McMurrich1914 Chapter 4
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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 IV. The Development of the External Form of the Human Embryo
In the preceding chapter descriptions have been given of human embryos representing the earlier known stages and the development of the general form of the human embryo has been traced up to the time when the mesodermic somites have made their appearance. It will now be convenient to continue the history of the general development up to the stage when the embryo becomes a fetus.
In the earlier stages, that is to say up to that represented by the Eternod embryo (Fig. 43), the embryonic disk may be described as floating upon the surface of the yolk-sac, and while this description still holds good for the Eternod embryo a distinct groove may be seen in that embryo between the peripheral portions of the embryonic disk and the upper part of the sac. This groove marks the beginning of the separation or constriction of the embryo from the yolk-sac, the result of which is the transformation of the discoidal embryonic portion of the embryonic disk into a cylindrical structure. Primarily this depends upon the deepening of the furrow which surrounds the embryonic area, the edges of this area being thus bent in on all sides toward the yolk-sac. This bending in proceeds most rapidly at the anterior end of the body, as shown in the diagrams (Fig. 52), and less rapidly at the posterior end where the bellystalk is situated, and produces a constriction of the yolk-sac, the portion of this structure nearest the embryonic disk becoming enclosed within the body of the embryo to form the digestive tract, while the remainder is converted into a pedicle-like portion, the yolk-stalk, ' at the extremity of which is the yolk-vesicle. The further continuance of the folding in of the edges of the embryonic area leads to an almost complete closing in of the embryonic ccelom and reduces the opening through which the yolk-stalk and bellystalk communicate with the embryonic tissues to a small area known as the umbilicus.
In the Kromer embryo Klb (Fig. 44) this separation of the embryo proper from the yolk-sac has proceeded to such an extent that both extremities of the embryonic disk are free from the yolk-sac, and the anterior extremity is bent ventrally almost at a right angle to the rest of the disk, producing what is termed the vertex bend, a feature characteristic of all later embryos. The marked development in this embryo of the medullary folds and the occurrence of mesodermic somites have already been mentioned (p. 72).
Fig. 52. - Diagrams Illustrating the Constriction of the Embryo from the Yolk-sac. A and C are longitudinal, and B and D transverse sections. B is drawn to a larger scale than the other figures.
Somewhat more advanced is the Bulle embryo described by Kollmann and shown from the side and dorsally in Fig. 53, the greater part of the yolk-sac having been removed as well as the most of the amnion. The embryo measured about 2.5 mm. in length and showed a considerable increase in the number of mesodermic somites, there being about fourteen of them on either side. Posteriorly the medullary groove has become converted into a medullary canal by the medullary folds meeting over it and fusing, but anteriorly it is still open. The vertex bend is well marked and immediately behind the tip of the head, on the ventral surface of the body, there may be seen a well-marked depression, the oral fossa, between which and the anterior surface of the yolk-sac is a rounded elevation due to the formation of the heart. Attention may be called to the fact that the position of this organ is far forward of that which it will eventually occupy, so that it must undergo a marked retrogression during later development.
Fig. 53. - Embryo 2.5 mm. Long. om, Amnion; B, belly-stalk; h, heart; M, closed, and M', still open portions of the medullary groove; Om, vitelline vein; OS, oral fossa; Y, yolk-sac. - (Kallmann.)
Fig. 54. - Embryo Lr, 4.2 mm. Long. am, Amnion; au, auditory capsule; B, belly-stalk; h, heart; LI, lower, and Ul, upper limb; Y, yolk-sac. - (His.)
As an example of a later stage. of development the embryo Lr of His, measuring 4.2 mm. in length, may be taken (Fig. 54). In this the constriction of the yolk-sac has progressed so far that its proximal portion may now be spoken of as the yolk-stalk. The mesodermic somites have undergone a further increase and have almost reached their final number, the vertex bend has become still more pronounced and the medullary groove, throughout its entire length, has been converted into the medullary canal, which, anteriorly, shows distinct enlargements and constrictions which foreshadow various portions of the future brain. The auditory organ, which made its appearance in earlier stages, has now become quite distinct, and a lateral bulging of the most anterior portion of the head indicates the position of the future eye.
In addition certain other important features have now appeared. Thus, about opposite the head a second bend, the nape bend, is becoming visible on the dorsal surface of the body and toward the posterior end a distinct sacral bend is evident. Secondly, a little posterior to the level of the nape bend a slight elevation is to be seen on the side of the body; this is the limb bud for the upper limb and a corresponding, though smaller, elevation in the region of the sacral bend represents the lower limb.
