Human Embryology and Morphology 6

From Embryology

Keith, A. Human Embryology And Morphology (1921) Longmans, Green & Co.:New York.

Human Embryology and Morphology: 1 Early Ovum and Embryo | 2 Connection between Foetus and Uterus | 3 Primitive Streak Notochord and Somites | 4 Age Changes | 5 Spinal Column and Back | 6 Body Segmentation | 7 Spinal Cord | 8 Mid- and Hind-Brains | 9 Fore-Brain | 10 Fore-Brain Cerebral Vesicles | 11 Cranium | 12 Face | 13 Teeth and Mastication | 14 Nasal and Olfactory | 15 Sense OF Sight | 16 Hearing | 17 Pharynx and Neck | 18 Tongue, Thyroid and Pharynx | 19 Organs of Digestion | 20 Circulatory System | 21 Circulatory System (continued) | 22 Respiratory System | 23 Urogenital System | 24 Urogenital System (Continued) | 25 Body Wall and Pelvic Floor | 26 Limb Buds | 27 Limbs | 28 Skin and Appendages | Figures


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Chapter VI. The Segmentation of the Body

At the end of the 3rd week, as we have already seen (p. 18), the paraxial mesoderm, lying at each side of the neural tube, becomes divided from before backwards into somites or primitive segments, their demarcation becoming evident first in the occipito-cervical region of the body. By the end of the 4th week the process has reached the 1st coccygeal segment, there being then 3 occipital and 30 body somites. The occipital somites disappear prematurely but those of the body, although they become specialized and broken up, can still be recognized in the adult. In the preoccipital region of the head, parts are also arranged on a segmental plan, one which is older than the vertebrate segmentation of the trunk and can best be identified by the visceral or gill arch system of the pharynx.

Fig. 64 Some of the structures derived from the 11th Dorsal Segment of the Right Side.

Fig. 64. Some of the structures derived from the 11th Dorsal Segment of the Right Side.

Segmentation of the Body

The human body or trunk consists of 33 or 34 segments.[1]Each segment is fundamentally of the same type, but the resemblance is obscured owing to extensive modifications which the somites undergo to form the cervical, dorsal (thoracic), lumbar (abdominal), sacral (pelvic) and caudal regions of the body. The outgrowth of the limbs also renders it difficult to recognize in the adult the simple system of segments which can be seen in the embryo at the end of the third week (Fig. 19).


Until lately the segmentation of the human body was a matter of only speculative importance, but recent advances in our knowledge of the distribution of nerves have shown that it has a direct bearing on diagnosis and treatment.


Constitution of a Typical Segment (11th Dorsal)

It is better to study the development of a typical body segment, and from that the student will be able to note for himself the modifications which have taken place in the more highly differentiated segments of the body. As already explained, the process of segmentation affects chiefly the paraxial block of mesoderm which lies on each side of the neural canal and notochord, and also, to a lesser degree, the intermediate cell mass. In Figs. 65, A, B, a body-segment is represented in the adult and in the embryonic condition. The following elements make up the 11th dorsal segment : (1) Its skeletal basis ; (2) Muscular element ; (3) Renal element ; (4) Vessels ; (5) Nerves ; (6) Neural segment ; (7) Cutis plate. Although the ectoderm and entoderm are never segmented, yet a definite area of each is associated with every body segment. The origin of each element will be taken separately.

Fig. 65 Transverse Section showing the Elements of the 1st Lumbar Segment in the Adult.

Fig. 65, A. A Transverse Section showing the Elements of the 1st Lumbar Segment in the Adult. B. A corresponding Section of an Embryo about the end of the 4th week (diagrammatic).

I. The skeleton

The skeleton of the 11th dorsal segment is represented by the adjacent halves of the 11-12 dorsal vertebrae and the disc between them, for, as already pointed out, the vertebrae are intersegmental in their development (Fig. 55, B). The transverse processes, the spinous processes and 11th and 12th ribs are also formed in the septa in front of and behind the 11th segment (Fig. 64). The septum in the rectus muscle a little below the umbilicus represents the intersegmental septum corresponding to the 11th rib. Sometimes another septum occurs in the rectus, midway between the pubes and umbilicus, marking the lower limit of the 11th segment. The linea alba separates the segments of the two sides.


In the linea alba or ventral median line of the thoracic region, the sternum is developed. The intersegmental septa are well marked in the thoracic region ; the ribs and their cartilages are developed in them. In the neck the septa are almost lost ; the intermediate tendon of the omo-hyoid and the septa occasionally found in the sterno-hyoid and -thyroid, complexus and trachelomastoid muscles are the only representatives of them in the cervical region.

