Paper - Measurements on a Human Embryo 30 mm Long
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Blaisdell FE. Measurements on a human embryo 30 mm long. (1914) J Anat Physiol., 48(2): 182-209. PMID 17232990
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- 1 Measurements on a Human Embryo 30 mm Long
- 1.1 Introduction
- 1.2 Condition of the Embryo
- 1.3 Method of Preparation
- 1.4 Measurements and Age of the Embryo
- 1.5 Derivatives of the Epiblast
- 1.6 Derivatives of the Epidermis
- 1.7 Derivatives of the Neural Epiblast
- 1.8 Derivatives of the Mesoblast
- 1.9 List of Measurements
- 1.10 References
Measurements on a Human Embryo 30 mm Long
By Frank E. Blaisdell, sen.
(From the Division of Anatomy of the Department of Medicine of Stanford University.)
The present observations are based on an imperfect human embryo obtained without data. They constitute an inquiry into the relative size of organs and the degree of development attained in this embryo, measuring 30 mm from vertex to breech after ﬁxation in alcohol.
So far, embryological measurements have been conﬁned to external measurements, and it does not seem improbable that a series of careful measurements on the chief internal organs may form a far better criterion for estimating the age of embryos. It is to be hoped that a group of embryos of approximately similar external size may be subjected to corresponding measurements, with the idea of securing such a basis and of throwing some light on the rate of development in the various viscera, in the same and in different embryos.
Condition of the Embryo
The posterior parietal and anterior cervical regions, portions of the abdomen and lower extremities, were lacerated. The injury to the head had involved the lateral wall of the occipital portion of the right cerebral hemisphere. All of the other parts of the body were perfect and in excellent condition. The embryo, which is apparently normal, is No. 9 of the collection.
Method of Preparation
The embryo was ﬁxed in alcohol, embedded in parafﬁn, and cut serially for class work. The head was cut sagittally and the body transversely. One-half of the head, as cut, constitutes a sagittal series of one hundred and ninety-ﬁve sections (Series 9). From the transverse series of the neck and thorax twenty—ﬁve sections were selected from different levels, and these constitute Series 9A of the collection. The sections were cut 16 micra in thickness. Both series were stained with Mallory’s connective tissue stain.
Measurements and Age of the Embryo
The damaged condition made it necessary more or less to approximate the length of the lower extremities. According to Mall’s method of estimating the standing height, the measurements for Embryo No. 9 are as follows :— A vertex-breech length of 30 mm., a hip-knee (hk) length of 3'0 1nm., and a knee-heel (kh) length of 60 n1m., with a standing height of 39'0 mm. The head of No. 9 is somewhat large in proportion to the body, the upper extremities aregﬂexed, and the forearm and hand lie transqersely across the thorax; the arms are placed somewhat vertically in relation to the antero-lateral thoracic wall, with a well-marked bend at the elbows. The ﬁngers are all free. The eyelids are completely closed and adherent. It is evident that the embryo is older than its size indicates.
Unfortunately, the lack of a menstrual history prevents any discussion of the question from that standpoint, and recourse must therefore be had to a comparison with embryos of a known age and development. Probably 32 mm. vertex-breech measurement is nearer the truth, and the age about seventy days.
The measurements made from a median sagittal section of the head of No. 9 are as follows :— The diameter on a plane cutting a point between the occipital cartilage and atlas to the anterior nasal spine is 100 mm. The greatest cephalic diameter, which lies parallel to this line, measures 125 mm., while a line drawn at right angles to the ﬁrst gives the height of the head as 110 mm. The height, as measured from the occipital cartilage at the anterior margin of the foramen magnum to the vertex, is 125 mm. The transverse diameter through the shoulders just cephalad to the margin of the cartilaginous manubrium and close to the articulationes humeri was 100 mm., and the sagittal diameter in the same plane 6'5 mm. The transverse diameter of the thorax, on a plane through base of the heart, 95 mm. ; and the sagittal diameter in the same plane, 8'0 mm. The transverse diameter of the thorax on a plane through the mammary anlagen was 9'5 mm., and the sagittal diameter in the same plane 7'5 mm. The length of the forearm and hand, measured from a point over the cartilaginous olecranon to the tips of the ﬁngers, 7'0 mm.
All micrometrical determinations have been made withla Zeiss ocular micrometer.
Derivatives of the Epiblast
Epiderrmis.— Embryo No. 9 exhibits considerable variation in the thickness of the epidermis. In the region of the scalp the epidermis is two—layered, and one or both layers are absent in places, showing that they are easily detached. The peridermal cells are fusiform, and here and there two cells deep. The cells of the stratum germinativum, which havenuclei somewhat centrally placed, are short and cuboidal, varying more or less in form from mutual pressure. In the lateral pectoral region, and especially lateral to the mammary region, the epidermis is distinctly two-layered, with larger cells than those on the scalp. Over the dorsal and anterior thoracic regions the stratum intermedium is present. Here the peridermal cells are fusiform in places and seem thicker, but there is so much denudation that it is hazardous to make a positive statement. Over the brachium the basal cells are rather more columnar than cuboidal. This is especially true medially where they are distinctly columnar, with nuclei apically situated. Over the face, mammary region, forearm, and hand, the stratum intermedium is well developed. The cells of the superficial stratum are larger, thicker, with straighter sides and larger oval nuclei. It is difficult, however, to judge the proper focal plane, for the cells are transparent and the picture changes with the focal plane. Hence an endeavour has been made to judge and consider only the cell layers where the line of sectioning has fallen perpendicular to. the surface. Commencing just above the supraciliary region, the epidermis becomes three and four cells thick, especially in the region of the mouth. On the lips the cells of the stratum germinativum gradually become high columnar, the nuclei are more elongated, and closely placed to each other so as to form a regular, heavily stained nuclear line, all these characteristics becoming more prominent as the buccal walls are approached. The same structural characteristics are present in the basal layers or stratum germinativum between the adherent palpebrae, as well as on the volar surface of the hands, where the nuclei are very deeply stained.
Note.—A Zeiss microscope has been used, except when otherwise stated. Tube length of 15, and the drawing surface raised 30 mm. above that of the table.
Derivatives of the Epidermis
Hair germs are present in the supraciliary region, upper and lower lips, but not elsewhere on the head. The incomplete series of sections of the neck and thorax contain none in those regions.
