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| {{Header}}
| | #REDIRECT [[Paper - A presomite human embryo (Shaw) with primitive streak and chorda canal with special reference to the development of the vascular system (1941)]] |
| {{Ref-GladstoneHamilton1941}}
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| {| class="wikitable mw-collapsible mw-collapsed"
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| ! Online Editor
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| |-
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| | [[File:Mark_Hill.jpg|90px|left]] The author suggests an "estimated age is 31 days", that would be Carnegie stage 14. The appearance of the upper and lower limb buds are more like a later [[Carnegie stage 16]] embryo occurring in [[Week 6|week 6]], 37 - 42 days, CRL 8 - 11 mm.
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| |}
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| {{Historic Disclaimer}}
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| =A Presomite Human Embryo (Shaw) with Primitive Streak and Chorda Canal with special reference to the Development of the Vascular System=
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| By R. J. Gladstone And W. J. Hamilton
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| Department of Anatomy, St Bartholomew’s Hospital Medical College
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| ==Introduction==
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| THE embryo was presented to the Anatomy Department of the Medical
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| College of St Bartholomew’s Hospital, London, by Dr Wilfred Shaw. In
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| appreciation of the gift we have associated the name of Dr Shaw with this
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| embryo. ‘
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| The embryo shows many features of interest in the development of the
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| angioblastic tissue and blood cells. These features have been described at some
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| length, since they represent an intermediate stage of development which
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| throws considerable light on the relations between angiogenesis and blood
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| formation. In the chorionic mesoderm, including that of the villi, the outstanding feature is the presence in the mesenchyme of angioblastic strands of
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| nucleated protoplasm which, by vacuolation, are becoming transformed into
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| sinuses lined by endothelium and capillary vessels, of which the majority appear
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| to be empty and contain at this stage of development no cellular elements. In
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| the yolk sac haemopoiesis dominates and angiogenesis appears to be secondary.
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| This intermediate stage bridges over a gap in the history of our knowledge of
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| the development of the vascular system in the human subject and confirms the
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| opinion of Streeter (1920), Ingalls (1921), and other authors with regard to the
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| early and advanced stages of angiogenesis in the chorionic mesoderm as compared with blood formation, while in the yolk sac the blood islands often appear
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| before the endothelial walls which are subsequently developed around them.
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| A short description of this embryo was published by Dr Shaw in 1932, and
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| the following account of the clinical history of the case is based on his original
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| article. ’
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| The embryo was obtained at a subtotal hysterectomy performed by Dr
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| Shaw, on a woman aged 32, for the relief of a right cornual uterine myoma.
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| The uterus was opened immediately after removal. A small prominence, about
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| 1 cm. in diameter, was observed on the posterior wall. This was excised and
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| fixed in Camoy-sublimate solution for 1 hr. and then passed through alcohol,
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| xylol, benzene and wax in the usual way. Dr Shaw cut a complete series of
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| sections of the whole block of tissue in which the embryo was embedded, at
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| 10 p. thickness. The histological condition of the specimen proved to be exceptionally good. and the embrvo was neither diseased nor macerated.
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| 10 R. J. Gladstone and W. J. Hamilton
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| The woman had had two previous pregnancies. One child had been born at
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| term in 1922, and this was followed by menorrhagia, but the sequence of the
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| menstrual cycle was unaltered; in 1926 the patient had a miscarriage during
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| the fourth month. On 17 February 1930 she came to St Bartholomew’s
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| Hospital complaining of an abdominal tumour and abdominal pain. She had
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| last menstruatedl between 12 and_l7 January 1930. Her menstrual period was
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| one of 28 days; the next period should accordingly have started on 9 February,
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| 8 days before she came to the hospital.
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| Rising from the pelvis was an abdominal tumour 4 by 3 in. in diameter,
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| which on bimanual‘ examination was located in the right comu of the uterus.
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| The patient was admitted into the hospital, and the operation of subtotal
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| hysterectomy was performed 2 days later, on 19 February. The left ovary was
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| removed and found to contain a corpus luteum.
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| In discussing the data bearing upon insemination, ovulation and fertilization, Dr Shaw ascertained that the probable date of insemination was
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| 24 January. A discrepancy in the history occurred between the patient’s
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| assertion that the February period was due on the 12th and the calculated
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| period 9 February, when the period should have appeared in conformity with
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| her normal period of 28 ‘days. The dating of the specimen, calculated from
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| 19 February, the date on which the operation was performed, is as shown in
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| Table 1.
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| Table 1
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| Number of days
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| preceding the
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| Accredited dates of menstruation, insemination, ovulation, operation date,
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| and assumed age of embryo 19 Feb.
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| From onset of last menstruation (12 Jan.)' 38
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| From assigned date of insemination (24 Jan.) 26
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| From assumed period of ovulation (25-28 Jan.) 25 to 22
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| From calculated date of expected menstruation (9 Feb.) 10
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| From onset of next menstruation expected by patient (12 Feb.) 7
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| Assumed ‘fertilization age’ of embryo 23 to 20
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| Assuming that fertilization took place within 48 hr. of ovulation, the age
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| 'of the embryo, according to the data given above, would be from 20 to 23 days.
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| Presomite embryos belonging to group E of Bryce’s classification (1924) orto
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| groups V or VI of Hertig’s classification (1935), with which this -embryo
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| corresponds, namely, embryos with a completed notochordal process‘(head
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| process of many authors) and chordal canal (archenteric canal of many authors),
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| but before the development of neural folds, are usually thought to be younger
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| than this, e.g. the Heuser (1932) presomite embryo (Carnegie, no. 5960) has
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| an estimated ‘ovulation age’ of 18-19 days, as contrasted with that of the
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| present Shaw embryo of 20-23 days.
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| In discussing the relation of ovulation to fertilization, Dr Shaw, after
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| stating that the general belief is that ovulation is limited to the intermenstrual
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| phase of the cycle, mentions that, in his opinion, ‘ovulation is limited to
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| A presomite human embryo 11 | |
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| between the 13th and 16th days of the menstrual cycle, the first day of the
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| period of bleeding being taken as the first day of the cycle’, and that, ‘in the
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| human subject it is uninfluencedi by such external factors as coitus’ (see also
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| Shaw, 1925). | |
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| Further, he stated that, in his view, ‘since it is obvious that fertilization
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| cannot precede ovulation, the union of spermatozoon and ovum must be
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| regarded as being restricted to the latter half of the menstrual cycle, but this
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| view does not imply that insemination before the date of ovulation must be
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| sterile, there being much statistical and clinical evidence that coitus may be
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| fertile during the first half of the menstrual cycle (Siegel, 1916). ‘
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| Allen et al. (1930), who have recovered, living ova in washings from the
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| uterine tube, and who have studied the young corpora lutea, believe that
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| ovulation takes place on or about the 14th day of the cycle. The recovery of
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| ova is the most convincing method of demonstrating that ovulation has
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| occurred.
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| The work of Knaus (1934), Venning & Browne (1937), and others confirms
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| the view that ovulation occurs around the 14th day of the cycle.
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| It is possible that this difference in the degree of development of the Shaw
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| embryo, as compared with others, may be explained by retardation of development due to the existence of a myoma of the uterus, although, judging from
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| the sound condition of its tissues, the embryo itself does not seem to have been
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| diseased. Other possibilities are: (a) ovulation had occurred at a later date in
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| the cycle than between 25 and 28 January (see Flew, 1941), (b) a delay in
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| fertilization had taken place—in other words, there was an increase in the
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| period between ovulation and fertilization, and (c) the date of fertile insemination was not 24 January, as the patient admitted. that coitus interruptus
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| had been frequently practised during January and February. '
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| GENERAL DESCRIPTION AND DIMENSIONS OF
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| THE SPECIMEN
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| The chorionic vesicle which is embedded in the compact layer of the
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| decidua is almost completely covered by a decidua capsularis. "It belongs to
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| group E of Bryce’s classification (1924«)——in which a primitive streak and cloacal
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| membrane are differentiated and a complete chorda canal has been formed;
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| in the case of the Shaw embryo the canal appears to be a blind diverticulum.
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| The intrachorionic rudiment (amnio-ectodermic and yolk-sac vesicles) is surrounded by the chorion with its villi and has the embryonic plate included
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| within theeopposed walls of the two vesicles of which the yolk sac is the larger.
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| The intrachorionic rudiment is connected with the internal aspect of the wall
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| of the chorionic vesicle by a connecting stalk which consists of two parts, a
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| basal ‘amnio-embryonal’ segment, and a proximal ‘umbilical’ segment containing an allanto-enteric diverticulum (Text-fig. 3).
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| The principal measurements of the specimen are given in Table 2.
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| 12 R. J. Gladstone and W. J. Hamilton
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| Some of these measurements have been made directly from the sections
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| mounted on the slides, others have been estimated indirectly by counting the
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| number of sections (10 p. thick) in which a particular part appears in the series.
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| Owing to shrinkage and the obliquity of the sections to the longitudinal axis
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| of the embryonic plate the measurements must be regarded as approximate
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| (see Text-fig. 1). .In the case of an elliptical disc obliquity of the sections
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| relative to the axis-will cause a diminution in the estimated length of the
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| longitudinal axis, and an increase in the width, which will be proportional to
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| the degree of deviation of the plane of the sections from that of the embryonic
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| disc; the estimation will also be influenced by the difierenceybetween the length
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| of the longitudinal or axial plane of the ellipse and the width of the ellipse.
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| Table 2
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| ‘ Approximate measurements ' mm.
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| Maximum horizontal diameter of infiltrated zone around the 15
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| implantation cavity
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| Depth of infiltrated zone, including the closing plate 5
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| Chorionic vesicle: maximum external diameter, including villi 11
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| Chorionic vesicle: vertical external diameter 4-04
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| Chorionic vesicle: maximum internal diameter . 8
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| Chorionic vesicle: vertical internal diameter 3
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| Embryonic rudiment » 1-66 x 1-34
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| Length of embryonic disc 1~o5
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| Body stalk 0-61
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| Amnion 1-35 x l~34 x 0-32
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| Yolk sac 1-21 x 1-35 x 040
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| Allanto-enteric diverticulum 0-35
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| Cloacal membrane 0-09
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| Definitive _ 0-05
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| Caudal 0-04
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| Primitive streak ' 0-43
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| Preblastoporic segment of disc 0-62
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| Hensen’s knot 0-04
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| Notochordal process A 0-17‘
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| \
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| Another well-known circumstance which renders the measurement of
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| embryos difficult is the development of the normal sagittal and transverse
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| curvatures of the embryo, andflthe occasional presence of abnormal twists and
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| tilts: Towards the end of the presomite phase of development the ventral
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| flexure of the caudal end of the embryo has a marked effect upon the estimate‘
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| of the length of the primitive streak, and at a. later period when the cephalic
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| flexure is being formed estimation of the length of the embryo by the simple
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| method of enumerating the number of sections is still more profoundly affected.
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| In the specimen under consideration, the ventral bend of the caudal segment
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| of the primitive streak renders any measurement of its length, except upon a
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| model, extremely difficult, since a very considerable extent of its total length
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| is included in two or three sections. This is due to the plane of the sections
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| which pass through the terminal ventrally flexed part of the primitive groove
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| A presomite human embryo 13
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| being somewhat in the longitudinal axis of the groove, instead of transversely
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| as in its anterior part.
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| In many specimens, owing to twists around the longitudinal axis and bends
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| in a lateral direction, measurements based on linear reconstructions are also
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| liable to be fallacious. An indication of the amount of error that may occur by
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| simple enumeration of sections, even in a perfectly ellipsoidal or approximately
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| Text-fig. 1. AOBD is an ellipse representing the outline of an embryonic disc in which it is
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| assumed that the primitive streak and groove occupy the lower half of the major axis of the
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| ellipse, and that the serial sections have been cut at right angles to the major axis. A’0’B’D’
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| represents an embryonic disc of corresponding size and form the sections of which have been
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| cut at an angle of 30° with the major axis of the ellipse. The maximum length of the disc, if
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| calculated from the number and thickness of the sections which include the major axis,
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| would be shortened by 0-22 mm. and the primitive groove would also be proportionately
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| shortened by 0-11 mm. Y in the interrupted line X Y Z represents the position in which the
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| primitive groove would appear in relation to the edges X, Z of the embryonic disc ina section
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| cut in the position which is indicated in the figure. .
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| circular embryonic disc, will be gained by a reference to Text-fig. 1, and also
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| the advantage which is gained by an attempt to make a reconstruction model
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| will be appreciated, even though an orientating or basal line may be absent.
