven.IV, Fourth ventricle. X 35 diams.

The light line, which marks the boundary between the adjacent neuromeres, is produced by the comparative absence of nuclei. As to the number of neuromeres our knowledge is still defective; nor have we yet succeeded in making sure of their exact relation to the nerves of the head, though such a relation evidently exists. Thus we find, for example, that the facial nerve is always connected with neuromere 2 of our figure, and the glosso-pharyngeal nerve with neuromere 4.

Our second section is very near the end of the same series. Owing to the curvature of the posterior end of the body of younger embryos (compare Fig. 99; pig 10 mm.), sections taken in the plane which we call transverse strike the lumbar region so as to give longitudinal sections of the spinal cord and primitive segments. Figure 135, therefore, shows the cavity of the spinal cord, Sp. c, cut for a very long distance. At the upper and lower ends of the section the dorsal side of the spinal cord is cut, and accordingly we see at these levels sections of the ganglia, G, on either side of the spinal cord. In the middle of our section the ventral portion of the spinal cord is cut, and here, therefore, the ventral roots, V. R, of the nerves are displayed. The segments are clearly marked by the external configuration of the embryo, the ectoderm, Ec, forming an arch over the outside of each segment. Each mesodermic segment shows three distinct parts : next to the ectoderm the broad, epithelioid cutis plate, within which comes the spindle-shaped section of the inner portion of the segment, muse, the anlage of the skeletal muscles ; and, third, an expanding mass of mesenchyma, Scler, which is sometimes termed the sclerotome. This term, however, is not wholly felicitous, because this mesenchyma forms not only the segments of the skeleton, but the connective tissue of the whole region about the spinal cord in the dorsal part of the embryo. The figure shows very clearly that the ganglia and ventral nerve-roots are arranged in exact conformity to the segments, and it can be easily observed, by following through the series of sections, that for each segment there is one ganglion and one ventral root. It also shows that the ventral roots reach directly to the muscle plate. The muscle plate is histologically partlv differentiated, for its cells have already elongated in a direction parallel with the longitudinal axis of the embryo, and their nuclei also have become much larger than any other nuclei in the neighboring parts of the embryo, being perhaps three times as large as the mesenchymal nuclei of the sclerotome. They are oval in form, contain many fine, and usually one or two somewhat larger granules, the larger ones being deeply stained, but the nuclei, as a whole, are stained more lightly than their neighbors. Each segment is very clearly separated from its neighbors, and between the ends of the adjacent muscle plates there is a small, clear space entirely free from cells and extending outward to the epidermis. Just inside of this space in every case is a small blood-vessel, the intersegmental artery, A . is. The intersegmental arteries are small branches which arise in symmetrical pairs from the dorsal aorta.

Fig. 135. -Pig, 6.0 mm. Transverse Series 9, Section 519.

A. is, Intersegmental artery. C«, Cutis plate. Ec, Ectoderm. C, Ganglion. muse, Muscle plate. Scler, Sclerotome, and. Sp.<; Spinal cord. /'. R, Ventral nerve-root. X 5° diams.

Pig Embryo of 17 mm

Since the pig of 12 mm. contains the anlages of perhaps every important part of the body, sufficiently advanced in development to be clearly recognized, we find in the immediate subsequent development that we have to do not so much with an introduction of new parts as with the differentiation of those which have already commenced. Embryos of 17 mm. are convenient for the study of the differentiations referred to. Particularly important for the student to note are the advances in the development of the vertebra?, of the lungs, of the Wolffian bodies and genital glands, and of the kidneys. These points are illustrated in figures 136 to 138, representing portions of three transverse sections of a 17 mm. embryo.

Transverse Section through the Lungs. — The epidermis of the embryo has become more distinct owing to its growth in thickness, which is accomplished by the increase of the number of layers of cells. The growth is very marked at the sides of the section about the level of the vertebra. At these points it can be clearly seen that upon the outside the epidermis has a very thin layer of flattened cells, the nuclei of which are themselves also somewhat flattened. This single layer of cells is known as the epitrichium, because the hairs are developed entirely underneath it. Where the epidermis is thickest, one can observe that the layers of cells next to the mesoderm are closely packed together with round nuclei. They represent the commencing formation of the basal layer of the adult epidermis. Between the basal layer and the epitrichium the cells are more loosely placed, forming the initial stage of the mucous layer. The mesenchyma is very much developed and occupies a large territory in the dorsal region of the embryo. It carries the nerves and blood-vessels and shows at various points accumulations of more darkly stained cells, which are of two kinds: first, groups of mesenchymal cells proper, the anlages of portions of the skeleton ; and, second, groups of mesothelial muscle cells, the anlages of the various skeletal muscles. There is little differentiation otherwise in the mesenchyma, but we may note the following changes in it : (1 ) The anlage of the vertebra, Vert, which is now quite well defined ; around the edge of it the cells have assumed an elongated form and have elongated nuclei; the elongation is parallel with the surface of the anlage. These cells result from the commencing differentiation of the perichondrium, which at this stage merges on the one side into the anlage of the vertebrae, and on the other into the surrounding mesenchyma. The cells of the vertebra have changed into young cartilage cells. They are now distinctly separated from one another by a well-developed matrix. Each cell occupies a separate space or capsule in the matrix. The protoplasm of the cell having almost completely disappeared, only the nucleus remains distinct. It stains readily, has a distinct outline, and contains a number of dark granules, one or two of which are conspicuous by their greater size and irregular shape. The nucleus itself, in most of the cells, is somewhat irregular in outline, as if distorted by shrinkage. Toward the center of the anlage the cytomorphosis is most advanced. Toward its outer surface the cells are less changed, lie nearer together, and have more regularly shaped nuclei. In the center of the vertebra lies the round notochord, Nch, the sheath of which has increased considerably in thickness, and, being unstained, appears as a clear space between the cells of the notochord and those of the enclosing vertebra. The nuclei in the notochord are numerous and somewhat crowded together. (2) The costal processes, Cost, of the vertebra, which are rod-like and extend quite far down into the somatopleure. The histogenetic changes in these processes are similar to those in the vertebra, but less advanced. They have progressed somewhat more in the proximal than in the distal portion of the rib. (3) Around the central nervous system the pia mater has become more distinct, and the arachnoid membrane is indicated by the wide separation of its cells and the length of the processes connecting them. Its differentiation is most easily recognized at the sides of the spinal cord. The outer limit of the arachnoid is shown by a slight condensation of the mesenchyma which marks the first step in the differentiation of the dura mater, the anlage of which is further defined by the elongated form of the mesenchymal cells, by which they differ from the mesenchymal cells on both sides. (4) There is a distinct layer of condensed mesenchyma around the aorta, Ao. The layer thus formed consists of elongated cells, and perhaps corresponds only to the muscular coat of the vessel. (5) About the oesophagus, CE, the mesenchyma forms two distinct layers. The inner, next to the epithelium, is of looser texture, and is the anlage of both the mucous and submucous layers of the adult. The outer layer is denser and consists chiefly of young smooth muscle cells, which are merely modified mesenchymal cells, characterized by the greater development of their protoplasm and by their elongated form. Traces of the differentiation of the outer layer into the inner circular muscular coat and the outer longitudinal coat of the adult are clear in the section. The spinal cord, Sp. c, has changed its outline in section, being broadest in the ventral zones, which have also begun to expand ventralwards so that the outline of the cord shows on its inferior side a concavity, the first indication of the ventral fissure. The three layers of the spinal cord are very distinct. The change in form, however, it can be clearly seen, is due chiefly to the growth of the gray layer, cin, especially in the ventral zone. The gray layer in the dorsal zone is still very slightly developed. From the dorsal zone descends on either side the dorsal nerve-root, D. R, which presently joins the ganglion, G. The ganglion now occupies a much lower position than in the earlier stages (compare Fig. 122, G). From the ventral zone springs the ventral root which unites with the dorsal at the lower tip of the ganglion. From the nerve-trunk thus formed there is given off almost immediately the dorsal branch, R. D, which soon ramifies in the midst of a dark mass of tissue, the anlage of the dorsal musculature. The main nerve-trunk descends ventralwards and sends off at the level of the vertebra a sympathetic branch, R. sym, which runs obliquely downward and inward toward the aorta, and there terminates in the anlage of the sympathetic chain, Sym, which consists partly of nerve-fibers, partly of ganglion cells which have migrated along the nerve and taken up their position at its end. These cells are easily recognized by their very dark staining. Their nuclei arc ci little lighter than those of the neighboring mesenchymal cells, but, owing to their deep coloration, stand out conspicuously, even when the section is examined only with the low power. The sympathetic anlage comes in close contact with a portion of the cardinal vein, card, near the aorta. The main nerve-trunk, R. V, continues obliquely downward and presently forks into an upper and a lower branch. The cardinal veins, card, lie on either side of the aorta, but they are almost completely obliterated by the ingrowth of the Wolffian tubules, which subdivide the vein into numerous smaller channels or sinusoids. The section also shows two branches, Ve', and Ve", of the subclavian vein. The identity cf these branches lias not yet been determined. The aorta, Ao, is a very large vessel a little to the left of the median plane. It has a well-developed muscular coat. Beneath it follows the (esophagus, the lumen of which is much smaller than that of the aorta. Its epithelium has the general characteristics of the epithelial entoderm at this stage, being a rather thick cylinder epithelium. As above mentioned, the differentiation of the mucous and muscular layers of the oesophagus shows clearly. Ik-low the (esophagus lie the two large vagus nerves, N. TO, and then follow the sections of the two lungs, Lu. Kaeh lung is a lobe of tissue connected with its fellow across the median line of the embryo and projecting laterally far into the pleural cavity, PI. cos. Each lung consists chiefly of a large accumulation of dense mesenchyma in which the epithelial bronchi, bro, ramify. Each bronchus has a central lumen and its walls are formed by a moderately thick layer of cylinder entodermal cells. The surface of each lung is covered by mesothelium, which is shown as a distinct line in the engraving. The mesothelium can be followed to the root of the lung, where it is reflected on to the outer wall of the pleural chamber. The pleural cavity, PL cce, is thus everywhere bounded by mesothelium which persists throughout life, being known in the adult as the pleural epithelium.