Fig. 2 shows the degree of development reached by the hair germs. A very few may possibly possess a slight kiln-like arrangement of the cells, but since they are heavily stained the exact details are obscured. Several of the hair germs have an aggregation of heavily stained nuclei in the corium in close proximity to them. These aggregations represent differentiating papillaa. Section 49, from which the camera lucida tracing Was made (ﬁg. 2), shows the basal cells of the epidermis in the supraciliary region to be of the high columnar type. Such a pronounced columnar form may be due partly to an obliquity of sectioning, although other sections do not show evidence of this. At (0,), ﬁg. 2, a germ is cut through the side and the nuclei of the stratum germinativum immediately adjoining are shown as assuming a straight course. It has not been possible to count all the hair germs in the supraciliary region, but as many as forty—three have been counted and numerous others in earlier stages of development are present.
Fig. 2. Sagittal section of two hair germs in the right supraciliary region. At (a) the line of sectioning is through the side of a. germ. Drawn with the aid of the camera lucida. Series 9A, section 49. B. & L., 0c. 1 in. ; obj. 4} in. Reduced).
The mammary anlagen are more or less oval in outline. The right gland consists of two closely approximated divisions. The maximum diameter of these divisions measured transversely is‘166'5 micra, and the depth to which they penetrate the mesoblast, as measured from the epidermal surface, is 2164+ micra. . The thickness of the overlying epidermis is 24'2+ micra. The formation of a connective-tissue sheath is indicated by the condensation of.the surrounding mesenchyma and the crowded state of the nuclei.’ Fig. 1 shows the abundance of the latter, as contrasted with their sparseness in the undifferentiated corium lateral to the mammary region.
A differentiated corium and tela subcutanea are not yet discernible. The connective—tissue reticulum is made up of ﬁne delicate ﬁbrillae, and the deep surface of the developing glands with their enveloping sheaths rest upon a stratum of Wavy parallel ﬁbrillae, that are more condensed and heavily stained. The latter is the developing pectoral fascia. The mesenchymal tissue in the vicinity of the glands has a much greater depth than at some distance from it.
Fig. 3. — Section through the point of junction of the lacrimal cords with the nasolacrimal cord. Camera lucida drawing. 0c. 3; obj. E. Reduced §.
The lacrimal apparatus consists of solid epithelial cords. The lacrimal canal cords end abruptly against the deep surface of the conjunctival epithelium, and are therefore ‘not in continuity with it. In all sections of the series a distinct and rather heavily stained line marks the basal line of the conjunctival epithelium. Both lacrimal cords are largest Where they are in juxtaposition to the conjunctival epithelium, their diameter being 68'1‘+ micra. From that point each gradually diminishes to 4.«5'4s+ micra opposite the medial border of the lacus lacrimalis, again to increase in size as their junction with the nasolacrimal cord is approached, the diameter of the superior cord being 48'/14+ micra, and that of the inferior 53'0+ micra. The average diameter of the nasolacrimal cord is about 6l'0+ micra. That portion of the cord which corresponds to the position of the lacrimal sac has a diameter of 606+ micra, which increases to 757+ micra; thence the diameter decreases to 424+ micra as the cord enters the nasolacrimal canal, to increase again to about 54.«'5+ micra as the nasal epithelium is approached.
The central cells of the lacrimal and nasolacrimal cords have stained less strongly than the peripheral cells, but there is no distinct evidence of their degeneration to form a lumen. Fig. 3 is a camera lucida drawing of a section through the point of junction of the lacrimal cords with the nasclacrimal cord. In the cords the nuclei of the basal cells are slightly more elongated than elsewhere, and placed more closely and regularly, so as to form a more or less regular line. The nuclei of the intermediate and centrally placed cells are more oval. The basal part of the peripheral and more or less columnar cells shows as a narrow, clear, and unstained line. The regular row of elongated nuclei of the basal cells shows most plainly in the thickened distal extremities of the lacrimal cords just where they approach the conjunctival epithelium.
The mesenchymal tissue immediately surrounding the cords has undergone considerable condensation and differentiation to form a sheath. This sheath is less evident in the palpebral mesoblast, where there is a general condensation with a marked increase in the number of nuclei.
The nasolacrimal cord passes downward between the centre of ossiﬁcation for the superior maxilla and the lateral wall of the cartilaginous nasal capsule. At the superior extremity of the developing lacrimal groove, the centre of ossiﬁcation of the lacrimal bone is in contact with the nasal capsule. The nasolacrimal cord is surrounded by a thin sheath of condensed mesenchyma, and the accompanying blood-vessels are sufﬁciently abundant in the mesoblast occupying the pathway of the nasolacrimal canal.
The conjunctival sac in sagittal section appears as an arcuate space between the palpebrae and the bulbus oculi. The greatest distance between the superior and the inferior fornices is 1.6+ mm., the measurement having been made in a straight line between the two points named. At the internal angle of the sac, the rudimentary semilunar fold, which has a mesoblastic core, projects inwards to the distance of about 52 micra. The cells of the palpebral and bulbar conjunctival epithelium possess spherical nuclei, which lie about three deep. However, theioutlines of the cells are not evident, and the basal layer does not form a regular row. The basal layer of columnar cells so evident in the inter-palpebral epithelium passes quickly into low columnar, then cuboidal, and becomes lost as a distinct layer after attaining the conjunctiva] surface of the palpebrae. At many points and in different sections the epithelium appears to be from four to six layers deep, but this thickness is undoubtedly due in part to an obliquity of sectioning.
The anlagen of the ducts of the right lacrimal gland are six in number. The epithelial cords have grown dorsad and somewhat laterad into the mesoblast, from the superior and lateral angle of the conjunctival sac. The longer cords are slightly tortuous, and the longest one has penetrated the surrounding mesenchyma to a distance of 4828+ micra from the surface of the conjunctival epithelium (ﬁg. 8, D, and 14 13). The longest cords are lateral in position, and exhibit the most irregularity and curve medialward (cords D and F, ﬁgs. 7, 8, 9, 10, 11, 12).
Figs. 4-12. Diagrammatic camera lucida. drawings of serial sections, showing the anlagen of the ducts of the glandula lacrimalis dextra. The order given is medial to lateral. a, saccus conjunctivalis ; b,_epithelium tunicae palpebrarum ; c, epithelium tunicaa bulbi; d, epithelium intrerpalpebrale; e, epithelium fornicis laterelis.
