McMurrich1914 Chapter 3
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McMurrich JP. The Development Of The Human Body. (1914) P. Blakiston's Son & Co., Philadelphia, Pennsylvania.
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Chapter III. The Medullary Groove, Notochord, and Mesoderms Somites
In the preceding chapter the development of the mammalian ovum has been described up to and including the formation of the three germinal layers. The earlier stages of development there described are practically unknown in the human ovum, but for the stages subsequent to the establishment of the germinal layers human material is available, and it will, therefore, now be convenient to consider the structure of the younger human ova at present known and to trace in them the appearance and development of such structures as the primitive streak, the head process and the gastral mesoderm.
The youngest human ovum at present known is that described by Bryce and Teacher, but, unfortunately, it presents certain features that are evidently abnormal, so that it becomes doubtful how far it may be accepted as representing the typical condition. The trophoblast, which was very thick and clearly differentiated into two layers, enclosed a space whose diameter was about 0.63 mm. and which was largely occupied by a loose syncytial tissue, presumably mesoderm. Toward the center of this was an irregular cavity in which were two vesicles, quite separate from one another and probably together representing the embryo, the smaller one being the amniotic cavity and the larger one the yolk-sac (Fig. 36). The separation of these two structures is apparently an abnormality and it is possible that the cavity in which they lie is, as Bryce and Teacher suggest, an artefact produced by contraction of the syncytial mesoderm during the preservation of the ovum.
If comparison of this ovum with those of other mammals is warranted, it may be likened to that of the bat as shown in Fig. 29, C, with the difference that the mesoderm that lines the trophoblast in that ovum has become much more voluminous and forms the syncytial mass in which the ovum is supposed to have been imbedded, a condition that may be "represented diagrammatically as in Fig. 38, A.
Somewhat older are the ova described by Peters, Fetzer, Jung and Herzog. The Peters ovum was taken from the uterus of a woman who had committed suicide one calendar month after the last menstruation, and it measured about 1 mm. in diameter. The entire inner surface of the trophoblast (Fig. 37, ce) was lined by a layer of mesoderm (cm), which, on the surface furthest away from the uterine cavity, was considerably thicker than elsewhere, forming an area of attachment of the embryo to the wall of the ovum. In the substance of this thickening was the amniotic cavity (am), whose roof was formed by flattened cells, which, at the sides, became continuous with a layer of columnar cells forming the floor of the cavity and constituting the embryonic ectoderm (ec). Immediately below this was a layer of mesoderm (m) which split at the edge of the embryonic disk into two layers, one of which became continuous with the mesodermic thickening and so with the layer of mesoderm lining the interior of the trophoblast, while the other enclosed a sac lined by a layer of endodermal cells and forming the yolk-sac (ys). The total length of the embryo was 0.19 mm., and so far as its ectoderm and mesoderm are concerned it might be described as a flat disk resting on the surface of the yolk-sac, though it must be understood that the yolk-sac also to a certain extent forms part of the embryo.
Fig. 36. From a Reconstruction of the Bryce-Teacher Ovum. - (Bryce-Teacher .)
Fig. 37. - Section of Embryo and Adjacent Portion of an Ovum of i mm.
am, Amniotic cavity; ce, chorionic ectoderm; cm, chorionic mesoderm; ec, embryonic ectoderm; en, endoderm; m, embryonic mesoderm; ys, yolk-sack. - (Peters.)
This embryo seems to be in an early stage of the primitive streak formation, before the development of the head process. On comparing it with the stage of development represented in Fig. 38, A, it will be seen to present some important advances. The cavity (Fig. 38, B, C) into which the yolk-sac projects is unrepresented in Fig. 38, A. How this cavity is formed can only be conjectured, but it seems probable that it arises by the splitting of the layer of cells which lines the interior of the trophoblast in the earlier stage (or perhaps by the vacuolization of the central cells of this layer) and the subsequent accumulation of fluid between the two mesodermal layers so formed. However that may be, it seems clear that the size of the human ovum is due mainly to the rapid growth of this cavity, which, as future stages show, is the extra-embryonic portion of the body-cavity, the splitting or vacuolization of the mesoderm by which it is probably formed being the precocious appearance of the typical splitting of the mesoderm to form the embryonic body-cavity which, as will be seen in a subsequent chapter, takes place only at a later stage of development. From now on the trophoblast and the layer of mesoderm lining it may together be spoken of as the chorion, the mesoderm layer being termed the chorionic mesoderm.