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| In the present case, the absence of a base-line was overcome by making use of
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| a portion of the wall of the chorionic vesicle in the region of the embryonic
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| rudiment.
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| 14 R. J. Gladstone and W. J. Hamilton
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| The choriomlc vesicle
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| The chorionic vesicle is flattened in such a way that its major axes lie
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| parallel to the membrane lining the posterior surface of the uterine cavity in
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| which the vesicle was embedded. The external surface of the sac is covered
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| throughout its whole extent by villi, and these are more differentiated and
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| larger at the circumference than at the embryonic pole (future placental site)
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| or at the abembryonic pole (next the lumen of the uterus). '
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| The epithelial layer of its wall shows a distinct subdivision into cytotrophoblast and syncytiotrophoblast '(Pl. 3, figs. 7, 8). The extent and relations
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| of the latter to the uterine mucosa will be described in a subsequent publication.
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| The cytotrophoblast has the usual characters of this layer in embryos at this
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| stage of development. .Mitotic figures are occasionally visible in it, and connexions with the underlying mesenchyme and angioblastic strands appear to be
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| present; these are discussed in the description of angiogenesis and haemopoiesis.
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| The chorionic mesoderm consists of a thin stratum of mesenchymal tissue,
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| composed of a network of laminated fibres (Pl. 3, figs. 7, 8), with nuclei at the
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| nodes of the reticulum and enclosing intercellular tissue spaces. Within the
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| mesenchymal tissue there are also strands of angioblastic tissue, ‘many of
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| which contain vacuoles, which by fusion give rise to intracellular spaces; the
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| latter eventually, by further fusion and extension, become the lumina of
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| capillary vessels and sinuses.
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| The spaces are, therefore, of two types: ( 1) intercellular and (2) intracellular.
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| The intercellular spaces are irregular in form and represent the meshes of the
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| network (Pl. 3, figs. 7, 8). They contain occasional free cells of ailymphoid or
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| phagocytic type. The intracellular spaces at this stage of development usually
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| contain no formed elements except near the attachment of the connecting '
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| stalk (Pl. 4, fig. 9). During life they are probably filled by embryonic plasma,
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| secreted by the endothelium. The mesenchymal tissue, on its internal aspect, is
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| limited by a flattened mesothelial stratum which is in relation with the magma
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| reticulare. The mesothelial stratum, \however, is in some places incomplete,
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| and the mesenchymal tissue becomes continuous with delicate syncytial
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| strands of the magma reticulare. The latter contains, in addition to the protoplasmic strands, groups of cells of both mesodermal and epithelial types. There
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| are also isolated cells showing various stages of degeneration, and some free
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| pycnotic nuclei. The epithelial cells appear to be of both entodermal and
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| ectodermal origin. Some of the formerare found in close relation with the yolk
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| sac and possibly may have originated from a yolk-sac diverticulum or stalk, while
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| the ectodermal cells are found in the vicinity of the body stalk and amnion.
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| The embryomic disc
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| The embryonic disc is 1-05 mm. in length and 1-84 mm. in breadth. Including the body stalk the length of the whole embryonic rudiment is 1-66 mm.
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| The primitive streak part of the disc is 0-43-mm. in length and the preblastoporic segment is 0-62 mm.
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| A presomite human embryo - T 15
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| The disc is somewhat oval in outline (Text-fig. 2). In the coronal plane it_ is
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| curved with a slight dorsal convexity, but at the margin of the disc the convexity is replaced by a slight concavity (Text-fig". 4).
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| In'the median axis of the disc, in its posterior.part, there is a distinct
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| primitive groove which overlies the primitive streak "(Text-fig. 4). A blastopore
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| is situated at the front part of the primitive groove, and in front of the blastepore there is an elevation produced by its anterior lip. The disc in front of this
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| dips away in a. moderately steep slope to the anterior edge (Text-fig. 3).
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| \ i “"4vN;§!7:i§$ULm'
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| Text-fig. 2. Wax plate reconstruction of the embryo made at a magnification of 100 diameters
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| and reduced to a magnification of 50 in the reproduction. Viewed from above and in front.
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| The ectoderm is composed of a pseudo-stratified layer of tall cells (Pl. 2,
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| fig. 4 and‘ P1. 8, fig. 6) which become flattened at the edge of the disc, where
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| they are continuous with the ectodermal cells of the amnion (Text-figs. 4-6);
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| they have a distinct basement membrane except in the regions of Hensen’s
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| knot and the primitive streak. The nuclei of these cells are round or oval
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| and contain a distinct nucleolus.’ Numerous mitotic figures are present in
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| the ectodermal cells of the ‘primitive groove and near its margins. These
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| P“
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| s§
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| 8‘
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| §
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| §.
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| §
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| 5
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| m
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| §.
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| §
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| Text-fig. 3. Idealized reconstruction of 9. median sagittal section of the embryo showing the right half of the embryonic
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| A presomite human embryo 17
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| proliferating ectodermal cells give rise, on each side of the primitive streak, to
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| intra-embryonic mesoderm. Towards the anterior part of the disc the number
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| of nuclear layers in the ectodermal cells becomes fewer—-compare Text-figs. 7-9
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| with Text-fig. 4 and P1. 3, fig. 6.
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| In the region of the disc in front of Hensen’s knot the surface of the
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| ectodermal epithelium is irregular and some of the cells appear to be undergoing degenerative changes (Pl. 2, fig. 4). Whether this is the result of tension
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| in a rapidly growing area or the result of fixation cannot be determined.
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| .At the posterior part of the primitive streak there is great proliferation of
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| the cells of the ectoderm. Some of these cells contribute to the formation of the
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| mesoderm of the connecting stalk and pass around the side of the cloacal
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| membrane into the stalk (Pl. 3, fig. 5). '
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| There is no evidence yet of the limitation of the medullary plate, nor is
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| there as yet any sign of the formation of the medullary groove.
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| Cloacal membrane
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| This area is represented by the fusion of ectoderm and entoderm behind
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| the primitive streak. It involves the posterior part of the embryonic disc and
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| the proximal portion of the allanto-enteric diverticulum. The extent of these
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| two parts is seen in Table 2 and is shown schematically in the graphic recon- .
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| | |
| struction (Text-fig. 3). The ectodermal cells of the embryonic disc and amniotic
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| wall which form the ectodermal part are cuboidal or columnar and have
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| rounded nuclei. The entodermal cells which come into contact with the ectoderm form a somewhat conical mass; their nuclei are oval in shape (Pl. 3,
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| fig. 5). The mesodermal cells which border the sides of the membrane are
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| separated from the entodermal cells.
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| Primitive streak and mesoderm
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| The extent of the primitive streak is given in Table 2 (see also Text-fig. 3).
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| The mesoderm cells pass laterally from the sides of the streak as a layer which
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| divides into two at the edge of the disc (Text-fig. 4). The amount of mesodermal tissue is much greater at the posterior half of the disc where it forms a
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| compact mass of cells, than in the anterior half where it is only represented by
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| scattered cells and does not form a continuous sheet (compare Text-fig. 4 with
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| Text-figs. 8, 9). Whether any mesoderm arises from the side of the chordal
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| process could not be determined owing to the obliquity of the sections. The
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| entoderm forms a distinct layer below the mesoderm except in the middle line
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| below the primitive streak where it is intimately related to the mesodermal
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| tissue.
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| Hensen’s knot and chorda canal
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| There is great proliferation of the ectoderm in the region of Hensen’s knot
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| which is raised above the general level of the ectoderm (Text-fig. 3 and P1. 2,
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| fig. 4). The posterior edge of the knot is sharply delimited from the anterior
| |
| extremity of the primitive streak. A blastopore leads into a chorda canal; at
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| | |
| Anatomy 76 2
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| Text-fig. 4. General .view of the embryonic disc at the anterior part of the primitive streak and
| |
| groove. Blood islands in an early stage of development are seen to the left and right; in the
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| centre, endothelial-lined spaces, some of which contain free blood corpuscles mostly of
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| primitive erythroblast type. Section no. 57, the sections having been numbered from the
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| anterior end of the embryonic disc. x 100. .
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|
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| Text-fig. 5. General view of the embryonic disc in the. region of Hansen's knot; the section is to
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| one side of the blastopore (compare with Pl. 2, fig. 4). Vascular spaces containing a few fi'ee
| |
| developing blood corpusoleslare visible in the lower part of the section; they are lined on
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| their distal aspect and sides by endothelioid cells with rounded or lens-like nuclei; on the
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| central side, however, the mesothelial layer is often defective and the wall of the space is
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| formed by entoderm. Section no. 47. x 100.’
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| A presomite human embryo. . 19
| |
| | |
| its commencement the latter is a cavity, but farther forward it is only a potential
| |
| space, and its anterior end appears to be solid (Text-figs. 3, 5). Thelcells in the
| |
| roof of the canal are tall, columnar, have a clear cytoplasm, and their boundaries
| |
| are mostly well defined. The nuclei of the cells are deeply stained and are at,
| |
| or, near, the base of the cells. The cells of the floor of the canal are intimately
| |
| connected to the underlying entoderm, which~in this region does not appear as
| |
| a distinct layer (Pl. 2, fig. 4). The chorda canal, as far as can be determined,
| |
| does not communicate with the yolk sac. Owing to the obliquity of the
| |
| sections it is not possible to determine accurately the anterior end of the
| |
| notochordal process, nor is it possible to state whether mesoderm arises from
| |
| the sides of the notochordal process.
| |
| | |
| Prochordal plate
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| | |
| In the present specimen, the prochordal plate is irregular in shape, two
| |
| horns projecting backwards from its posterior edge on each side of the anterior
| |
| end of the notochordal process. The plate is not sharply delimited from the
| |
| surrounding entode_rm and it is composed, for ‘the most part, of cuboidal cells
| |
| in some of which are chromophilic granules (Pl. 3, fig. 6). Dorsal to the plate,
| |
| in some sections, there is some cell detritus and many chromophilic granules.
| |
| The plate is separated from the ectoderm, and only a few mesodermal cells are
| |
| interposed between it and the ectoderm.
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| | |
| Connecting‘ stalk
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| | |
| Following the description of Florian (1930), the connecting stalk may be
| |
| divided into two parts, the amnio-embryonal stalk and an umbilical, or body,
| |
| | |
| stalk. The latter makes an angle of 80° withpthe embryonic disc. The whole .
| |
| | |
| connecting stalk is composed of a mass of mesenchymal cells, projecting into
| |
| which are the allanto-enteric diverticulum and a diverticulum from the domecaudal part of the amnion (? amniotic duct) (Text-figs. 2, 3). The mesenchymal
| |
| tissue at the base of the stalk (Pl. 4, fig. 9) consists of a wide-meshed reticulum
| |
| containing intercellular spaces and enclosing angioblastic strands and developing capillary vessels; in the region of the allanto-enteric diverticulum
| |
| (Pl. 2, fig. 3) the cells are rounded and have an oval or spherical nucleus. The
| |
| cells are thus more compactly arranged around the allanto-enteric diverticulum
| |
| than in the more basal part of the stalk. The allarito-enteric diverticulum arises
| |
| from the right side of the yolk sac. This is probably due to the fact that the
| |
| embryo has been twisted upon the stalk as is so commonly found in young
| |
| embryos. The umbilical stalk is bounded in front by the amniotic ectoderm.
| |
| There is some irregularity in that part of the amniotic wall which comes into
| |
| | |
| .contact with the mesoderm of the stalk. Some of the amniotic ectodermal
| |
| | |
| cells show degenerative changes, and some have become freed into the amnion.
| |
| The appearance is similar to that described by Florian and, suggests the enlargement of the posterior part of the amniotic cavity at the expense of the
| |
| | |
| 2-2 20 y R. J. Gladstone and W. J. Hamilton
| |
| | |
| stalk tissue. Chromatic particles are present in some of the ectodermal, mesodermal, and entodermal cells in the anterior part of the stalk (Pl. 2, fig. 3).
| |
| Well-formed blood vessels and blood islands are present in the stalk; their
| |
| appearance is described under angiogenesis and blood formation.
| |
| | |
| Yolk sac and allanto-enteric diverticulum
| |
| | |
| The measurements of the yolk sac are given in Table 2.
| |
| There is an infolding of the abembryonic part of the sac (Text-figs. 3, 6),
| |
| and in our opinion this is an artefact. Further, the sac does not lie symmetri~
| |
| | |
| .cally below the embryonic disc but more to the right side of it, possibly as
| |
| | |
| the result of the twisting of the upper part of the embryo (Text-fig. 2).