Fig. 136. — Pig, 17.0 mm. Transverse Series 51, Section 464. Ao, Aorta, bro, Entodermal bronchus, card. Posterior cardinal vein, cin, Neurone layer (cinerea) of spinal cord. Cost, Anlage of ribs. D.R, Dorsal root., Ectoglia. G, Ganglion. /./, Liver. Lit, Lung, muse, Dorsal musculature. -Y./o, Vagus nerve. Nch, Notochord. GS, (Esophagus. Pica:, Pleural coelom. R.D, Ramus dorsalis. R. /", Ramus ventralis., Ramus sympathicus. Sfi.c, Spinal cord. Sym, Sympathetic ganglion. IV, Ve" , Branches of the subclavian vein. Vert, Vertebra. X 22 diams.

Section through the Wolffian Body and Genital Gland. — The general characteristics of the ectoderm, mesenchyma, and nervous system are nearly the same as in the section last described. On one side the section shows a thickening of the ectoderm, the anlage of a mammary gland, mam (compare page 249). The branches of the nerves are not so well shown in this section as in the previous one. The level of our section corresponds to the lower end of the vena cava inferior, which is marked at this stage by the two large mesonephric veins, V. msn, which come from the Wolffian bodies and by their union constitute the lower end of the vena cava. The mesonephric veins are, strictly speaking, portions thereof. The Wolffian bodies are the most conspicuous structures shown in the section. They consist chiefly of a great number of tubules, W.t, very much crowded together. On the median side of the organ appear the large glomeruli, Glo, and on their ventral side we have the section of the longitudinal Wolffian duct. The tubules of the Wolffian body are formed by a more or less nearly cuboidal epithelium, the nuclei of which are decidedly larger than those of the mesenchymal cells. The nuclei themselves stain deeply, have well-marked outlines, and very distinct granules in their interior. The protoplasm of the cells also stains somewhat with cochineal, carmine, hematoxylin, etc. There is very little mesenchyma in the organ, but each tubule is closely invested by vascular endothelium; hence the tubules are separated from one another only by blood spaces, which, morphologically speaking, are portions of the cavity of the cardinal vein. These blood spaces are highly characteristic and are typical sinusoids. The intertubular circulation of the Wolffian body is, so far as known, always sinusoidal. The aorta, Ao, is seen in the figure to give off a small branch, art, which runs toward the Wolffian body. There are numerous such branches, each one of which may be traced to a glomerulus of the mesonephros. Each glomerulus has a capillary circulation, and the blood on leaving the glomerulus is supposed to be emptied into the venous sinusoids. More exact investigation of this point is needed. The mesonephros is covered by a layer of mesothelium, msth, underneath which is a thin layer of mesenchyma. The two together constitute the anlage of the peritoneal covering of the organ. To the median side of the Wolffian body is appended the large anlage of the genital gland, Gen, which has a constricted connection with the Wolffian body. Each gland is covered by mesothelium and extends until

Fig. 137. — Pig art. Glomerular artery

In. Li. nisi.

17.0 mm. Transverse Series 51, Section 651.

Ao, Dorsal aorta, art. Glomerular artery. Gv, coelom., Ectoglia. G, Ganglion. Gen, Genital gland. Glo, Glomerulus of Wolffian body. In, Intestine. Li, Liver, mam, Mammary anlage. nut. Mesentery, inst/i, Mesothelium. N, Ventral nerve. Nch, Notochord., Ramus sympathicus of nerve. R. V ', R. V" , Iiranches of the ventral ramus of the spinal nerve. Sent, Somatopleure. Sp.c, Spinal cord. Sym, Sympathetic ganglion. I'.msn, Vena mesonephrica. II. D, Wolffian duct. II'..'. Wolffian tubule. X 22 diams.

it comes in contact with the mesentery, mst. The gland contains two kinds of tissue, one, the anlage of the medullary, the other of the cortical portion of the gland. The medullary tissue resembles the neighboring mesenchyma and occupies only a small territory about the stalk of the organ. The cortical tissue contains cells with much larger nuclei and clearly developed protoplasmic bodies. It occupies by far the larger part of the gland. Comparison with figure 122 will show that the genital anlage at this stage occupies the same topographical relation to the Wolffian body as at earlier stages. It differs now from the earlier condition chiefly by its growth in size and by its advancement in histological differentiation. Below the genital gland the intestinal canal is cut several times. One portion of the intestine is seen in the section to be connected by means of the mesentery, mst, with the median dorsal tissues of the embryo. The intestine is formed by a small tube of entoderm with a small cavity. The entoderm is a rather thick cylinder epithelium. The greater part in bulk of the walls of the intestine is constituted by mesenchyma. The external surface is covered by a thin mesothelial layer. The mesenchyma is beginning to show the differentiation of the external muscular from the internal mucous coat. There is at this stage no trace whatever of the development of any folds or glands on the inside of the intestinal canal.

Section through the Kidney. — This section being much nearer the caudal end of the embryo, we find, as throughout all the early stages, that the differentiation of the tissues is less advanced than nearer the head. We have accordingly, so to speak, an earlier stage in the development of the spinal cord, Sp. c, of the nerves, and of the vertebra. In the median line is the large aorta, Ao, about which the mesenchyma is only slightly condensed. Near the aorta are the conspicuous anlages of the sympathetic system, Sym, which appear at this level in a very characteristic hook-shaped pattern. At the dorsal end of the hook the nervefibers are much more numerous than in the ventral portion of the anlage. The sympathetic cells themselves are extremely conspicuous, owing to the depth of their stain. On either side is situated the anlage of the permanent kidney. Each anlage consists of an irregularly branching space bounded by a thick layer of epithelium, which has somewhat the appearance of the intestinal entoderm at this stage. If the series of sections be followed through further toward the tail of the embryo, the epithelial space will be seen to contract to a relatively small tube, the ureter, which opens into the Wolffian duct of the same side. The expanded portion of the cavity shown in our figure corresponds in part to the pelvis of the adult organ. Its irregular shape is due to the fact that it is forming a series of outgrowths, which are to give rise to the tubules of the kidney. Around the ends of the branches of the renal pelvis is a darker tissue, in which the cells are very much crowded. The nature of this tissue has been much debated. Two divergent interpretations of it have been offered. According to one view, it is the material out of which the glomeruli and convoluted tubules of the kidney are to be differentiated. By a secondary process these tubules are supposed to become united with the branches from the renal pelvis, the branches forming only the collecting tubules of the adult organ. According to another view, the condensed tissue is partly mesenchyma and partly an outgrowth from the walls of the renal epithelium. This interpretation makes the collecting tubules arise as branches of the pelvis, and the convoluted tubules and glomeruli as branches of the collecting tubules. The origin of the renal anlage may easily be followed in earlier stages. It is found that from the pelvic end of each Wolffian duct there develops a dorsal outgrowth, which is lined by epithelium. This outgrowth elongates in a head ward direction. Its end expands; the narrow portion is the ureter, the expanded portion the anlage of the pelvis. The pelvis becomes irregular in shape and forms outgrowths. Around it appears the condensed tissue just referred to. On the ventral and lateral sides of the kidneys in our section appear the ends of the Wolffian bodies, W. b. From the ventral and inner edge of each Wolffian body is a projecting lobe of tissue in which the Wolffian duct, II*. D, is lodged. The walls of the Wolffian duct are a rather thin, cuboidal epithelium, surrounded by mesenchyma in which there is no very clear evidence of specialization. Between the Wolffian bodies is suspended the large intestine. It has a small canal formed by entoderm and very thick mesodermic walls. Attached to the ventral side of the body-wall of the embryo is the allantois, All, the cavity of which is quite large, somewhat irregular in shape, and lined by a cuboidal epithelium, a portion of the entoderm. By following through the sections it can be seen that the allantois and large intestine join at the cloaca. The entodermal allantois is surrounded by mesenchyma, which is very much looser in texture than that of the intestine proper. On either side of the allantois is a projecting lobe of tissue in which the umbilical artery, A. urn, is lodged. The two arteries pass upward to the umbilicus, then outward to the placenta. Downward they continue to the level of the cloaca, there pass to the dorsal side of the embryo, and unite with the end of the median dorsal aorta.

