1897 Human Embryology 21

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Minot CS. Human Embryology. (1897) London: The Macmillan Company.

Human Embryology: Introduction | The Uterus | General Outline of Human Development | The Genital Products | History of the Genoblasts and the Theory of Sex | The Germ-Layers | Segmentation | Primitive Streak | Mesoderm and the Coelom | Germ-Layers General Remarks | The Embryo | The Medullary Groove, Notochord and Neurenteric Canals | Coelom Divisions; Mesenchyma Origin | Blood, Blood-Vessels and Heart Origin | Urogenital System Origin | The Archenteron and the Gill Clefts | Germinal Area, the Embryo and its Appendages | The Foetal Appendages | Chorion | Amnion and Proamnion | The Yolk Sack, Allantois and Umbilical Cord | Placenta | The Foetus | Growth and External Development Embryo and Foetus | Mesenchymal Tissues | Skeleton and Limbs | Muscular System | Splanchnocoele and Diaphragm | Urogenital System | Transformations of the Heart and Blood-Vessels | The Epidermal System | Mouth Cavity and Face | The Nervous System | Sense Organs | Entodermal Canal | Figures | References | Embryology History
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Chapter XXI. The Mesothelial Muscles

The muscle fibres fall into two main classes, the smooth or mesenchymal fibres, which have been already considered, p. 4l7, and striated or mesothelial muscles. The latter fall into three groups; 1, the skeletal muscles ; 2, the branchial muscles ; 3, the cardiac muscles. The first are developed from the epithelial muscle plates, the origin of which from the mesothelial primitive segments has been already described, p. 205 ; the second are developed from the mesothelium of the branchial coelomatic cavities (head cavities of Balfour) see p. 478 ; the latter are developed from the mesothelial wall of the heart of the embryo and constitute the so-called " muscular heart " {Muskelherz), see p. 227.


The Segmental or Skeletal Muscle Fibre. — Remak, 60.1, was the first, if I am not mistaken, to show that the primitive segment, or as he termed it the "protovertebra," forms lx)th the anlage of the axial mesenchyma and of the muscles ; he also thought that the '* proto vertebra" formed the spinal ganglion, an error which was corrected by His, 68.1. To the myotome, or the two layers of the mesothelium remaining after the differentiation of the periaxial mesenchyma (Van Wijhe's sclerotome), Remak applied the term ' Riickenplatte, After Remak (1850) followed a series of investigations and discussions as to the histogenesis of the striated muscle fibre. The chief differences of opinion were as to whether, as originally maintained by Remak, each fibre is developed from a single cell, or, as suggested by Theodore Schwann, out of the fusion of several cells. The latter view was advocated by Margo, 59.1, in 1859; Margo studied the muscle corpuscles, terming them sarcoplasts, and regarding them as so many separate cells which had united to form the muscle fibre. That Remak was right was maintained by Kolliker in 1857, 57. 1, on the basis of his own observations, and also by Max Schultze in a masterly essay, 61.1, which at the same time laid the foundation of the modem doctrine of cells, and anticipated Heitzmann's observations on the union of the cells by over twenty years. In the same year, 18G1, appeared Deiters' paper, 61.1, and the year after, F. E. Schulze's, 62.1, who together with Max Schultze conclusively established Remak's opinion as correct. Nevertheless we find the Schwann-Margo hypothesis reap|)earing from time to time, although it has never had any sound observational basis to rest upon. A synopsis of various pa|)ers upon the development of striated muscle fibres is given by Calberla, 76.1, and more fully by G. Bom in his dissertation, 73.1. That the striated muscles have an epithelial origin was first emphasized by th(» two Hertwigs, 81.1, G1-G6, who demonstrated at the same time that only the inner layer of the myotome forms muscle, not both plates as had been wrongly stated by Balfour, " Comp. Embryology," II., to be the case in elasmobranchs. Since then it has been ascertained beyond question that the outer layer gives rise to the dermis (compare p. 206), Kaestner's contrary conclusions, 90.1, being attributable, in my judgment, to his imperfect observation.