| |
| | |
| .“"V’.‘.‘..,k uuwv cowpg.-ug.§.~“.‘
| |
| v
| |
| | |
| D
| |
| aw” . . ‘we.
| |
| | |
| ' Text-fig. 6. General view of the embryonic disc showing the relations of the thickened entodermal
| |
| | |
| cells forming the prochordal plate. Typical blood islands are seen in the wall of the yolk sac
| |
| on the left side of the photograph. Vascular spaces are visible in the lower part of the figure.
| |
| They contain a few free corpuscles, or are quite empty, vasculogenesis being more advanced
| |
| in the abembryonjc region than at the circumference. Section no. 31. x 100.
| |
| | |
| At the anterior end of the embryo there is an infolding from the front and,
| |
| to a lesser extent, at the sides of the wall of the yolk sac to form what we
| |
| interpret as the foregut (Text-figs. 7-9).
| |
| | |
| _ At the posterior end of the embryo there is a. folding of the posterior part
| |
| of the primitive streak and embryonic part of the cloacal membrane to form
| |
| the beginning of the hindgut (Text-fig. 3). The allanto-enteric diverticulum
| |
| A presomite human embryo T 21
| |
| | |
|
| |
| | |
| Text-fig. 7. Section through the anterior part of the" embryonic rudiment showing amnion,
| |
| embryonic plate, and wall of the yolk sac. The wide cavity beneath the embryonic plate is
| |
| believed to be the foregut. The mesoblastic tissue between the embryonic ectoderm andthe
| |
| entodcrm is here scanty and in the section appears incomplete, this section lying in the pre-_
| |
| sumptive region of the buccopharyngeal membrane. The mesenchymal cleft to the left in
| |
| the figure, and the enclosed space in the corresponding situation on the right, correspond in
| |
| position to die future pericardial cavity. Apart of the anterior wall of the yolk sac is seen in
| |
| the lower part of the photograph. Empty spaces lined by endothelial cells are present in the
| |
| wall of the yolk sac and embryonic plate. Section no. 6. x 100.
| |
| | |
|
| |
| | |
| Text-fig. 8. The section passes through the anterior boundary of the anterior intestinal--portal
| |
| and shows the junction of the lateral folds of the yolk sac wall. Blood islands and empty
| |
| endothelial-‘lined spaces are present in the wall of the yolk sac, andin the floor and roof of the
| |
| foregut. Section no. 10. x 100. '
| |
| 22 R. J. Gladstone and W. J. Hamilton
| |
| | |
| arises below the level of the hindgut, passes to the right and upwards into the
| |
| body stalk, and its tip is in close relationship with a detached mass of entodermal cells.
| |
| The entodermal layer of the yolk sac is formed of cells the shape and size
| |
| of which difi'er considerably in different parts of the sac. The entodermal cells
| |
| below the embryonic disc are flattened, except in the prochordal plate region
| |
| (Text-fig. 6 and P1. 3, fig. 6) where they are cuboidal, and in the floor of the
| |
| foregut where they become rounded (Text-fig. 7). In the region of thecloacal
| |
| membrane and allanto-enteric diverticulum they are again cuboidal. In the
| |
| abembryonal pole of the yolk sac, where the vascular spaces are well developed,
| |
| | |
| . 5 “V .-~
| |
| $‘.,,§-v(i4s.§.. .~§,_‘ _‘
| |
| ~ X...‘
| |
| ;
| |
| | |
|
| |
| | |
| Text-fig. 9. A section just behind the anterior intestinal portal. The lateral infoldings of the
| |
| yolk sac wall, and the marginal clefts in the mesenchyme of the embryonic plate in the region
| |
| of the pericardium, are visible. The medullary plate which in Text-figs. 7 and 8 consists of a
| |
| single layer of flattened eotodermal cells is now considerably thicker and two or three layers
| |
| of nuclei are visible. Section no. 12. . x 100. '
| |
| | |
| some of the entodermal cells are columnar and swollen, others contain large
| |
| vacuoles in their free surfaces, while still others show a ragged edge or are
| |
| rounded, the vacuoles apparently having discharged their contents into the
| |
| | |
| yolk-sac cavity (Pl. 1, figs. 1, 2). The entoderm is covered by the mesodermal '
| |
| | |
| cells. The amount of this tissue present in different areas of the yolk sac is
| |
| subject to wide variation. Close to the embryo it is composed of flattened cells
| |
| which form a definite layer, the splanchnic mesothelium, which is intimately
| |
| related to the underlying entoderm. In other areas the two layers are separated
| |
| | |
| ' by mesenchymal tissue; in this tissue, and in the spaces enclosed by it, lie
| |
| | |
| haemocytoblasts and other cells. At the abembryonic pole hit is composed of
| |
| many layers of cells in which are seen various stages in the formation of endothelial-lined spaces and developing blood cells (see under Angiogenesis, p. 29). _
| |
| A presomite human embryo 23
| |
| | |
| Amnion
| |
| | |
| _The dimensions of the amnion are 1-35 by 1134 mm., and its height is
| |
| 0-32 mm. It extends caudally to form a. diverticulum which leads into the
| |
| connecting stalk (Text-figs. 2, 3). The lining cells of the amnion are flattened
| |
| and elongated in type except where it comes into contact with the mesoderm
| |
| of the connecting stalk; here the cells are cuboidal and the surface is irregular.
| |
| The nuclei of the cells are near the amniotic cavity. Some degenerated ecto
| |
| N dermal cells and cell detritus are found in the amniotic cavity. The free surface
| |
| | |
| of the amnion, in the greater part of its extent, is covered with rather flattened
| |
| | |
| . mesodermal cells. In places the mesodermal cells are raised to form small
| |
| | |
| blebs or vesicles. , .
| |
| ANGIOGENESIS AND I-IAEMOPOIESIS
| |
| | |
| The early stages of haemopoiesis in the human subject have recently been
| |
| investigated by Bloom & Bartelmez (1940), who have distinguished the
| |
| following morphological grades in the development of corpuscles and defined
| |
| them under the following names:
| |
| | |
| (1) Haemocytoblasts.
| |
| | |
| (2) Primitive blood cells.
| |
| | |
| (3) Lymphoid wandering cells.
| |
| (4) Primitive erythroblasts.
| |
| (5) Definitive erythroblasts.
| |
| (6) Definitive erythrocytes.
| |
| | |
| Additional types of cells are described by them in both intravascular and
| |
| extravascular zones of haemapoietic organs or tissues, namely macrophages,
| |
| giant cells, megakaryocytes, and granular leucocytes.
| |
| | |
| The characteristics of these cells are defined by Bloom & Bartelmez as‘
| |
| | |
| indicated below, and as the characteristics agree closely with those which We
| |
| have observed in the‘ Shaw embryo, we propose to adopt the nomenclature
| |
| which they have employed, in so far as the types described are encountered
| |
| at the stage of development which has been reached by the Shaw embryo.
| |
| In adopting this nomenclature, however, we do not wish it to be inferred that
| |
| we use the names in any other than a descriptive sense, and also it should be
| |
| understood that by so doing we do not associate ourselves with certain current
| |
| theories as to the pluripotentialities of some of the stem cells which are
| |
| described.
| |
| | |
| Some of the principal features of the types as defined by Bloom &. Bartelmez
| |
| are: ,
| |
| (1) H aemocytoblasts. ‘ Spherical, or slightly polygonal cells, often provided
| |
| with amoeba-like processes ’ which often occur intra- and extravascularly.
| |
| ‘The cytoplasm is deeply basophil and may contain a few vacuoles.’ ‘The
| |
| relatively very large nuclei are spherical or slightly indented.’ ‘ The very large
| |
| and acidophil nucleoli may be single or multiple and are often very angular.’
| |
| ‘ The finely granular chromatin is aggregated in larger masses along the nuclear
| |
| 24 R. J. Gladstone and W. J. Hamilton
| |
| | |
| membrane in some cells.’ ‘ These cells are identical in appearance not only with
| |
| the primitive blood cells but also with the large, free, basophil stem cells seen
| |
| in all haematopoietic foci of both embryonic and adult man.’ .
| |
| | |
| (2) Primitive blood cells. ‘These are the first free precursors of blood cells
| |
| and are morphologically identical with haemocytoblasts ’ (which may be found
| |
| in bloo_d-forming centres in older embryos, foetuses, and adult man).
| |
| | |
| (3) Lymphoid wandering cells. ‘This name was given by Maximow (1909)
| |
| to cells of lymphocyte, that is haemocytoblast, type, which he found moving
| |
| in the mesenchyme.’
| |
| | |
| (4) Primitive erythroblasts. ‘This is the largest group of cells in the blood
| |
| ‘cell forming areas. The youngest ones closely resemble, and are connected by
| |
| | |
| many transition forms with, the haemocytoblasts. The nuclei become smaller,
| |
| the nucleoli smaller and often more numerous, and the chromatin particles
| |
| more prominent as the cells mature. The cytoplasm remains about the same
| |
| size but loses its basophilia, the colour gradually changing as haemoglobin
| |
| accumulates within it.’ ‘
| |
| | |
| (5) Definitive erythroblasts (e.g. thosefound in older embryos (20 mm.) in
| |
| the wall of the yolk sac, and other situations in foetal and adult life, and which
| |
| are commonly known as normoblasts). ‘The cytoplasm becomes paler and the
| |
| chromatin assumes a more regular distribution, with the nucleoli smaller and
| |
| more numerous; this is often spoken of as the checker board nucleus.’ ‘The
| |
| nucleus becomes progressively smaller and pycnotic and is finally extruded to
| |
| produce the mature definitive erythrocyte.’ Proliferation by mitosis occurs in
| |
| definitive erythroblasts as in primitive erythroblasts.
| |
| | |
| (6) Definitive erythrocytes. The presence of erythrocytes is first mentioned
| |
| in an 18-somite human embryo (H.E., H. 1516) in which a few extravascular
| |
| primitive erythrocytes, singly or in pairs, were noted in the yolk-sac mesoderm.
| |
| The greater number of the developing blood cells seen at this stage were,
| |
| however, intravascular haemocytoblasts, and primitive erythroblasts. (In
| |
| this embryo it is noted that the circulation had commenced and the
| |
| authors state that the vessels of the villi contain polychromatophil primitive
| |
| erythroblasts at various stages of maturity and an occasional haemocytoblast;
| |
| and further, that there wasno haemopoiesis in thefinesenchyme of the villi.)
| |
| | |
| A few words of explanation are perhaps necessary here in order to distinguish the exact meaning which is implied by the use of the qualifying
| |
| adjectives ‘primitive ’. and ‘definitive’; thus the primitive blood cells are
| |
| stated to be ‘morphologically identical with haemocytoblasts’—the difference
| |
| here lies in the circumstance that the primitive blood cells are the first haemocytoblasts to appear whereas the name haemocytoblast is a more general term
| |
| than primitive blood cell, and is applicable to all developing blood cells having
| |
| the special characters of haemocytoblasts (e.g. basophil cytoplasm and
| |
| amoeboid processes) wherever and whenever these cells are recognizable. Thus
| |
| | |
| _they may be present in mesenchymal connective tissue, lymphoid tissues, or
| |
| | |
| the medulla of adult bone; whereas the use of the term primitive blood cell is
| |
| A presomite human embryo A 25
| |
| | |
| restricted to the parent cell of the early embryonic stage when blood is commencing to be formed on the yolk sacror vascular area. The terms primitive
| |
| and definitive, when used in descriptive embryology, usually denote respectively the first or undifferentiated and the final stage of development of
| |
| any particular cell _or organ. When, however, it is implied that successive
| |
| independent generations of developing cells follow one another, there is apt to
| |
| be confusion unless this is clearly stated. I
| |
| Our observations on angiogenesis and haemopoiesis in the Shaw embryo
| |
| will be described under the following headings:
| |
| (1) The chorionic mesoderm and villi.
| |
| (2) The connecting stalk.
| |
| (3) The yolk sac.