Fig. 138. — Pig, 17.0 mm. Transverse Series 51, Section 759. All, Allantois. Ao, Aorta., Umbilical artery, card. Branch of cardinal vein. Cat, coelom. G, Ganglion. A'i, Kidney. N', N", Nerves. Nch, Notochord. P.L, Posterior limbs. Rtct, Large intestine. Sp.c, Spinal cord. Sym, Sympathetic ganglion. W.b, Wolffian body. W.D, Wolffian duct. X '7 diams.

Frontal Section of the Umbilical Cord of Embryo of 17 mm

Since the umbilical cord projects from the abdomen, we get in frontal series of the embryo sections of the umbilical cord which are more or less nearly transverse. Such sections are instructive (Fig. 139). In the illustration we see that the umbilical cord is formed chiefly by mesenchyma, mes. It contains four blood-vessels; two umbilical veins, of which the left, U. V. S, is enlarged, while the right, U. V. D, is diminished in size. The arteries, Art, are almost symmetrical in position and alike in size. They are much smaller than the veins, but the mesenchymal tissue about them is somewhat condensed, so that they are provided with an imperfectly differentiated muscular coat. The body-cavity of the embryo is prolonged into the cord, forming a central space, Coe, in which are lodged the loops of the intestine and the prolongation of the allantois. The intestine is cut twice, the section on the left passing through the ileum, and on the right through the jejunum, which is much larger than the ileum, having both a smaller entodermal portion and a thicker mesenchymal part. The two segments of the intestine are joined together, and in the part between them are two blood-vessels, one the vitelline artery, A . vi, and the other the vitelline vein. The mesenchyma of the intestine and of the bit of mesentery between them consists of very crowded cells, so that the tissue appears darkly stained, and offers in this respect a striking contrast to the allantois, All, the mesenchymal walls of which contain only loosely scattered cells. The entoderm of the allantois is quite thin. The external surfaces of the intestines and of the allantois, and the outer surface of the coelom, are all lined by a distinct layer of mesothelium. The ectoderm, Ec, is thin, and consists, for the most part, of a single layer of cells, although the formation of a second outer layer seems to be beginning.

Pig Embryo of 20 mm

Sections of this stage are figured. In the practical laboratory work embryos a little larger or smaller may serve equally well to illustrate the developmental conditions of this stage.

Transverse Section through the Snout.— The parts shown are the same as in figure 146, to the description of which reference is made. The present figure 140

In.. 139.— Fig, 17.0 mm. Frontal Series 39, Section 63.

All, Allantois. Art, Umbilical artery., Vitelline artery. Ca, Ccelorn. Ec, Ectoderm. //, Ileum, ma, Mesenchyma. C.V.D, Right umbilical vein. U. !'. S, Left umbilical vein. X 35 diams.

is added to illustrate the development of the palate shelf, Pal. The palate shelf is a large protuberance on the inner side of the maxillary process. Its inner edge abuts against the tongue, Ton, and its upper edge underlies the maxillo-turbinal fold, max. th, and its lower edge forms part of the roof of the oral cavity, Or. At this stage it consists of a large mass of undifferentiated mesenchyma, covered by a layer of epithelium. The two palate shelves continue to grow toward one another until they meet in the median line below the nasal septum. As they approach one another the tongue descends. Ultimately the tw T o palate shelves unite with one another and with the overlying nasal septum. The epithelium of the two shelves concresces and forms for a time a partition, which marks the point of union of the two shelves, both with one another and with the nasal septum. This partition persists for a short time only, for it soon disappears by resorption. The union of the palate shelves separates definitely the nasal and oral cavities from one another. Their union is gradual, beginning in front and gradually extending backward. It is a not infrequent anomaly that the palate shelves fail to unite perfectly. When this occurs, there results the condition known as cleft palate.

Fig. 140. — Pig, 20 mm. Transverse Series 59, Section 522. Jk.o, Jakobson's organ, lat, Lateral ethmoid cartilage, max. id, Maxillo-turbinal fold. Afk, Meckel's cartilage. nas.tb, Naso-turbinal fold. Or, Oral cavity. Pal, Palate shelf. Sept, Cartilage of nasal septum. Ton, Tongue. X 22 diams.

Transverse Section through the Lower Part of the Neck (Fig. 141). — -The spinal cord, Sp.c, shows a very great enlargement of the ventral zones, which now project downward so as to enclose between them a distinct groove in the median ventral line, which can be identified as the commencing anterior fissure of the cord. In this groove runs a small, longitudinal blood-vessel, the arteria sulci, which from time to time gives off small branches, which enter the substance of the spinal cord. In the ventral zone the ependymal layer has become quite thin and the middle or gray layer has acquired great thickness, chiefly owing to the growth of the neuroblasts, many of which, especially toward the outside of the cord, can now be readily identified as young nerve-cells. The ectoglia or outer neuroglia layer has increased in thickness. Many of the processes of the neuroglia cells can be readily distinguished, running, for the most part, more or less nearly perpendicular to the surface of the cord. Between the neuroglia fibers are numerous fine dots which are the cut ends of the nerve-fibers running longitudinally. Although about these nerve-fibers there are as yet no medullary sheaths developed, it is, nevertheless, proper to speak now of the ectoglia as the external white matter of the cord. Immediately beneath the entrance of the dorsal root the external outline of the cord shows a concavity which disappears in later stages. The dorsal zones are very much smaller than the ventral. The differentiation of their three primary layers is being completed by the development of a distinct middle layer. The ectoglia of the dorsal zone resembles that of the ventral zone in structure and thickness. The spinal ganglia, G, have descended from their original position, so that they now lie on a level with the lower edge of the spinal cord, and the nerve-root, by which each ganglion is connected with the dorsal zone of the cord, has correspondingly elongated. The lower edges of the ganglia come in contact with the lateral processes of the vertebra. Between the spinal cord and the vertebra is an area of loose mesenchyma which may be regarded as a portion of the arachnoid membrane. Close to the upper surface of the vertebra, bounded dorsally by the tissue just mentioned, are two symmetrically placed blood-vessels. The vertebra, Vert, is distinctly cartilaginous, though not yet fully differentiated, and is surrounded by a distinct fibrous layer, the perichondrium. In the median line below the vertebra lie the oesophagus, CE, and trachea, Tra, both tubes lined by entoderm. The cavity of the oesophagus is somewhat crescent-shaped, that of the trachea triangular. About the oesophagus the mesoderm forms two layers, an inner lighter layer and an outer muscular layer, the cells of which are already elongated. The mesenchyma about the trachea is more condensed, especially on the sides and below, and the condensed tissue is in close contact with the epithelium. On the dorsal side of the trachea close to the entoderm is a thin layer of transversely elongated cells. The sympathetic nervous system, Sym, appears symmetrically placed near the trachea and oesophagus. In section the sympathetic is round and contains numerous nerve-fibers and characteristic young sympathetic nerve-cells, by which it is readily recognized. Close to the ventral side of the sympathetic is the section of the large jugular vein, V. jug, a branch of which, V. br, lies laterad from the main vessel. This branch receives blood-vessels from the facial region, and is perhaps the facial vein, but its identity is not certain. Between the main jugular and its branch are some lymphatic spaces, somewhat irregular in form, and lined by a thin endothelium so that they present a close resemblance to veins in their structure. Close to the medial wall of the jugular vein is situated the large trunk of the vagus nerve, N. to. At a little lower level than the vagus nerves and in the median line lies the anlage of the thyroid gland, which, owing to its darker staining, is somewhat conspicuous. The cells of the thyroid form an irregularly shaped branching mass. The spaces between the branches are chiefly occupied by cavities lined by endothelium and which probably belong to the lymphatic system. The arrangement of these cavities and the relation of their endothelium to the cells of the organ recall the blood sinusoids of the liver and of the suprarenal capsule. The thyroid cells are compactly arranged without distinct cell-boundaries, but with protoplasm which stains somewhat and with nuclei of rounded form, distinct outline, and granular appearance, the granules being decidedly more conspicuous than the granules in the nuclei of the neighboring mesenchymal cells Just ventral to each jugular vein is a small darker body, consisting of closely compacted cells, resembling in appearance those of the thyroid. The body has a very distinct external outline and is actively growing, for several of its nuclei are in mitosis. The bodies in question are the parathyroid glands. The rest of the section is mainly occupied by mesenchyma and numerous darker masses, muse, the anlages of the various muscles of the neck and throat. On each side is shown a small piece of the cartilaginous scapula, Scap. At the lower corner of the section is an indication of the anterior limb, A . L, and of its vein, Ve".