Cord F is not shown to join the conjunctival epithelium, and this fact strongly suggests the loss of a section at that point. The smallest cords are more medial as a rule. Cord B, ﬁgs. 8 and 13, has a diameter of 484+ micra. In the cords that have been cut longitudinally, the central cells are clearer and sparser than those at the periphery. Cord F, ﬁg. 12 (section 128), shows a well-marked line bounding the apical surfaces of the medial layer of the peripheral cells, which are at least two layers deep. The central part of this cord has a homogeneous and hyaline appearance, with an apparent coagulum which has stained more or less deeply. The conjunctiva} epithelium _is compact and in continuity at the point of junction’ with the cords.
Fig. 13. Sagittal section through the anlagen of three cords of cells of the right lacrimal gland.
Fig. 14. Drawing of a reconstruction of the developing parotid gland. Reduced 0.5. D, ductus parotldeus ; 1, 2, 8, primary rami; a, b, c. d, a, secondary divisions or buds.
The surrounding mesenchymal tissue is condensed and crowded with nuclei, but there is no evidence of a distinct diﬁerentiating sheath. As usual, a heavily stained line marks the basal attachment of the cells to the mesoblast. Most of the cords have shrunken away from the surrounding mesoblast, leaving a clear and unstained space between the two, which is crossed by ﬁne ﬁbres connected with the base of the epithelial cells. Nerves and capillary blood-vessels are present among the anlagen, which ﬁg. 13 shows in greater detail. A nerve is in close proximity to cord D (mlde ﬁg. 8). ’
In sagittal section, and as it appears in the series of sections, the lens is regularly oval in outline. Its antero-posterior diameter is 731‘2+ micra., and its vertical diameter 877'5 micra. At the anterior pole the epithelium is about one-fortieth of the antero-posterior diameter of the lens, or 181 + micra. '
The parotid gland has been reconstructed (ﬁg. 14) after the methods of Schaffer and Born. It is less developed than the submaxillary glands, for they have Well-developed connective-tissue sheaths surrounding the ducts and acini, and as a whole stand out distinctly among the other and less conspicuous structures (ﬁg. 16).
The parotid gland arises from the buccal epithelium. In embryo No. 9 the epithelial cord can be followed laterad to the masseter, around which it curves, being in close relation with it. Just after leaving the lateral border of the muscle a division into what may be termed its three primary rami takes place, and these in turn possess buds which represent secondary divisions. Ramus 1 (ﬁg. 14), which is the oldest, or perhaps more correctly a direct continuation of the main cord, lies exceedingly close to the carotid artery at its distal extremity; but it does not come in contact with it, nor with any of its larger branches. The veins occupy a similar relation, and the adult relationship has not yet become apparent. The course and relations of the cylindrical cord and its divisions may best be indicated by considering it in four portions as follows:—
1st. It is straight for a. very short distance as it passes laterad from the buccal epithelium and beneath the external maxillary artery.
2nd. Then follows a very slightly tortuous portion as it passes through the buccal mesenchyma to the medial margin of the masseter.
3rd. It then arches across, passing external to the masseter to its lateral margin.
4th. The ﬁnal portion enters the region between the masseter and the ramus of the mandible medially and the external auditory meatus laterally. In this portion division and budding occur, producing the various rudimentary ducts and acini. The mesenchyma, which immediately surrounds the duct, has become condensed, and later forms the connective-tissue elements of the duct and sheath. This differentiation is most evident at the proximal end of the duct and gradually less so to the point of ramiﬁcation ; for the fourth portion is surrounded by undifferentiated mesenchymal tissue. To be sure, changes are going on in the mesoblast, but none are recognisably concerned in the development of the sheath of the gland.
Where the duct leaves the buccal epithelium it has a diameter of 696+ micra, with a lumen 23'1+ micra. Fig. 15 represents a transverse section of the duct where it approaches the masseter; although the section is very slightly oblique, it gives a correct picture of the relative proportions of the Walls to the lumen. The lumen is closed in the superﬁcial portion of the buccal epithelium, for it can be traced only partly through it. Just before the primary divisions the duct has a diameter of 454+ micra, and the lumen, although less sharply deﬁned, because more or less ﬁlled with broken-down cells, can nevertheless be traced into the rami as far as the buds. The epithelial cells, forming the walls of the duct and its divisions, are irregular in outline and possess large nuclei. Fig. 15 shows that the basal cells are not arranged in a regular layer, and do not exhibit any evident differentiation. Their bases are marked by a heavily stained line. Since the epithelial rami have shrunken away from the surrounding mesoblast to a greater or less extent, a clear unstained interval separates the two. "This space is crossed by very ﬁne strands, some of which are incomplete and project from the mesoblastic surface or from the bases of the peripheralepithelial cells. Ramus 1 (ﬁg. 14) has a diameter of about 454+ micra at base, and a distinct lumen 9'0 micra in diameter. Ramus 2, which also has a distinct lumen, has a diameter of ' 454+ micra. Ramus 3, which is probably the youngest of the three, has a diameter of 484+ micra at the base, the only point where it possesses a lumen. Bud a, which has no lumen, has a diameter of about 62'1+ x81'8+ micra at the middle, and 348+ micra at the base. Bud b has a very narrow base. Bud c, which is 68'] + micra in diameter at the base, is large, terminal, and knotlike, and apparently was beginning to develop two smaller outgrowths from its medial surface. Its greatest diameter is 106'0+ micra. Bud d has a much greater diameter at the base than bud (a), and a lumen has also apparently begun to appear.
Fig. 15. Transverse section of the ductus parotideus. 0c. 3 ; obj. E. Reduced 0.5.
Fig. 16. Section of the submaxillary gland. 0c. 3 ; obj. A. Reduced 0.5.
In sections 437 to 51 a solid cord of cells, that ends blindly and has no connection with the parotid duct, is present. From its position near the medial border of the masseter, it is to be considered the branch observed by Chievitz. M‘Murrich states that the signiﬁcance of this structure is at present uncertain (Humor/n Embrg/ology, vol. ii., 1912, p. 348).
At a short distance from the frenulum linguae, in the ﬂoor of the embryonic mouth, the submaxillary duct has a diameter of 568+ micra, It has a distinct lumen 233+ micra in cross—section, with walls about 16'6+ micra thick. The outlines of the cells are not evident and the nuclei are about three deep. In the four sections studied there is some variation in the relative proportions of the Walls and lumen of the duct. One section shows the lumen a mere ﬁssure with the walls practically in contact, suggesting that this condition may be accidental. With the high moist power it can be seen that the basal cells have separated from the contiguous mesenchyma, and that very ﬁne strands pass between the two.