Fig. 38. - Diagrams to show the Probable Relationships of the Parts in the Embryos Represented in Figs. 29, C, and 37. Ac, Amniotic cavity; C, extra-embryonic body-cavity; Me, (in figure to the left) mesoderm, (in figure to the right) somatic mesoderm; Me, splanchnic mesoderm; D, digestive tract; En, endoderm; T, trophoblast. The broken line in the mesoderm of the figure to the left indicates the line along which the splitting of the mesoderm occurs.
A little older again than the Peters and Herzog ova are those described by Strahl and Beneke and by von Spee (Embryo v. H.), the chorionic cavity of the former two having an average diameter of about 2.4 mm., while the corresponding size of the latter two was somewhat less than 4.0 mm. Notwithstanding the considerable increase in the size of these older ova, due to the continued increase in the size of the extra-embryonic ccelom, the embryos are but little advanced beyond the stage shown by the Peters embryo. The thickening of the chorionic mesoderm that encloses the amniotic cavity has increased in size and now forms a pedicle, known as the belly-stalk (Fig. 39, 6), at the extremity of which is the yolk-sac (d). Furthermore, the amniotic cavity (a) now lies somewhat excentrically in this pedicle, being near what may be termed its anterior surface, and the entire embryo projects like a papilla from the inner surface of the chorion into the extra-embryonic ccelom. Fig. 40 is from a model of the Beneke embryo, detached from the chorion by cutting through the belly-stalk, and with the roof of the amniotic cavity removed. The dorsal surface of the embryo, thus exposed, is an oval disk, resting, as it were, on the yolk-sac, and quite smooth except for a slight longitudinal groove upon its posterior portion. This is the primitive groove and sections passing through it show the primitive streak, consisting of a sheet of mesoderm interposed between the ectoderm and endoderm, as in the Peters embryo, and but poorly defined from the other two layers. From its anterior edge a median process extends forward for a short distance and is the head process (see p. 56). In front and to the sides of this there is as yet no mesoderm intervening between the ectoderm and endoderm.
Fig. 39. - The Embryo v. H. of von Spee. The Left Half of theT Chorion has been Removed to show the Embryo. a, Amniotic cavity; b, belly-stalk; ch, chorion; d, yolk-sac; e, extra-embryonic ccelom; k y embryonic disk; 2, chorionic villus. - (von Spee.)
Fig. 40. - Embryo from the Beneke Ovum, the Roof of the Amniotic Cavity having been Removed. From a model, b, Belly-stalk; p.g., primitive groove; y, yolk-sac - (Strahl and Beneke.)
Fig. 41. - Embryo from the Frassi Ovum, the Roof of the Amniotic Cavity having been removed. From a model, b, belly-stalk; p.g., primitive groove; mg, medullary groove; n, neurenteric canal. - (Frassi.)
The embryonic disk of the Beneke embryo measured 0.75 mm. in length. That of an embryo described by Frassi (Fig. 41) was 1. 1 7 mm. in length, and in correspondence with its greater size, it presents some advances in structure that are of interest. As in the younger embryo one sees a distinct primitive groove on the posterior portion of the embryonic disk, but the groove terminates anteriorly at a distinct pore (w) , which perforates the disk and opens ventrally into the yolk-sac. This is the neurenteric canal (see p. 58) and in front of it a groove extends forward in the median line almost to the anterior edge of the embryonic disk and is evidently the first indication of the medullary groove, whose walls are destined to give rise to the central nervous system. Sections passing through the region of the medullary groove show, lying beneath it, the head process (Fig. 42, hp), already fused with the endoderm (compare p. 57), and on each side of the process is a plate of mesoderm (gm), representing the gastral mesoderm of lower forms (see Figs. 28 and 34) , but not as yet showing any indications of splitting into the two layers that bound the embryonic ccelom (see p. 59).
Fig. 42. - Section through the Frassi Embryo just in Front of the Neurenteric Canal. am, Amniotic cavity; gm, gastral mesoderm; hp, head process; mp, medullary plate; ys. yolk-sac. - (Frassi.)