Thirdly, three grooves having a dorso-ventral direction have appeared on the sides of what will be the future pharyngeal region. These are representatives of a series of branchial clefts, structures that are of great morphological importance in the further development inasmuch as they determine to a large extent the arrangement of various organs of the head region. They represent the clefts which exist in the walls of the pharynx in fishes, through which water, taken in at the mouth, passes to the exterior, bathing on its way the gill filaments attached to the bars or arches, as they are termed, which separate successive clefts. Hence the name "branchial" which is applied to them, though in the mammals they never have respiratory functions to perform, but, appearing, persist for a time and then either disappear or are applied to some entirely different purpose. Indeed, in man they are never really clefts but merely grooves, and corresponding to each groove in the ectoderm there is also one in the subjacent endoderm of what will eventually be the pharyngeal region of the digestive tract, so that in the region of each cleft the ectoderm and endoderm are in close relation, being separated only by a very thin layer of mesoderm. In the intervals between successive clefts a more considerable amount of mesoderm is present (Fig. 55).
Fig. 55. - Floor of the Pharynx of Embryo B, 7 mm. Long. Ep, Epiglottis; Sp, sinus prsecervicalis; t 1 , tuberculum impar; t 2 , posterior portions of the tongue; I, II, III, and IV, branchial arches. - (His.)
In the human embryo four clefts and five branchial arches develop on each side of the body, the last arch lying posteriorly to the fourth cleft and not being very sharply denned along its posterior margin.
As just stated, the clefts are normally merely grooves, and in later development either disappear or are converted into special structures. Occasionally, however, a cleft may persist and the thin membrane which forms its floor may become perforated so that an opening from the exterior into the pharynx occurs at the side of the neck, forming what is termed a branchial fistula. Such an abnormality is most frequently developed from the lower (ventral) part of the first cleft; normally this disappears, the upper portion of the cleft persisting, however, to form the external auditory meatus and tympanic cavity.
A further stage in the differentiation of these clefts and arches is shown by the embryo represented in Fig. 56. The nape bend has now increased to such an extent that the whole anterior part of the body is bent at a right angle to the middle part and the entire embryo is coiled in a spiral manner. The limb buds are much more distinct than in the previous stage and four branchial arches are now present; the second and third have become more defined and a strong process has developed from the dorsal part of the anterior border of the first one, which has thus become somewhat <3 -shaped. The anterior limb of each V is destined to give rise to the upper jaw, and hence is known as the maxillary process, while the posterior limb represents the future lower jaw and is termed the mandibular process.
Fig. 56. - Embryo Backer, 7.3 mm. in Length. X5. - (Keibefand Ehe.)
In the stage represented by this embryo the closing in of the embryonic ccelom has progressed to such a degree that only a small opening is left in the ventral body-wall of the embryo through which the yolk-stalk and its accompanying vessels and the belly-stalk pass. Indeed the margins of the umbilicus may have begun to be prolonged outward over these structures, enclosing them in a cylindrical investment, the first stage of what will later be the umbilical cord being thus established.
Leaving aside for the present all consideration of the further development of the limbs and branchial arches, the further evolution of the general form of the body may be rapidly sketched. In an embryo (Fig. 57) from Ruge's collection, described and figured by His and measuring 9.1 mm. in length,
- the prolongation of the margins of the umbilicus has increased until more than half the yolk-stalk has become enclosed within the umbilical cord. The nape and sacral bends are still very pronounced, although the embryo is beginning to straighten out and is not quite so much coiled as in the preceding stage. At the posterior end of the body there has developed a rather abruptly conical tail filament, in the place of the blunt and gradually tapering termination seen in earlier stages, and a well-marked rotundity of the abdomen, due to the rapidly increasing size of the liver, begins to become evident.
- This measurement is taken in a straight line from the most anterior portion of the nape bend to the middle point of the sacral bend and does not follow the curvature of the embryo. It may be spoken of as the nape-rump length and is convenient for use during the stages when the embryo is coiled upon itself.
Fig. 57. - Embryo 9.1 mm. Long. LI, Lower limb; U, umbilical cord; Ul, upper limb; Y, yolk-sac. - (His.)