Derivatives of the Neural Epiblast
The two layers of the ophthalmic cup are not in contact, but are separated by a considerable distance; the pigment layer is closely approximated to the undifferentiated chorioid as far forward as the anterior margin of the cup. The retinal layer, which becomes gradually thinner from the position of the developing ora serrata to the margin of the cup, is irregular where the ciliary processes develop later. At the periphery of the lens the retinal layer has a diameter of about 4.«9'9+ micra, and is about four or ﬁve cells thick. The pigment of the external layer does not extend beyond the angle of the margin. This layer has a width of about 238+ micra. The optical part of the retinal layer at the fundus of the cup is 1165 + micra thick where the line of sectioning is perpendicular to the surface. The pigment layer at the same place has a thickness of 121+ micra. The margin of the ophthalmic cup, as observed in the sections, lies at the periphery of the lens, at about the junction of its anterior and middle thirds. The length of the ciliary and iridal portions of the ophthalmic cup, as measured in a straight line from the ora serrata to the anterior margin, is about 406'2+ micra.
The optic nerve, the vertical diameter of which is 357 '5 micra,'is solid and shows no evidence of its previous hollow state.
The greatest length of a hemisphere, measured from the frontal to the occipital pole, is 90 mm. The thickness of the hemisphere varies according to the degree of growth and differentiation in different regions. Measurements made on the larger medial sections, where the plane of sectioning is quite perpendicular to the surface, gives the following results :—
Superiorly and at the frontal pole the thickness of the wall varies from 260 to 1787+ micra, and similarly at the occipital pole the thickness varies from 138'1+ to 1056+ micra. That part of the anterior olfactory lobe, from which the bulbus and tractus olfactorius are developed, appears as an evagination directed vertically downward, the apex of which is in relation with the membranous lamina cribrosa, The ﬁla olfactoria can be seen entering at the dorsal part of the apex. The evagination, which is 12676 micra long, was measured from a line cutting the base at its junction with the hemisphere. The length of the cavity is 1'0+ mm.
The walls and cavity have (section No. 2, Series IX. A) the following dimensions in micra :—
Anterior wall. Cavity. Posterior wall. At base . . . 162'5 81'2 + 3737 + At middle. . . 2437 + 130'0 292'5 At apex . . . V 357‘5 130'0 146‘2+
Ganglia of the Dorsal Roots
The dorsal ganglia vary more or less in size and shape. In the lower cervical and thoracic regions they are situated in the intervertebral foramina.
The twenty-ﬁve transverse sections, which constitute Series IX. A of the lower cervical and thoracic regions were selected from different levels. Four sections have been selected as best representing the degree of development in the cord. At the level of the foramen magnum it has a sagittal diameter of 18+ mm. (Series IX. A, section 5.)
Fig 17-20. Transverse sections of the medulla spinalis, as observed under low power. x70. Reduced to 0.25.
Fig. 17, section 33, represents a section made transversely on a plane through the lower cervical region, cutting the trachea just below the larynx, the lateral lobes of the thyroid gland, and the upper part of the articulationes humeri. It is throughithe cervical enlargement, and shows that the myelon is nearly oval in section at that point. The surface line of the funiculi cuneati, as studied on a horizontal plane, is arcuate, and continues the line of the lateral funiculi, the latter being less arcuate in outline. On each side of the posterior median septum the funiculi graciles are very prominent, and consequently these funiculi form two ridges that are continued downward as far as the lower thoracic region at least, and there disappear. In section 33 the funiculi graciles are slightly divergent, and the re-entrant surfaces between them form an acute angle———the posterior median ﬁssure. The surface line of the ventral funiculi is less arcuate than the dorsal, and a slight angle is evident ventro-laterally. The surface is slightly sunken just ventral to the posterior roots (ﬁg. 17, a), and is most pronounced in the cervical and superior thoracic portions of the cord. The anterior median ﬁssure is broad and rounded at the bottom. A commissura anterior is present. The central canal is large and pointed ovate in shape.
The posterior and anterior grey columns are large and have broad connexions on each side. Across the median plane a posterior grey commissure is more or less distinct dorsal to the degenerating ependymal cells. The posterior grey columns are noticeably larger than the anterior.
The measurements of the cord are as follows (fig. 17) :
- Sagittal diameter 1446.2+ micra.
- Transverse 1251.2+ micra.
- Central Canal
- Sagittal diameter 503.7 +micra.
- Transverse 211.2+ micra.
- Vertebral Canal
- Sagittal diameter 1787.5 micra.
- Transverse 1727.5 micra.
Fig. 18, section 48, represents a section made transversely on a plane through the base of the neck just above the cartilaginous manubrium, below the thyroid gland and articulationes humeri and through the upper part of the thymus gland. At this level the cord is less oval and somewhat pentagonal in outline. The angle at the junction of the dorsal and lateral-surfaces is more marked. The sides are less arcuate, more parallel, and arch into the surface of the ventral funiculi. The ‘surface of the funiculi cuneati is straighter and somewhat oblique, giving an ogival effect to the outline. The prominence of the funiculi graciles is about the same as at a little higher level, but the re—entrant surface between them is less acutely angled. The surface just ventral to the dorsal roots is the same as at a higher level.
The measurements are (ﬁg. 18) :—
- Sagittal diameter 1300.0 micra.
- Transverse 1202.5 micra.
- Central Canal
- Sagittal diameter 471.2+ micra.
- Transverse 195'0+ micra.
- Vertebral Canal
- Sagittal diameter 1706.2+ micra.
- Transverse 1576.2+ micra.
Fig. 19, section 35, represents a section made on a plane through the atrio-ventricular oriﬁces of the heart. At this level the cord is still more pentagonal in section. The lateral surfaces are straighter ventro-dorsally Measurements on a Human Embryo 30 mm. long 195
and the surface lines but slightly arcuately re-entrant. They are marked both dorso-laterally and ventro-laterally by somewhat evident angles from the surface of the cuneate and ventral funiculi respectively. The surfaces of the ventral funiculi are arcuate, and the anterior median ﬁssure is more acutely angular at the bottom. The dorsal funiculi are shaped about the same as at the higher level at the base of the neck, but the funiculi graciles
FIG. .l 9. F19, 20,
Transverse sections of the medulla. spinalis, as observed under low power. x 70. Reduced to 1.
are less prominent; the re-entrant surface between them is correspondingly shallower and broader. The central canal is more rhomboidal in crosssection than at the level previously described. The sunken surface ventral to the dorsal roots is not noticeable, and there is still greater disparity between the grey columns.