This is just beginning to appear in an embryo, also described by von Spee and known as embryo Gle. It measured 1.54 mm. in length and is closely similar, in general appearance, to an embryo described by Eternod and measuring 1.34 mm. in length (Fig. 43). It differs from the Frassi embryo most markedly in that the posterior portion of the embryonic disk, that is to say the primitive streak region, is bent ventrally so. as to lie almost at a right angle with the anterior portion. As a result the belly-stalk arises from the ventral surface of the embryo instead of from its posterior extremity, near which the opening of the neurenteric canal (Fig. 43, nc) is now situated, almost the whole length of the surface seen in dorsal view being occupied by the medullary groove (m), which, in the embryo Gle, is bounded laterally by distinct ridges, the medullary folds.
Fig. 43. - Embryo 1.34 mm. Long. al Allantois; am, amnion; bs, belly-stalk; h, heart; m, medullary groove; tic neurentenc canal; pc, caudal protuberance; ps, primitive streak; ys, yolk-stalk. - (Eternod.)
In the Kromer embryo Klb (Fig. 44), measuring i.8 mm. in length, a new feature has made its appearance. The medullary folds have become quite high, and lateral to them there is on each side a series of five or six oblong elevations, which represent what are termed mesodermic somites and are due to divisions of the underlying mesoderm.
Fig. 44. - Model of the Kromer Embryo Klb seen from the Dorsal Surface, the Roof of the Amniotic Cavity having been Removed. - (Keibel and Elze.) Instead of proceeding with a description of the external form of still older embryos it will be convenient to consider the further development of certain structures whose appearance has already been noted, namely, the head process, the medullary folds and the mesodermic somites, and first of all â€¢ the medullary folds may be considered.
The Medullary Folds
The two folds are continuous anteriorly, but behind they are at first separate, the anterior portion of the primitive streak lying between them. In forms, such as the Reptilia, which possess a distinct blastopore, this opening lies in the interval between the two, and consequently is in the floor of the medullary groove, and in the mammalia, even though no well-defined blastopore is formed, yet at the time of the formation of the medullary fold an opening breaks through at the anterior end of the primitive streak in the region of Hensen's node, and places the cavity lying below the endoderm in communication with the space bounded by the medullary folds. The canal so formed is termed the neurenteric canal (Figs. 43 and 45, nc) and is so called because it unites what will later become the central canal of the nervous system with the intestine (enteron). The significance of this canal has already been discussed (p. 58) ; it is of very brief persistence, closing at an early stage of development so as to leave no trace of its existence.
Fig. 45. - Diagram of a Longitudinal Section through the Embryo Gle, Measuring 1.54 mm. in Length. al, Allantois; am, amnion; B, belly-stalk; ch, chorion; h, heart; nc, neurenteric canal; V, chorionic villi; Y, yolk-sac. - (vonSpee.)
As development proceeds the medullary folds increase in height and at the same time incline toward one another (Fig. 44), so that their edges finally come into contact and later fuse, the two ectodermal layers forming the one uniting with the corresponding layers of the other (Fig. 46). By this process the medullary groove becomes converted into a medullary canal which later becomes the central canal of the spinal cord and the ventricles of the brain, the ectodermal walls of the canal thickening to give rise to the central nervous system. The closure of the groove does not, however, take place simultaneously along its entire length, but begins in what corresponds to the neck region of the adult and thence proceeds both anteriorly and posteriorly, the extension of the fusion taking place rather slowly, however, especially anteriorly, so that an anterior opening into the otherwise closed canal can be distinguished for a considerable period (Fig. 53).
Fig. 46. - Diagrams showing the Manner of the Closure of the Medullary Groove.
The Noto chord
While these changes have been taking place in the ectoderm of the median line of the embryonic disk, modifications of the subjacent endoderm have also occurred. This endoderm, it will be remembered, was formed by the head process of the primitive streak, and was a plate of cells continuous at the sides with the primary endoderm and extending forward as far as what will eventually be the anterior part of the pharynx. Along the line of its junction with the primary endoderm it gives rise to the plates of gastral mesoderm (Fig. 28), while the remainder of it produces an important embryonic organ known as the notochord or chorda dorsalis and on this account is sometimes termed the chorda endoderm.