The single muscle fibre arises from a single epithelial (mesothelial) cell of the muscle plate or inner wall of the myotome. In the amphibia each cell elongates in a direction paraUel with the axis of the body until, as shown by F. E. Schultze, 62.1, it stretches the entire length of the segment ; it seems to me that each cell extends the entire length (cephalo-caudal) of the segment in sharks and chick embryos also, but I have not studied the point sufficiently. Paterson, 87.1, asserts that in chicks the cells lengthen but remain shorter than the segment. In amphibia the cells are crowded with yolk granules, which, however, are gradually resorbed ; thus in the frog they at first hide the nuclei, but by the fourth day are sufficiently reduced to allow the nuclei to be seen easily in the fresh unstained specimen (Calberla, 76.1); in amniota, on the other hand, there are exceedingly few yolk grains left in the muscle \^c plate. The first evidence of striation appears in the frog toward the end of the fifth day, on 07ie side of the cell, Fig. 225, as first recorded by F. E. Schultze, 62.1, and since frequently confirmed (e.g., by Calberla and Ranvier, **Traite technique d'Histologie," 510). The side upon which the striation first appears has been observed in elasmobranchs by C. Rabl, 89.2, 239, to be the side toward the notochord, or farthest from the cavity of the myotome. In Petromyzon, A. Goette, 90.1, 50, the fibrillso are found to form a peripheral layer so very early that it is doubtful whether they first appear on one side of the cell only or not. The striation continues to develop until it passes completely around the cell, forming a peripheral layer (Deiters, 61.1, Kollikor, "Ge- Fig. 255. —isolated webelehre," Gte Aufl., p. 401) as illustrated in Fig. g^^^-o^^X ""shoSiSg 25G. At about this time, perhaps sooner in some yoik grains cpartiy reforms and later in others, the nucleus divides, and ?dth the*mu«:ui^^trial by repetitions of the divisional process the cell ^^^^^l^ appearing on soon becomes multinucleate. C. Rabl, 89.2, 242, directs attention to the fact that the nuclei of the muscle-plate in sharks stain more lightly than the mesenchjrmal nuclei and contain an elongated chromatine granule ; in the chick I have observed the same nuclear peculiarity. Later the nuclei lose this main granule and have instead a number of smaller ones. Fig. 25G, mn. The muscle fibre ac(iuires its membranous sheath, sarcolemma, some time later. As to the exact time I have found no positive data; but authorities are agreed that the fibre remains naked for a considerable period. During the early stages of their differentiation, the muscle fibrea retain the epithelial arrangement, that is, remain closely packed; not long after the appearance of the fibrillae and striation, the fibres begin to separate and connective tissue grows in between them. During their separation the fibres become massed in bundles instead of in epitheliaHayers, The central portion of the young muscle fibre is granular, and contains not only the nuclei and the remains of the yolk material, but also a considerable quantity of glycogen (Kanvier, "Traite techniqued'Histologie," 515). Astbis substance is very readily dissolved out, it is probable that the clear empty appearance of the central portion of the fibres, which is so striking in sections of hardened embryos, see Fig. 2.ifJ, A, is due to the loss of the glycogen. The mantle of striated muscle-substance gradually incresises until the whole fibre is fibrillatod and the muscle no longer appears lioUuw. The time at which the muscle fibres become " solid " varies from embryo to embryo bnd from muscle to muscle. In the human embryo at the end of the fifth or beginning of the sixth month most of the fibres of the upper extremity are solid, hut it is not until the seventh month that those of the lower extremity become solid, \V. Felix, 89.1, 23'2. The nuclei have at first an axial position, but toward the end of the third month some of them lie iu the mantle, compare also p. -17-1.


Fro. SSK.-A. TninBveni?; B. r.oii(rltii<lliuil SecClon of MuwlR _

BuinBD Eiubryu o' alitf-three U> olitj'-i'lElK Iiays. UlDOt Collrvtioa. N». 1^. m.ni.ni. jiuhcii? fibres; mil. uiuwle flbrea BhoKiQg tha ceDtrul nui'liil; tn», mntvacbymol uuck'l. Ax about 7fO


The size of the fibres is smaller in the embryo than in the adult, bat Felix, 89.1, 2;(:t, points out that up to latter part of the third month in man the fibres increase in size, many having at two and one-half months a diameter of from 13-19At, but Latter they are smaller owing probably to the division of the fibres, and it is not until birth that the same diameter is again reached by the single fibres.