The measurements are (ﬁg. 19) :—
- Sagittal diameter 1015.6 + micra.
- Transverse 1121.2 + micra.
- Central Canal
- Sagittal diameter 406.2 micra.
- Transverse 95.0 micra.
- Vertebral Canal
- Sagittal diameter 1348.7 micra.
- Transverse 1592.5 micra.
Fig. 20 was drawn from section 31, which was on a horizontal plane through the liver and diaphragm, at the caudal extremity of the sternum. At this level the myelon is less pentagonal and more quadrate in section. The surface lines of the lateral funiculi are straighter and more parallel; the dorso-lateral and ventro-lateral angles are marked, the latter more rounded. The surface of each ventral funiculus is arcuate from side to side, and the anterior ﬁssure shallower and less acutely angled at the bottom. The surfaces of the funiculi cuneati are straighter on each side, and the dorsal outline less ogival. The funiculi graciles are less prominent and the intervening surface scarcely re-entrant. The central canal is rhomboidal.
The measurements are (ﬁg. 20) :—
- Sagittal diameter .991.2 micra.
- Transverse 1137.5 micra.
- Central Canal
- Sagittal diameter 4062+ micra.
- Transverse 219.3+ micra.
- Vertebral Canal
- Sagittal diameter 1332.5 micra.
- Transverse 1462.5 micra.
For the sake of accuracy it should be stated that the sections are not cut on a perfectly horizontal plane, but slightly diagonally. However, this obliquity is scarcely enough seriously to affect the diameters.
The lumen of the central canal has undergone considerable obliteration between the dorsal zones. The canal is also evenly curved from side to side across the ﬂoor—plate. If any angles have existed they have become obliterated, and this is a probable explanation of the deeply stained ependymal cells observed each side of a centrally located clearer area in the floor-plate.
Derivatives of the Mesoblast
The cartilaginous skeleton is well developed and centres of ossiﬁcation for the following bones have appeared :—
The cartilaginous outline is already involved in ossiﬁcation.
Mandible and Maxilla
The most conspicuous centre is that for the lower border and front of the alveolar process. None of the sections show that the distal end of Meckel’s cartilage is involved in the process of ossiﬁcation, and there are two other small centres which have not been clearly identiﬁed. One of these may be the centre for the coronoid process. The other consists of two punctiform areas lying between and below the proximal end of Meckel’s cartilage and the tympanum. A well-developed centre is present for the maxilla.
A distinct centre lies over the cartilaginous nasal capsule.
A thin delicate centre lies over the side of the nasal capsule.
Two centres are apparently present.
Ossification is Well under way in the orbital plate and lower portion of the vertical part.
There is no evidence of centres in the parietal and temporal membranes or cartilages.
The twenty-ﬁve sections constituting the selected series of transverse sections, Series IX. A, give negative results for centres of ossiﬁcation in the lower cervical and thoracic regions.
The sclera is well deﬁned from the surrounding orbital connective tissue. The chorioid—sclera has a thickness of 832+ micra posteriorly, at a point where it is sharply deﬁned by being in contact With a rectus muscle. The ﬁbrils of the sclera, as viewed in sagittal section, run parallel to each other, and are more or less undulating. The meshes which are elongately fusiform vary in width and size. The nuclei are elongate and have their long axes parallel to the ﬁbrils, with which they are in close contact. In the equatorial region, at the places of muscular insertions, and in the ciliary zone as well, they become much more abundant, and a large number of them are shorter, thicker, and more oval. A moderately large vessel, probably a Vena vorticosa, is occasionally present in crosssection in the sclera. No muscular tissue has yet differentiated in the ciliary body.
The corneal tissue is more condensed, and structurally resembles the sclera, into which it passes imperceptibly. The rmeshes between the ﬁbrils are narrower. The thickness of the substantia propria corneae is 63'6+ micra at the periphery, and 318+ micra at the middle. The corneal epithelium is two or three layers thick. The cells of the basal layer appear cuboidal, and the nuclei are relatively large, round, or slightly oval. The superﬁcial cells are ﬂattened and appear more or less fusiform in section. The epithelium has a thickness of 151+ micra. The sclera passes quite gradually, although somewhat abruptly, into the surrounding orbital connective tissue. Its ﬁbrils become farther apart, ﬁnally less parallel, with shorter and more irregular meshes.
The distance between the internal surface of the cornea and the anterior surface of the lens is about 339 + micra. This space is ﬁlled at the periphery with mesoblast, which disappears in the tissue of the sclera opposite the lenticular zone of the ophthalmic cup. Except for a very attenuated layer extending over the lens, and that constituting the corneal endothelium, which is 3'9+ micra thick, the mesoblast is absent centrally, leaving a space, the cavity of the aqueous humor.
The basal cells of the corneal epithelium rest upon a rather deeply stained and ﬁne line, that marks the anterior limit of the substantia propria corneae, but such a line is not evident at the posterior limit.
The chorioid, which is not clearly differentiated from the sclera, can only be recognised by the few capillary vessels lying immediately adjacent to the pigment layer of the ophthalmic cu p. These capillaries are less evident in the fundus, and are most distinct in the region of the ciliary body.
The internal layer of the ophthalmic cup is not in contact with the external or pigment layer, and the diameter of the vitreous chamber has not been measured. The greatest internal diameters of the eye are as .follows :——the vertical diameter between the internal surfaces of the chorioid-sclera is 1'8 mm; the antero—posterior diameter, measured as nearly as possible in the ‘optic axis, is approximately 20 mm. The measurement was made between the internal surface of the chorioid-sclera posteriorly and the internal surface of the corneal endothelium anteriorly.
The voluntary muscles are well developed, and those that are in longitudinal section show distinct striations. Much of the muscular tissue is stained either very slightly or very heavily, hence the striations could not be studied in all.
A7-m.—The diameters of the cartilaginous shaft of the humerus in the middle third are 6500 x 6450 micra, while the transverse diameter through the epieondyles is 1'4 +, mm.
F0/rea,rm.—The cartilaginous radius has a length of 38+ mm. The diameter of the head is O‘5 mm.