| |
| | |
| Chorionic mesoderm and villi
| |
| | |
| This presents the usual appearance of embryonic mesenchyme at the presomite stage of development, namely, a loose reticular tissue, formed by a
| |
| delicate syncytial network of feebly stained protoplasmic strands and laminae,
| |
| which enclose irregular intercellular spaces. Basophil nuclei of varying size
| |
| and shape which occasionally are seen to be undergoing mitosis are present at
| |
| the nodes of the network. The loose reticular stratum is limited, on its inner
| |
| side, by a thin membranous layer of flattened cellular elements. This is
| |
| incomplete in certain places where the chorionic mesenchyme becomes continuous with protoplasmic strands of the magma reticulare. In those places
| |
| where the membrane is well developed it forms a definite mesothelium which
| |
| resembles the splanchnic mesothelium covering the basal part of the body
| |
| stalk and the yolk sac, but the cellular elements of the chorionic membrane
| |
| are more delicate and flattened than in the yolk-sac mesothelium. Branching
| |
| strands of angioblastic tissue are present throughout the Whole extent of the
| |
| chorionic mesenchyme and the central mesodermal cores of the villi. They
| |
| are readily distinguished by their diffuse pale pink colour and finely granular
| |
| cytoplasm. It is difficult to determine the exact relations of the strands but
| |
| they appear to be connected with (1) the surrounding mesenchymal tissue,
| |
| (2) the chorionic mesothelium, and (3) the cytotrophoblast. Vacuoles are
| |
| frequently visible in the cytoplasm and, by fusion of these with one another,
| |
| intracellular spaces or lumina are formed in the larger and more differentiated
| |
| strands which are becoming transformed into capillary vessels and vascular
| |
| sinuses bounded by an endothelial wall. .
| |
| | |
| The spaces in the mesenchyme are thus of two kinds: (1) the irregular,
| |
| intercellular spaces or meshes of the reticulum, and (2) vascular spaces lined by
| |
| endothelium.
| |
| | |
| The vascular sinuses and developing capillary vessels are mostly empty
| |
| (Pl. 3, figs. 7, 8); very occasionally, however, various stages in the ‘rounding
| |
| up’ of the primarily flattened cell elements of the wall of a vascular sinus are
| |
| seen, until the stage is reached of a round bud with a constricted neck pro-I
| |
| 26 R. J. Gladstone and W. J. Hamilton
| |
| | |
| jecting into the lumen of the sinus. Within the lumen of the sinus (Pl. 4, fig. 9),
| |
| and in the Vicinity of these buds, free cells having all the characters of the
| |
| endothelial cell elements forming the buds are frequently visible. These
| |
| liberated cells arising from the endothelium vary in size, but are on the .whole
| |
| smaller than the erythroblasts in the blood islands and vascular spaces of the
| |
| yolk sac. Occasionally the free cells are seen to be undergoing mitosis; on the
| |
| other hand, some show degenerative changes.
| |
| | |
| The vascular spaces and developing capillaries are seen in the chorionic
| |
| mesenchyme and central cores of the villi throughout the whole extent of the
| |
| vesicle, but spaces and capillaries containing free endothelial cells appear more
| |
| frequently in the chorionic mesenchyme in the vicinity of the body stalk. The
| |
| transformation of theangioblast by canalization into sinuses and vessels is
| |
| more advanced on the placental side of the chorionic vesicle, and at the equator
| |
| where the villi are largest, than at the abembryonic pole. ‘
| |
| | |
| Free spherical cells, or groups of such cells, ‘of mesothelial type are also seen
| |
| | |
| in the magma reticulare, and more particularly in the neighbourhood of the I
| |
| | |
| body stalk.
| |
| | |
| Fibroblasts and small round cells of a lymphoid type are also occasionally
| |
| seen in the intercellular spaces of the chorionic mesenchyme; the nuclei of
| |
| some of the fibroblasts show mitosis. I
| |
| | |
| The appearance of the angioblastic strands and vessels in the mesenchyme
| |
| forming the central core of the chorionic villi is similar to that in the parietal
| |
| mesenchyme, though, on the whole, the development is less advanced than it
| |
| is in that layer. The development of angioblastic tissue and vessels in the villi
| |
| is taking place throughout the whole extent of the vesicle but, as in the parietal
| |
| mesenchyme, the largest and most differentiated vessels are found in villi near
| |
| the attachment of the connecting stalk. As in the parietal mesenchyme, the
| |
| vessels arise by vacuolation of the protoplasmic strands and the fusion of
| |
| vacuoles to form an intracellular lumen which contains, as a rule, no formed
| |
| elements. The appearance of one of these vessels and the relation of its wall to
| |
| the surrounding mesenchyme are well seen in the transverse section shown
| |
| in P1. 3, fig. »8, in which it will be noted that the upper wall of the vessel, as
| |
| seen in the photograph, is formed by angioblast, whereas the lower is completed by a delicate film or membrane which is continuous with both the
| |
| angioblast above and with the surrounding mesenchyme. The vessels of a
| |
| villus are often continuous at the base of the villus with a vessel in the parietal
| |
| layer of the chorion, sometimes by a T-shaped junction suggesting outward
| |
| growth into the villus of a sprouting bud from a previously formed vessel in
| |
| the mesenchyme. Other strands, however, appear to be formed independently
| |
| by transformation of the mesenchyme, although it is possible, as claimed by
| |
| Hertig (1935), since the strands often appear to be continuous with the tropho—
| |
| blastic cells, that they may have arisen at an earlier stage by delamination
| |
| from the cytotrophoblast, but the end-product, namely, the endothelium lining
| |
| a blood sinus or capillary vessel, is so similar -to that which occurs in the
| |
| A presomite human embryo 27
| |
| | |
| "formation of the blood vessels in the yolk-sac mesoblast and other mesodermal
| |
| | |
| tissues, that we are inclined to think that the vessels arise by the transformation
| |
| or differentiation of mesodermal cells. When once the angioblastic tissue has
| |
| become differentiated it reproduces itself by budding out processes which
| |
| spread by proliferation of endothelial cells and extension, as shown by Clark &
| |
| Clark (1932 and 1937) and also Clark (1909) in their accounts of observations
| |
| upon the growth and extension of new blood and lymphatic capillaries from
| |
| pre-existing capillaries in the liver, rabbit ear, and in amphibian and mammalian
| |
| material respectively.
| |
| | |
| Connecting stalk
| |
| | |
| This region must be considered as consisting of two segments: (1) the basal
| |
| part at the chorionic end of the stalk, where the supporting tissue consists of a
| |
| loose mesenchymal reticulum continuous with the parietal chorionic mesenchyme, and (2) the proximal, or embryonic, end—-the umbilical stalk—which
| |
| encloses the allanto-enteric diverticulum and in which the supporting tissue
| |
| | |
| consists of closely packed, rounded, embryonic cells continuous with the yolk- .
| |
| | |
| sac mesoblast. In the former (Text-fig. 10) a large capillary vessel is seen
| |
| which extends into the parietal mesenchyme. Its endothelial wall shows budlike projections into the lumen, and some free rounded cells lying in the
| |
| lumen. These cells are smaller than the haemocytoblasts and early erythroblasts found in the blood islands in the wall of the yolk sac (cf. Pl. 4, fig. 9, with
| |
| P1. 4, figs. 10, 11), and they differ considerably in appearance, having compact
| |
| deeply stained spherical nuclei surrounded by a relatively small amount of
| |
| cytoplasm. In contrast with these isolated cells, definitely arising by a process
| |
| of budding from a preformed endothelial wall, are the cells found in the region
| |
| of the allanto-enteric diverticulum, which lie in the mesoblast in close relation
| |
| with the entodermal wall of the diverticulum. These cells closely resemble those
| |
| of the blood islands of the yolk sac, with which they are strictly homologous,
| |
| and with the blood-vascular system of which they will later become continuous,
| |
| when an endothelial wall has been differentiated from the mesoblast surrounding them, and the two systems become joined. A connexion is also
| |
| formed, at a later stage, of the plexus of vessels which gives rise to the umbilical
| |
| arteries and the umbilical vein with the independently formed vessels in the
| |
| chorionic segment of the stalk. The time at which this occurs appears to be
| |
| variable; thus, in Ingalls’s human embryo at the beginning of segmentation
| |
| (three somites) the author (1921) states that ‘the distal portion of the stalk
| |
| is -occupied by a few small blood vessels through which a narrow connexion
| |
| is set_ up with the vessels of the chorion’.
| |
| | |
| In contrast with this we find in Stieve’s 13}-day human embryo Hugo (1926)
| |
| —total length of embryonic rudiment 0-75 mm. and length of embryonal shield
| |
| 0-57 mm.—the development of solid blood islands and blood vessels in the stalk
| |
| unusually far advanced considering the early stage of development of this
| |
| embryo. Stieve regards the mesoderm which contains the blood islands as
| |
| 28 l R. J. Gladstone and W. J. Hamilton
| |
| | |
| arising from the ‘sickle node’ and, therefore, axial in origin. He also believes
| |
| that the cells forming the islands in the stalk, like those of the islands in the
| |
| yolk sac, may originate, ‘in the human subject, from both entoderm and
| |
| | |
| Text-fig. 10. The photograph shows the junction of the connecting stalk with the chorionic
| |
| mesoderm, andthe advanced stage of development of the vessels in the stalk, adjacent
| |
| parietal mesenchyme, and mesenchymal tissue of the In the cavity of the chorionic
| |
| vesicle isolated groups of degenerate mesenchymal cells and free nuclei are visible. The connexions of the villi with the decidua, and of the intervillous spaces with the basal sinus, are
| |
| also clearly shown. Section no. 144. x l0Q.
| |
| | |
| mesoblast, as in other species of mammalian embryos. The mesenchyme at the
| |
| chorionic end of the stalk, and the chorionic mesenchyme, which contain blood
| |
| vessels but no islands, are regarded by him as ‘morula mesoderm’, which
| |
| A presomite human embryo 29
| |
| | |
| difference, we believe, may explain the difference in type of angiogenesis in the
| |
| two parts of the stalk. It is not until the vessels are fully formed in both parts
| |
| and are connected that, when the heart commences to beat, blood flows from
| |
| the umbilical arteries into the chorionic or placental vessels.
| |
| | |
| Yolk sac
| |
| | |
| There are three types of blood island on the surface of the yolk sac :'
| |
| | |
| (1) Haemocytoblasts and early primitive erythroblasts not enclosed inlan
| |
| endothelial covering.
| |
| | |
| (2) Solid masses of haemocytoblasts enclosed in an endothelial wall.
| |
| | |
| (3) Endothelial-lined cysts or vessels containing a few free haemocytoblasts
| |
| and other cell types.
| |
| | |
|
| |
| | |
| Text-fig. 11. The photograph shows the early formation of intercellular and intracellular spaces.
| |
| Some of the intercellular spaces are lined by entoderm (upper layer in the photograph) on
| |
| one side and lay mesenchymal tissue on the other. Some haemocytoblasts and primitive
| |
| erythroblasts (cells with clearer cytoplasm) are lying free in the spaces. Section no. 46.
| |
| | |
| x 560. ‘
| |
| | |
| Intermediate stages between these three types of island are frequent, and
| |
| intercellular spaces not lined by endothelium are also present in the mesoblastic
| |
| tissue between the splanchnic mesothelium and the yolk-sac entoderm (Textfig. 11).
| |
| | |
| Angiogenesis and haemopoiesis are most advanced at the distal or abembryonic pole of the sac (Pl. 1, fig. 1) where, presumably, the blood islands first
| |
| commenced to be formed in the zone corresponding to the vascular area of
| |
| lower types of Mammalia; these two processes are less advanced in the circumferential zone near the margin of the embryonic disc where the earlier stages
| |
| 30 R. J. Gladstone and W. J. Hamilton
| |
| | |
| of smaller solid islands not yet enclosed in endothelium are best studied (Pl. 1,
| |
| fig. 2). In this region the yolk-sac wall consists of two well-defined layers, the
| |
| entoderm and splanchnic mesothelium, between which in the loose mesoblastic
| |
| tissue are empty spaces, some of which are interstitial and others are lined by
| |
| | |
| endothelium. There are also small solid masses of proliferating cell elements, '
| |
| | |
| haemocytoblasts and primitive erythroblasts, which represent the earliest stage
| |
| of blood formation (Pl. 4, fig. 10). Sometimes the blood cells project from one
| |
| side into the lumen of an endothelial-lined space into which at a later developmental stage the freed blood cells will be discharged (Pl. 4-, fig. 11). The cell
| |
| outlines of the future haemocytoblasts are, in the earliest stages, absent or
| |
| only very indistinctly visible (Pl. 1, fig. 2). In the more advanced stages the
| |
| cell elements of these plasmodial masses differentiate and become separated
| |
| from each other and are finally liberated in the form of free haemocytoblasts
| |
| showing irregular protoplasmic processes. These by further division form the
| |
| first generation of embryonic red blood cells or primitive erythroblasts. Some
| |
| of the cells show signs of degeneration, e.g. the formation of vacuoles, the
| |
| contents of which when discharged may contribute to the formation of the
| |
| embryonic plasma.