Fibrillce. — Before discussing the origin of the fibrillse it is necessary to remove an unfortunate confusion which has prevailed in the use of the term. By fibrilhe is sometimes meant the longitudinal threads of protoplasm, but more often is meant the material between adjacent longitudinal threads. Corresponding to the two usages of the word "fibrilla," there are two essentially different conceptions of the structure of the adult muscle fibre. According to the older view, which is currently repeated in text-books of anatomy and histology, the " primitive fibrillse " into which a muscle fibre may be mechaiiically divided under certain conditions are the contractile portions of the fibre ; the ends of the " primitive fibrillse" are the socalled Cohnheim's areas, and the " sarcous elements" are the divisions of the " primitive fibrilla?. " Henle (** AUgemeine Anatomic," 1841, p. 580) recognized that there was substance left between the fibrillse; later Leydig (Muller's Arch., 1850, 15G) and Kolliker (Ze/f . Wiss. ZooL, VIII., 316) pointed out its general occurrence. Max Schultze, 61.1, 3, shows that this material was the derivative of the protoplasm of the muscle cells. L. Gerlach * was the first, so far as I know, to demonstrate that this interfibrillar material formed a reticulum, but he regarded it as the prolongation of the nerve. The reticulate structure appears to have been recognized also by G. Thin {Quart. Jonrn. Microsv. Sci.y 1876, XVI. 251). No special significance seems to have been attributed to all these observations until 1881, when Retzius, 81.2, proposed the new conception of the structure of muscle fibres, according to which the material between the so-called primitive fibrilke" is the essential part of the fibre; this material is part of the protoplasmatic network of the cell which makes the muscle. According to Retzius, the essential feature of the muscle fibre is the peculiar and characteristic arrangement of this network, by which the striation is caused. The fibrillse of embryologists are threads of protoplasm and are not the same as the " primitive fibrillse " of histologists, but are characterized by staining readily. That the fibrillation was developed by a metamorphosis of the protoplasm of the young muscle cell has long been the conception of embryologists, see for example Max Schultze's article, 61.1, published in 1861. L. Bremer, 83. 1, was the first to place this conception upon a firm basis of observation, by tracing out further than had been done before the transformation of the protoplasmatic network of the developing fibre. Retzius' results w^ere extended and made the basis of a theory of the structure and contraction of the muscle fibre by B. Melland,86.1, and C. F. Marshall, 87.1, 90.1, both working in A. Milnes Marshall's laboratory at the Owens College; compare also Biitsclili und Schewiakoff, 91.1. This series of investigations render it necessary to accept Retzius' view — although KoUikerin the sixth edition of his " Gewebelehre " throws the weight of his great authoritA^ against it. As it now stands Retzius' view may be summarized thus: fibrillsB and sarcous elements are postmortem effects due to the cleavage of the matrix ; the muscle fibre really consists of a homogeneous matrix which is traversed by a ver3* regular reticulum, made of longitudinal threads connected at regular inter\'als by transverse threads, corresponding in position to Krause's membrane; at the nodes, where the cross and long threads unite, there are little thickenings. The thickenings correspond to the balls of Schafer's dumb-bells, the handles of which are the long threads, compare Schafer, Philosophical Transactions, 18T3. Between every two Krause's membranes are numerous fine cross threads, which cause the appearance of the dark bands and therefore of the transverse striation.


  • Tlach, "Das V<'irlialtiiiss dor NVrvfu zu dou Muskelu der Wirbelthifre," Leipzig, 1874. See also Arch. f. Microsc. Aiiat., xiii.. 1877, 3U7.



the transfornuition of the reticulum of the multinucleate cell of the my<^t(>ine into the network of the adult muscle fibre has been most carefully described by L. Bremer, 83.1, whose results may be sumniarize<l as follows : The nucleus of the muscle fibre, together with the protoplasm surrounding it, constitutes the so-called muscle corpuscle; the corpuscle is much more prominent in young than in old muscle, for its protoplasm is gradually differentiate<l into muscular substance ; a small number of corpuscles enter into the fonnation of each fibre ; the substance of the muscle forms a network, which was first partially recognized by Heitzmann (Wien. Sitzungsber., XVII., Al)th. :5, 1873); the meshes of this network appear polygonal in transverse — rectangular in longitudinal, sections; the network is a modification of the protoplasmatic network of the cor• puscles, and is so arranged that there are alternating rows, both transverse and longitudinal, of fine knots and large knots (corresponding to the fine and brojul striae) ; the fine knots are connected by fin(» thn»ads, and the large knots by coarse threads ; hence there is a fine and a coarse net.