Thoracic Aorta.—The thoracic aorta is of uniform size and quite cylindrical in section. It is in close relation with the ventral surface of the cartilaginous centra of the thoracic vertebrae, being on an average about 211'2+ micra ventral to the same. The total diameter is 4062+ micra. The lumen measures 211'2+ micra, and the walls have a thickness of 866 + micra.
Contents of the Carotid Sheath.—The common carotid has a diameter of 178‘7+ micra, with a lumen of 766+ micra, and a wall of about 96'5-| micra thick. The internal jugular vein, which varies in size, measures 4«22‘5x739'3+ micra, and is distended with blood. The vagus nerve, although epiblastic, may be mentioned as having a diameter of about 133'2 X 2331 micra.
In one of the transverse sections the heart is cut longitudinally and through the atrio-ventricular oriﬁces. At the apex a broad and wellmarked sulcus longitudinalis is present. One cusp of the tricuspid valve has a thickness of 454+ micra, but the others are thicker and evidently have been sectioned obliquely. The left ventricular Wall has the following thickness in micra:—near the base, 2275; atimiddle, 276'2+; at apex, 178'7+. Similarly for the right ventricular wall:—near the base, 260'0; at middle, 1625; at the apex near thesulcus longitudinalis, 976. The septum at the middle has a thickness of 4876 micra, being thinner near) the base and apex. The above measurements do not include the trabeculae carneae.
DERIVATIVES or THE MESOBLAST AND HYPOBLAST.
(Esophagus.—-Three tunics are recognisable in the oesophagus—an epithelial, a muscular, and an intervening connective-tissue layer. The peri-oesophageal connective-tissue tunic is slightly denser in the neck than in the mediastinum; in the latter region it is delicate, loose, and stains very lightly. The oesophagus is largest at its commencement, and gradually diminishes in diameter from above downward, to a point slightly above the bifurcation of the trachea (ﬁg. 21, C to J), where it becomes smaller, more cylindrical, and nearly uniform in diameter and size, and continues so to the oesophageal opening in the diaphragm, the diameters being 4225 x 4.650 micra. Just below the termination of the pharynx the oesophagus has a diameter of § mm. (ﬁg. 21, C). It is contiguous to the trachea throughout the entire length of the latter, except for a short distance opposite the lower end of the larynx and the beginning of the trachea (ﬁg. 21, C and D). The interval of separation is about 1666 micra wide. Fig. 21, A and B, two sections through the lower extremity of the pharynx.
The epithelium has a mean thickness of 373 micra. The cells of the basal layer have become distinctly columnar and are clear and unstained at their bases. The other layers of the mucosa have not differentiated from the subepithelial mesenchyma, which forms a wellmarked tunic varying in thickness as a result of longitudinal folds which extend through the whole length of the oesophagus, below a point just above the bifurcation of the trachea (ﬁg. 21, J to X). Above this point the lumen is more open (ﬁg. 21, C to I) and the form of the oesophagus is transversely oval for a short distance, as a result of sagittal ﬂattening (ﬁg. 21, H and I).
Fig. 21. Outlines of transverse sections of the oesophagus and pharynx, at different levels, serially arranged. x 70. (Zeiss 0c. 3 ; B. & L., obj. 1 in.) Reduced to 0.33. A, through the termination of the pharynx; B, through the transitional point; C and D, (esophagus slightly separated from the larynx; E to J, oesophagus contiguous to the trachea; K, immediately below the bifurcation oi the trachea; K to M, oesophagus in relation to the aorta, beyond which point the latter is too distant to be shown in the figures; N to X, oesophagus in relation with the nervi vagi (above N the latter is too distant to be shown) ; X, a short distance above the hiatus cesophageus.
Rugae are present above a point opposite the bifurcation of the trachea. Since they are more irregular in development the lumen is more irregular in outline. The subepithelial mesenchyma, which has a mean thickness of about 81'0+ micra, is thinnest between rugae. Opposite them the thickness is much greater, being 66'6 + and 14.«1'7 + micra. The tunica muscularis has an average thickness of about 37‘8+ micra, but is a little thicker opposite the larynx.
The lumen varies in shape in different parts of the oesophagus, and the epithelial Walls are in contact nowhere. It is larger and more broadly open in the ﬁrst part, where it is irregular and not affected -by distinct rugae. In the lower part of the pharynx and the beginning of the oesophagus Where it is connected to the larynx by condensed mesenchymal tissue, two distinct and well—marked parallel prominenccs, which project into the lumen, are present in the ventral wall (ﬁg. 21, A and B). At the beginning of the oesophagus the lumen has a transverse diameter of about 3576 micra (ﬁg. 21, C); and as the oesophagus becomes more distinctly cylindrical in the region of the tracheal bifurcation, the three longitudinal rugae become distinct. The largest is the dorsal and more or less slightly sinistral in position. The other two, which are ventro-lateral in position, are subequal, and scarcely one-half as prominent as the dorsal ruga. Consequently the lumen is tri-radiate in form as seen in transverse section; narrowest below the tracheal bifurcation, and becomes slightly larger as the diaphragm is approached.
The relation of the oesophagus to the trachea and aorta already indicates the adult condition. In determining their relative positions, a line was drawn on the camera lucida outlines of the sections, to represent the median sagittal plane passing through the anterior median ﬁssure and the posterior median septum of the spinal cord, and the chorda dorsalis. In some sections, especially those of the upper thorax, this line cut the sternal cartilage at the middle, but lower down where some asymmetry exists it passed sinistrad to the sternum. The right side of the thorax appears slightly more prominent than the left. At the termination of the pharynx the oesophagus is directly dorsal to the larynx; lower it becomes gradually and slightly more sinistral, while still lower it appears to be in the median plane, with the trachea distinctly more dextral down to its point of division. At its beginning the thoracic aorta is sinistral to the median plane, and the oesophagus median on a plane ventral to it. That is, a frontal plane passing between the two would touch the ventral wall of the aorta and the dorsal wall of the oesophagus, which are about 0'06 mm. distant from each other. At a lower level the aorta becomes slightly more medial; the oesophagus more ventral, or rather ventro-dextral and distant. As the diaphragm is approached both are sinistral in position as regards the median plane, the oesophagus being distinctly ventral to the aorta. A short distance above the oesophageal opening in the diaphragm the oesophagus is 0'5 mm. distant from the aorta. At its commencement it lies about 34]'2+ micra ventral to the vertebral column; below the tracheal bifurcation it is about 520'O micra distant, and but a short distance above the diaphragmatic opening it is 1'1+ mm. ventral to the same.