Section through the Lungs (Fig. 142). — The spinal cord shows very clearly in the differentiation of the three primary layers of the medullary wall. Its structure is similar to that shown in figure 1 4 1 , and need not be again described. The vertebra, Vert, is now distinctly young cartilage. On its ventral side its boundary is quite distinct, the formation of the perichondrium having there begun. Laterally it merges into a dense mesenchyma, by which it is united without demarcation with the rib, cost', and indirectly with the vertebral arch, V. ar, both of which are cartilaginous. The cells of the vertebral cartilage occupy rounded cavities, each of which is marked by a distinct capsule. The matrix between the capsules is homogeneous, stains slightly, and has acquired a greater density than in earlier stages. The cells themselves exhibit traces of their protoplasmic bodies and have deeply stained nuclei which are quite irregular in shape and very granular.

Immediately around the notoehord the spaces occupied by the cells are the largest and the capsules most distinct, the nuclei most altered. Proceeding toward the periphery of the cartilage, the cells appear in successively earlier and earlier stages, until at the very periphery we have normal nuclei and a transition to mesenchyma. The cells of the notoehord are beginning to degenerate, and in place of the notochordal sheath there seems to be only a space between the notochordal cells and the vertebral cartilage. Immediately below the vertebra are the conspicuous anlages of the sympathetic system, Sym. They overlie the sections of the posterior cardinal veins, card. These are now quite small vessels, the vena cava inferior having become the main channel for the return of the blood from the abdominal region to the heart. The two cardinal veins are not quite symmetrically placed, that on the left side lying a little lower than that on the right. Between them is situated the median aorta, Ao, with a relatively thick and welldeveloped muscular coat, the deeper staining of which makes it conspicuous even with low powers. The oesophagus, CE, and trachea, Tra, are not in the median line, but are both displaced toward the right of the embryo. As compared with earlier stages, both structures show an advance, first, by the growth of the entoderm; and, second, by the differentiation of the surrounding mesenchyma. In both oesophagus and trachea the entoderm is a ring of cylinder epithelium, the tracheal ring being much larger than the oesophageal. The mesenchyma about the oesophagus forms two distinct layers, an inner looser layer and an outer denser muscular layer. Around the trachea the mesoderm is much condensed. On the dorsal side of the trachea the cells form next to the epithelium a special layer characterized by the elongated form of the cells. Between the oesophagus and trachea are situated the vagus nerves, that of the right side, N. 10, occupying a higher position than that on the left, so that the nerves are not symmetrically placed. The cardinal veins, the aorta, the oesophagus, the vagus nerve, and the trachea are all imbedded in mesenchyma, which, together with these structures, forms the so-called mediastinum by which the right and left pulmonary cavities, PL d, PI. s, are separated from one another. On its ventral side the mediastinum joins on to the veins entering the heart. On either side of the mediastinum at the level of the trachea may be seen a projecting line. That on the left side shows clearly the division of the organ into a dorsal lobe, Lu. d, and a ventral lobe, Lu. v. Each lung consists at this stage chiefly of mesenchymal tissue and is covered by a layer of mesothelium which forms the boundary of the pleural coelom. Within the mesenchyma appear several sections of the branches of the entodermal bronchi. Each bronchus is lined at this stage by a rather thick entodermal layer of cylinder cells. The union of the lung with the mediastinum const itutes the so-called root of the lung. In the root of the lung-is seen the small pulmonary artery, A. pul. The two arteries join a little nearer the head and on the left side of the embryo to form a single trunk, the main pulmonary artery. Originally the pulmonary arteries arise symmetrically as branches from the fifth aortic arch. They soon unite, however, throughout the greater part of their extent, forming a single vessel. The two arteries shown in our figure represent the two original symmetrical vessels where they are about to enter the lungs. On the ventral side of the section various cardiac structures are shown, but so cut that the picture is not very instructive. It will suffice to refer to the explanation of the figure for the identification of the parts.

Section through the Posterior Limbs (Fig. 143). — Although this section is from a transverse series, yet, owing to the curvature of the body, it shows the spinal cord cut very obliquely. The three layers of the cord, the ependymal, epen, the cinerea or neurone layer, Cin, and the ectoglia are well marked. Something of the dorsal root, D. R, and of the ganglia, G, of a lumbar nerve are also shown in the section. The nerves have already joined together to form a very complex lumbar plexus, sections of portions of which appear at various points. These are all indicated by the reference letter N in the figure, it being thought not desirable to attempt an identification of each component of the plexus. The plexus is more or less symmetrically placed on the right and left, at about the level of the intestine, Rect. The limbs are large projections extending downward and containing in their interior the cartilaginous anlages, cart', cart", of the skeleton of the limb ; and, around these, darker masses of tissue, the developing muscle-fibers. At the lower edge of each limb is a blood-vessel, V. p, the so-called border or peripheral vein, which extends completely around the edge of the developing hand and foot. When the digits are developed, this vein becomes broken up, and out of its divisions are formed the digital vessels. The section also passes through the penis, Pen, in the center of which is the urethra, Ur. It shows here as a narrow epithelial band entirely without any cavity, except a very small one at its external dorsal end. The band is lighter in the center, owing to the fact that the nuclei are grouped chiefly close to the two surfaces of the band. At the base of the limb is situated the irregularly shaped section of the iliac vein, V. il. In the median line may be noted the following structures. Immediately underneath the nervous system is the arteria sulci, A. sul. The vertebra, Vert, and notochord, Nch, resemble corresponding structures in the section last described, except that their cytomorphosis is slightly less advanced. Below the vertebra lie the paired anlages of the sympathetic nervous system, Sym, between which is the small median caudal artery. The intestine, Red, has its transverse diameter somewhat increased, so that it appears oval in the section. Around it is beginning the differentiation of the mucosa muscularis.

Fig. 143 — Pig; 20 mm. Transverse Series 59, Section 1253. ara cH, Arachnoid membrane. A,< Arteria sulci. «# ,,„V", Cartilaginous anlages^f e,en,en. f t he skeleton of the limb. On, Neurone layer of spinal cord. D.R, Dorsal root Be Ectoderm, epen, Iliac vein. T./.Border vein of the limb. X 22 diams.

Section through the Mammary Anlage. — Figure 144 represents a section through the somatopleure of the embryo in the region of a mammary gland. The ectoderm, Ec, covers the external surface of the somatopleure, as does the mesothelium, msth, the inner surface, the space between the two covering layers being occupied by various mesodermic structures. The ectoderm consists of two or three layers of cells, the external one of which, Eptr, the epitrichium, is very thin. To form the mammary anlage, Mam, the ectoderm suddenly thickens and projects somevyhat upward and still more downward into the mesoderm. The epitrichium passes continuously over the thickening, in the production of which it takes no share. The inner edge of the ectoderm is marked by a very distinct line or basement membrane, b, against the underlying mesoderm. The cells of the anlage form two groups, one a band next to the basement membrane, in which the cells present a somewhat radial arrangement, and the other a central group of cells, many of which are elongated in a direction somewhat parallel to the surface of the anlage, so that they form curving lines. The elongated cells in later stages gradually cornify and fall out, so that the anlage becomes hollow, but its excavation proceeds very slowly, and in man is not usually completed until after birth. Soon after the hollowing out of the anlage has begun, it sends out a series of buds from its inner surface. These buds become elongated, somewhat twisted cords of cells, and offer at this stage resemblance to embryonic sweat-glands. The outgrowths subsequently branch and develop central cavities, and are ultimately transformed into the secretory portion of the gland.

Fig. 144. — Pic, 20.0 mm. Transverse Series 59, Section 1043. 6, Basement membrane of epidermis, cost, Costal anlage. Cu, Cutis. Ec, Ectoderm. Eptr, Epitricbium. Mam, Mammary anlage. mes, Mesenchyma. msth, Mesotbelium. Pan, Tunica panchoroidea. per. m, Peritoneal mesoderm, ve, Blood-vessel. X 2 5° diams.