Multiplication of Muscle Fibres. — That the muscle fibres multiply (luring embr^'onic life can hanlly be questioned at present. Two nn^tlioils of accounting for the multiplication have been advocated, the first that it is effecrted by the intervention of sarcoplasts (Margo), the second that it is by a direct longitudinal fusion of the fibre (Weismann, 61.1). I consider the latter view the correct one.


1. Manjo's Theory. — Bremer's results, 83.1, on this question are as follows: The |X)st-embryonic multiplication of fibres takes place by means of the structures described by Margo (69.1, *^2*.)) under the name of ,s(ircoplasfen : these are lines or chains of muscle corpuscles, united by the protoplasm net, and derived by proliferation from the (*orpusc]es of the original fibres; the sarcoplast gradually separat(»s from the parent fibre, undergoing muscular differentiation meanwhilis and also Incoming connecteil with the nerve. The growth of the fibre is initiated by a multiplication of the corpuscles; the sarcolemma is not present at first, but apjwars later, being pn)bably formtnl by the fuse<l cell membranes of the corpuscles, to which ap|K»ai*s to be ad<led a coat of conne<'tiv(» tissu(», and also around the motor plate lK»tween the two sarcolemmic coats aj)pears an extension of Henle's sheath of the nerve. Paneth has n^cently renewtnl, 85. 1, Margo's observations, 59.1, giving a can^ful description of the sarcoplasts and maintaining that they are the ag(»nts of fibre multiplication. Sigmund Mayer, 86.1, attacked Paneth, because he found muscle corpuscles abundant in the fibres of the tail in tadpoles during the process of resorption, and hence concluded thatt he corpuscles are muscle destroyers (sarcolytes). This opinion has been accepted by Barfurth, 87.1, but the mere presence of the corpuscles, while the muscle fibres are becoming destroyed, is, as Paneth justly replied, 87.1, no evidence whatever that they have a sarcolytic function. There remains, however, another hypothesis which has been advanced by Felix, 89.1, 253, namely, that the so-called sarcoplasts represent muscle fibres partly degenerated. Felix's interpretation is the one which has most commended itself to me.


2. WeismaniVs Theory, — Felix's conclusions are, that from the middle of the third month until the end of foetal life there are, in the muscles, fibres with multiplied nuclei, which are arranged in rows. These fibres with multiple nuclei are of two kinds, those with a single and those with several rows of nuclei. In the first kind the nuclei are central, color deeply, lie transversely, and differ but little from one another ; fibres of this kind do not divide though they may grow ; some of them degenerate and form Marge's sarcoplasts. The second kind of fibres have several rows of nuclei in the mantle or fibrillar layer ; in the middle part of the rows the nuclei are closely crowded and compressed into all possible forms; this crowding probably marks the centre of proliferation. The fibre divides into daughter fibres, one for each row of nuclei. The fibre lxK?omes enveloped in a sheath, rich in nuclei and vessels, and this sheath persists while the fibre is dividing; afterwards it disappears. The daughter fibres may also divide, but apparently usually into two onlv.


The areas, in which the nuclei are crowded together, have long been known, though imperfectly described. They are usually termed Muskelknospen or Muskelspindel by German writers, and they mark the point where the union with the nerve is established. They were known to Weismann in isril, 61.1, and were sliorth' after described by KoUiker, 62.1, in amphibia. Von Franque, 90.1, records some observ-ations ujKm them and gives references to the scattered observations upon them made by a number of writers.


The Muscle Plates. — The development of the muscle plates has already been described. There is unfortunately little to be added at present concerning their later history. When the outer leaf of the myotome is changed into the mesenchyma of the dermis, the cells nearest the muscle plate on all sides retain for a considerable period their epithelial arrangement, and appear to act as a growing layer, and presumably contribute Ix^th to the mesenchyma on one side (compare Fig. 257) and the muscle plate on the other. Certainly the muscle plate continues to gi'ow in all directions, but most rapidly dorsalward over the medullary canal and ventralward into the somatopleure. At the siime time the muscle jjlates not only lengthen out as the whole tnmk lengthens, but eaf'li one gn)ws forward under the one in front, and thus is pr<)<luced the stage so characteristic of fishes, with the muscular segments oblique and the hind border of each overlapping the segment next behind. That the imbrication is produced as stated seems to me clear from the study of shark embryos, in which the original position of the segment is indicated by the ner\'es, ganglia, and inter-segmental arteries; the hind edge of the muscle plate coincides with posterior limit of its Regment thus determined, while the anterior edge ia clearly within the territory of tho next segment in front.