| |
| | |
| In the later stages of development seen at the abembryonic pole of the
| |
| yolk sac, when the endothelial walls of the blood spaces are completed (Pl. 1,
| |
| fig. 1), the vessels are much increased in size and communicate with each other,
| |
| forming the capillary plexus from which the vitelline vessels will afterwards be
| |
| formed in the early stages of somite development, as has been demonstrated
| |
| in the Ingalls (1921) three-somite human embryo. In these vessels we can
| |
| recognize the following types of cell (Pl. 1, fig. 1): haemocytoblasts, transitional stages between these and primitive erythroblasts, primitive erythroblasts, intermediate erythroblasts, late prirnitive erythroblasts, degenerating
| |
| erythroblasts, non-nucleated primitive erythrocytes, and binucleated giant
| |
| cells. '
| |
| | |
| DISCUSSION
| |
| | |
| It is proposed to compare the present embryo only with those that are
| |
| nearly related to it in development. It is at approximately the same stage of
| |
| development as W9. 17 (Grosser, 1931) and would, therefore, be at a. later
| |
| stage than the Manchester embryo (Florian & Hill, 1935) and younger than the
| |
| Pehl-Hochstetter (Rossenbeck, 1923).
| |
| | |
| If we accept the patient’s history of coitus, the embryo has a coital age of
| |
| 26 days, much older than the corresponding embryos of this stage of development. The embryo, indeed, appears to be much younger than its supposed
| |
| coital age, being at about the 17th or 18th day of development, and as it was
| |
| | |
| procured as the result of an operation there is no reason to suppose that it ,
| |
| | |
| had been dead for some time.
| |
| A presomite human embryo 31
| |
| | |
| Embryonic disc
| |
| | |
| After comparing the shape of the dorsal views of graphic reconstructions,
| |
| Hill & Florian (1931) have suggested that the embryonic discs of embryos may
| |
| be divided into two varieties, as Rabl (1915) has done for the rabbit, a narrow
| |
| type, e.g. the embryos Bi 24 (Florian, 1934), Manchester (Florian & Hill, 1935),
| |
| Wa 17 (Grosser, 1931), and Dobbin (Hill & Florian, 1931), and to these may
| |
| be added the embryos, Kl 13 (Grosser, 1913), Ingalls (1918), Carnegie 5960
| |
| (Heuser, 1932), and HR1 (Johnston, 1940), and a broader type, e.g. the Hugo
| |
| (Stieve, 1926), Scho (Waldeyer, 1929) and Pehl-Hochstetter (Rossenbeck,
| |
| 1923) embryos; to this list may be added the Thompson-Brash (1923) and H0
| |
| (Fahrenholz, 1925) embryos.
| |
| The embryo Shaw would belong to the latter group. In this connexion it 1
| |
| | |
| may be pointed out that the head process of this embryo is at an early stage
| |
| of development, and this may partly account for the fact that the disc is
| |
| broad; on the other hand, the Pehl-I-Iochstetter embryo has a well-developed
| |
| head process.
| |
| | |
| The distinct dip on the embryonic disc in front of Hensen’s knot is a
| |
| | |
| feature of this embryo, and it- is partly due to the proliferation of the cells at’
| |
| | |
| the node. In 'the HO embryo (Fahrenholz, 1925) there is also a distinct
| |
| elevation at the knot. In the Scho embryo (Waldeyer, 1929) there is an
| |
| | |
| - elevation in front of Hensen’s knot and the disc slopes gently away towards
| |
| | |
| the anterior edge. The Carnegie 5960 (Heuser, 1932) embryo shows a distinct
| |
| slope some distance in front of the node. There is no marked convexity of the
| |
| disc in thesagittal plane as described in HR, by Johnston (1940); this author
| |
| is of the opinion that this convexity is the result of abnormal growth. The
| |
| concavity in the Shaw embryo, in our opinion, is normal and is the result of
| |
| the rapid proliferation at Hensen’s knot. It may, however, be an artefact due
| |
| to fixation. The arrangement of the cells in the ectoderm is typical for an
| |
| embryo of this stage of development.
| |
| | |
| Primitive streak, cloacal membrane, allanto-enteric diverticulum
| |
| and yolk we
| |
| | |
| The arrangement of the layers in the primitive streak is essentially similar
| |
| | |
| to that described in embryos of a corresponding stage of development and will‘
| |
| | |
| not be discussed further.
| |
| | |
| The cloacal membrane is easily recognized in this embryo. It involves the
| |
| posterior part of the embryonic disc and the proximal part of the allantoenteric diverticulum. The junction between the_two parts makes a distinct
| |
| angle as in the Pehl-Hochstetter (Rossenbeck, 1923) embryo. There is no
| |
| indication of the separation of the cloacal membrane into two parts by the
| |
| appearance of mesoderm between ectoderm and entoderm as described by
| |
| Florian (1930) in the Bi II and Sternberg (1927) embryos, and by Wybum
| |
| (1987) in the McIntyre embryo. The present embryo does not throw any light
| |
| 32 R. J. Gladstone amid W. J. Hamilton
| |
| | |
| on whether the cloacal membrane first appears between theamniotic ectoderm
| |
| and entoderm, as in the Beneke embryo (Florian & Beneke, 1930-1) and HR1
| |
| embryo (Johnston, 1940), or within the embryonic disc, as in the Bi I and
| |
| Fetzer embryos (Florian, 1933). ' '
| |
| | |
| The proliferation of the ectoderm which gives rise to the primitive streak in
| |
| the Edwards-Jones-Brewer embryo (Brewer, 1938), which is well preserved,
| |
| seems to us to settle the controversy as to whether the primitive streak appears
| |
| before or after the cloacal membrane. In the Brewer embryo the primitive
| |
| streak is at the earliest stage described so far for the human subject, and there
| |
| is no indication yet of the cloacal membrane. Further, in this embryo the
| |
| primitive streak is a crescentic mass, as in the pig (Streeter, 1927) and the
| |
| ferret (Hamilton, 1937), and it lies at the posterior part of the disc and not at
| |
| some distance in front of the edge of the disc as shown by Florian (1933) for
| |
| the OP and W0 embryos of von Mollendorff, and the Fetzer embryo.
| |
| | |
| The mesodermal cells which arise from the posterior part of the primitive
| |
| streak in the Shaw embryo pass around the cloacal membrane to the connecting stalk. The cells are closely packed together as described by Florian &
| |
| Beneke (1930-1), and Hill & Florian (1931) in the Dobbin embryo.
| |
| | |
| It is not our intention to enter into a detailed discussion on the developmental history of the cloacal membrane since, in the present embryo, it is
| |
| already well developed. We are, however, unable to accept the interpretation
| |
| of Florian (1933) and Wyburn (1937) for this structure. Florian states: ‘ If we
| |
| realize that the cloacal membrane and the primitive streak represent parts of
| |
| the blastopore in lower vertebrates, we must accept the View that the homologue of the blastopore in Man originates in two separated areas which grow
| |
| together only during later development. This fact has perhaps caused the
| |
| difficulties in interpretation of the structure which I regard as cloacal membrane.’ Wyburn writes: ‘The appearance of the primordium of the primitive
| |
| streak representing the fused lips of the blastopore acclaims the formation of
| |
| secondary mesoderm separating the ectoderm and entoderm in this region’,
| |
| and ‘ Caudal to the fused lips of the blastopore and reaching to the attachment
| |
| of the stalk remains a relatively extensive area of contact, the anlage of the
| |
| cloacal membrane. . . .The cloacal membrane would thus be the homologue of
| |
| the ventral lip of the blastopore and adjacent area.’ There is apparently no
| |
| phylogenetic reason why the cloacal membraneshould be regarded as part of
| |
| the blastopore. In Amphiowus and Amphibia the blastopore is recognized as
| |
| an invagination and the cells around it are proliferating rapidly to give rise to
| |
| axial structures and mesoderm. In reptiles, birds and mammals the ventral
| |
| lip of the blastopore is represented by the primitive streak and, as such, gives
| |
| rise to mesoderm. The cloacal membrane in man and mammals, on the other
| |
| hand, is a region where ectoderm and entoderm are in contact, or actually
| |
| fused, and gives rise neither to axial structures nor to mesoderm. It is diflicult,
| |
| therefore, to see why it should be regarded as part of the primitive streak or of
| |
| the blastopore. If Rabl’s (1915) opinion is accepted, that gastrulation (formation
| |
| A presomite human embryo 33
| |
| | |
| of head process and mesoblast) is a process of growth which allows certain
| |
| presumptive organs to reach their deflnitive positions, then it seems to us. that
| |
| the cloacal membrane must be regarded as lying outside this field of growth.
| |
| We are _rather of the opinion of Keibel (1896) that the cloacal membrane is a
| |
| secondary structure. The matter is only to be decided by experimental methods
| |
| as it has been in lower vertebrates. Whether the area in the connecting stalk
| |
| where mesoderm is in contact with the amniotic ectoderm can be regarded as
| |
| partof the blastopore, as it gives rise to mesoderm, is another question. The
| |
| origin of the primitive mesoblast from trophoblast, as described by Hertig
| |
| (1935), introduces further and fundamental dilficulties in accepting the view
| |
| that the area described by Florian is part of the blastopore.
| |
| | |
| N otochordal process and chorda, canal
| |
| | |
| The presence of a chorda canal has been described in embryos at approximately the same stage of development as the present embryo.
| |
| | |
| The youngest embryo described with a head process (length 0-06 mm.) is
| |
| the Meyer (1924) embryo; according to Florian (1928) the structure identified
| |
| as the head process was erroneously interpreted. Johnston (1940) describes a.
| |
| head process (length 0-04 mm.) consisting of a mass of cells derived from what
| |
| he interprets as Hensen’s knot in the HR1 embryo. It should be pointed out
| |
| that Florian does not recognize a head process in the HR1 embryo (see
| |
| Johnston’s paper). As far as can be’ ascertained from the photographs published in J ohnston’s paper we are of the opinion that this author’s interpretation is the correct one. One of us (Hamilton, 1937) has found appearances
| |
| similar to that described and figured by Johnston in the early stages of the
| |
| development of the head process in the ferret.
| |
| | |
| An undoubted head process is present in the embryos H. Schm. 10 (Grosser,
| |
| 1931), Hugo (Stieve, 1926) and Bi 24 (Hill & Florian, 1931; Florian, 1934).
| |
| In the Hugo embryo the process (length 0-07 mm. according to Hill & Florian
| |
| (1931), and 0-09‘ mm. according to Stieve) is composed of irregular cells which
| |
| are never columnar; they are, however, in continuity with the intra-embryonic
| |
| mesoderm. The entoderm is missing underneath the head process. In the
| |
| H. Schm. 10. the head process (length 0-1 mm.) is composed, in one section, of
| |
| columnar cells which surround a space which Grosser recognized as the chorda
| |
| (Lieberkuhn’s) canal. Thus at an early stage /the cells are typical of later stages
| |
| of development when a well-defined canal is established, as in the embryo
| |
| described by Heuser (1932). In the Bi 24 embryo the head process (length
| |
| 0-1 mm.) is more differentiated than in the Hugo. It consists of a thickened
| |
| median cord ‘and two. thinner lateral wings of mesoderm. The underlying
| |
| entoderm is indistinct andis represented in some places only by. scattered cells,
| |
| many of which have pycnotic nuclei. The pycnosis of the nuclei, in our opinion,
| |
| may be regarded as the first sign of the disappearance of this entoderm. The
| |
| median cord of cells ends in continuity with thickened entoderm which is
| |
| recognized as the prochordal plate. The next stage in the development of the
| |
| | |
| Anatomy 76 V 3
| |
| 34 R. J. Gladstone and W. J. H (tmilton
| |
| | |
| head process (length 0-12 mm.) is found in the Manchester embryo (Florian &
| |
| Hill, 1935); it has a definite Hensen’s knot extending forward from which is a
| |
| short head process which terminates in the prochordal plate; no mention is
| |
| made of the presence of a canal in this head process." In the embryo Wa 17
| |
| (Grosser, 1931) there is the beginning of a chorda canal in the head process
| |
| (length 0-18 mm.). Whether a ventral opening connects the chorda canal with
| |
| the yolk sac appears to be doubtful. In the Shaw embryo, with a head process
| |
| 0-18 mm. in length and, therefore, at approximately the same stage of development as the Wa 17, the dorsal opening of the chorda canal and the caudal part
| |
| of the canal are distinct. We have not been able to find a ventral opening into
| |
| the yolk sac. If entoderm is underlying Hensen’s knot and the head process it
| |
| is indistinguishable from the ‘tissue of the knot and head process. In the
| |
| embryo Thompson-Brash (1923) the head process (length 0-3 mm.) has not
| |
| yet acquired a lumen, but the dorsal cells are arranged fanwise so that the
| |
| appearance of a lumen is imminent. In the Pehl-Hochstetter embryo (Rossenbeck, 1923) the head process (length 0-6 mm.) has a distinct canal. There is
| |
| some doubt as to whether there are one or more ventral openings into the yolk
| |
| sac (see Hill & Florian, 1931). In the.Kl 13 embryo (Grosser, 1913) the chorda
| |
| canal (length 0-2 mm.) communicates by means of several openings with the
| |
| yolk sac. This embryo is smaller, but apparently more differentiated, than the
| |
| Pehl-Hochstetter, as is also the Dobbin embryo (Hill & Florian, 1931). There
| |
| | |
| ‘is, therefore, considerable variation in the time of appearance of the canal. It is
| |
| | |
| distinct in the Shaw embryo, with a head process length of 0-18 mm., and
| |
| has not yet appeared in the Thompson-Brash (head process length 0-3 mm.-).