Figure 144 also illustrates some important points in regard to the differentiation of the somatopleure. Parallel to the ectoderm, and some distance from it, is a layer, Pan, which is marked out by numerous blood-vessels. This is the pan-choroid layer. There is a slight but unmistakable difference in the mesoderm within and without this layer, for in the region between the pan-choroid and the ectoderm the cells are somewhat more crowded. They probably represent the anlage of the cutis, Cm, and of the cutis only. Within the vascular layer the mesodermic cells, mes, are less near to one another, and their processes, by which they are connected, are more slender. Toward the mesothelium is a broad band of denser tissue, cost, the rudiment of a rib, the inner boundary of which is further marked by several blood-vessels, ve. Between the costal anlage and the mesothelium is a layer of embryonic connective tissue, the cells of which are more crowded toward the mesothelium, so that we may say that there are already imperfectly differentiated two layers of mesenchyma within the rib. The denser layer next the mesothelium is destined to become still more marked and to transform itself into the connective-tissue layer of the peritoneum. With the overlying mesothelium it develops into the peritoneal membrane of descriptive anatomy.

Sagittal Section.— Through the Right Luna ami Kidney (Fig. 145).— The lungs occupy a position in the upper part of the figure and are easily recognized by the conspicuous entodermal bronchi, bro, which resemble in microscopic structure the bronchi of earlier stages. The branches are widely separated from one another by the voluminous mesenchyma of the organ. The lung is covered by mesothelium, msth, and projects into the pleural cavity, Pleu. c, which is lined by a continuation of the mesothelium of the lung itself. The pleural cavity can be followed downward past the Wolffian body, W. b' , and liver, and from there past the genital gland, Gen, and so on to the lowest part of the abdominal cavity, Ab. cce. The pleural cavity at this stage is entirely separated from the pericardial, but it is still directly continuous with the abdominal cavity. On the ventral side (in the figure, to the right) of the pleural cavity are the great veins, the duct of Cuvier, D. C, descending from above, and the ductus venosus, Du. V, rising from below. The pleural cavity is separated from the duct of Cuvier by a lamina of the mesoderm, x, and from the ductus venosus by a similar but thinner lamina, y. Both laminae are bounded on the pleural side by the mesothelium, and on the venous side by the endothelium of the vessel. The opening of the veins into the right auricle, Au. d, does not appear in this section, though a small bit of the left valve, v. s, which guards this opening is shown. The Wolffian body is divided into two parts, an upper, W . b' , on a level with the liver, and a lower, W. b", toward the pelvic end of the abdomen. The lower part is larger than the upper. The two parts are separated from one another chiefly by the mesonephric vein, V. mm, which is the principal vessel to take the blood from the Wolffian body. It delivers the blood to the lower end of the vena cava inferior. The separation of the two parts of the Wolffian body is, however, further accented by the position of the genital gland, Gen, and of the kidney, Ki. The structure of the latter organ does not differ much from that of earlier stages, except that the diameter of the tubules has increased. The genital gland (testis) is remarkable for its large size. It is covered by a layer of mesothelium, underneath which is a rather broad layer of elongated mesenchymal cells, the anlage of the tunica albuginea. The interior of the organ contains a number of contorted epithelioid cords of cells in which there are a certain number of so-called primitive ova, cells which are distinguished by their larger size, rounded form, greater transparency, and spherical nuclei. The bands of cells are known as the sexual cords, and they are separated from one another by loose mesenchymal tissue. The cords frequently anastomose with one another. They are the solid anlages of the seminiferous tubules. The question of the origin of these cords has been much debated, but cannot be considered as yet settled. As to both the origin and the ultimate fate of the primitive ova in the gland we have no satisfactory information. The cords remain solid throughout embryonic life, not acquiring a central cavity until after birth. The kidney is well defined and is similar in structure to the kidney in the 17 mm. pig. (page 237), but is somewhat more advanced in its organization, especially as regards the formation of the glomeruli and convoluted tubules. The liver is a very voluminous organ permeated everywhere by sinusoidal blood-vessels, which. offer the greatest possible variety in size. In the figure only the larger of these blood-vessels have been drawn in. A large proportion of the smaller sinusoids are crowded with nucleated red blood-corpuscles, the nuclei of which are small and deeply stained ; hence each cluster of corpuscles stands out as a darker spot in the liver, for the liver cells themselves stain lightly and have nuclei which, though three or four times the size of the nuclei of the blood-corpuscles, yet appear relatively pale in the stained specimen. The bloodcorpuscles which form the clusters in the liver differ somewhat from those in the active circulation, for they are smaller and show less of the characteristic hemoglobin color. It is believed that the liver at this stage furnishes sites for the multiplication of the blood-corpuscles, and the clusters, which are so conspicuous in the organ, correspond not to blood-corpuscles in active circulation, but rather to corpuscles which have found a lodging-place in the liver and are there proliferating. Our knowledge of the blood-forming function of the embryonic liver is imperfect. Above the liver is the septum transversum or diaphragm, Diaph' , Diaph" , which is formed chiefly by mesenchyma. On the lower side of the liver is another broad accumulation of mesenchyma, mes, in which is lodged the gallbladder, a small section, G. bl, of the entodermal lining of which is included. The intestine, In', In", In'", is cut several times, because at this stage the intestinal canal forms several coils in the abdominal cavity below the liver and on the ventral side of the Wolffian bodies. Below the intestines appear the curious mesothelial villi, All. vi, of the allantois (compare page 222). At this stage the villi are little more than large vesicles of mesothelium, which contain in their interior some coagulum and a very few mesenchymal cells, associated with which are a few fibers — whether true connective-tissue fibers, or not, is undetermined. The mesothelium of the villi is a very thin layer of flattened cells.

FIG. 145.

Frontal Sections of the Head. — The three sections to be described are from a special series of the head. The plane of the series was made as nearly as possible transverse and at right angles to the plane of the roof of the mouth. They illustrate some important points in regard to the development of the facial region and of the fore-brain. In all of the sections the differentiation of the mesoderm around the brain is clearly demonstrated. The pia mater is very distinct. In those parts of the sections where the brain-wall is cut obliquely, it can be distinguished only by a somewhat careful observation, as the tissues of the pia mater and of the brain overlap. All about the brain is the broad zone of the arachnoid (Figs. 147 and 148, arach), easily distinguishable even with a low power by its light coloration. It consists of widely separated cells connected together by very distinct processes, and is permeated by a number of small blood-vessels running in various directions through the layer. Its external boundary is now very distinct, being marked by a layer of somewhat crowded, elongated cells which merge on the side toward the ectoderm into the general surrounding mesenchyma. Out of this denser layer (Figs. 146 and 147, Sk) arise both dura mater and the membrane bones of the skull.

Fig. 145. — Pig, 20.0 mm. Sagittal Series 60, Section 213. Ab.cce, Abdominal coelom., Mesothelial villi of the allantois. Au.d, Right auricle, bro, Entodermal bronchus. Ca',Cce", coelom. D. C, Duct of Cuvier. Diaph', Diaph", Diaphragm. Du.v, Ductus venosus., Gall-bladder. Gat, Genital gland. In' , In" , In"', Intestine. Ki, Kidney. Li, Liver. nits, Mesenchyma. msth. Pleural mesothelium., Pleural coelom. Pltu.e, Pleural cavity. Ve, Vein of liver. V. mm, Vena mesonephrica. v.s, Valvula sinistra. IV.b', W.b" , Wolffian body, x, Partition separating the pleural cavity from the duct of Cuvier. }', Partition separating the pleural cavity from the ductus venosus. '/ 22 diams.

Fig. 146. — Pig, 20.0 mm. Frontal Section of Head. Series 40, Section 68. //, Cerebral hemisphere. Jk.o, Jakobson's organ. Max, Maxillary process. max.tb, Maxilloturbinal fold.

J//-, Mandible. Mx.sup, Superior maxillary nerve. nns.tb, Nasoturbinal fold. St, Mesenchymal anlage of the dura mater and skull. X '8 diams

Sept, nasal septum.