Trachea.—Immediately caudal to the larynx the trachea is oval in section and the sagittal diameter is greater than the transverse. The oval shape may in part be due to an obliquity of sectioning, although a short distance farther caudad it becomes cylindrical. A short distance cephalad to the bifurcation it is slightly transversely oval. Cartilaginous rings are present. It has a uniform shape between the points above mentioned. In its cylindrical portion the diameters are 6337 +-X601'2+ micra; with a lumen of 4«38'7+ ><373‘7+ micra, and an epithelium 22'7+ micra thick. In the transversely oval portion, just above the bifurcation the transverse diameter is 6500 micra, and the sagittal diameter is 438'7+ micra.
In sections 12 and 13, Series_IX. A, the right bronchus is transversely and the left bronchus longitudinally cut. In the right bronchus, cartilages are present. The diameters are 487‘5x325'O micra, and that of the lumen about 268'1+ ><228‘5 micra. The thickest part of the Wall with the cartilage is 156‘5+ micra.
In the left bronchus the cartilages disappear before the hilum of the lung. The sagittal diameter is 2925 micra, and the thickest part of the Wall with cartilage is about 111‘5+ micra.
The greatest diameters of a lateral lobe are 568'7+ X 4876 micra. The largest parathyroid, which is quite cylindrical in section, measures 1864+ x 1332 micra in diameter.
Both lobes taken together have a transverse diameter of 10+ mm., and a sagittal one of 4875 micra. They gradually taper as they ascend into the neck to a short distance above the cartilaginous manubrium, Where the apices appear equal and cylindrical in section.
List of Measurements
|Vertex to breech||30 mm|
|Hip to knee||3.0 mm|
|Knee to heel||6.0 mm|
|Estimated standing height||39.0 mm|
|Head||Median sagittal section of head|
|(a) Diameter on a plane cutting a point between the occipital cartilage and atlas, through the anterior nasal spine||10.0 mm|
|(b) Greatest sagittal diameter||12.5 mm|
|(c) Height of the head. Obtained by drawing a line at right angles to that from which (a) was made.||11. 0 mm|
|(d) Height as measured from the occipital cartilage at the anterior margin of the foramen magnum to the vertex||12.5 mm|
|Transverse diameter through the shoulders just cephalad to the margin of the cartilaginous manubrium and close to the articulationes humeri||10.0 mm|
|Sagittal diameter in the same plane||6.5 mm|
|Transverse diameter on a plane through the base of the heart||9.5 mm|
|Sagittal diameter in the same plane||8.0 mm|
|Transverse diameter on a plane through the mammary anlagen||9.5 mm|
|Sagittal diameter in the same plane||7.5 mm|
|Forearm and hand|
|Length from a point over the cartilaginous olecranon to the tips of the ﬁngers||7.0 mm|
|Maximum transverse diameter of the right anlage||166.5 micra|
|Depth to which they penetrate the mesoblast, from the surface||216.4 + micra|
|Thickness of epidermis over the anlage||246.2 + micra|
- The lacrimal cords, which are largest where they are in juxtaposition to the conjunctival epithelium, measure . . . . . . . . . 68.1 + micra
- From that point each gradually diminishes to 45.4+ micra
- opposite the medial border of the lacus lacrimalis, to again increase in size as their junction with the nasolacrimal cord is approached, the diameter of the superior cord being 48.4 + micra
- And that of the inferior cord 53.0 + micra
- The average diameter of the nasolacrimal cord is about 61.0 + micra
- That portion which corresponds to the position of the lacrimal sac, has a diameter of 60. 6 + micra
- It increases to 75.7 + micra
- And decreases to 42.4 +micra
- as the cord enters the nasolacrimal canal, and increases again to about 54.5 + micra
- as the nasal epithelium is approached. The greatest distance between the superior and inferior fornices of the conjunetival sac is 1.6 + mm
- The rudimentary semilunar fold projects into the sac to the distance of approximately 52.0 micra
- The longest developing duct of the lacrimal gland has penetrated the surrounding mesenchyma to the distance of 482.8+ micra
from the surface of the conjunctival epithelium.
- Cord B (ﬁg. 14) has a diameter of . . . . 48.4 + micra
- The lens has an antero-posterior diameter of . . 7312 + ,,
- and a vertical one of . 877'5+ ,,
- At the anterior pole of the lens the epithelium is about 181 + , in thickness, being approximately one-fortieth of the total antero-posterior diameter.
- The retinal layer at the periphery of the lens has a diameter of about . . . . 49'9+ micra
- and the external or pigment layer is about . 23'3+ ,, thick
- The optical part of the retinal layer at the fundus is about. . . . . . . 116'5+ ,, ,,
- and the "pigment layer . . . . . 121 + ,,
- The length of the ciliary and iridal portions as measured in a straight line from the ora serrata to the anterior margin of the cup is about . . . . . . . 4.«06‘2+ ,,
- The vertical diameter of the nerve is . . . . 375'5 micra
- The greatest length, as measured from the frontal to the occipital pole, is . . . . . . 90 mm.
- Superiorly and at the frontal pole the thickness of the wall varies from . . . . . 2600 to 178'7+ micra
- but at the occipital pole it varies from 138'1 + to 1056+ ,,
- Measured from a line cutting the base at its junction with the hemisphere at the surface has a length of 1267'5 ,,
- The total length of its cavity as measured from a. point on a line with the inner surface of the wall of the hemisphere is . . . . . . . 1 0+ mm.
|The Walls and cavity have the following dimensions in micra|
|Anterior wall||Cavity||Posterior wall|
|At base||162.5||81.2 +||373.7 +|
|At middle||243.7 +||130.0||292.5|
|At apex||357.5||130.0||146.2 +|
Measurement of cross sections in micra :—
Plane of section. Object cut. Sagittal. Transverse.
At level of foramen magnum . Cord 1'8+ mm.