| |
| There is also variation as to the stage of development when it communicates
| |
| with the yolk sac. .
| |
| | |
| The line of demarcation between.the posterior edge of Hensen’s knot and
| |
| the anterior part of the primitive streak can be recognized in the present
| |
| specimen. The cells in the roof of the chorda canal are tall and columnar, with
| |
| clear cytoplasm towards the lumen, and are, therefore, typical for embryos of
| |
| this stage of development (compare Rossenbeck, 1923; Heuser, 1932; Johnston,
| |
| 1940). The obliquity of the section in the present embryo does not permit us
| |
| to give an opinion as to the amount of mesoderm, if any, arising from the side
| |
| of the head process. That the head process becomes intimately connected with
| |
| the underlying entoderm in man is, however, borne out by the present
| |
| specimen. _
| |
| | |
| The prochordal plate
| |
| | |
| There is now an extensive literature dealing with the prochordal plate in
| |
| man and in mammalian forms (Hubrecht, 1890; Bonnet, 1901; Adelmann,
| |
| 1922; Bryce, 1924; Hill & Tribe, 1924; Hill & Florian, 1931; Heuser, 1932;
| |
| Hamilton, 1937; and others).
| |
| | |
| In the present embryo the thickened entodermal cells form,a horseshoeshaped plate resembling somewhat that described by Florian & Beneke ( 1930-3 1 )
| |
| in the Beneke embryo. The anterior tip of the head process lies between the
| |
| A presomite human embryo 35
| |
| | |
| two horns which project backward. The chromophilic granules first described
| |
| | |
| by Bonnet (1901) in the prochordal (completion) plate in the dog have been '
| |
| | |
| recognized in the cells of the entoderm in that part of the disc that we interpret
| |
| as prochordal plate. We have not found pouches or_ ‘cell groups’ in the prochordal plate, as described by Bryce (1924) in the McIntyre embryo (Sternberg,
| |
| 1927; Heuser, 1932).
| |
| | |
| Connecting stalk
| |
| | |
| As stated earlier, we have followed Florian (1930) in recognizing two parts
| |
| in the connecting stalk, an amnio-embryonal and an umbilical part. We find
| |
| in the Shaw embryo, as in nearly all human embryos at this stage of development, that the amniotic cavity runs into a distinct amniotic tip at its dorsocaudal part where it comes into contact with the mesoderm in the basal part
| |
| (i.e. near the chorion) of the umbilical stalk. In some of the cells of the mesoderm, entoderm, and ectoderm in the anterior part of the umbilical stalk
| |
| there are conspicuous chromatic granules. Florian regards these granules as a
| |
| sign of degeneration in the cells. After discussing at some length the changes
| |
| occurring in the stalk tissue he comes to the» conclusion that the amniotic
| |
| cavity extends backwards into the umbilical stalk at the expense of the tissue
| |
| of the stalk, as first described by von Mollendorff (1921), and that the cell
| |
| dissolution is a preliminary to this extension. ,
| |
| | |
| We find that there is some irregularity of the ectodermal amniotic cells in
| |
| the front of the umbilical stalk which would support the hypothesis of back
| |
| ward extension; we did not find, however, the sequestration of a large mass of
| |
| mesoderm as described by Florian.
| |
| | |
| The vascular primordia found in the stalk are discussed under angiogenesis.
| |
| | |
| Allanto-enteric diverticulum
| |
| | |
| The allanto-enteric diverticulum in the present embryo is found to be
| |
| associated at its tip with a solid mass of cells which we interpret as entodermal.
| |
| | |
| In the Mateer embryo.Streeter (1920) describes the allantois as separated
| |
| into two parts. In the proximal part of the distal separated portion there is a
| |
| lumen. In the Sternberg embryo (Sternberg, 1927) a separated epithelial
| |
| vesicle is associated with the allantois. Hill & Florian (1931) found, in the
| |
| Dobbin embryo, that the terminal part of the allantois passed into a terminal
| |
| vesicle. We are in agreement with the opinion expressed by them that the
| |
| allantois may undergo separation into two parts during‘ development, the
| |
| distal part being subject to early degeneration. In the present specimen the
| |
| cavity, if ever present, has disappeared.
| |
| | |
| Yollc sac
| |
| The details of the histological appearances of the cells in the yolk sac are
| |
| | |
| discussed under Angiogenesis. The formation of the foregut by anterior, and
| |
| partially by lateral, foldings is clearly shown. A folding is shown in the Heuser
| |
| | |
| 3-2
| |
| 36 R. J. Gladstoneiaml W. J. Hamilton
| |
| | |
| (1932) embryo but the formation is regarded as a fold in the yolk sac; Heuser
| |
| states that a recognizable foregut has not yet developed. ‘
| |
| | |
| Angiogenesis and haemapoiesis
| |
| | |
| The study of this specimen affords evidence in support of certain conclusions which have previously been reached from observations made on other
| |
| human embryos of approximately the same age and stage of development.
| |
| It also throws light upon the following problems which are still sub judice:
| |
| | |
| (1) The origin of the angioblastic tissue of the primary or chorionic mesoderm. V .
| |
| | |
| (2) The origin of the first haemocytoblasts and primitive blood cells.
| |
| | |
| (3) The relation of the blood islands of the yolk sac and umbilical segment
| |
| of the body stalk to the endothelial walls of the vessels which surround them.
| |
| | |
| (1) The origin of the angioblastic tissue of the primary or chorionic mesoderm.
| |
| It is almost universally accepted that both blood cells and blood vessels arise
| |
| from (the mesenchyme. This View is based chiefly on the observations of
| |
| Maximow (1909, 1927), Bloom (1938) and Bloom & Bartelmez (1940).
| |
| | |
| It seems best, in our opinion, to approach the subject of the early stages of
| |
| blood-cell formation and development of blood vessels in the human subject
| |
| and other vertebrate animals by an examination of the first stage of the
| |
| differentiation of these elements rather than to work backward from later
| |
| stages of blood formation, when differentiation of other cells from morphologically similar cells is taking place at the same time in different situations.
| |
| Apart from the difiiculty of deciding whether cells are developing intra— or
| |
| extra-vascularly, the examination of later stages is complicated by the fact
| |
| that there is a mixing in the general blood stream of cells which may be derived
| |
| from different sources.
| |
| | |
| This difficulty has to a large extent been overcome by Stockard’s (1915)
| |
| experimental work on the origin of blood and endothelium in Fundulus larvae.
| |
| Heconcludes that the origin of the blood cells and endothelial lining of the
| |
| heart, aortic arches and dorsal aortae is independent. Stockard succeeded in
| |
| rearing Frmdulus larvae in weak solutions of alcohol; although a heart and
| |
| intra-embryonic blood vessels lined by endothelium were developed in these
| |
| larvae, the connexion of the vitelline veins with the heart was never established
| |
| and consequently no circulation of either blood cells or plasma could take place.
| |
| Moreover, the blood developed in the intermediate cell mass at the posterior
| |
| end of the embryo, and, found in the blood islands in the vascular area of the
| |
| yolk sac, remained in its original situation and was not drawn into the heart
| |
| and intra-embryonic vessels. Furthermore, neither the heart with its endothelial lining nor any portion of the aortae were, at any stage of development,
| |
| seen to contain an erythroblast or erythrocyte. Stockard believes that the
| |
| blood islands in Fundulus embryos are formed by wandering mesenchymal
| |
| cells which migrate from the intermediate cell mass which lies at the posterior
| |
| end of the embryo, between the notochord and the caudal end of the gut; he
| |
| A presomite human embryo 37
| |
| | |
| further states‘ that the only mesodermal portion of the yolk sac in Fundulus
| |
| | |
| . is made up of disconnected wandering mesenchymal cells, some of which group
| |
| | |
| themselves to form the blood islands, others give rise to the yolk vessel endothelium, and still other wandering cells develop into the chromatophores.
| |
| Stockard thus ranges himself alongside those authors, such as Maximow, Bloom
| |
| and Bartelmez, who believe that both erythroblasts and endothelial cells arise
| |
| from the mesenchyme. He does not, however, believe that endothelial cells
| |
| once differentiated are capable of producing erythroblasts and erythrocytes,
| |
| since, in his series of embryos which developed without a circulation, he never
| |
| found that the endothelial-lined heart or the vessels leading from it, such as
| |
| the ventral aorta, aortic arches and dorsal aortae, contained developing blood
| |
| cells; nor does he believe that the mesenchyme in general, that is to say in other
| |
| places than the known haemopoietic foci, gives origin to developing red
| |
| corpuscles.
| |
| | |
| In the chorionic mesenchyme, including that of the villi and that at the
| |
| chorionic end of the connecting stalk, in the human subject, specialized angioblastic tissue appears at an early stage of development. This tissue gives rise,
| |
| by vacuolation of primarily solid protoplasmic strands, to endothelial-lined
| |
| spaces and vessels which before the circulation is established contain no
| |
| primitive blood cells, although these are abundant in the blood islands of the
| |
| yolk sac and are present in the umbilical stalk in the vicinity of the allanto—
| |
| enteric diverticulum. The formation of vessels in the chorionic ‘mesenchyme
| |
| has been specially studied in primates and in the human subject by Hertig
| |
| (1935). He concluded that there is ‘a simultaneous origin of angioblasts and
| |
| primary mesoderm by a. process of delamination and differentiation from the
| |
| chorionic trophoblast of macaque and human ova’, and that the isolated
| |
| vascular primordia thus formed soon possess the power of independent growth
| |
| and lumen formation, resulting not only in the vascularization of the 'chorion
| |
| and primary villi but of secondary villi as well. Further, after discussing the
| |
| question of haemopoiesis in the chorionic blood vessels of early human embryos,
| |
| such as the Mateer (Streeter, 1920) and the Heuser (1932) presomite specimen,
| |
| Hertig points out that ‘blood elements begin to appear in the chorionic and
| |
| villous circulation with regularity only about the time the embryonic circulation is becomingofunctionally established’. The generally empty condition of
| |
| these chorionic vessels which we have observed in the Shaw embryo, and which
| |
| was also‘ noted by Stieve (1926) in his 13;-day human embryo, confirms
| |
| Hertig’s conclusions and, in the light of Stockard’s experimental work,‘is
| |
| significant. It may be noted herethat the few formed elements, which appear
| |
| to be arising from the endothelium of the vessels shown in P1. 4, fig. 9, differ
| |
| considerably in size and character from the primitive blood cells and erythroblasts formed in the blood islands of the yolk sac, and although they may be
| |
| interpreted as being a generation of developing red blood cells arising from
| |
| the endothelium of vessels which have been developed in the chorionic
| |
| mesenchyme at the base of the connecting stalk, it is by no means certain that
| |
| 38 . R. J. Gladstone and W. J. Hamilton
| |
| | |
| they are such. Apart from this question it appears that angioblastic tissue is
| |
| differentiated at a very early stage from the mesenchyme in which it lies, and
| |
| which arises by delamination, either directly or indirectly, from an ectodermal
| |
| layer, namely, the cytotrophoblast.