In the region of the limbs the muscle plates send in elii»mohranohs buds into the limbs to ]}roduce their muscles, as discovered by Balfour, "Comp. Embrjologj-," II., 073. According to Dohrn, 84.1, 103, this budding tiikes place after all the gill-defta have become open, and the cartilage is just begining to appear in the branchial arches; each myotome produces an anterior and jiosterior bud, which both point outward and downward ; the buds have at first a spherical form, but soon sepurato from the parent muscle plate, and elongate, and later divide each into two, a dorsal and ventral secondary l)ud, so that from each myotome there are produced four buds. The main muscle plate continues its growth into the abdominal somatoplenre. The number of myotomes wliich contribute to the limbs is uncertain, but there are several. It is probable that in all amniota the myotomes also send buds to form the muscles of the limbs. Van Bemmelen, 89.1, 242, has shown that in snake embiyos with the fifth gill-clett just formed, the myi)tomc of the swoiid to tenth post-occipital segments send downgro^vths into the limbs, and continue on in the somatopleure ventralward. Of the eight segments the first three have their outgrowths oblique to enter the limbs. Paterson, 87. 1, has expressly denied this origin for the chick, but as he was able to distinguish only a confused mass of mesoderm in the yoimg limbs, his opinion cannot carry weight, but must be considered based upon imperfect observation.


Abortion. — A certain number of muscle plates disappear during early embryonic life. Thus Froriep has shown, 86.1, that in the m cow embryo there are four rudimentary muscle plates in the region of the occiput or hypoglossus, which, however, all disappear very early. It is probable that there were once other muscleplates in the head which have now disappeared, compaie p. 200. Further, it is probable that in man there are rudimentary muscle plates in the embryonic tail, which has been shown by Fol to contain at least nine rudimentary segments, some of which may advance into the muscleplate stage.


Myotomic Muscles. — There is no part of embryology" so obscure at present as the development of the muscular system. Scarcely the most elementary (luestions have been answered. Not only has the development of the single muscles from the mesothelial plates scarcely been studied, but also the very significance and the arrangement of these plates in the head is wrapped in uncertainty, see p. 200.


The following points in regard to the cephalic myotomes have been ascertained. Of Van Wijhe's nine myotonies, seen in elasmobranchs, the first comes to lie against the optic vesicle and gives rise to the rectus superior, rectus inferior, and obliquus inferior of the eye; the second produces the obliquus superior, and the third the rectus extemus; a good figure of the three myotomes which form the eye muscles, as observed in an elasniobranch embryo, is given by A. Froriep, Anat, Anzeiger^ N. 50, see ako Miss Piatt's figures 91.2; the fourth, fifth, and sixth disappear; the eighth, ninth, and tenth produce muscles running from the skull to the shoulder girdle. Froriep,86. 1, has shown that there are four myotomes in the occipital hyix)glossal region of mammals, which early become rudimentary, but Van Bemmelen has observed, 89.1, 241, that these four myotomes together with that of the first cervical (atlas) segment grow obliquely ventralward, so as to meet and unite into a single cord which descends behind the last (in reptiles the fifth) gill-cleft, accompimied by the hypoglossal nerves, and then curving forward grows into the tongue and there produces the lingual musculature. This explanation of the origin of the muscles of the tongue is probably correct, but it differs from that offered by Froriep, 86. 1, and still more from that of His ('*Anat. menschl. Embryonen," III., U2). According to Van Wijhe, 82. 1, the coracohyoid muscle of sharks arises, like the mammalian lingual muscles, from the downgrowth of the posterior cephalic and anterior cervical myotomes.

As regards the development of the muscles of the rump and limbs we possess, so far as I am aware, practically no information beyond the little wliicli has been noticed in connection with the history of the muscle plates, p. 200.