Section through the Anterior Part of the Snout (Fig. 146). — On the dorsal side appear the two hemispheres, H, cut separately and each showing the cavity of its lateral ventricle. On the ventral side the mandible, Mdb, is cut separately and is separated by the oral fissure from the rest of the section. The maxillary processes, Max, are large, and each is furnished with an inward prolongation extending toward the median line. From the oral fissure there extend upward two irregular cavities, the nasal chambers. The two cavities are separated from one another by a broad mass of tissue, the nasal septum, the ventral edge of which at this stage forms a portion of the roof of the mouth-cavity. In the center of the nasal septum is a broad band, Sept, of denser mesenchymal tissue, the anlage of the cartilaginous septum of the nose. On either side of the nasal septum are the two irregularly shaped nasal cavities, which open into the mouth between the ventral edge of the nasal septum and the inner edge of the maxillary process. The medial side of each nasal cavity is comparatively regular, but the external side shows two prominences, each of which is formed by a mass of mesenchymal tissue covered by epithelium. The upper of these projections, nas. tb, is the anlage of the naso-turbinal fold, and the lower, max. tb, the anlage of the maxillo-turbinal fold. In the nasal septum itself are two oval rings of epithelium, sections of Jakobson's organs. This organ is an evagination of the epithelial lining of the nasal cavity which opens anteriorly and extends backward some distance in the nasal septum. In the maxillary process may be observed the maxillary nerve. The number of cells in the nerve has increased, and consequently the division of the nerve-fibers into distinct bundles has become more marked as compared with the pig embryo of 12 mm.

Section through the Middle of the Snout (Fig. 147). — The relations are very similar to those described in the previous section, so that it will suffice to note the three most important differences : First, the absence of Jakobson's organ ; second, the appearance of the tongue, Ton, and third, of the olfactory nerve, N. olf. The tongue is a protuberance attached to the lower jaw, Mdh. Its connection with the jaw is rather narrow and corresponds to the frenum. The tongue extends upward between the maxillary processes until it is almost or quite in contact with the lower edge of the nasal septum. It is formed by a somewhat dense mass of tissue in which there is no very evident histological differentiation, and is covered by a layer of epithelium of moderate thickness and which is probably entirely derived from the entoderm, for the tongue first appears as a small median protuberance on the ventral floor of the pharynx, between the first gill pouches. The olfactory nerve, N. olf, can be seen joining the lower part of the inner side of the brain-wall and extending down toward the nasal cavity and branching. Under the part of the nerve near the brain-wall numerous cells are mingled with the fibers, and bytheir crowding render the nerve conspicuous in stained sections. The fibers of the olfactory nerve differ from all other nerve-fibers known in vertebrates. They arise as prolongations of certain of the epithelial cells of the olfactory region of the nose and grow from these cells into the brain, where they have their termination in the glomeruli of the olfactory bulb. All other nerve fibers arise from nerve-cells either of the ventral nervous system or of the ganglia. Morphologically, therefore, the olfactory nerve takes a unique place, and is not directly comparable with any other nerve of the brain. The cells which accumulate in the course of the olfactory nerve do not, so far as known, have any direct share in the production of the nerve-fibers. Nor do they result in the formation of the medullary sheaths, as they do in other nerves, the olfactory nerve-fibers remaining, as it is termed, naked throughout life.

Fig. 147. — Pig, 20.0 MM. Frontal Section ok Head. Series 40, Section S4. araih, Arachnoid membrane, H, Cerebral hemispheres. Max.tb, Maxilloturbinal fold. MJb, Mandible. Mx. sup, Superior maxillary nerve. Nas.tb, Naso-turbinal fold. N.olf, Olfactory nerve. Sept, Cartilaginous septum of the nose. Sk, Mesenchymal anlage of the dura mater and skull. Ton, Tongue. > 18 diams.

Section through the Fore-brain and Eyes (Fig. 148). — The section passes behind the nasal cavities, no part of which is shown. The maxillary and mandibular processes are united and the pharynx, Ph, appears as a closed cavity. On the dorsal side of the section the fore-brain stands out conspicuously, both from its dark staining and from being surrounded by the lightly stained broad zone of the arachnoid, arach. The cavity of the fore-brain has two lateral expansions, L. V, the lateral ventricles, which extend outward and upward. The walls, H, of the lateral ventricles are much thinner than the walls of the lower part of the forebrain and are the anlages of the cerebral hemispheres. In the median plane the hemispheres include between themselves a partition, Fx, of mesodermic tissue which can at once be identified as the falx. From the base of the falx there extends on each side a fold, Plx, which projects into the cavity of the lateral ventricle. This fold contains in its interior a prolongation of the mesodermic tissue of the falx, and it is covered by a continuation of the wall of the hemispheres. The covering layer of the fold is much thinner than any other portion of the brainwall shown in the section, and shows no differentiation into layers. It retains throughout life an epithelial character and is already to be termed ependyma. The ependyma of the two folds is connected across the median line, and it forms the median dorsal boundary of the cavity of the fore-brain. The two folds are the anlages of the lateral choroid plexus. They are destined to grow much in size and in complexity of form, but they always remain morphologically what they now are, vascularized mesenchyma covered by ependyma. The choroid plexus protrudes into the cavity of the brain in the same way in which the viscera may be said to protrude into the abdominal cavity. The cavity of the brain is bounded by the brain- wall or ependyma, just as the abdominal cavity is bounded by the peritoneum. The vascular tissue of the choroid plexus is outside of the cavity of the brain, in the same way that the tissue of the kidney is outside the cavity of the abdomen. Throughout life the choroid plexus springs, as it does from the start, from the medial wall of the hemispheres, and it is only at that point that it can receive its blood-supply. The lateral walls of the hemispheres, H, gradually thicken as they continue ventralwards, and on the ventral side of the brain form in part the lateral boundary of the medial portion of the braincavity, as an especial thickening of the brain-wall which projects far into the cavity. The thickening, C. sir, is the corpus striatum. Between the summit of the corpus striatum and the choroid plexus is an open passage through which we may pass from the median portion of the brain-cavity into the lateral ventricle, L. V . The passage is the foramen of Munro, which we learn from this section is bounded above by the choroid plexus, and below by the corpus striatum. On the dorsal and middle sides of the hemispheres, the ectoglia, ec. gl, is already clearly differentiated. There is, however, at this stage, no clear indication of the cortex cerebri, although in the slightly older stages it will begin to develop by the accumulation of neuroblasts immediately beneath the ectoglia. The notochord does not appear between the brain and the pharynx, the section being too far forward. The notochord stops near the hypophysis. The eyes are not cut quite symmetrically. They show the lens, L, and retina, Ret, clearly, and the left eye of the embryo shows also the entrance of the optic nerve. On the right side of the embryo, near the eye, are three areas which are somewhat more darkly stained than the surrounding mesenchyma. These are the anlages of the muscles of the eye. They have not yet been studied sufficiently to make their identification certain, but it seems probable that the uppermost of these anlages, m. rec. sup, is the rectus superior, that the middle one, m. rctr. b, is the retractor bulbi, and that the lowest one, m. r. lot, is the rectus lateralis. Until a reconstruction is made these identifications can be recorded as tentative only. The pharynx, Ph, appears as a yolk-shaped slit lined throughout by entoderm. From its median ventral floor rises the great mass of the tongue, Ton, over which the dorsal roof of the pharynx forms a closely fitting arch. A portion of the epithelium of the tongue is loosened from the underlying tissue, probably owing to defective preservation. Upon the lower side of the tongue extend downward the .anlages of the hyoglossal muscles, hy. gl, between which are situated the lingual arteries, art. On either side, in the part of the section corresponding to the mandible, appears Meckel's cartilage, Mk, a somewhat conspicuous and easily identified structure, owing to its dark staining. Meckel's cartilage is the primitive skeletal element of the mandibular arch, and is homologous with the cartilaginous jaw of the lower fishes. It is, for the greater part, a purely embryonic structure, the mandible of the adult being a secondary bone. By referring to figure 99 (pig IO mm.), it can be seen that the mandibular arch extends upward toward the otocyst and forms the boundary of the first gill cleft, the outer division of which becomes the meatus auditorius externus. In other words, the upper portion of the mandibular arch is in close proximity to the otocyst and the anlage of the tympanum or middle ear. Meckel's cartilage is a rod-like structure extending the entire length of the arch. Its upper end is, therefore, close to the future tympanum. While the greater part of Meckel's cartilage disappears during later development, the upper end persists and takes a direct share in the formation of the malleus. A little outside of Meckel's cartilage in our section is the inferior maxillary nerve, Mx.i, and still further lateralwards is a small, darkly stained body, x, which has not yet been identified with certainty.

Fig. 14S.— Pig, 20 mm. Frontal Section of Head. Series 40, Section 123. arach, Arachnoid zone, art, Lingual arteries. C.slr, Corpus striatum,, Ectoglia. Fx, Falx cerebri. H, Cerebral hemisphere,, Hyoglossal muscle. L, Lens. L. V, Lateral ventricle. Mk, Meckel's cartilage, m.rec.sup, Musculus rectus superior, m.retr.h, Musculus retractor bulbi., Musculus rectus lateralis (</. text). Mx.i, Inferior maxillary nerve. P/i, Pharynx. Fix, Plexus choroideus lateralis. Ret, Retina. Sk, Anlage of membranous skull. Ton, Tongue, x, Unidentified structure. X 18 diams.