After the separation of the plates of gastral mesoderm the chorda endoderm, which is at first a flat band, becomes somewhat curved (Fig. 47, A), so that it is concave on its under surface, and, the curvature increasing, the edges of the plate come into contact and finally fuse together (Fig. 47, B), the edges of the primary endoderm at the same time uniting beneath the chordal tube so formed, so that this layer becomes a continuous sheet, as it was at its first appearance.
Fig. 47. - Transverse Sections through Mole Embryos, showing the Formation of the Notochord. ec, Ectoderm; en, endoderm; m, mesoderm; nc. notochord. - (Heape.)
The lumen which is at first present in the chordal tube is soon obliterated by the enlargement of the cells which bound it, and these cells later undergo a peculiar transformation whereby the chordal tube is converted into a solid elastic rod surrounded by a cuticular sheath secreted by the cells. The notochord lies at first immediately beneath the median line of the medullary groove, between the ectoderm and the endoderm, and has on either side of it the mesodermal plates. It is a temporary structure of which only rudiments persist in the adult condition in man, but it is a structure characteristic of all vertebrate embryos and persists to a more or less perfect extent in many of the fishes, being indeed the only axial skeleton possessed by Amphioxus. In the higher vertebrates it is almost completely replaced by the vertebral column, which develops around it in a manner to be described later.
The Mesodermic Somites
Turning now to the middle germinal layer, it will be found that in it also important changes take place during the early stages of development. The probable mode of development of the extra-embryonic mesoderm and body-cavity has already been described (p. 67) and attention may now be directed toward what occurs in the embryonic mesoderm. In both the Peters embryo and the embryo v.H described by von Spee this portion of the mesoderm is represented by a plate of cells lying between the ectoderm and endoderm and becoming continuous at the edges of the embryonic area with both the layer which surrounds the yolk-sac and, through the mesoderm of the belly-stalk, with the chorionic mesoderm (Fig. 37). It seems probable, since there is in these embryos no indication as yet of the formation of the chorda endoderm, that this plate of mesoderm corresponds to the prostomial mesoderm of lower forms. In older embryos, such as the embryo Gle of Graf Spee and the younger embryo described by Eternod (Fig. 43), the mesoderm no longer forms a continuous sheet extending completely across the embryonic disk, but is divided into two lateral plates, in the interval between which the ectoderm of the floor of the medullary groove and the chorda endoderm are in close contact (Fig. 48). These lateral plates represent the gastral mesoderm, whose origin has already been described (p. 59), and which apparently supplants the original prostomial mesoderm, whose fate in the human embryo is at present unknown. The changes which now occur have not as yet been observed in the human embryo, though they probably resemble those described in other mammalian embryos, and the phenomena which occur in the sheep may serve to illustrate their probable nature.
It has been seen that in the stage represented by the Frassi embryo a plate of mesoderm has formed on either side of the chorda endoderm, and that in a later stage, represented by the Kromer embryo Klb, a differentiation occurs in these plates leading to the formation of mesodermic somites. These make their appearance in what will later be the cervical region of the embryo and their formation proceeds backward as the body of the embryo increases in length. A longitudinal groove appears on the dorsal surface of each lateral plate of mesoderm, marking off the more median thicker portion from the lateral parts (Fig. 48), which from this stage onward may be termed the ventral mesoderm. The median or dorsal portions then become divided transversely into a number of more or less cubical masses which are termed the protoverlebrce or, better, mesodermic somites (Fig. 48, ms). The cells of the somites and of the ventral mesoderm, are at first stellate in form, but later become more spindle-shaped, and those near the center of each somite and those of the ventral mesoderm arrange themselves in regular layers so as to enclose cavities which appear in these regions (Fig. 48). Each original lateral plate of gastral mesoderm thus becomes divided longitudinally into three areas, a more median area composed of mesodermic somites, lateral to this a narrow area underlying the original longitudinal groove which separated the somite area from the ventral mesoderm and which from its position is termed the intermediate cell-mass (Fig. 48, 1) , and, finally, the ventral mesoderm. This last portion is now divided into two layers, the dorsal of which is termed the somatic mesoderm, while the ventral one is known as the splanchnic mesoderm (Fig. 48, so and sp; and Fig. 49) , the cavity which separates these two layers being the embryonic body-cavity or pleuroperitoneal cavity (coslom) , which will eventually give rise to the pleural, pericardial and peritoneal cavities of the adult as well as the cavity of each tunica vaginalis testis.
