Difference between revisions of "Paper - Contributions to the embryology of the marsupialia 4-6"

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==Chapter VI.  -  General Summary and Conclusions==
 
==Chapter VI.  -  General Summary and Conclusions==
  
The observations recoi'ded in the pi'eceding pages and the  
+
The observations recoi'ded in the pi'eceding pages and the conclusions deducible therefrom may be summarised as follows ;
conclusions deducible therefrom may be summarised as  
 
follows ;  
 
  
(a) Ovum.  -  The uterine ovum of Dasyurus is characterised  
+
===(a) Ovum===
(1) by its large size relatively to those of Eutheria; (2) by  
+
The uterine ovum of Dasyurus is characterised (1) by its large size relatively to those of Eutheria; (2) by the presence externally to the zona of a layer of albumen and a shell-membrane, both laid down in the Fallopian tube and homologous with the corresponding structures in the Mouotreme ovum, the shell-membrane, like the shell of the latter, inci'easing in thickness in the uterus; (3) by its marked polarity, its lower two thirds consisting of formative cytoplasm, dense and finely granular in appearance, owing to the presence of fairly uniformly distributed deutoplasmic material, and containing the two pronuclei, its upper third being relatively clear and transparent, consisting as it does of a delicate reticulum of non-formative cytoplasm, the meshes of which are occupied by a clear deutoplasmic fluid. Study of the pi'ocess of vitellogenesis in ovarian ova demonstrates that this fluid represents surplus deutoplasmic material which has not been utilised in the upbuilding of the formative region of the ovum.
the presence externally to the zona of a layer of albumen and  
 
a shell-membrane, both laid down in the Fallopian tube and  
 
homologous with the corresponding structures in the Mouotreme ovum, the shell-membrane, like the shell of the latter,  
 
inci'easing in thickness in the uterus; (3) by its marked polarity, its lower two thirds consisting of formative cytoplasm, dense and finely granular in appearance, owing to the  
 
presence of fairly uniformly distributed deutoplasmic material,  
 
and containing the two pronuclei, its upper third being  
 
relatively clear and transparent, consisting as it does of a  
 
delicate reticulum of non-formative cytoplasm, the meshes of  
 
which are occupied by a clear deutoplasmic fluid. Study of  
 
the pi'ocess of vitellogenesis in ovarian ova demonstrates that  
 
this fluid represents surplus deutoplasmic material which has  
 
not been utilised in the upbuilding of the formative region of  
 
the ovum.  
 
  
The fate of the clear non-formative portion of the ovum is
 
a very remarkable one. Prior to the completion of the first
 
cleavage, it is separated off from the formative remainder of
 
the ovum as a spherical mass or yolk-body, Avhich takes no
 
direct part in development, though it becomes enclosed iu the
 
blastocyst cavity on completion of the blastocyst wall at the
 
upper pole. Its contained deutoplasmic fluid is to be regarded
 
as the product of an abortive attempt at the formation of a
 
solid yolk-mass, such as is found in the Monotreme ovum.
 
By its elimination the potentially yolk-laden telolecithal ovum
 
becomes converted into a secondarily homolecithal, holoblastic
 
one. All the evidence is held to support the conclusion that
 
the Marsupials are descended from oviparous ancestors with
 
ineroblastic ova.
 
  
(b) Cleavage.  - Cleavage begins in the uterus, is total, and
+
The fate of the clear non-formative portion of the ovum is a very remarkable one. Prior to the completion of the first cleavage, it is separated off from the formative remainder of the ovum as a spherical mass or yolk-body, Avhich takes no direct part in development, though it becomes enclosed iu the blastocyst cavity on completion of the blastocyst wall at the upper pole. Its contained deutoplasmic fluid is to be regarded as the product of an abortive attempt at the formation of a solid yolk-mass, such as is found in the Monotreme ovum. By its elimination the potentially yolk-laden telolecithal ovum becomes converted into a secondarily homolecithal, holoblastic one. All the evidence is held to support the conclusion that the Marsupials are descended from oviparous ancestors with ineroblastic ova.
at first equal and of the radial type. The first two cleavage  
 
planes are meridional and at right angles to each other.
 