In later stages the enclosure of the yolk- and belly-stalks within the umbilical cord proceeds until finally the cord is complete through the entire interval between the embryo and the wall of the ovum. At the same time the straightening out of the embryo continues, as may be seen in Fig. 58 representing the embryo xlv (Br 2 ) of His, which shows also, both in front of and behind the neck bend, a distinct depression, the more anterior being the occipital and the more posterior the nape depression; both these depressions are the indications of changes taking place in the central nervous system. The tail filament has become more marked, and in the head region a slight ridge surrounding the eyeball and marking out the conjunctival area has appeared; a depression anterior to the nasal fossae marks off the nose from the forehead; and the external ear, whose development will be considered later on, has become quite distinct. This embryo had a nape-rump length of 13.6 mm.
Fig. 58.' - Embryo B r 2 , 13.6 mm. Long. - (His.)
In the embryos xxxv (S 2 ) and xcix (L 3 ) (Fig. 59, A and B) of His' collection the straightening out of the nape bend is proceeding, and indeed is almost completed in embryo xcix, which begins to resemble closely the fully formed fetus. The tail filament, somewhat reduced in size, still persists and the rotundity of the abdomen continues to be well marked. The neck region is beginning to be distinguishable in embryo S 2 and in embryo L 3 the eyelids have appeared as slight folds surrounding the conjunctival area. The nose and forehead are clearly defined by the greater development of the nasal groove and the nose has also become raised above the general surface of the face, while the external ear has almost acquired its final fetal form. These embryos measure respectively about 15 and 17.5 mm. in length.*
Finally, an embryo - again one of those described by His, namely, his lxxvti (Wt), having a length of 23 mm. - may be figured (Fig. 60) as representing the practical acquisition of the fetal form. This embryo dates from about the end of the second month of pregnancy, and from this period onward it is proper to use the term fetus rather than that of embryo. The changes which have been described in preceding stages are now complete and it remains only to be mentioned that the caudal filament, which is still prominent, gradually disappears in later stages, becoming, as it were, submerged and concealed beneath adjacent parts by the development of the buttocks. The incompleteness of the development of these regions in embryo Wt is manifest, not only from the projection of the tail filament, but also from the external genitalia being still largely visible in a side view of the embryo, a condition which will disappear in later stages.
Fig. 59. - A, Embryo S 2 , 15 mm. Long (showing Ectopia of the Heart); B, Embryo L 3 , 17.5 mm. Long. - (His.)
- The embryo S 2 presents a slight abnormality [in the great projection of the heart, but otherwise it appears to be normal.
Fig. 60. - Embryo Wt, 23 mm. Long. - (His.)
The Later Development of the Branchial Arches, and the Development of the Face. - In the embryo shown in Fig. 56, the four branchial clefts and five arches which develop in the human embryo are visible in surface views, but in the Ruge embryo (Fig. 57) it will be noticed that only the first two arches, the first with a welldeveloped maxillary process, and the cleft separating them can be distinguished. This is due to a sinking inward of the region occupied by the three posterior arches so that a triangular depression, the sinus pracervicalis, is formed on each side of what will later become the anterior part of the neck region. This is well shown in an embryo (Br 3 ) described by His which measured 6.9 mm. in length and of which the anterior portion is shown in Fig. 61. The anterior boundary of the sinus (ps) is formed by the posterior edge of the second arch and its posterior boundary by the thoracic wall, and in later stages these two boundaries gradually approach one another so as first of all to diminish the opening into the sinus and later to completely obliterate it by fusing together, the sinus thus becoming converted into a completely closed cavity whose floor is formed by the ectoderm covering the three posterior arches and the clefts separating these. This cavity eventually undergoes degeneration, no traces of it occurring normally in the adult, although certain cysts occasionally observed in the sides of the neck may represent persisting portions of it.
Fig. 61. - Head of Embryo of 6.9 mm. na, Nasal pit; ps, precervical sinus. - (His.)
Fig. 62. - Face of Embryo of 8 mm. mxp, Maxillary process; np, nasal pit; os, oral fossa; pg, processus globularis. - (His.)
A somewhat similar process results in the closure of the ventral portion of the first cleft,* a fold growing backward from the posterior edge of the first arch and fusing with the ventral part of the anterior border of the second arch. The upper part of the cleft persists, however, and, as already stated, forms the external auditory meatus, the pinna of the ear being developed from the adjacent parts of the first and second arches (Figs. 58 and 59).
- See page 91, small type.