Fig. 48. - Transverse Section through the Second Mesodermic Somite of a Sheep Embryo 3 mm. Long. am, Amnion; en, endoderm; I, intermediate cell-mass; mg, medullary groove; ms, mesodermic somite; so, somatic and sp, splanchnic layers of the ventral mesoderm. - (Bonnet.)
Fig. 49. - Transverse Section of an Embryo of 2.5 mm. (See Fig. 53) showing on either side of the medullary canal a mesodermic somite, the interMEDIATE Cell-mass, and the Ventral Mesoderm. - (vonLenhossek.) Beginning in the neck region, the formation of the mesodermic somites proceeds posteriorly until finally there are present in the human embryo thirty-eight pairs in the neck and trunk regions of the body, and, in addition, a certain number are developed in what is later the occipital region of the head. Exactly how many of these occipital somites are developed is not known, but in the cow four have been observed, and there are reasons for believing that the same number occurs in the human embryo.
In the lower vertebrates a number of cavities arranged in pairs occur in the more anterior portions of the head and have been homologized with mesodermic somities. Whether this homology be perfectly correct or not, these head-cavities, as they are termed, indicate the existence of a division of the head mesoderm into somites, and although practically nothing is known as to their existence in the human embryo, yet, from the relations in which they stand to the cranial nerves and musculature in the lower forms, there is reason to suppose that they are not entirely unrepresented
Fig. 50. - Transverse Section of an Embryo of 4.25 mm. at the Level of the Arm Rudiment. A, Axial mesoderm of arm; Am, amnion; il, inner lamella of myotome; M, myotome; me, splanchnic mesoderm; ol, outer lamella of myotome; Pn, place of origin of pronephros;^ sclerotome; S 1 , defect in wall of myotome due to separation of the sclerotome; st, stomach ; Vu, umbilical vein. - (Kollmann.)
The mesodermic somites in the earliest human embryos in which they have been observed contain a completely closed cavity, and this is true of the majority of the somites in such a form as the sheep. In the four first-formed somites in this species, however, the somite cavity is at first continuous with the pleuroperitoneal cavity and only later becomes separated from it, and in lower vertebrates this continuity of the somite cavities with the general bodycavity is the rule. The somite cavities are consequently to be regarded as portions of the general pleuroperitoneal cavity which have secondarily been separated off. They are, however, of but short duration and early become filled up by spindle-shaped cells derived from the walls of the somites, which themselves undergo a differentiation into distinct portions. The cells of that portion of the wall of each somite which is opposite the notochord become spindleshaped and grow inward toward the median line to surround the notochord and central nervous system, and give rise eventually to the lateral half of the body of a vertebra and the corresponding portion of a vertebral arch. This portion of the somite is termed a sclerotome (Fig. 50, S), and the remainder forms a muscle plate or myotome (M) which is destined to give rise to a portion of the voluntary musculature of the body. The outer wall of the somite has been generally believed to take part in the formation of the cutis layer of the integument and hence has been termed the cutis plate or dermatome, but it seems probable that it becomes entirely transformed into muscular tissue.
The intermediate cell-mass in the human embryo, as in lower forms, partakes of the transverse divisions which separate the individual mesodermic somites. From one portion of the tissue in most of the somites (Fig. 50, Pri) the provisional kidneys or Wolffian bodies develop, this portion of each mass being termed a nephrotome, while the remaining portion gives rise to a mass of cells showing no tendency to arrange themselves in definite layers and constituting that form of mesoderm which has been termed mesenchyme (see p. 61). These mesenchymatous masses become converted into connective tissues and blood-vessels.