. . . C d ’ 14«4«6‘2+ 1251‘2 Fig. 17.—Lower cervical regioni Cggtral canal 5037 + 2112: 3"“ °"““d"‘d ‘° the 1‘“‘-‘>’ ‘‘x’ l Vertebral canal 1787-5 1527-5 . C d 13000 12025
Fig 18 JuSt ceplialad to the] Cziitral canal 4712 + 1950 manubnum sterm Vertebral canal 17062 + 15762 + . . C d lO15'6+ ll21'2+ F13‘ 19: ‘ Th"°“gh the ”"t”°‘ Cgiitral canal 4062 195-0 "°n”‘°“1""' °riﬁ°eS Vertebral canal 1348-7 1592-5 . C d 9912 1137'5 F‘g‘ 2°‘-Thr°“‘-3h °”’“d"“1 ex‘ Cziitral canal 4062+ 2193+ t”e'“ity °f the sternum Vertebral canal 1332-5 1462-5
- At the point where it leaves the buccal epithelium the parotid duct has a diameter of 696+ micra, with a lumen of . . . . . 231 + ,,
- Just before the primary divisions it has a diameter of . . . . . . 4-5'5+ ,,
- Ramus 1 (ﬁg. 14) has a diameter of about . 454+ ,, with a lumen. . . . . . 9'0 ,,
- Ramus 2 has a diameter of . . . . 45'4.«+ ,,
- Ramus 3 ,, ,, . . . . 4«8'4.«+ ,, at the base
- Bud a has a diameter of abou . 62'1+ X 81'8+ ,,at the middle, and . .
Bud b has a. very narrow base. Bud c which is large has a diameter of about . 681 + ,, ,, and a greatest diameter of . . . 106‘O+ ,,
- The duct at some distance from the frenulum linguae has a diameter of . . . . 56'8+ micra
- with a lumen of . . . . . . 23'3+ ,,
- The walls are about . . . . . . 16'6+ ,, thick
34«'8 + v a
- The chorioid-sclera has a thickness of 83.2+ micra posteriorly at a point where it is sharply deﬁned through contact with a rectus muscle.
The thickness of the substantia propria corneas is . 63'6+ ,, at the periphery, and . . . . . . 318 + ,, at the middle.
The corneal epithelium has a thickness of . . . 15'] + ,,
The corneal endothelium . . 3'9+ ,,
and the distance between the internallsurface of the cornea and the interior surface of the lens is about . . . . . . . . 33'9+ ,,
Posterior chamber— The greatest vertical diameter between the internal
surfaces of the chorioid-sclera is . . . . 1'8+ mm. The antero-posterior diameter, measured as nearly as possible in the optic axis, is approximately . . 2'0 ,,
It is made between the internal surface of the corneal endothelium anteriorly and the internal surface of the chorioid-sclera posteriorly.
The diameters of the cartilaginous shaft of the humerus in the middle third are . . . . . 6500 x 645 micra And the transverse diameter through the epicondyles is 1'4+ mm. Forearm— The cartilaginous radius has a length of . . . 3'8 + mm. The diameter of the radial head is . . . . 0'5
Thoracic aorta he average distance ventral to the cartilaginous
centra of the thoracic vertebrae is . . . . 211'2+ micra The total diameter is about . . . . . . 406'2+ ,, The lumen measures . . . . . . . 211'2+ ,, And the walls have a thickness of . . . . 86'5+ ,, Contents of the carotid sheathThe common carotid artery has a diameter of . . 178'7+ ,, with a lumen . . . V . . 76'5 +_ ,,
and walls with a uniform thickness of about . 96‘5+ ,,
The internal jugular vein, which varies in size and is distended with blood, measures . 4226 x 739'3+n1icra The vagus nerve is about . 1332 X 2331 T ,,
A cusp of the tricuspid valve has a thickness of . 454+ ,, The ventricular walls have the following thickness in micra: Left wall near base 2276 at middle 276'2+ at apex . . 178'7+ Right wall near base . . 2600 at middle . . . . . . 1625 at apex near the sulcus longitudinalis . ._ 975 The interventricular septum at the middle has a . thickness of . . . 487'5 micra
but is thinner near the base and apex. ‘The above measurements do not include the trabeculae carneae.
A nearly uniform size is maintained between a point opposite the bifurcation of the trachea and a short distance above its passage through the diaphragm.
The diameters at this point are and just below the termination of the pharynx it measures . . . . . . .
The maximum distance of separation from the trachea, at a point opposite the lower end of the larynx, is
. . 422'5 X 4550 ,,
The tunica presents the iollowing variations in thickness :—
Epithelium, maximum thickness . minimum thickness . mean thickness . . . . . The muscularis has an average thickness of about The sub-epithelial mesenchyma has a minimum thickness between the rugae of . . . A maximum thickness at the largest ruga of at the smallest with a mean thickness of . . . . . The transverse diameter of the lumen. of the oesophagus at the beginning is . . . . . and below the bifurcation of the trachea about
$1 mm. 166'5 micra 43'9 ,, 30'3 ,, 37‘3 ,, 37 ‘8 + ,, 34-"3 + ,, l4l'7 + ,, 666 + ,, 810 + ,, 357'5 ,, 199'3 ,.
The distance of separation from the thoracic aorta at the beginning of the descending portion is . . 95'4«+ micra and a short distance above the oesophageal opening in the diaphragm it is . . . . . . 0'5 mm.
The distance of the oesophagus from the vertebral column at the beginning is . . . . . 3412+ micra below the tracheal bifurcation, about . . . 520'0 ,, and a short distance above the opening in the diaphragm it is . . . . ‘ . . . 1'1 + mm.
In its cylindrical portion the diameters of the trachea are . . . . . . . 6337 + x 601'2+ micra and the lumen measures . . . 438‘7+ X 373'7+ ,,
The thickness of the epithelium is. . . . . 22'7+ ,,
Above its bifurcation the diameters are as follows :—
Sagittal . . . . . . . . 4387+ ,, Transverse . . . . . . . 650'0 ,,
- The right bronchus measures . . . . 4875 X 3250 ,,
- the lumen . . . . . . . 268'1+ X 2285 ,,
- and the wall with cartilage about . . . 156'5+ ,,
- The sagittal diameter of the left bronchus is . . 292‘5 ,,
- and the wall including the cartilage is about 111'5+ micra thick
- The greatest diameters of a lateral lobe are . 568'7+ X 4875 micra
- And the largest parathyroid measures Q . 186‘4s+ x 1332 ,,
- The transverse diameter through both lobes is . . 1'0+ mm.
- and the sagittal diameter of a lobe is . . . 4875 micra
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Cite this page: Hill, M.A. (2020, June 3) Embryology Paper - Measurements on a Human Embryo 30 mm Long. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Paper_-_Measurements_on_a_Human_Embryo_30_mm_Long
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