The region immediately in front of the first arch is occupied by a rather deep depression, the oral fossa, whose early development has already been noticed. In an embryo measuring 8 mm. in length (Fig. 62) the fossa (os) has assumed a somewhat irregular quadrilateral form. Its posterior boundary is formed by the mandibular processes of the first arch, while laterally it is bounded by the maxillary processes (mxp) and anteriorly by the free edge of a median plate, termed the nasal process, which on either side of the median line is elevated to form a marked protuberance, the processus globular is (pg). The ventral ends of the maxillary processes are widely separated, the nasal process and the processus globulares intervening between them, and they are also separated from the globular processes by a deep and rather wide groove which anteriorly opens into a circular depression, the nasal pit (np).
Fig. 63. - Face of Embryo after the Completion of the Upper Jaw. - (His.)
Later on the maxillary and globular processes unite, obliterating the groove and cutting off the nasal pits - which have by this time deepened to form the nasal fossae - from direct communication with the mouth, with which, however, they later make new communications behind the maxillary processes, an indication of the anterior and posterior nares being thus produced.
Occasionally the maxillary and globular processes fail to unite on one or both sides, producing a condition popularly known as "harelip." At the time when this fusion occurs the nasal fossa? are widely separated by the broad nasal process (Fig. 63), but during later development this process narrows to form the nasal septum and is gradually elevated above the general surface of the face as shown in Figs. 58-60. By the narrowing of the nasal process the globular processes are brought nearer together and form the portions of the upper jaw immediately on each side of the median line, the rest of the jaw being formed by the maxillary processes. In the meantime a furrow has appeared upon the mandibular process, running parallel with its borders (Fig. 59); the portion of the process in front of this furrow gives rise to the lower lip and is known as the lip ridge, while the portion behind the furrow becomes the lower jaw proper and is termed the chin ridge.
The Development of the Limbs
As has been already pointed out, the limbs make their appearance in an embryo measuring about 4 mm. in length (Fig. 54) and are at first bud-like in form. As they increase in length they at first have their long axes directed parallel to the longitudinal axis of the body and become somewhat flattened at their free ends, remaining cylindrical in their proximal portions. A furrow or constriction appears at the junction of the flattened and cylindrical portions (Fig. 57), and later a second constriction divides the cylindrical portion into a proximal and distal moiety, the three segments of each limb - the arm, forearm, and hand in the upper limb, and the thigh, leg, and foot in the lower - being thus marked out. The digits are first indicated by the development of four radiating shallow grooves upon the hand and foot regions (Fig. 58), and a transverse furrow uniting the proximal ends of the digital furrows indicates the junction of the digital and palmar regions of the hand or of the toes and body of the foot. After this stage is reached the development of the upper limb proceeds more rapidly than that of the lower, although the processes are essentially the same in both limbs. The digits begin to project slightly, but are at first to a very considerable extent united together by a web, whose further growth, however, does not keep pace with that of the digits, these thus coming to project more and more in later stages. Even in comparatively early stages the thumb, and to a somewhat slighter extent the great toe, is widely separated from the second digit (Figs. 59 and 60).
While these changes have been taking place the entire limbs have altered their position with reference to the axis of the body, being in stages later than that shown in Fig. 57 directed ventrally so that their longitudinal axes are at right angles to that of the body. From the figures of later stages it may be seen that it is the thumb (radial) side of the arm and the great toe (tibial) side of the leg which are directed forward; the plantar and palmar surfaces of the feet and hands are turned toward the body and the elbow is directed outward and slightly backward, while the knee looks outward and slightly forward. It seems proper to conclude that the radial side of the arm is homologous with the tibial side of the leg, the palmar surface of the hand with the plantar surface of the foot, and the elbow with the knee.
The limbs are, however, still in the quadrupedal condition, and they must later undergo a second alteration in position so that their long axes again become parallel with that of the body. This is accomplished by a rotation of the limbs around axes passing through the shoulders and hip-joints, together with a rotation about their longitudinal axes through an angle of 90 degrees. This axial rotation of the upper limb is, however, in exactly the opposite direction to that of the lower limb of the corresponding side, so that the homologous surfaces of the two limbs have entirely different relations, the radial side of the arm, for instance, being the outer side while the tibial side of the leg is the inner side, and whereas the palmar surface of the hand looks ventrally, the plantar surface of the foot looks dorsally. In making these statements no account is taken of the secondaryposition which the hand may assume as the result of its pronation; the positions given are those assumed by the limbs when both the bones of their middle segment are parallel to one another.