The ventral mesoderm in the neck and trunk regions never becomes divided transversely into segments corresponding to the mesodermic somites, differing in this respect from the other portions of the gastral mesoderm. In the head, however, that portion of the middle layer which corresponds to the ventral mesoderm of the trunk does undergo a division into segments in connection with the development of the branchial arches and clefts (see p. 90). A consideration of these segments, which are known as the branchiomeres, may conveniently be postponed until the chapters dealing with the development of the cranial muscles and nerves, and in what follows here attention will be confined to what occurs in the ventral mesoderm of the neck and trunk.
Its splanchnic layer (Fig. 51, vm), applies itself closely to the endodermal digestive tract, which is constricted off from the dorsal portion of the yolk-sac, and becomes converted into mesenchyme out of which the muscular coats of the digestive tract develop. The cells which line the pleuroperitoneal cavity, however, retain their arrangement in a layer and form a part of the serous lining of the peritoneal and other serous cavities, the remainder of the lining being formed by the corresponding cells of the somatic layer; and in the abdominal region the superficial cells, situated near the line where the splanchnic layer passes into the somatic, and in close proximity to the nephrotome of the intermediate cell-mass, become columnar in shape and are converted into reproductive cells.
The somatic layer, if traced peripherally, becomes continuous at the sides with the layer of mesoderm which lines the outer surface of the amnion (Fig. 50) and posteriorly with the mesoderm of the belly-stalk. That portion of it which lies within the body of the embryo, in addition to giving rise to the serous lining of the parietal layer of the pleuroperitoneum, becomes converted into mesenchyme, which for a considerable length of time is clearly differentiated into two zones, a more compact dorsal one which may be termed the somatic layer proper, and a thinner, more ventral vascular zone which is termed the membrana reuniens (Fig. 51). In the earlier stages the somatic layer proper does not extend ventrally beyond the line which passes through the limb buds and it grows out into these buds to form an axial core for them, in which later the skeleton of the limb forms. The remainder of the mesoderm lining the sides and ventral portions of the body-wall is at first formed from the membrana reuniens, but as development proceeds the somatic layer gradually extends more ventrally and displaces, or, more properly speaking, assimilates into itself, the membrana reuniens until finally the latter has completely disappeared.
It is to be noted that no part of the voluntary musculature of the lateral and ventral walls of the neck and trunk is derived from the somatic layer; it is formed entirely from the myotomes which gradually extend ventrally (Fig. 51) and finally come into contact with their fellows of the opposite side in the mid-ventral line.
Fig. 51. - Diagrams Illustrating the History of the Gastral Mesoderm.
dM, dorsal portion of myotome; gr, genital ridge; I, intestine; M, myotome, mr, membrana reuniens; N, nervous system; SC, sclerotome; Sm, somatic mesoderm; vm, splanchnic mesoderm; vM, ventral portion of myotome; Wd, Wolffian duct.
Whether the voluntary musculature of the limbs is also derived from the myotomes is at present doubtful. It has been very generally believed that the myotomes in their growth ventrally sent prolongations into the limb buds which invested the axial core of mesenchyme and eventually gave rise to the voluntary muscles. The actual existence of the prolongations of the myotomes and their conversion into the limb musculature has, however, not yet been observed and it is quite possible that the limb musculature may be derived from the axial core of somatic mesoderm from which the limb skeleton develops.
The appearance of the mesodermic somites is an important phenomenon in the development of the embryo, since it influences fundamentally the future structure of the organism. If each pair of mesodermic somites be regarded as a structural unit and termed a metamere or segment, then it may be said that the body is composed of a series of metameres, each more or less closely resembling its fellows, and succeeding one another at regular intervals. Each somite differentiates, as has been stated, into a sclerotome and a myotome, and, accordingly, there will primarily be as many vertebra? and muscle segments as there are mesodermic somites, or, in other words, the axial skeleton and the voluntary muscles of the trunk are primarily metameric. Nor is this all. Since each metamere is a distinct unit, it must possess its own supply of nutrition, and hence the primary arrangement of the blood-vessels is also metameric, a branch passing off on either side from the main longitudinal arteries and veins to each metamere. And, further, each pair of muscle segments receives its own nerves, so that the arrangement of the nerves, again, is distinctly metameric.