The resulting four equal-sized blastomeres lie disposed radially
 
around the polar diameter like those of the Monotreme (not
 
in pairs at right angles to each other as in Eutheria), and
 
enclose a segmentation cavity open above and below, their
 
upper ends partially surrounding the yolk-body. The third
 
cleavage planes are again meridional, each of the four blastomeres becoming subdivided equally into two. The resulting
 
eight cells form an equatorial ring in contact with the inner
 
surface of the sphere formed by the egg-envelopes. They
 
  
 +
===(b) Cleavage===
 +
Cleavage begins in the uterus, is total, and at first equal and of the radial type. The first two cleavage planes are meridional and at right angles to each other. The resulting four equal-sized blastomeres lie disposed radially around the polar diameter like those of the Monotreme (not in pairs at right angles to each other as in Eutheria), and enclose a segmentation cavity open above and below, their upper ends partially surrounding the yolk-body. The third cleavage planes are again meridional, each of the four blastomeres becoming subdivided equally into two. The resulting eight cells form an equatorial ring in contact with the inner surface of the sphere formed by the egg-envelopes. They contain deutoplasmic material, which is, however, located mainly in their lower halves. The ensuing fourth cleavages are equatorial, and in correlation with the just-mentioned disposition of the deutoplasm, are unequal and qualitative, each of the eight blastoraeres becoming subdivided into an upper smaller and clearer cell, with relatively little deutoplasm fairly uniformly dispersed through the cytoplasm, and a lower larger, more opaque cell Avith much deutoplasm, mainly located in a broad zone in the outer portion of the cell-body. A 16-celled stage is thus produced in which the blastomeres are characteristically arranged in two superimposed rings, each of eight cells, an upper of smaller, clearer cells next the yolk-body, and a lower of larger, denser cells. The former is destined to give origin to the formative or embryonal region of the blastocyst wall, the latter to the non-formative or extra-embryonal region of the same.
  
THE EARLY DEVELOPMENT OF THE MARSUPIALIA. . »1
 
  
contain deutoplasmic material, which is, however, located
+
===(c) Formation of the Blastocyst===
mainly in their lower halves. The ensuing fourth cleavages
+
There is in the Marsupial no morula stage as in Butheria, the blastomeres proceeding directly to form the wall of the blastocyst. The cells of the two rings of the 16-celled stage divide at first meridionally and then also equatorially, the division planes being always vertical to the surface. The daughter-blastomeres so produced, continuing to divide in the same fashion, gradually spread towards opposite poles in contact with the inner surface of the fii-m sphere formed by the zona and the thickened shell-membrane. Eventually they form a complete cellular lining to the said sphere and it is this which constitutes the wall of the blastocyst. The latter is accordingly unilaminar at its first origin, and it remains so in Dasyurus until it has attained, as the result of active gi'owth accompanied by the imbibition of fiuid from the uterus, a diameter of 4-5 mm. It consists of two parts or regions, distinct in origin and in destiny, and clearly marked off from each other in later blastocysts by a definite junctional line approximately equatorial in position, viz. an upper, embryonal or formative region derived from the upper cell-ring of the 16-celled stage, and a lower, extra-embryonal or non-formative region derived from tlie lower cell-ring of the same stage.
are equatorial, and in correlation with the just-mentioned
 
disposition of the deutoplasm, are unequal and qualitative,  
 
each of the eight blastoraeres becoming subdivided into an
 
upper smaller and clearer cell, with relatively little deutoplasm fairly uniformly dispersed through the cytoplasm, and
 
a lower larger, more opaque cell Avith much deutoplasm,  
 
mainly located in a broad zone in the outer portion of the
 
cell-body. A 16-celled stage is thus produced in which the
 
blastomeres are characteristically arranged in two superimposed rings, each of eight cells, an upper of smaller, clearer
 
cells next the yolk-body, and a lower of larger, denser cells.
 
The former is destined to give origin to the formative or  
 
embryonal region of the blastocyst wall, the latter to the
 
non-formative or extra-embryonal region of the same.  
 
  
(c) Formation of the Blastocyst.  -  There is in the
 
Marsupial no morula stage as in Butheria, the blastomeres
 
proceeding directly to form the wall of the blastocyst. The
 
cells of the two rings of the 16-celled stage divide at first
 
meridionally and then also equatorially, the division planes
 
being always vertical to the surface. The daughter-blastomeres so produced, continuing to divide in the same fashion,
 
gradually spread towards opposite poles in contact with
 
the inner surface of the fii-m sphere formed by the zona and
 
the thickened shell-membrane. Eventually they form a complete cellular lining to the said sphere and it is this which
 
constitutes the wall of the blastocyst. The latter is accordingly unilaminar at its first origin, and it remains so in
 
Dasyurus until it has attained, as the result of active gi'owth
 
accompanied by the imbibition of fiuid from the uterus, a
 
diameter of 4-5 mm. It consists of two parts or regions,
 
distinct in origin and in destiny, and clearly marked off from
 
each other in later blastocysts by a definite junctional line
 
approximately equatorial in position, viz. an upper, embryonal
 
or formative region derived from the upper cell-ring of the
 
16-celled stage, and a lower, extra-embryonal or nonVOL. 56, PART 1. NEW SERIES. 6
 
  
 +
===(d) Later History of the Two Regions of the Blastocyst Wall===
 +
(for details see pp. 72-74).
  