II. The Muscles of the 11th Dorsal Segment

All the muscles of this segment are developed from the muscle plate (myotome) of the primitive segment (see Figs. 64 and 65). There is a cavity, which probably arises as a diverticulum of the coelom, in each primitive segment (Fig. 39, p. 39). The cells of the mesoderm on the inner side of the segmental cavity become columnar and form the muscle plate (Figs. 51, 64). Each segment has its own muscle plate. The cells or myoblasts of each plate increase rapidly in number, forming a fused mass or syncytium ;[2] they spread into the somatopleure, and form the muscles of the body wall and limbs. In the myosyncytium fibrillae and fibres are formed ; each fibre becomes elongated and directed across its segment from septum to septum. The intercostal muscles retain this arrangement, but in the abdominal region the fibres fuse with those of neighbouring segments to form muscular sheets — -the external oblique, internal oblique, transversalis and rectus. In the foetus of the fifth month traces of these septa may be seen ; Bardeen found that the intercostal nerves retained their segmental distribution in the muscles of the belly wall. In fishes the embryonic segmental arrangement of the musculature persists. The manner in which the final groups of muscles are derived from the muscle plates is not accurately known, but in the typical segment with which we are at present dealing it will be seen that the musculature falls into two groups (see Fig. 65, A) : (1) epaxial, the erector spinae, etc. ; and (2) ventro-lateral or body-wall muscles (intercostals, rectus, oblique muscles, etc.). The musculature of the limbs is derived from the ventro-lateral group (Figs. 446, p. 423 ; 455, p. 432).


The ventro-lateral sheet separates into a ventral longitudinal band and a lateral transverse-oblique stratum. Each of these divides into an inner and outer primary layer ; the outer and inner secondary layers arise as delaminations of the primary layers, thus making four in all. The internal oblique and transversalis, and internal intercostal are derived from the internal primary layer ; the external oblique and external intercostal from the external primary layer.[3] The rectus abdominis represents the deeper of the two layers derived from the external primary. Parts of the deepest layer of the lateral sheet, represented in the adult by the transversalis, have migrated inwards to form the subvertebral or hypaxial muscles — the quadratus lumborum, crura of the diaphragm, longus colli, rectus capitis anticus major and minor, and the levator ani. When muscles migrate they invariably carry with them the nerves of the body segments in which they are developed. Hence the nerve supply affords the clue to the segments from which a muscle or part of a muscle arises. The middle layer of the lumbar fascia is developed between the epaxial and ventrolateral musculatures.

Many of the ventro-lateral muscles (trapezius, rhomboids, and latissimus dorsi), migrate dorsalwards over the epaxial muscles, and take origin from the spines of the vertebrae (Fig. 65, A).

Fig. 66 The distribution of a typical Segmental Artery.

Fig. 66. The distribution of a typical Segmental Artery.

Muscular fibrillae begin to form in the 5th week, appearing in the protoplasmic matrix, in which the nuclei of the myoblasts are embedded. The fibrillae group themselves in bundles or muscle fibres, the nuclei with some of the myoplasm being applied to the surface of the completed fibre. New fibre production goes on rapidly until the 5th month, when the complement for each muscle is nearly complete. Thereafter muscles grow in size, chiefly, it is believed, by an increase in the size of the individual fibres. Although voluntary muscle fibres atrophy when their nerve is cut, yet myoblasts will develop into muscles when separated from nerve cells (Ross Harrison), or when grown in artificial media outside the body.

III. The Arteries of the 11th Segment

(Fig. 66).— The 11th intercostal is the artery of the segment.[4] It gives off a dorsal branch to supply the epaxial muscles, the spinal column, spinal cord and membranes, and skin.


The segmental artery joins at its termination with a ventral longitudinal vessel, the deep epigastric. The primitive arrangement in vertebrates appears to have been one with a dorsal and ventral longitudinal vessel, the segmental artery passing from the dorsal to the ventral vessel. The vertebral, ascending cervical, deep cervical, ascending lumbar and lateral sacral arteries are examples of the anastomoses that may arise between segmental arteries.


Segmental arteries also arise from the aorta to supply the structures formed from the intermediate cell mass (the kidney, testis, ovary, etc., Fig. 66). As a rule only one renal segmental artery persists, but frequently accessory renals are seen. These may be persistent embryonic vessels of several segments of the intermediate cell-mass in which the Wolffian body and kidney arise. The splanchnopleure shows no certain traces of segmentation ; hence its vessels (coeliac axis and mesenteric) if of segmental origin have become profoundly modified. Lately Broman has demonstrated that the splanchnic arteries have the appearance of a segmental arrangement in the embryo (Fig. 25). During the 4th week there are right and left aortae, each giving off splanchnic branches ; in the 5th week fusion of the aortic trunks sets in ; later the right and left splanchnic branches unite.