| |
| | |
| (2) The origin of the first haemocytoblasts and primitive blood cells. The
| |
| results of an investigation of the early stages of development of the blood
| |
| vessels and of the heart were published by Wang (1918). This research was on
| |
| ferret embryos and commenced with the stage in which the embryo measured
| |
| 1-15 mm. and in which no intra—embryonic blood vessels could be recognized,
| |
| nor any indication of a heart rudiment. In this paper not only are the earliest
| |
| stages of development of the vessels, heart and pericardium described, but
| |
| there is a description of the author’s observations on the earliest stages of
| |
| development of the blood cells, preceded by a discussion of the literature on
| |
| the origin of blood vessels and blood cells. This discussion is so complete that
| |
| it will be quite unnecessary to do more" than refer readers to Wang’s paper
| |
| and we need only’ give a summary here of some of his conclusions. At that
| |
| time there .was a diversity of opinion with regard to the germ layer from which
| |
| the vascular endothelium and blood cells arise. In 1887 Ziegler expressed the
| |
| view that ‘the system of blood vessels and that of- the lymphatic vessels are
| |
| produced in their first fundaments from remnants of the primary body cavity
| |
| (blastocoel) which at the general distribution of the primitive tissue (mesoderm)
| |
| remain behind as vessels’. This view, therefore, harmonizes with B1'itschli’s
| |
| hypothesis that in all Metazoa the blood-vascular system originates from the
| |
| blastocoel. This theory, which may be referred to as the origin of the bloodvascular system from the primary mesoderm, has ‘received a considerable
| |
| amount of support from recent writers such as Hertig (1935) although, as
| |
| mentioned under the first heading of our discussion, Hertig limits his deductions to the origin of the blood vessels (angioblast) only, and he traces that
| |
| back to the simultaneous origin of the vascular rudiments and the chorionic
| |
| mesenchyme‘ from the cytotrophoblast by a process of delamination. It is
| |
| worth while here to draw attention to the statement made by Wang ‘that in
| |
| no case has an author stated that the blood cells and vascular endothelium are
| |
| derived from different germ layers’, and he furnishes evidence; based on his
| |
| observations on early ferret embryos, in support of his opinion that ‘whilst
| |
| blood cells and vascular endothelium are closely related to each other, and are
| |
| found invariably between the mesoderm and endoderm, there is evidence to
| |
| show that, in the ferret, the origins of these two vascular elements are separate
| |
| and distinct—the blood cells arising from the endoderm, and the vascular endothelium from the mesoderm ’. ‘ Blood cells develop at first extra-embryonically
| |
| in the area vasculosa in the form ‘of clusters of spheroidal cells which are
| |
| | |
| provided with large and round nuclei and with a comparatively small amount ~
| |
| | |
| of protoplasm. These are for the most part found adherent to the endoderm in
| |
| the neighbourhood of their origin, before they are engulfed by the endothelium,
| |
| and are in most instances identical in structure with the endodermal cells
| |
| Apresomite human embryo 39
| |
| | |
| where the contact is intimate.’ ‘On the other hand, the cells which form the
| |
| endothelial rudiment are mesodermal in origin.’ I
| |
| | |
| Wang’s_microphotographs of sections of early ferret embryos show features
| |
| which are very similar to those of the early stages in the development of the
| |
| blood islands in the presomite phase of human embryos, and we believe that
| |
| it may be safely concluded that the process of blood formation is essentially
| |
| the same in the ferret as it is in man, and that any differences in the conclusions which may be drawn are due to differences in interpretation rather
| |
| than to differences in species.
| |
| | |
| Another supporter of the entodermal origin of the first embryonic blood
| |
| cells is Piney (1927), who bases his argument in its favour on the following
| |
| facts: (1) ‘at the time of earliest blood formation thereare no formed organs
| |
| in the embryo and, therefore, transport of the yolk substances is very essential’.
| |
| It may be noted that in the Shaw embryo vacuoles are frequently visible in
| |
| the cytoplasm of the early blood cells as well as of the cells forming the walls of
| |
| the blood islands and the entodermal cells: (2) ‘The endodermal masses become
| |
| surrounded by mesodermal endothelium which, therefore, takes no part in this
| |
| form of haematopoiesis.’ (3) It is very striking that this extra—embryonic
| |
| form of haematopoiesis is later seen in the liver of .the embryo.’ With reference
| |
| to Maximow’s claim that these first-formed blood cells are the ancestors of all
| |
| the types that develop later, Schridde and Piney contend that ‘they are all
| |
| haemoglobiniferous elements’. Maximow holds that some of these primitive
| |
| cells become haemoglobiniferous and act as temporary carriers of oxygen
| |
| (primitive erythroblasts), while others retain all their haemopoietic potencies,
| |
| whereas Piney considers that ‘these first formed cells have only a short life
| |
| in the embryo but that they are all nucleated red cells (megaloblasts) which
| |
| normally persist longest in the liver of the embryo from which they disappear
| |
| before birth’. Piney agrees with Sabin $1920) that certain amoeboid cells
| |
| develop from, or near, the primitive endothelium, and quotes Ferrata as
| |
| stating that ‘all the white cells of primitive blood in the embryo are of such
| |
| endothelial origin (haemohistioblasts), i.e. are not derived from ‘the first
| |
| formed intravascular islet cells’. The marked difference in the shape and
| |
| character of the nuclei belonging to the early or primitive blood cells in the
| |
| yolk sac of a human embryo from those of the endothelial cells of the vessel
| |
| which encloses them is evident in his figure showing mitoses of the blood
| |
| cells. The nuclei of the blood cells are rounded in form, whereas those of the
| |
| endothelial cells are elongated and spindle or lens—shaped. The difference is seen
| |
| to be even more pronounced in the figure showing the cross-section of a vessel
| |
| of the area vasculosa of a rabbit embryo (five somites) by Maximow & Bloom
| |
| (1934), who, however, explain the difference as being due to rounding off of
| |
| the endothelial cells and their transformation into primitive blood cells. This
| |
| ‘rounding off’ of cells which are afterwards set free in the lumen of the vessel,
| |
| as seen-in Pl. 4, fig. 9, certainly appears to take place in the mesenchyme at the
| |
| chorionic end of the connecting stalk, and the appearances seem to support the
| |
| 40 R. J. Gladstone and W. J. Hamilton
| |
| | |
| View expressed by Piney that ‘ the embryo is supplied for a short time with two
| |
| varieties of blood; one derived from the entoderm, and composed purely of
| |
| haemoglobiniferous cells, and the other of mesenchymal origin and consisting
| |
| of both red and white cells’. T
| |
| | |
| Quite recently a description of haemopoiesis from the clinical standpoint
| |
| has been published by Gilmour (1941). He has made a special study of early
| |
| presomite and somite embryos prior to the establishment of the circulation,
| |
| as well as of older embryos, foetuses, and infants up to 21 days after birth.
| |
| From his study of the earlier stages of development he concludes that ‘vessels
| |
| arise from mesodermal cells independently in three areas, the yolk sac, the
| |
| chorion—perhaps at first limited to the body stalk—and the embryo. The
| |
| vessels in each of these areas unite to form nets or systems. The three systems
| |
| later unite with each other and the complete circulation is established.’
| |
| ‘ Blood islands form constantly in "the yolk sac and consist of separate vascular
| |
| units containing blood cells. The cells are first haemocytoblasts which arise
| |
| from the vessel wall. Later, the cells are almost entirely primitive erythroblasts but a few haemocytoblasts persist and a few differentiate into histiocytes.’
| |
| | |
| It may be noted here that this account differs from the usual text—book
| |
| description in which it is stated that the primarily uniform mass of rounded
| |
| cells which forms a ‘blood island’ or ‘angioblastic cord’ in the earliest stage
| |
| of development becomes differentiated into a superficial layer of flattened
| |
| cells—the endothelial wall of the blood vessel—which becomes separated by
| |
| a cleft containing embryonic plasma from the central cells, which retain their
| |
| rounded form and become the primitive blood cells.
| |
| | |
| Space will not permit us to review adequately this paper as a whole; we
| |
| may state, however, that we are in agreement with Gilmour’s conclusions
| |
| regarding the ipdependent development from mesoderm of vessels in the
| |
| three areas mentioned and their subsequent union prior to the establishment
| |
| of the circulation. _We cannot, however, wholly concur with his statement
| |
| that the first haemocytoblasts arise from the vessel wall, as there are numerous
| |
| instances of small islets which are not enclosed in an endothelial wall, the cells
| |
| of which cannot have arisen from differentiated endothelium by transformation,
| |
| although, as Piney suggests, this does not exclude the formation of other cells
| |
| of a different type from this source.
| |
| | |
| (3) The relation of the blood islands of the yolk sac and the umbilical stalk to
| |
| the endothelial walls of the vessels which are formed around them. The frequent
| |
| appearance of syncytial masses, or small groups .of rounded cells, which lie in
| |
| the interval between the entoderm and the mesoderm of the yolk-sac wall
| |
| (Pl. 4, fig. 10), and which are either isolated or in intimate relation with the
| |
| entoderm, and also of spaces which are bounded, on the inner side, by a single
| |
| layer of entodermal cells (Text-fig. 11), and, on the outer side, by undifferentiated mesenchyme, suggests that the cells which comprise these groups, and
| |
| which are often indistinguishable from the entodermal cells, may be primarily
| |
| A presmnite human embryo 41
| |
| | |
| derived from the entoderm. The vessel wall, on the other hand, appears to be
| |
| developed by differentiation of the neighbouring mesenchyme into endothelium
| |
| either before or subsequently to the formation of the island, or the primitive
| |
| blood cells may grow outward as a rounded mass into a mesodermalspace the
| |
| waH of which is simultaneously becoming transformed from undifferentiated
| |
| mesoblast into a. continuous endothelial wall.
| |
| | |
| This embryoals'o presents some interesting features in connexion with the
| |
| early ‘development of the heart and its relationship to the anterior part of the
| |
| wall of the yolk sac. The description and discussion of these in conjunction
| |
| with the development of the pericardium, floor .of the foregut, and intraembryonic vessels, we propose to defer for consideration in a later communication. A
| |
| | |
| SUMMARY
| |
| | |
| 1. A description is given of a well-fixed presomite human embryo at a
| |
| stage of development with a blastopore, notochordal process and chorda canal.
| |
| | |
| 2. The age of the embryo is discussed in relation to embryos at approximately the same stage of development. V \
| |
| | |
| 3. A well-defined cloacal membrane is formed which involves the posterior
| |
| part of the embryonic disc and the proximal part of the allanto-enteric
| |
| diverticulum.
| |
| | |
| 4. A description is given of the formation of blood vessels "from specialized
| |
| angioblastic tissue in the ehorionic mesenchyme. The majority of these vessels
| |
| at this stage of development contain no blood cells.
| |
| | |
| 5. The vascular spaces are partly developed by the fusion of small vacuoles,
| |
| which are formed in solid angioblastic cords (intracellular spaces), and partly
| |
| by direct transformation of mesodermal cells into flattened endothelium, which
| |
| may either enclose the blood islands of the yolk sac, or form the walls of
| |
| vascular spaces, which atfirst empty and incomplete, become secondarily
| |
| filled with blood cells and enclosed by a continuous membrane.
| |
| | |
| 6. The earliest generation of blood cells (haemocytoblasts and primitive
| |
| erythroblasts) are formed in the wall of the yolk sac and in the umbilical
| |
| segment of the connecting stalk in close connexion with the entoderm of the
| |
| allanto-enteric diverticulum, and in the situation of the future umbilical
| |
| vessels. A few rounded cells of endothelial origin were, however, found in the
| |
| | |
| . mesenchyme at the base, or amnio-embryonic segment, of the connecting
| |
| | |
| stalk. These differ in type from the former cells which arise in close association
| |
| with the entoderm of the yolk sac and its diverticulum.
| |
| | |
| It is a pleasure to record our indebtedness to Mr A. K. Maxwell for the
| |
| beautiful natur-treu figures reproduced in P1. 1, and for Text-figs. 1+3. We wish
| |
| to thank our technicians, Messrs Westwood and Park, for their invaluable
| |
| assistance. ‘
| |
| 42
| |
| | |
| R. J. Gladstone and W. J. Hamilton
| |
| | |
| REFERENCES
| |
| | |
| ADELMANN, H. B. (1922). Amer. J. Amt. 31, 55.
| |
| ALLEN,’E., PRATT, J. P., NEWELL, Q. U. & BLAND, L. J. (1930). Contr. Embryol. Carney.
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| Inatn, 22, 45. ‘ '
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| | |
| BLOOM, W. (1938). Handbook of Haematology. Ed. by Downey. N.Y. : Hoeber. Vol. 2.
| |
| BLOOM, W. & BARTELMEZ, G. W. (1940). Amer. J. Amt. 67, 21.