82
+
From the embryonal region are derived the embryonal ectoderm and the entire entoderm of the vesicle. I conclude^ therefore, that it is tlie homologue of the inner cell-mass or embryonal knot of the Eutherian blastocyst. The extra-embryonal region directly furnishes the outer extra-embryonal layer of the vesicle wall, i. e. the outer layer of the omphalopleure and chorion of later stages. Assuming, as the facts of comparative anatomy and palaeontology entirely justify us in doing, that the Mammals are monophyletic and of reptilian origin, and further assuming that the foetal membranes are homologous structures throughout the Amniotan series (also in my view a perfectly justifiable assumption)^, then the homologies of this extraembryonal region of the Marsupial blastocyt are not far to seek. It is clearly the homologue of the extra-embryonal ectoderm of the Sauropsidan and Monotreme egg, and the homologue also of the outer enveloping layer of the Eutlierian blastocyst, to which Hubi'echt has given the special name of “ trophoblast.” In my view the trophoblast is none other than extra-embryonal ectoderm which in the viviparous mammals, in correlation with the intra-uterine mode of development, has acquired a special significance for the nutrition of the embryo.
  
  
J. P, HILL.  
+
These, then, are my conclusions, and to me they seem on general grounds perfectly obvious, viz. : (1) that the embryonal or formative region of the unilaminar Marsupial blastocyst is the homologue of the inner cell-mass or imposed cell-viiigs, respectively and non-formative cell-rings of formative (emlDryonal) and non- the Metatherian. formative (extra-embryonal) in significance.
  
  
formative region derived from tlie lower cell-ring of the same
+
* How Assheton can maintain (09, p. 266) that the amnion of the rabbit is not more homologous to the amnion of the Sauropsidan than the homy teeth of Ornithorhynchns ai-e homologous to the true teeth of the mammal or reptile, which they have supplanted,” how he can hold this view and yet proceed to utilise the presence of the amnion as one of the leading charactei-s distinguishing the Amniota from the Anamnia, I fail to comprehend. Surely the presence of a series of purely analogous structures in a group is of no classificatory value.
stage.  
 
  
(d) Later History of the Two Region s of the Blastocyst Wall (for details see pp. 72-74). -  From the embryonal
 
region are derived the embryonal ectoderm and the entire
 
entoderm of the vesicle. I conclude^ therefore, that it is tlie
 
homologue of the inner cell-mass or embryonal knot of the
 
Eutherian blastocyst. The extra-embryonal region directly
 
furnishes the outer extra-embryonal layer of the vesicle wall,
 
i. e. the outer layer of the omphalopleure and chorion of later
 
stages. Assuming, as the facts of comparative anatomy and
 
palaeontology entirely justify us in doing, that the Mammals
 
are monophyletic and of reptilian origin, and further assuming
 
that the foetal membranes are homologous structures throughout the Amniotan series (also in my view a perfectly
 
justifiable assumption)^, then the homologies of this extraembryonal region of the Marsupial blastocyt are not far to
 
seek. It is clearly the homologue of the extra-embryonal
 
ectoderm of the Sauropsidan and Monotreme egg, and the
 
homologue also of the outer enveloping layer of the Eutlierian
 
blastocyst, to which Hubi'echt has given the special name of
 
“ trophoblast.” In my view the trophoblast is none other
 
than extra-embryonal ectoderm which in the viviparous
 
mammals, in correlation with the intra-uterine mode of
 
development, has acquired a special significance for the
 
nutrition of the embryo.
 