IV. The Nerve Elements of the 11th Segment

(Fig. 67).— Although the spinal cord during development of the human embryo shows no clear sign of being definitely divided into segments corresponding to those of the body, yet from what we know of its condition in embryos of other animals and from clinical evidence there can be little doubt that such a segmentation does take place, and that it possesses segments corresponding to those of the body. From each segment four groups of cells arise : (1) Somatic motor, (2) somatic sensory, (3) splanchnic motor, (4) splanchnic sensory. The motor groups for the greater part remain within the spinal cord, but many enter the sympathetic ganglia ; the sensory groups form ganglia outside the cord. The nerve fibres connected with the somatic groups have a diameter varying from 9-18µ ; those with the splanchnic, 2-9µ. The somatic motor group, in the anterior horn, sends out processes to all the muscles of the primitive body segment in which it is situated. The anterior root of a spinal nerve is formed by the somatic motor fibres. The splanchnic motor cells, in the lateral horn, send out processes within the splanchnopleure which reach viscera through the white rami communicantes and sympathetic system (Fig. 67, A). It is probable that, as Elliot Smith has suggested, some of the splanchnic motor cells emigrate from the cord and take up a position in the prevertebral ganglia.


At the point where the medullary plates are cut ofi from the ectoderm to form the neural canal, a crest, the neural crest, grows out on each side (Fig. 67, B) composed of the cells which formed the junctional line between medullary plates and ectoderm. A group of these neuroblasts— the somatic sensory group — grows into each segment and forms the posterior root ganglion. Each neuroblast within the ganglion sends out a process which bifurcates, one branch or fibre growing into the cord and ending in the posterior column and cells of the posterior horn, the other passing to the skin, muscles, etc., of the segment. The posterior nerve root is thus formed by the ingrowing processes from the cells of the posterior root ganglion, and the body segment in which the outgrowing processes are distributed is thereby brought into sensory communication with the central nervous system (see also p. 84). The anterior and posterior roots unite to form a spinal or segmental nerve. Like the artery, it divides into a posterior division for the epaxial part of the segment and an anterior for the ventro-lateral part (Fig. 67, A).

Fig. 67 Diagram of the Nerve System of the 11th Dorsal Segment.

Fig. 67. A. Diagram of the Nerve System of the 11th Dorsal Segment. B. A diagram showing the derivation of the Parts of the Nerve System of the 11th Segment in the Embryo.

The splanchnic sensory groups[5] are situated in the posterior root-ganglia, and probably also in the various ganglionic masses of the sympathetic system. These sympathetic cells are derived, with the posterior root ganglion, from the neural crest, and at first form a continuous paravertebral column (in 5th week). From the paravertebral column are differentiated :

(a) The prevertebral ganglion situated on the vertebra (in the gangliated chain), ventral to the exit of the spinal nerve ;

(b) A group to the intermediate cell mass (renal ganglion and adrenal body) ;

(c) Another to the splanchnopleure (in the semilunar ganglia) ;

{d) To the viscera (cells of Auerbach's plexus, etc.).

Groups (c) and {d) show no trace of segmentation in their arrangement, but, clinically, evidence is to be found that every viscus or part of a viscus is connected with certain segments of the spinal cord. The cells of the sympathetic ganglia throw out axis-cylinder processes, which are connected with the spinal cord by fibres in a white ramus communicans and posterior root, and act as sensory pathways from the viscera. The distal end of the axis-cylinder process terminates in a viscus. In this manner certain segments of the spinal cord are brought into touch with certain parts of the viscera. The vaso-motor supply of each body segment passes to it from the sympathetic ganglion by a grey ramus communicans.