Pig Embryo of 24 mm

Section through the Eye. — In the pig of 24 mm. the anlages of all the parts of the adult eye may be said to be present, with the exception of the pigment layer of the iris, which arises somewhat later by a forward growth of the retina and pigment layer. The origin of the retina and lens is illustrated by the chicken embryo (Figs. 155, 156), and in a more advanced stage by the pig of 12 mm. (Fig. u 6). There is added here figure 149, from a section of the eye of a rabbit embryo of thirteen days, in order to facilitate the comparison between the 12 mm. stage and the 24 mm. stage of the pig. In figure 149 the ectoderm, Ec, forms an arch over the eye and indicates the commencing formation of the cornea, the layer of ectoderm being destined to become' the external epithelium of the cornea. Between the lens and the retina there has been an ingrowth of tissue accompanied by blood-vessels, which form a more or less distinct covering over the surface of the lens and constitute the so-called tunica vasculosa, tu. v. The space between the retina and lens will increase during the following stages and will become occupied by a very clear tissue containing a minimal number of cells. This clear tissue is the commencement of the vitreous humor. Between the lens and the overlying ectoderm the mesenchyma has begun to penetrate. This mesenchyma will ultimately furnish the connective tissue of the cornea and of the iris. About the eyeball as yet there is no distinct condensation of tissue such as will appear later to develop the anlages of the choroid and scleral coats of the eyeball.

Fig. 149. — Rabbit Embryo of Thirteen Days ; Section ok the Eyb. L, Lens, mes, Mesenchyma. A 7 , Anlage of optic nerve. /', Pigment layer. tu.v. Tunica vasculosa lentis.

R, Retina.

In the pig of 24 mm. (Fig. 150) we encounter a marked advance in the differentiation of all parts of the eye. Above and below the eye the anlages of the eyelids, L.sup, L.inf, ha ve appeared. The anlage is at this stage merely a projecting fold of the ectoderm filled with mesenchyma and extending a short distance over the projecting cornea. The folds will continue to grow until the eyelids meet in the middle of the eye, covering it completely. The ectoderm of the two lids where they meet unites. The union of the two lids occurs in all mammals, and in some cases they do not separate again until after birth, in which case the animals are said to be " born blind." The ectoderm, Ec, of the cornea describes a wide, high arch, underneath which is a broad band of embryonic connective tissue, corn, which forms the main thickness of the cornea. Between the connective tissue of the cornea and the anterior surface of the lens is a clear space, an. ch, which we can identify as the anterior chamber of the eye, which in the adult is filled only with the aqueous humor. On the corneal side the anterior chamber is bounded by a distinct layer of cells, Ep, the internal epithelium of the cornea. This layer is, however, formed from the mesenchyma, the cells of which develop into the internal epithelioid covering of the cornea. At the upper and lower edge of the cornea there is a separate forward growth, Ir, of the connective tissue between the cornea and the lens. It is the anlage of the connectivetissue layer of the iris. In later stages it will grow still further over the lens from all sides, leaving a central opening, the pupil, and it will acquire a special pigmented layer on its side nearest the lens. At the base of the iris anlage is a small blood-vessel, Schl, which is commonly designated in the adult as the canal of Schlemm. The retina has increased in thickness and is closely covered by a pigment layer, Pig. The separation which appears on the inner side of the eyeball between the retina and pigment layer in figure 150 is probably artificial, the result of shrinkage during the preservation of the specimen. At its outer edge the retina, Ret, suddenly thins out and passes over into the external pigment layer, which is heavily loaded with dark, uniform, pigment granules, especially crowded together on the side of the layer nearest the retina. In later stages the pigment layer grows forward on the inner side of the iris, making a fold, so that the iris is covered on the inside by a double layer of pigmented epithelium, the uvea. The retina resembles closely in structure the brain-wall in an earl)- stage, for it has on its outer surface a thin layer corresponding to the ectoglia, and within a broad, nucleated zone. The mitotic figures are found only next to the surface of the retina nearest the pigment layer. Since the space between the pigment layer and the retina corresponds to the cavity of the brain, .it is evident that the position of the mitotic figures is homologous with their position in the medullary wall elsewhere. The section of the lens clearly reveals its vesicular structure. The external wall of the lens vesicle, L" , is a comparatively thin epithelial layer which stains quite readily and therefore stands out clearly in the section. Toward the edges of the lens the outer layer slightly thickens and then passes over quite abruptlv into the inner layer of the vesicle, L', which is very thick and constitutes by far the greater part of the bulk of the organ and gives to the lens its characteristic shape. The outer and inner walls of the lens are in close contact so that there is no actual cavity present. The epithelial cells of the inner wall have elongated enormously, so much that they might perhaps already be termed "fibers." Each cell is supposed to extend through the entire thickness of the inner wall. The nuclei are placed somewhat irregularly in the middle portion of the long cells so that they constitute a more or less distinct band in the section. Toward the edge of the lens the nuclear band becomes more distinct, and where the inner wall merges into the outer, the band becomes narrow and the nuclei are much crowded together. The nuclei of the lens fibers are oval, being slightly elongated in the same direction as the fibers, and each nucleus contains usually a distinct nucleolus. Between the lens and the retina is the vitreous humor, Vit, which has become quite voluminous. It contains a few mesenchymal cells and a few small bloodvessels, and when examined with a high power it is seen to be permeated by a fine network which is probably to be interpreted as a modification of the protoplasmic threads of the mesenehyma. There are also a very few cells of rounded form and distinct outline, with a single small granular nucleus, which are probably leucocytes. Against the surface of the lens there is a delicate homogeneous hyaloid membrane, which can usually be better seen where by shrinkage it has been loosened from the surface of the lens, as is apt to occur. Against the hyaloid membrane are a number of small blood-vessels, more numerous than those elsewhere in the vitreous humor, and forming a fairly distinct vascular membrane around the lens. The membrane, tit. v, is called the tunica vasculosa lentis. The blood-vessels of the vitreous humor are chiefly, possibly at this stage exclusively, branches of the central artery of the retina. The artery enters the eye through the optic nerve, and sends branches throughout the vitreous humor. The space originally occupied in the humor by the stem of the central artery persists, and is called the hyaloid canal. The muscles of the eye are already differentiated, but their relations cannot be properly understood without a reconstruction.

Fig. 150.— Pig, 24.0 mm. Transverse Series 62, Section 428., Anterior chamber of eye. corn, Corneal mesoderm. Ec, Ectoderm (epidermis). Ep, Inner epithelium of cornea. Ir, Mesodermal anlage of iris. U , Outer layer of lens. L", Inner layer of lens. L.inf, Inferior eyelid. L.sup, Superior eyelid. N.j, Oculomotor nerve. N.op, Optic nerve. Pig, Pigment layer. Ret, Retina. Schl, Canal of Schlemm. tu.v, Tunica vasculosa lentis. Vit, Vitreous humor. X 5° diams.

Median Sagittal Section (Fig. 1 5 1 ) . — The section figured is very nearly median for the region of the head, but in the body it passes to the left of the median plane. The area occupied in the section by the neck and head of the embryo is almost as great as that occupied by the rest of the body. The great size of the head at this stage is characteristic. Attention is especially directed to the sharp angle which the medulla oblongata, Md. ob, makes with the spinal cord, Sp. c, and to the very great bend formed by the floor of the mid-brain, Ar. hab, in consequence of which the floor of the hind-brain above the medulla oblongata and the floor of the forebrain are brought quite close together and run in almost parallel directions. The cavity of the brain is very large. Its walls in the median plane are, for the most part, thin. From the roof of the diencephalon, Dien, there runs off a small evagination, Ephys, a shallow pocket or diverticulum of the medullary wall. It is the anlage of the epiphysis or pineal organ of the adult. It is an important landmark in the topography of the brain, for its position is always at the extreme limit of the fore-brain. In the wall of the mid-brain, behind the epiphysis, for some distance the ectoglia shows considerable thickening and contains a very large number of nerve-fibers running transversely. They constitute the posterior commissure, which morphologically belongs to the mid-brain. In later stages the opening of the epiphysis and the anterior boundary of the posterior commissure are separated by a narrow band of ependyma. Immediately in front of the epiphysis, close to the external surface of the medullary wall, is another tract of nerve-fibers which is very small and is known as the superior commissure. The superior commissure develops much later than the posterior, and is much smaller in size. The two commissures are found in vertebrates of all classes and are exceedingly constant anatomical features of the brain. Anterior to the epiphysis the dorsal roof of the diencephalon forms a broad arch which descends in the figure vertically until it ends in a small inward projection, Plx, of the brain-wall, the anlage of the choroid plexus. Below this point the brain-wall is continued, forming the lamina terminalis. It then makes a bend almost at right angles and runs in a horizontal direction toward the dorsal side of the embryo. This portion of the brain- wall shows a considerable thickening, the optic chiasma. Behind the optic chiasma the brain-wall forms a short evagination, the infundibular gland, which bends over so as to lie close to the dorsal side of the hypophysis, Hyp. The hypophysis, which in earlier stages appears clearly as an evagination of the oral epithelium (Fig. 123), is now entirely separated from the mouth, and is an epithelial vesicle with an irregular cavity. The epithelium has sent out, especially on its anterior side, a number of solid outgrowths. The infundibular gland and hypophysis constitute the pituitary body of the adult. They are surrounded by loose mesenchymal tissue. The sella turcica, in which the pituitary body of the adult is lodged, is already marked out, because the chondrification, which is to form the sphenoidal cartilages, has already begun about these structures. The sphenoidal cartilage is continuous, on the one hand, with that of the nasal septum, Sept; and, on the other, with that of the vertebral column, Vert.