  
These, then, are my conclusions, and to me they seem on
+
embryonal knob of the Eutberiaii blastocyst ; and (2) that the extra-embryonal or noii-formative region of the same is the homologne of the extra-embryonal ectoderm of the Sauropsida and Monotremata and of the trophoblast of the Eutheria.
general grounds perfectly obvious, viz. : (1) that the embryonal or formative region of the unilaminar Marsupial
 
blastocyst is the homologue of the inner cell-mass or
 
  
* How Assheton can maintain f 09, p. 266) “ that the amnion of the
 
rabbit is not more homologous to the amnion of the Sauropsidan than
 
the homy teeth of Ornithorhynchns ai-e homologous to the true teeth
 
of the mammal or reptile, which they have supplanted,” how he can
 
hold this view and yet proceed to utilise the presence of the amnion as
 
one of the leading charactei-s distinguishing the Amniota from the
 
Anamnia, I fail to comprehend. Surely the presence of a series of
 
purely analogous structures in a group is of no classificatory value.
 
  
 +
As regards conclusion (1) there is not likely to be much difference of opinion, but as regards (2), whilst perhaps the majority of embryologists support the obvious, not to say common-place view which I here advocate, it seems certain that it will prove neither obvious nor acceptable to those mammalian embryologists (I refer specifically to my friends Professor A. A. W. Hubrecht and Mr. R. Asshetou) who, with only Selenka^s account of eaidy Marsupial ontogeny before them, have formulated other and quite divergent views as to the morphological nature of the outer enveloping layer of the Eutherian blastocyst. It is therefore necessary to discuss this question further, though I would fain express my conviction that had the observations recorded in this paper been earlier available, much vain speculation as to the phytogeny of the trophoblast might possibly have been avoided.
  
THE EARLY DEVELOPMENT OP THE MAllSUPIALIA
 
 
 
83
 
 
 
 
imposed cell-viiigs, respectively and non-formative cell-rings of
 
formative (emlDryonal) and non- the Metatherian.
 
formative (extra-embryonal) in
 
significance.
 
 
 
84
 
 
 
J. P. HILL
 
 
 
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W
 
 
 
THE EARLY DEVELOPMENT OP THE MARSUPIALIA.
 
 
 
85
 
 
 
embryonal knob of tlie Eutberiaii blastocyst ; and (2) that the
 
extra-embryonal or noii-formative region of the same is the
 
homologne of the extra-embryonal ectoderm of the Sauropsida and Monotremata and of the trophoblast of the
 
Eutheria.
 
 
As regards conclusion (1) there is not likely to be much
 
difference of opinion, but as regards (2), whilst perhaps the
 
majority of embryologists support the obvious, not to say
 
common-place view which I here advocate, it seems certain
 
that it will prove neither obvious nor acceptable to those
 
mammalian embryologists (I refer specifically to my friends
 
Professor A. A. W. Hubrecht and Mr. R. Asshetou) who, with
 
only Selenka^s account of eaidy Marsupial ontogeny before
 
them, have formulated other and quite divergent views as to
 
the morphological nature of the outer enveloping layer of the
 
Eutherian blastocyst. It is therefore necessary to discuss
 
this question further, though I would fain express my conviction that had the observations recorded in this paper been
 
earlier available, much vain speculation as to the phytogeny
 
of the trophoblast might possibly have been avoided.
 
  
 
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{{Hill1910 footer}}

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Hill JP. The Early Development of the Marsupialia, with Special Reference to the Native Cat (Dasyurus Viverrinus). (1910) Quart. J. Micro. Sci. 56(1): 1-134.

  Contents: 1 Review of Previous Observations | 2 The Ovum of Dasyurus | 3 Cleavage and Blastocyst | 4 Blastocyst Growth Ectoderm Entoderm | 5 Early Stages of Perameles and Macropus | 6 Summary and Conclusions | 7 Early Mammalia Ontogeny | Explanation of Plates
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Eastern quoll
Eastern quoll
Mark Hill.jpg
This historic 1910 paper by James Peter Hill describes marsupial development in the native cat (Dasyurus Viverrinus)



Note that native cat, eastern native cat, are historic names for the eastern quoll Dasyurus Viverrinus (D. viverrinus). The eastern quoll is a medium-sized carnivorous marsupial native to Australia.

  • Dasyurus - "hairy tail"

dasyurid

Modern Notes:

Australian Animal: echidna | kangaroo | koala | platypus | possum | Category:Echidna | Category:Kangaroo | Category:Koala | Category:Platypus | Category:Possum | Category:Marsupial | Category:Monotreme | Development Timetable | K12
Historic Australian Animal  
Historic Embryology: 1834 Early Kangaroo | 1880 Platypus Cochlea | 1887 Monotremata and Marsupialia | 1910 Eastern Quoll | 1915 The Monotreme Skull | 1939 Early Echidna

The Hill Collection contains much histology of echidna and platypus embryonic development.