It will thus be seen that all the parts of a segment— body wall (somatopleure), kidney (intermediate cell mass), and a part of the abdominal or thoracic viscera (splanchnopleure) are connected by nerves to a corresponding segment of the spinal cord. In diseased conditions of any part of a body segment, the corresponding spinal segment of the cord is disturbed. Such a disturbance is referred along the somatic sensory fibres, for the brain has no power to assign to their source, impressions travelling inwards by the splanchnic sensory fibres. Thus, for instance, a stone in the pelvis of the kidney (which is supplied from the 10th, 11th, and 12th, dorsal segments) is frequently accompanied by pain which the brain refers along the 11th and 12th intercostal nerves. The skin supplied by these nerves may become hyper-aesthetic. In the central nerve system as in the muscular, the primary simple segmental arrangement has been disturbed by enormous changes which have occurred in the process of evolution. In order to secure a harmonious co-operation of the various segments of the body, communications have been established, by means of nerve tracts, between the various segments of the spinal cord and between the segments of the cord and the higher centres of the brain. These communications have obliterated well nigh all traces of the primitive segments, and yet we see in the ganglia of the posterior roots and in the prevertebral ganglia of the sympathetic chain clear evidence that each segment of the body was originally provided with its own semi-automatic nerve mechanism. Clinical observation has supplied evidence that certain viscera — -such as the heart, the liver, the kidneys — have a nervous correlationship with certain segments of the body, and we may infer that these organs have been evolved in connection with certain definite segments of the body.

Fig. 68 Cervical and dorsal parts of the Spine of a Human Foetus showing irregularities of segmentation.

Fig. 68. Cervical and dorsal parts of the Spine of a Human Foetus showing irregularities of segmentation.

Segments from which Splanchnic Fibres Escape

The small medullated or splanchnic fibres dp not arise from every spinal segment. Bishop Harman found that in man such fibres escape only by the roots of the dorsal nerves and first lumbar ; occasionally splanchnic fibres come out in the roots of the last cervical and second lumbar. These fibres enter the gangliated chain, and are distributed to the viscera. Splanchnic fibres also escape by the 3rd sacral, frequently too from the 2nd or 4th, to form the nervi errigentes for the pelvic viscera. The greater part of the 9th, lOtli and 11th cranial nerves is made up of splanchnic fibres. There are thus three visceral areas — an anterior or medullary, a middle or thoracic, and a posterior or sacral. How these centres came to be thus separated is not known. It is also remarkable that the nerve centres which regulate or constrict arterioles are situated in the middle or thoracic area.


Abnormal Segmentation

In certain pathological conditions the process of segmentation is disturbed, with the result that an irregular and asymmetrical separation of the segments takes place. In Fig. 68 part of the spinal column and ribs are shown of a foetus in which the effects of such an irregularity are well illustrated. The vertebrae of the 3rd and 4th cervical segments are fused on the left side ; the succeeding segments show many abnormalities of a similar kind. The bodies of the 1st and 2nd ribs of the right side are fused. In the same foetus the pectoral muscles were imperfectly developed. In such foetuses one or both of the shoulders are placed high in the neck (congenital elevation of the scapula). Imperfect separation of two adjacent vertebrae or ribs is occasionally seen — abnormalities due to a lesser irregularity of segmentation.



  1. For papers on segmentation see : G. van Rynberk, Ergebnisse der Anat. 1908, vol. 18, p. 353 ; A. L. J. Sunier, Onderzoekingen verricht in het Zoolog. Lab. Univ. Groningen, 191 1, Leyden (Differentiation of Myotome). See also references : — Bar leen, p. 54 ; under W. H. Lewis, p. 426 ; Watt, p. 42; Barniville, p. 47.
  2. See Prof. J. Cameron, Trans. Roy. Soc. Canada, 1918, vol. 11, p. 81.
  3. See Prof. T. Walmsley, Journ. Anat. 1916, vol. 50, p. 165.
  4. For segmentation origin of arteries see : I. Broman, Ergebnisse der Anat. 1906, vol. 16, p. 639.
  5. See Gaskell's original paper in Journ. of Phy.siol. 1886, vol. 7, p. 1.


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 and Morphology: 1 Early Ovum and Embryo | 2 Connection between Foetus and Uterus | 3 Primitive Streak Notochord and Somites | 4 Age Changes | 5 Spinal Column and Back | 6 Body Segmentation | 7 Spinal Cord | 8 Mid- and Hind-Brains | 9 Fore-Brain | 10 Fore-Brain Cerebral Vesicles | 11 Cranium | 12 Face | 13 Teeth and Mastication | 14 Nasal and Olfactory | 15 Sense OF Sight | 16 Hearing | 17 Pharynx and Neck | 18 Tongue, Thyroid and Pharynx | 19 Organs of Digestion | 20 Circulatory System | 21 Circulatory System (continued) | 22 Respiratory System | 23 Urogenital System | 24 Urogenital System (Continued) | 25 Body Wall and Pelvic Floor | 26 Limb Buds | 27 Limbs | 28 Skin and Appendages | Figures