Fig. 151

Fig. 151. — Pig, 24.0 mm. Sagittal Series 63, Section 30. ^ A, Arachnoid space, in this specimen containing extravasated blood. A.Ao, Arch of the main aorta., Allantoic artery. Ao, Dorsal aorla. Ao.D, End of dorsal aorta. Ar.hab, Habenular arch (floor of midbrain). A.vert, Vertebral artery joining its mate to form the basilar artery. Bro, Main bronchus of lung. bro, Branch bronchus within the lung. Cbl, Cerebellum. Car, Crelom. Diap/i, Diaphragm. Dien, Diencephalon. Duo, Duodenum. Epen, Ependymal roof of hind-brain. Ephys, Epiphysis. G, Spinal ganglion. Hyp, Hypophysis. In, Intestine. Inl.v, Anlage of intervertebral ligament. La, Lateral wall of larynx. Li, Liver. Lit, Lung. M.b, Mid-brain. Md.ob, Medulla oblongata. Nch , Notochord. (E, Oesophagus. Pin, Penis. PA, Pharynx. Plx, Choroid plexus of fore-brain. Plx. IV, Choroid plexus of hind-brain. Stpt, Cartilaginous nasal septum. Sp.c, Spinal cord., Suprarenal capsule. St, Stomach. T',T", Tail. Tt, Testis. Ton, Anterior portion of tongue. Umb, Umbilical cord. Ve, Cardinal vein. Ven, Ventricle of the heart. Vtn.IV, Fourth ventricle, or cavity of the hind-brain. Vert, Vertebra, ll'.b, Wolffian body. X 8 diams.

From the opening of the infundibular gland the brain-wall ascends and joins the habenular arch, where it suddenly thickens. The arch forms the floor of the midbrain. The roof of the mid-brain, M. b, is quite thin, and forms the large arch in which the differentiation of the anterior and posterior corpora quadrigemina is not yet shown. At its posterior boundary the wall of the roof of the mid-brain bends inward, marking the constriction of the so-called isthmus. We now reach the cavity, Ven. IV, or fourth ventricle, of the hind-brain. This cavity is subdivided into an anterior and a posterior portion. The boundary is marked on the dorsal side by the inward projection of the ependymal roof of the ventricle to form the choroid plexus, Plx. IV, and on the ventral side by the angle formed by the union of the medulla oblongata, Md. ob, with the vertical peduncles of the brain. In front of the choroid plexus the arching brain-wall, Cbl, represents the median anlage of the cerebellum. The lateral portions of the cerebellum are much thicker. Behind the choroid plexus the roof, Epen, of the fourth ventricle is very thin. The medulla oblongata, Md. ob, is a thick mass of tissue which passes over abruptly into the spinal cord. The spinal cord is cut, as a whole, somewhat obliquely. In its upper part, where the reference line Sp. c is placed, the section is almost exactly median, and shows, therefore, the floor-plate or raphe of the spinal cord. In front of the cord is the vertebral artery, A. vert, which joins its fellow to form the basilar artery which runs in the median line the entire length of the hind-brain. The vertebral column is in the cartilaginous stage. It is an absolutely continuous uninterrupted rod of cartilage which merges at the neck with the cartilaginous skull. The entire continuous cartilaginous structure is termed the chondrostyle. Out of it both the cartilaginous skull and the vertebra are differentiated. More or less nearly in the center of the chondrostyle are found the remnants of the notochord, which, however, never extends anterior to the pituitary body, Hyp. The division of the chondrostyle into separate vertebrae is indicated by the modifications of the notochord and by the commencing differentiation of the intervertebral ligaments. The space occupied by the notochord expands in the region corresponding to the division between each two vertebrae. The notochord in the intervertebral expansions is partly degenerated, forming an enlarged mass of irregular strands of cells. From each such mass goes off a narrow extension of the notochord, through what is to become the body of the vertebra. Sometimes this extension is continuous with the intervertebral portions of the notochord, but more usually it forms a series of isolated fragments, for the notochord in the parts corresponding to the bodies of the vertebras is already in process of resorption. The diameter of the chondrostyle is nearly uniform in the vertebral region, but is a little smaller in the part corresponding to eacli body of a vertebra and a little wider in the parts corresponding to the intervertebral ligaments. The cartilage of the body of the vertebra continues past the intervertebral expansion of the notochordal cavity, but the external portion of the chondrostyle opposite each such expansion exhibits a modification of its cells, for they have become lengthened out in a direction parallel with the vertebral axis. The tissue thus produced is the anlageof the vertebral ligaments. The mouth and pharynx, Ph, form a narrow cavity, the floor of which is constituted by the tongue, Ton, the tip of which has already become free. The surface of the tongue forms a long arch, at the posterior end of which lies the epiglottis, a projecting fold of tissue which covers the opening of the trachea. The side of the trachea is marked by the longitudinal fold, La, which separates the trachea proper from the upper end of the oesophagus, CE. At the upper end of the oesophagus there is a small dorsal diverticulum. If the reference line CE be followed a short distance past the oesophagus, it will lead to the section of the main aorta. A little lower down is the section of the arch of the aorta, A. Ao. The heart shows chiefly its large ventricle, Yen. The section is not favorable for an exhibition of its structure or for that of the lungs, Lu. It does, however, — since in this part of the embryo the section passes to one side of the median plane, — show the main bronchus, Bro, coming off from the trachea to the lung, and some of the smaller entodermal bronchial branches, bro, in the lung itself. The heart and lung are separated from the abdominal cavity by the diaphragm, Diaph. It is only to the dorsal part of this diaphragm that the liver, Li, is attached. In earlier stages the liver is connected with the whole of the diaphragm (septum transversum). We now have a portion of the diaphragm without connection with the liver. Below the liver is the section of the stomach, St, the entoderm" of which is cut twice. Below the stomach lies the duodenum, Duo, extending from the dorsal side of the embryo and running toward the umbilicus. At the dorsal end of the duodenum is a group of clusters of darkly stained cells, marking the position of the anlage of the pancreas. Below the duodenum the loops of the intestine, In, are cut repeatedly. On the dorsal side of these loops is the section of the genital gland, in this specimen, testis, Te. Dorsal wards from the genital gland is the complicated anlage of the suprarenal capsule, Sp. ren, which is really a double organ, having one part derived from the sympathetic nervous system and another from a modification of mesenchymal cells. In a sagittal series the connection of the anlage with the sympathetic nervous chain of the abdomen can be readily made outIn the anlage the nerve-fibers and the sympathetic cells are irregularly distributed , although the cells are more or less grouped together. The sympathetic tissue constitutes the dorsal part of the anlage and gives rise to the so-called medulla of the adult organ. The ventral portion of the anlage, as seen in the section, consists of bands or cords of cells separated from one another by venous sinusoids.

The cells are much more closely compacted in this portion of the anlage than in the sympathetic, and they are further distinguished by having nuclei which stain much less deeply. The cords of cells, here seen, develop into the cortex of the adult organ. The fate of the medulla or sympathetic portion of the suprarenal in man is not known. The section passes through the side of the allantois, and, therefore, shows only one of the lateral arteries,. All. ar,but the allantois still bears a number of degenerating mesothelial villi (compare page 222). At the pelvic end of the abdomen a small bit of the Wolffian body, W. b, is displayed.

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A Laboratory Text-Book of Embryology: 1. General Conceptions | 2. Early Development of Mammals | 3. Human Embryo | 4. Pig Embryos | 5. Chick Embryos | 6. Blastodermic Vesicle and Ovum Segmentation | 7. Uterus and the Foetal Appendages in Man | 8. Methods | Figures | Second edition | Category:Charles Minot

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  1. The four reconstructions were made by Dr. F. T. Lewis, to whom the whole credit for them belongs. The two wax models referred to were made by Dr. John I.. Iiremer. The author hopes that both of these investigators will publish their results in full elsewhere.