Muscles of the Branchial Arches. — That these muscles all arise from the mesothelium of the arches is now generally believed although by no means rigidly demonBtrated, except for elasmobranchs; Van Wijhe, 82.1, states that the corocobranchialia and coracoloantlibularis muscles of sbarks are developed from the pericardial mesothelium. Anton Dohm, 84.1, lO'J-114, finds in selachians that the mesothelial tube lengthens with thewhole arch and by expanding in the transverse plane becomes a plate. Fig. 'ibti, math, which stretches across the arch between the nerve in front and theanlage of the cartilage behind the coelomatie cavity is obliterated except on the outer side of the arch the plate then di^ ides ciuse to the nerve. The , further history ie complicated and need not be presented here, as nothing defanite is known as to the lio/ mi logics of the branchial muscles of

A sharks with those of aniniota.

- ti His ( Anat. menschlicher Erabri' _JiZi - ofi^^ Heft III.. !)a)hHs endeavored indicate to which arches certain V L muscles belong, but has not worked

FioB8.-TramTerwhMti.nof BBnuicji out the Hctual development. Ho as t»l Wamenl' ^j^ n'^iMbPlli" "4 urSrr Blgns tllO palatoglossus, StjlogloSSUS

and levator palati mollis to the second arch (counting the manilibuliir as the first) ; the stylopharyngeus and perhaps both the palato-pharyngeus and hypoglossus to the third arch. Of the constrictors of the pharynx the upper probably belongs to the third arch, but the middle and lower to the fourth jirch.

C. Rabl, 87. 1, maintains that the myothelium of the hyoid arch forms the embryonic platysma, which spreads out in front of and behind the external ear (hyoid cleft) and breaks up into the individual superficial muscles of the face and epicraniuin. The !stai)ediuB muscle also belongs to the hyoid, according to Rabl.


Mandibular Muscles. — Their development in the chick has been studied by Kaczandor, 86.1. The muscles form at first a continuous mass, which grows by multiplication of the fibres. The mass is divided into separate muscles by the ingrowth (»f fibrillar connectivetissue partitions, and by the development of the osseous miUKlible, which sepanitcs the muscles attached to the connectiyc tissue from those having an insertion on the Meckel's cartilage. The change in the direction of the course of the fibres results from the muscles adapting themselves to changes in the form of the jaw. The insertion into tile mesenchymal aniage of the mandible remains unaltered when tin anla^e ossifies. there is n<) migration of th>' insertions.

Dolirn, 84.1, li:i, states that in sharks the developmental historjof the mandibular muscles is rpiite different from that of the muscles of the suciroetliug arches.


Muscles of the Heart, — The exact history of the genesis of the cartliac muscle fibre has still to lie worke<l out. In the rabbit (KfiUiker, "GnindrisH." 2te Aufl., .3H;J) the musculature of the heart appears the ninth day, and by the tenth or eleventh day is developed over the entire organ, including the bulbus aorta-. The muscles soon arrange themselves into n spongj- stnicture, each wob of the spongework being covered by eiidothelium, Fig, 2'JO, but during the third and fourth month the musculature gradually becomes more compact, so that at the beginning of the fifth montii the spongy structure is confined to the innermost layers of the muscular wall. The striations appear, according to A.C. Bemays, 76.1. 487, upon one side of the branching, protoplasmatic muscle cells (embryo calf of 12-16 mm.) and later around the periphery of the cells somewhat as in the myotomic muscle cell.



Historic Disclaimer - information about historic embryology pages 
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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)
Human Embryology: Introduction | The Uterus | General Outline of Human Development | The Genital Products | History of the Genoblasts and the Theory of Sex | The Germ-Layers | Segmentation | Primitive Streak | Mesoderm and the Coelom | Germ-Layers General Remarks | The Embryo | The Medullary Groove, Notochord and Neurenteric Canals | Coelom Divisions; Mesenchyma Origin | Blood, Blood-Vessels and Heart Origin | Urogenital System Origin | The Archenteron and the Gill Clefts | Germinal Area, the Embryo and its Appendages | The Foetal Appendages | Chorion | Amnion and Proamnion | The Yolk Sack, Allantois and Umbilical Cord | Placenta | The Foetus | Growth and External Development Embryo and Foetus | Mesenchymal Tissues | Skeleton and Limbs | Muscular System | Splanchnocoele and Diaphragm | Urogenital System | Transformations of the Heart and Blood-Vessels | The Epidermal System | Mouth Cavity and Face | The Nervous System | Sense Organs | Entodermal Canal | Figures | References | Embryology History



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