Embryology History | Historic Disclaimer

Other Marsupials  
Monito del Monte Development | Opossum Development
Historic Disclaimer - information about historic embryology pages 
Mark Hill.jpg
Pages where the terms "Historic" (textbooks, papers, people, recommendations) appear on this site, and sections within pages where this disclaimer appears, indicate that the content and scientific understanding are specific to the time of publication. This means that while some scientific descriptions are still accurate, the terminology and interpretation of the developmental mechanisms reflect the understanding at the time of original publication and those of the preceding periods, these terms, interpretations and recommendations may not reflect our current scientific understanding.     (More? Embryology History | Historic Embryology Papers)

Chapter VI. - General Summary and Conclusions

The observations recoi'ded in the pi'eceding pages and the conclusions deducible therefrom may be summarised as follows ;

(a) Ovum

The uterine ovum of Dasyurus is characterised (1) by its large size relatively to those of Eutheria; (2) by the presence externally to the zona of a layer of albumen and a shell-membrane, both laid down in the Fallopian tube and homologous with the corresponding structures in the Mouotreme ovum, the shell-membrane, like the shell of the latter, inci'easing in thickness in the uterus; (3) by its marked polarity, its lower two thirds consisting of formative cytoplasm, dense and finely granular in appearance, owing to the presence of fairly uniformly distributed deutoplasmic material, and containing the two pronuclei, its upper third being relatively clear and transparent, consisting as it does of a delicate reticulum of non-formative cytoplasm, the meshes of which are occupied by a clear deutoplasmic fluid. Study of the pi'ocess of vitellogenesis in ovarian ova demonstrates that this fluid represents surplus deutoplasmic material which has not been utilised in the upbuilding of the formative region of the ovum.


The fate of the clear non-formative portion of the ovum is a very remarkable one. Prior to the completion of the first cleavage, it is separated off from the formative remainder of the ovum as a spherical mass or yolk-body, Avhich takes no direct part in development, though it becomes enclosed iu the blastocyst cavity on completion of the blastocyst wall at the upper pole. Its contained deutoplasmic fluid is to be regarded as the product of an abortive attempt at the formation of a solid yolk-mass, such as is found in the Monotreme ovum. By its elimination the potentially yolk-laden telolecithal ovum becomes converted into a secondarily homolecithal, holoblastic one. All the evidence is held to support the conclusion that the Marsupials are descended from oviparous ancestors with ineroblastic ova.

(b) Cleavage

Cleavage begins in the uterus, is total, and at first equal and of the radial type. The first two cleavage planes are meridional and at right angles to each other. The resulting four equal-sized blastomeres lie disposed radially around the polar diameter like those of the Monotreme (not in pairs at right angles to each other as in Eutheria), and enclose a segmentation cavity open above and below, their upper ends partially surrounding the yolk-body. The third cleavage planes are again meridional, each of the four blastomeres becoming subdivided equally into two. The resulting eight cells form an equatorial ring in contact with the inner surface of the sphere formed by the egg-envelopes. They contain deutoplasmic material, which is, however, located mainly in their lower halves. The ensuing fourth cleavages are equatorial, and in correlation with the just-mentioned disposition of the deutoplasm, are unequal and qualitative, each of the eight blastoraeres becoming subdivided into an upper smaller and clearer cell, with relatively little deutoplasm fairly uniformly dispersed through the cytoplasm, and a lower larger, more opaque cell Avith much deutoplasm, mainly located in a broad zone in the outer portion of the cell-body. A 16-celled stage is thus produced in which the blastomeres are characteristically arranged in two superimposed rings, each of eight cells, an upper of smaller, clearer cells next the yolk-body, and a lower of larger, denser cells. The former is destined to give origin to the formative or embryonal region of the blastocyst wall, the latter to the non-formative or extra-embryonal region of the same.


(c) Formation of the Blastocyst

There is in the Marsupial no morula stage as in Butheria, the blastomeres proceeding directly to form the wall of the blastocyst. The cells of the two rings of the 16-celled stage divide at first meridionally and then also equatorially, the division planes being always vertical to the surface. The daughter-blastomeres so produced, continuing to divide in the same fashion, gradually spread towards opposite poles in contact with the inner surface of the fii-m sphere formed by the zona and the thickened shell-membrane. Eventually they form a complete cellular lining to the said sphere and it is this which constitutes the wall of the blastocyst. The latter is accordingly unilaminar at its first origin, and it remains so in Dasyurus until it has attained, as the result of active gi'owth accompanied by the imbibition of fiuid from the uterus, a diameter of 4-5 mm. It consists of two parts or regions, distinct in origin and in destiny, and clearly marked off from each other in later blastocysts by a definite junctional line approximately equatorial in position, viz. an upper, embryonal or formative region derived from the upper cell-ring of the 16-celled stage, and a lower, extra-embryonal or non-formative region derived from tlie lower cell-ring of the same stage.