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| | |
| Bommm, R. (1901). Amt. Hefte, 16, 231.
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| | |
| Bnnwnn, J. I. (1938). Ccmtr. Embryol. Carney. Imtn, 27, 87.
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| | |
| BRYCE, T. H. (1924). Trans. Roy. Soc. Edirtb. 53, 533.
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| | |
| B1'i'rscnL1, O. (1882). Morphol. Jb. 8, 474.
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| | |
| CLARK_, E. R. (1909). Amt. Rec. 3, 183.
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| CLARK, E. R. & CLARK, E. L. (1932). Amer. J. Anat. 4-9, 441.
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| | |
| —-—— — (1937). Amer. J. Amt. 60, 253.
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| FA1mENHoLz, C. (1925). Z. milcr. Amt. 8, 249. ,
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| | |
| FLEW, J. D. (1941). Brit. med. J. 11 Jan., p. 66.
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| | |
| FLOBIAN, J. (1928). Z. milcr. Amt. 13, 500.
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| | |
| —— (1930). J. Anat., Lond., 64-, 454.
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| —— (1933). J. Amt.,\Lond., 67, 263. ,
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| | |
| — (1934). Bratial. lelcdr. listy, 14-, 299.
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| | |
| FLORIAN, J. & BENEKE, R. (1930-31). Amt. Anz. Erg. 71, 229.
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| momm, J. & Hm, J. P. (1935). J. Anat., L<md., 69, 399.
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| | |
| GILMOUR, J. R. (1941). J. Path. Bact. 52, 25.
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| | |
| GBOSSEB, O. (1913). Amt. Hefte, 4-7, 649.
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| | |
| —— (l930—1). Anat. Anz. Erg. 71, 135. I
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| —— (1931). Z. ges. Amt. 1. Z. Amt. EntwGeach. 94-, 275.
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| HAmL'1'oN, W. J . (1937). Trans. Roy. Soc. Edinb. 59, 165.
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| | |
| HEBTIG, A. T. (1935). Contr. lfhnbryol. Carneg. Irwtn, 25, 39.
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| HEUSER, C. H. (1932). Contr. Embryol. Carney. Instn, 23, 251.
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| | |
| HILL, J . P. & FLORIAN, J . (1931). Philos. Tram. B, 219, 443.
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| | |
| HILL, J . ,P. & T3131}, M. (1924). Quart. J . micr. Sci. 68, 513.
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| | |
| HUBBECHT, A. A. W. (1890). Quart. J . micr. Sci. 31, 499.
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| | |
| INGALLS, N. W. (1918). Contr. Ernbryol. Carney. Inatn, 7, 111.
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| | |
| — (1921). Contr. Embryol. Carney. Instn, 11, 63.
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| | |
| JOHNSTON, T. B. (1940). J. Amt, Lond., 75, l.
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| | |
| KEIBEL, F. (1896). Arch. Anat. Phyaz'ol., Lpz., p. 250. _
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| KNAUS, H. (1934). Periodic Fertility and Sterility in Women. Vienna: W. Maudrich.
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| MAx1Mow, A. (1909). Arch. milcr. Amt. 73, 444. ‘
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| | |
| —: (1927). V. M6]1endo1'fl"s Handb. Miler. Anat. Merw, 2. Berlin.
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| MAXIMOW, A. & BLOOM, W. (1934). Textbook of Histology. London: Saunders.
| |
| MEYER, P. (1924). Arch. 122, 38. ‘
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| M5LL1a:NDonn-Ir, W. voN (1921). Z. gas. Amt. 1. Z. Amt. EntwGeach. 62, 406.
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| PINEY, A. (1927). Recent Advances in Haematology. London: Churchill.
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| RABL, C. (1915); Arch. mikr. Anat. 88, 1.
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| ROSSENBECK, H. (1923). Z. gee. Amt. 1. Z. Anat. EntwGeach. 68, 325.
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| SABIN, F. R. (1920). Contr. Embryol. Carneg. Irwtn, 9, 213.
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| | |
| SHAW, W. (1925). J . Physiol. 60, 193.
| |
| | |
| —— (1932). Brit. med. J. 5 Mar., p. 411.
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| | |
| SIEGEL, W. (1916). Munch. med. Wschr. 63, 748.
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| | |
| STEBNZBERG, H. (1927). Z. gee. Amt. 1. Z. Amt. EntwGeach. 82, 142,
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| STIEVE, H. (1926). Z. milcr. Amt. 7, 295.
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| STOCKARD, C. R. (1915). Amer. J. Amt. 18, 227.
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| Smnnmnn, G. L. (1920). Contr. Embryol. Cameg. Imtn, 9, 389.
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| | |
| — (1927). Contr. Embryol. Carney. Irwtn, 19, 73.
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| | |
| THOMPSON, P. & BRASH, J . C. (1923). J . Anat., Lond., 58, 1.
| |
| Fig.
| |
| | |
| Fig.
| |
| | |
| Fig.
| |
| | |
| Fig.
| |
| | |
| Fig.
| |
| | |
| Fig.
| |
| | |
| . A presomite human embryo 43
| |
| | |
| VENNING, C. H. & Bnowrua, J. S. L. (1937). Endocrinology, 21, 711.
| |
| WALDEYEB, A. (1929). Z. gas. Amt. 1. Z. Anal. E'n.twGeach. 90, 412.
| |
| WANG, C. C. (1918). J. Am1.i., L¢md., 52, 107.
| |
| | |
| WYBUBN, G. M. (1937). J. Anat., L¢md., 71, 201.
| |
| | |
| ZIEGLEB, H. E. (1887). Arch. milcr. Anal. 30, 596.
| |
| | |
| EXPLANATION OF PLATES 1-4
| |
| The photomicrographs in Pls. 2-4 have been reproduced without retouching.
| |
| PLATE 1
| |
| | |
| 1. A drawing of part of the wall of the yolk sac in the abembryonic region showing the
| |
| diiferent types of developing blood cells lying within, and in the neighbourhood of, a vascular
| |
| space which is lined with endothelium. Section no. 31, the sections having been numbered
| |
| from the anterior end of the embryonic disc. x c. 750. Cell no.: 1, haemocytoblast; 2, extravascular haemocytoblast; 3, small haemocytoblast; 4, transitional haemocytoblast; 5, early
| |
| primitive erythroblast; 6, intermediate primitive erythroblast (with indented nucleus);
| |
| 7, binucleated primitive erythroblast; 8, intermediate primitive erythroblast; 9, late primitive
| |
| erythroblast; 10, disintegrating erythroblast; ll, non-nucleated primitive erythrocyte;
| |
| 12, binucleate giant cell; 13, remains of early blood cell.
| |
| | |
| 2. A drawing of part of the wall of the yolk sac near to its attachment to the embryonic
| |
| disc. A well-formed blood island is seen in the right of the drawing. Cell no. 1 is a haemocytoblast and no. 9 a late primitive erythroblast. No. 14 shows the formation of an angiocyst
| |
| in relation with the mesothelium. No. 15 shows a cell undergoing vacuolation (? transfbrmation into an endothelial cell), the contents being discharged to contribute to embryonic plasma.
| |
| A large vacuole is formed in one of the entodermal cells; other entodermal cells have ‘ragged’
| |
| edges suggesting that they have discharged their vacuoles into the yolk sac. Section no. 31.
| |
| x c. 750.
| |
| | |
| - PLATE 2
| |
| 3. A section through the umbilical stalk in the region of the allanto-enteric diverticulum.
| |
| Chromatic particles are conspicuous in many of the mesodermal cells, and are also present
| |
| in some of the ectodermal and entodermal cells. Ventro-lateral to the allanto-enteric diverti
| |
| culum is a group of primitive erythroblasts some of which are undergoing Section
| |
| no. 108. x 560.
| |
| | |
| . 4. Section through the blastopore and beginning of the chorda canal. The anterior lip of the
| |
| | |
| blastopore lies to the left of the photograph. The cells are columnar both in the superficial
| |
| ectoderm and in the roof of the chorda canal. The nuclei are deeply stained and are, for the
| |
| most part, situated deeply, near the basement membrane. The cytoplasm is clear and the
| |
| cell boundaries are mostly well defined. Section no. 46. x 560.
| |
| | |
| PLATE 3
| |
| | |
| 5. A section of the definitive cloacal membrane. Itshows the continuity of ectoderm and
| |
| entoderm. The mesodermal cells pass around the side of the membrane. The amniotic cavity is
| |
| situated above in the figure. Section no. 96. x 560.
| |
| | |
| 6. A section through the prochordal plate. The entodermal cells are seen to be cuboidal.
| |
| Chromophilic granules are seen in some of the cells and between the plate and the ectoderm.
| |
| Section no. 28. x 700. .
| |
| | |
| 7. The section shows an empty capillary vessel, cut transversely, which is lying in the mesenchymal tissue of a chorionic villus; it is bounded on one side by angioblast which sends processes outward into the surrounding mesenchyme. An isolated angioblastic strand lies below
| |
| the vessel, but does not appear to take part in the formation of its wall. Section no. 93.
| |
| x 350.
| |
| | |
| . 8. In the upper part of the figure, maternal red blood corpuscles are seen in the intervillous
| |
| | |
| space. Below this is the trophoblast which shows a distinct division into plasmodial and
| |
| cytotrophoblastic layers. In the chorionic mesoderm angioblastic strands and an early stage
| |
| in the development of capillary vessels may be seen. Section no. 30. x 350.
| |
| 44
| |
| | |
| Fig.
| |
| | |
| Fig.
| |
| | |
| _Fig.
| |
| | |
| R. J. Gladstone and W. J. Hamilton
| |
| | |
| PLATE 4
| |
| | |
| 9. The section passes through the basal or chorionic end of the connecting stalk,‘ where the
| |
| supporting mesenchymal tissue resembles, and is continuous with, the parietal layer of the
| |
| chorionic mesoderm. A well-developed vascular channel crosses the lower part of the photograph. The endothelial wall of this‘ channel shows various stages in the ‘rounding up’ and
| |
| liberation of endothelial cell elements, some of which are seen lying free in the lumen of the
| |
| vessel. Note the contrast between these cells and those in P1. 4, Hg. 11. Note also extension
| |
| of solid angioblastic strands into the mesenchyme on the left. The group of epithelial cells
| |
| in the upper central part of the figure probably represent a degenerate remnant of the distal
| |
| portion of the allanto-enteric diverticulum. Section no. 144. x 350.
| |
| | |
| 10. Section through a portion of the wall of the foregut which is shown at a lower magnification in Text-fig. 8. A small blood island is seen between the entoderm lining the floor of the
| |
| foregut and the mesothelial layer covering it superficially. In the roof of the foregut opposite
| |
| the blood island is a triangular space which can be traced through a series of sections. The base
| |
| of the triangle is formed by entoderm; the inner and outer walls, which converge to the apex
| |
| of the triangle, closely resemble the angioblastic tissue found in the chorionic mesenchyme
| |
| and the basal segment of the connecting stalk. Section no. 10. x 560.
| |
| | |
| 11. Segment of wall of yolk sac showing entoderm on the right side and mesothelium on the
| |
| left. In the upper part of the figure the entodermal cells show signs of degeneration; in the
| |
| lower part the cells appear to have been shed, leaving the endothelial wall of a Vascular
| |
| space exposed. In the mesoblast is a blood island which is covered, in the greater part of its
| |
| extent, by endothelium; it projects below into the vascular space and appears to be breaking
| |
| through previous to discharging free erythroblasts into the lumen of the space. V x 560.
| |
| Journal of Anatomy, Vol. 76, Part 1 Plate 1
| |
| | |
|
| |
|
| |
|
| |
|
| |
| | |
| ’_ ENTODE_/?/‘1
| |
| | |
| ENDOTHEL/UN ~
| |
| NESENCHY/'1E
| |
| | |
| I 2'<‘3MM. .
| |
| | |
|
| |
| | |
| GLADSTONE AND HAMILTON——A rnmsomrm HUMAN EMBBYO
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| Journal of Anatomy, Vol. 76, Part 1 Plate 2
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| GLADSTONE AND HAMILTON———A PRESOMITE HUMAN EMBRYO
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| Journal of Anatomy, Vol. 76, Part 1 Plate 3
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| GLADSTONE AND HAMILTON—A PRESOMITE HUMAN EMBRYO
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| Journal of Anatomy, Vol. 76, Part 1 Plate 4
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| GLADSTONE AND HAMILTON——A PRESOMITE HUMAN EMBRYO
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| {{Footer}}
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| [[Category:Draft]]
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