It may be pointed out that the prevalent use of the physiological terms flexor and extensor to describe the surfaces of the limbs has a tendency to obscure their true morphological relationships. Thus if, as is usual, the dorsal surface of the arm be termed its extensor surface, then the same term should be applied to the entire ventral surface of the leg, and all movements of the lower limb ventrally should be spoken of as movements of extension and any movement dorsally as movements of flexion. And yet a ventral movement of the thigh is generally spoken of as a flexion of the hip-joint, while a straightening out of the foot upon the leg - that is to say, a movement of it dorsally - is termed its extension.
The Age of the Embryo at Different Stages
The age of an embryo must be dated from the moment of fertilization and from what has been said in preceding pages (pp. 27, 34) it is evident that it must be difficult to determine the exact date of this event from that of the cessation of the menses, or even when the date of the coition that resulted in pregnancy is known. And, furthermore, not only is the actual date of the beginning of development uncertain, but in the majority of known early human embryos the time of the cessation of development is also more or less uncertain, since so many of these embryos are abortions and their expulsion need not necessarily have immediately succeeded their death.
These various sources of uncertainty are of especial importance in the cases of embryos in the early stages of development, when a day more or less means much, and it seems probable that many of the estimated ages given for young embryos, based on the date of the last menstruation, are too low. This certainly is the case with the ages assigned to such embryos by His, who estimated embryos of 2.2 to 3.0 mm. to be two to two and one-half weeks old, those of 5.0 to 6.0 mm. to be about three and one-half weeks and those of 10.0 to 11.0 mm. to be about four and one-half weeks.
There are on record, however, a few cases in which the date of the fruitful coition is definitely known, and from these, few though they be, somewhat more definite information may be obtained. Thus it is fairly certain that the Bryce-Teacher ovum, with an embryo measuring about 0.15 mm. in length, was the result of a coition which took place sixteen days before the ovum was aborted, and one cannot be far astray in assuming the embryo to be about two weeks old. Similarly, an embryo described by Eternod and measuring 1.3 mm. in length was the result of a single coition occurring twentyone days previously and its age may be set at approximately three weeks or better at eighteen or nineteen days. A later embryo in which the nape bend and the coiling of the body had appeared and which measured 8.8 mm. in vertex-breech length, resulted from a single coitus that took place thirty-eight days before the abortion, so that the embryo may be regarded as having been somewhat more than five weeks old. These and two other similar cases may be combined into a table thus:
Length of embryo
Probable age in
i3- J 4
V. B. 8.8
V. B. 14.0
V. B. 25.0
If, on the basis of these figures, one may venture to estimate the age of embryos of other lengths those of 2.0 to 3.0 mm. may be supposed to belong to the fourth week of development, those of 5.0 to 6.0 vertex-breech length to the latter part of the fifth week, those of 10.0 mm. to the end of the sixth week and those of 25.0 to 28.0 mm. which are just passing into the fetus stage, to the end of the eighth week. As regards the later periods of development, the limits of error for any date become of less importance. Schroder gives the following measurements as the average: 3d lunar month 70-90 mm.
4th lunar month ' 100-170 mm.
5th lunar month 180-270 mm.
6th lunar month 280-340 mm.
7th lunar month 350-380 mm.
8th lunar month 425 mm.
9th lunar month 467 mm.
10th lunar month 490-500 mm.
The data concerning the weight of embryos of different ages are as yet very insufficient, and it is well known that the weights of newborn children may vary greatly, the authenticated extremes being, according to Vierordt, 717 grams and 6123 grams. It is probable that considerable variations in weight occur also during fetal life. So far as embryos of the first two months are concerned, the data are too imperfect for tabulation; for later periods Fehling gives the following as average weights: 3d month 20 grams.
4th month 120 grams.
5th month 285 grams.
6th month 635 grams.
7th month 1220 grams.
8th month 1700 grams.
9th month 2240 grams.
10th month 3 2 5Â° grams.
and the results obtained by Jackson are essentially similar.
In addition to the papers of Bryce and Teacher, Eternod, Fetzer, Frassi, Herzog, Peters, Von Spee and Strahl and Beneke cited in the preceding chapter, the following may be mentioned:
Bremer: "Description of a 4 mm. Human Embryo," Amer. Journ. Anal., v, 1906.
J. Broman: "Beobachtung eines menschlichen Embryos von beinahe 3 mm. Lange mit specieller Bemerkung uber die bei demselben befindlichen Hirnfalten," Morpholog. Arbeiten, v, 1895.