(d) Later History of the Two Regions of the Blastocyst Wall

(for details see pp. 72-74).

From the embryonal region are derived the embryonal ectoderm and the entire entoderm of the vesicle. I conclude^ therefore, that it is tlie homologue of the inner cell-mass or embryonal knot of the Eutherian blastocyst. The extra-embryonal region directly furnishes the outer extra-embryonal layer of the vesicle wall, i. e. the outer layer of the omphalopleure and chorion of later stages. Assuming, as the facts of comparative anatomy and palaeontology entirely justify us in doing, that the Mammals are monophyletic and of reptilian origin, and further assuming that the foetal membranes are homologous structures throughout the Amniotan series (also in my view a perfectly justifiable assumption)^, then the homologies of this extraembryonal region of the Marsupial blastocyt are not far to seek. It is clearly the homologue of the extra-embryonal ectoderm of the Sauropsidan and Monotreme egg, and the homologue also of the outer enveloping layer of the Eutlierian blastocyst, to which Hubi'echt has given the special name of “ trophoblast.” In my view the trophoblast is none other than extra-embryonal ectoderm which in the viviparous mammals, in correlation with the intra-uterine mode of development, has acquired a special significance for the nutrition of the embryo.


These, then, are my conclusions, and to me they seem on general grounds perfectly obvious, viz. : (1) that the embryonal or formative region of the unilaminar Marsupial blastocyst is the homologue of the inner cell-mass or imposed cell-viiigs, respectively and non-formative cell-rings of formative (emlDryonal) and non- the Metatherian. formative (extra-embryonal) in significance.


  • How Assheton can maintain (09, p. 266) that the amnion of the rabbit is not more homologous to the amnion of the Sauropsidan than the homy teeth of Ornithorhynchns ai-e homologous to the true teeth of the mammal or reptile, which they have supplanted,” how he can hold this view and yet proceed to utilise the presence of the amnion as one of the leading charactei-s distinguishing the Amniota from the Anamnia, I fail to comprehend. Surely the presence of a series of purely analogous structures in a group is of no classificatory value.


embryonal knob of the Eutberiaii blastocyst ; and (2) that the extra-embryonal or noii-formative region of the same is the homologne of the extra-embryonal ectoderm of the Sauropsida and Monotremata and of the trophoblast of the Eutheria.


As regards conclusion (1) there is not likely to be much difference of opinion, but as regards (2), whilst perhaps the majority of embryologists support the obvious, not to say common-place view which I here advocate, it seems certain that it will prove neither obvious nor acceptable to those mammalian embryologists (I refer specifically to my friends Professor A. A. W. Hubrecht and Mr. R. Asshetou) who, with only Selenka^s account of eaidy Marsupial ontogeny before them, have formulated other and quite divergent views as to the morphological nature of the outer enveloping layer of the Eutherian blastocyst. It is therefore necessary to discuss this question further, though I would fain express my conviction that had the observations recorded in this paper been earlier available, much vain speculation as to the phytogeny of the trophoblast might possibly have been avoided.


Historic Disclaimer - information about historic embryology pages 
Mark Hill.jpg
Pages where the terms "Historic" (textbooks, papers, people, recommendations) appear on this site, and sections within pages where this disclaimer appears, indicate that the content and scientific understanding are specific to the time of publication. This means that while some scientific descriptions are still accurate, the terminology and interpretation of the developmental mechanisms reflect the understanding at the time of original publication and those of the preceding periods, these terms, interpretations and recommendations may not reflect our current scientific understanding.     (More? Embryology History | Historic Embryology Papers)

Contents: 1 Review of Previous Observations | 2 The Ovum of Dasyurus | 3 Cleavage and Blastocyst | 4 Blastocyst Growth Ectoderm Entoderm | 5 Early Stages of Perameles and Macropus | 6 Summary and Conclusions | 7 Early Mammalia Ontogeny | Explanation of Plates


Cite this page: Hill, M.A. (2020, April 3) Embryology Paper - Contributions to the embryology of the marsupialia 4-6. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Paper_-_Contributions_to_the_embryology_of_the_marsupialia_4-6

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