It is to be noted that this metamerism is essentially resident in the dorsal mesoderm, the segmentation shown by structures derived from other embryonic tissues being secondary and associated with the relations of these structures to the mesodermic somites. The metamerism is most distinct in the neck and trunk regions, and at first only in the dorsal portions of these regions, the ventral portions showing metamerism only after the extension into them of the myotomes. But there is clear evidence that the arrangement extends also into the head, and that a portion of its mesoderm is to be regarded as composed of metameres. It has been seen that in the notochordal region of the head of lower vertebrates mesodermic somites are present, while anteriorly in the prechordal region there are headcavities which resemble closely the mesodermic somites, and are probably directly comparable to the somites of the trunk. There is reason, therefore, for believing that the fundamental arrangement of the dorsal mesoderm in all parts of the body is metameric, but though this arrangement is clearly defined in early embryos, it loses distinctness in later periods of development. But even in the adult the original metamerism is clearly indicated in the arrangement of the nerves and of parts of the axial skeleton, and careful study frequently reveals indications of it in highly modified muscles and blood-vessels.
In the head the development of the branchial arches and clefts produces a series of parts presenting many of the peculiarities of metameres, and, indeed, it has been a very general custom to regard them as expressions of the general metamerism which prevails throughout the body. It is to be noted, however, that they are produced by the segmentation of the ventral mesoderm, a structure which in the neck and trunk regions does not share in the general metamerism, and, furthermore, recent observations on the cranial nerves seem to indicate that these branchiomeres cannot be regarded as portions of the head metameres or even as structures comparable to these. They represent, more probably, a second metamerism superposed upon the more general one, or, indeed, possibly more primitive than it, but whose relations can only be properly understood in connection with a study of the cranial nerves.
In addition to many of the papers cited in the list at the close of Chapter II, the following may be mentioned: C. R. Bardeen: " The Development of the Musculature of the Body Wall in the Pig, etc.," Johns Hopkins Hosp. Rep., ix, 1900.
T. H. Bryce and J. H. Teacher: " Contributions to the Study of the Early Development and Imbedding of the Human Ovum," Glasgow, 1908.
A. C. F. Eternod: "Communication sur un ceuf humain avec embryon excessivement jeune," Arch. Ital. de Biologie, xxn, 1895.
A. C. F. Eternod: "II y a un canal notochordal dans l'embryon humain," Anat. Anzeiger, xvi, 1899.
Fetzer: "Ueber ein durch Operation gewonnenes menschliches Ei das in seiner Entwickelung etwa dem Peterssehen Ei entspricht," Verh. Anat. Gesellschaft, xxiv, 1910.
L. Frassi: "Weitere Ergebnisse des Studiums eines jungen menschlichen Eies in situ," Arch.f. mikr. Anat., lxxi, 1908.
W. Heape: "The Development of the Mole (Talpa Europaea)," Quarterly Journ. Microsc. Science, xxvn, 1887.
M. Herzog: "A Contribution to our Knowledge of the Earliest Known Stages of Placentation and Embryonic Development in Man," Amer. Journ. Anat., ix, 1909.
F. Keibel: "Zur Entwickelungsgeschichte der Chorda bei Saugern (Meerschweinchen und Kaninchen)," Archiv fur Anat. und Physiol., Anat. Abth., 1889.
S. Kaestner: "Ueber die Bildung von animalen Muskelfasern aus dem Urwirbel," Arch, filr Anat. und Phys., Anat. Abth., Suppl., 1890.
J. Kollmann: "Die Rumpfsegmente menschlicher Embryonen von 13 bis 35 Unvirbeln," Archiv filr Anat. und Physiol., Anat. Abth., 1891.
H. Peters: "Ueber die Einbettung des menschlichen Eies und das friiheste bisher bekannte menschliche Placentarstadium," Leipzig und Wien, 1899.
F. Graf von Spee: " Beobachtungen an einer menschlichen Keimscheibe mit offener Medullarrinne und Canalis neurentericus," Arch.f. Anat. u. Phys., Anat. Abth., 1889.
F. Graf von Spee: "Ueber friihe Entwicklungsstufen des menschlichen Eies," Arch.f. Anat. u. Phys., Anat. Abth., 1896.
H. Strahl and R. Beneke: "Ein junger menschlicher Embryo," Wiesbaden, 1910.
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McMurrich JP. The Development Of The Human Body. (1914) P. Blakiston's Son & Co., Philadelphia, Pennsylvania.
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