A. J. P. van den Broek: "Zur Kasuistik junger menschlicher Embryonen," Anal,. Hefte, xliv, 191 1.
J. M. Coste: " Histoire generale et particuliere du developpement des corps organises," Paris, 1847-1859.
W. E. Dandy: "A Human Embryo with Seven Pairs of Somites, Measuring about 2 mm. in Length," Amer. Joiirn. Anal., x, 1910.
A. Ecker: "Beitrage zur Kenntniss der ausserer Formen jiingster menschlichen Embryonen," Archiv fur Anat. und Physiol., Anat. Abth., 18S0.
C. Elze: " Beschreibung eines menschlichen Embryos von zirka 7 mm. grosster Lange," Anat. Hefte, xxxv, 1907.
C. Giacomini: "Un ceuf humain de 11 jours," Archives Hal. de Biologie, xxix, 1898.
V. Hensen: "Beitrag zur Morphologie der Korperform und des Gehirns des menschlichen Embryos," Archiv fur Anat. und Physiol., Anat. Abth., 1877.
W. His: "Anatomie menschlicher Embryonen," Leipzig, 1880.
F. Hochstetter: "Bilder der ausseren Korperform einiger menschlicher Embryonen aus den beiden Ersten Monaten der Entwicklung," Munich, 1907.
N. W. Ingalls: "Beschreibung eines menschlichen Embryos von 4.9 mm.," Arch. fiir mikr. Anat., lxx, 1907.
C. M. Jackson: " On the Prenatal Growth of the Human Body and the Relative Growth of the Various Organs and Parts," Amer. Journ. Anat., ix, 1909.
J. Janosik: "Zwei junge menschliche Embryonen," Archiv fiir mikrosk. Anat., xxx, 1887. H. E Jordan: "Description of a 5 mm. Human Embryo," Anat. Record, ill, 1909.
P. Jung: "Beitrage zur friihesten Ei-einbettung beim menschlichen Weibe," Berlin, 1908.
F. Keibel: "Ein sehr junges menschliches Ei," Archiv fiir Anat. und Physiol., Anat.
Abth., 1890. F. Keibel: "Ueber einen menschlichen Embryo von 6.8 mm. grosster Lange," Verhandl. Anatom. Gesellsch., xiii, 1899.
F. Keibel and C. Elze: " Normentafeln zur Entwicklungsgeschichte der Wirbeltiere," Heft viii, 190S.
J. Kollmann: "Die Korperform menschlicher normaler und pathologischer Embryonen," Archiv fur Anat. und Physiol., Anat Abth., Supplement, 18S9.
A. Low: "Description of a Human Embryo of 13-14 Mesodermic Somites," Journ. Anat. and Phys., xlii, 1908.
F. P. Mall: "A Human Embryo Twenty-six Days Old," Journ. of Morphology, V, 1891.
F. P. Mall: "A Human Embryo of the Second Week," Anat. Anzeiger, viii, 1893.
F. P. Mall: "Early Human Embryos and the Mode of their Preservation," Bulletin of the Johns Hopkins Hospital, XV, 1S94. C. S. Minot: "Human Embryology," New York, 1892.
J. Muller: " Zergliederungen menschlicher Embryonen aus friiherer Zeit," Archiv fiir Anat. und Physiol., 1830.
C. Phisalix: "Etude d'un Embryon humain de 11 millimeters," Archives de zoolog. experimentale et generale, Ser. 2, vi, 1888.
H. Piper: "Ein menschlicher Embryo von 6.8 mm. Nackenlinie," Archiv fiir Anat. und Physiol., Anat. Abth,, 1898.
C. Rabl: "Die Entwicklung des Gesichtes, Heft i, Das Gesicht der Saugetiere, Leipzig, 1902.
G. Retzitts: "Zur Kenntniss der Entwicklung der Korperformen des Menschen wahrend der fotalen Lebensstufen," Biolog. Untersuch., xi, 1904.
J. Tandler: "Ueber einen menschlichen Embryo von 38 Tage," Anat. Anzeiger, xxxi, 1907.
Allen Thompson: "Contributions to the History of the Structure of the Human Ovum and Embryo before the Third Week after Conception, with a Description of Some Early Ova," Edinburgh Med. and Surg. Journal, in, 1839. (See also Froriep's Neue Notizen, xiu, 1840.)
P. Thompson: "Description of a human embryo of twenty-three paired somites," Journ. Anat. and Phys., xli, 1907.
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McMurrich JP. The Development Of The Human Body. (1914) P. Blakiston's Son & Co., Philadelphia, Pennsylvania.
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