Text-Book of Embryology 2-1 (1919) - Revision history

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	=Chapter I - Segmentation, Gastrulation, and the Formation of the Germ Layers=		=Chapter I - Segmentation, Gastrulation, and the Formation of the Germ Layers=

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Z8600021: /* Chapter I - Segmentation, Gastrulation, and the Formation of the Germ Layers */

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Chapter I - Segmentation, Gastrulation, and the Formation of the Germ Layers

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	=Chapter I - Segmentation, Gastrulation, and the Formation of the Germ Layers=		=Chapter I - Segmentation, Gastrulation, and the Formation of the Germ Layers=

	~~THE Vertebrate~~ begins its individual existence in the form of a single cell, the Zygote or fertilized egg, which in turn originates in the process of fertilization by the fusion or conjugation of two gametes.‘ Of these the microgamete or spermatozoon, derived from the male parent, is of relatively insigniﬁcant bulk as compared with the macrogamete or unfertilized egg. As a consequence the more obvious features of the Zygote, such.as shape, size, and so on, are simply taken over from the macrogamete-in other words, they are of maternal origin. Such maternal features may remain obvious for some time during early stages of development, so long in fact as the maternal protoplasm remains predominant in bulk as compared with that elaborated under the control of the Zygote nucleus, but it seems unnecessary to assume that this fact has the important bearing upon questions connected with Heredity which has been claimed for it by some workers on Invertebrates.		The vertebrate begins its individual existence in the form of a single cell, the Zygote or fertilized egg, which in turn originates in the process of fertilization by the fusion or conjugation of two gametes. Of these the microgamete or spermatozoon, derived from the male parent, is of relatively insigniﬁcant bulk as compared with the macrogamete or unfertilized egg. As a consequence the more obvious features of the Zygote, such.as shape, size, and so on, are simply taken over from the macrogamete - in other words, they are of maternal origin. Such maternal features may remain obvious for some time during early stages of development, so long in fact as the maternal protoplasm remains predominant in bulk as compared with that elaborated under the control of the Zygote nucleus, but it seems unnecessary to assume that this fact has the important bearing upon questions connected with Heredity which has been claimed for it by some workers on Invertebrates.

	The Zygote is a typical cell, composed, so far as its living substance is concerned, of cytoplasm and nucleus, the cytoplasm containing a lesser or greater amount of food-material or yolk. In shape it is in the vast majority of cases approximately spherical. In the Myxinoids it is elongated, almost sausage-shaped, and in a certain number of cases, for example Amia, its sh e is literally “ oval.”

	The ~~macr0gamete——-and fore the Zygotemdiifers much in size in different Vertebrates~~, ~~ranging from about '1 mm. in diameter in Amphioacus to~~ as ~~much as 85 mm.~~, ~~or more~~, in the ~~case~~ of ~~the African Ostrich~~. In ~~some of the Sharks~~ the ~~size~~ of the ~~Zygote~~ is ~~also very great and this was doubtless the case too~~, ~~with that of‘ such extinct birds as Aepyozmls.‘~~-~~3 Such relatively huge Zygotes are~~ of ~~interest as being in bulk the largest single cells known in either the Animal or the Vegetable Kingdom~~. ~~The adjoining list based upon data obtained by various observers~~		The Zygote is a typical cell, composed, so far as its living substance is concerned, of cytoplasm and nucleus, the cytoplasm containing a lesser or greater amount of food-material or yolk. In shape it is in the vast majority of cases approximately spherical. In the Myxinoids it is elongated, almost sausage-shaped, and in a certain number of cases, for example Amia, its shape is literally “ oval.”


			The macrogamete — and fore the Zygote differs much in size in different vertebrates, ranging from about 1 mm. in diameter in Amphioacus to as much as 85 mm., or more, in the case of the African Ostrich. In some of the Sharks the size of the Zygote is also very great and this was doubtless the case too, with that of such extinct birds as Aepyozmls.

	* 1 A general account of the processes of gametogenesis and fertilization has already been given in Vol. I. and they are not further dealt with in this volume. I		* 1 A general account of the processes of gametogenesis and fertilization has already been given in Vol. I. and they are not further dealt with in this volume. I
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	* 2 Assuming that the Zygote of Aeyyormis bore the same ratio in size to its protective envelopes as does that of the Ostrich it would measure about 160 mm. in diameter.		* 2 Assuming that the Zygote of Aeyyormis bore the same ratio in size to its protective envelopes as does that of the Ostrich it would measure about 160 mm. in diameter.

			* 3 Such relatively huge Zygotes are of interest as being in bulk the largest single cells known in either the Animal or the Vegetable Kingdom. The adjoining list based upon data obtained by various observers M‘Intosh and Masterman, Bashford Dean, Boulenger, Budgett, Bles, Semon, Salensky and others~—-brings out in more detail the differences in the size of the Zygote amongst the lower Vertebrates.



	~~-—M‘Intosh and Masterman, Bashford Dean, Boulenger, Budgett, Bles, Semon, Salensky and others~—-brings out in more detail the differences in the size of the Zygote amongst the lower Vertebrates.~~

	===Approximate Diameter Of Egg (in millimetres) Of Various Fishes And Amphibians===		===Approximate Diameter Of Egg (in millimetres) Of Various Fishes And Amphibians===

Z8600021: /* Chapter I - Segmentation, Gastrulation, and the Formation of the Germ Layers */

2018-06-16T00:14:09Z

Chapter I - Segmentation, Gastrulation, and the Formation of the Germ Layers

← Older revision		Revision as of 11:14, 16 Haziran 2018
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	The Zygote is a typical cell, composed, so far as its living substance is concerned, of cytoplasm and nucleus, the cytoplasm containing a lesser or greater amount of food-material or yolk. In shape it is in the vast majority of cases approximately spherical. In the Myxinoids it is elongated, almost sausage-shaped, and in a certain number of cases, for example Amia, its sh e is literally “ oval.”		The Zygote is a typical cell, composed, so far as its living substance is concerned, of cytoplasm and nucleus, the cytoplasm containing a lesser or greater amount of food-material or yolk. In shape it is in the vast majority of cases approximately spherical. In the Myxinoids it is elongated, almost sausage-shaped, and in a certain number of cases, for example Amia, its sh e is literally “ oval.”

	The macr0gamete——-and fore the Zygotemdiifers much in size in different Vertebrates, ranging from about '1 mm. in diameter in Amphioacus to as much as 85 mm., or more, in the case of the African Ostrich. In some of the Sharks the size of the Zygote is also very great and this was doubtless the case too, with that of‘ such extinct birds as Aepyozmls.‘-3 Such relatively huge Zygotes are of		The macr0gamete——-and fore the Zygotemdiifers much in size in different Vertebrates, ranging from about '1 mm. in diameter in Amphioacus to as much as 85 mm., or more, in the case of the African Ostrich. In some of the Sharks the size of the Zygote is also very great and this was doubtless the case too, with that of‘ such extinct birds as Aepyozmls.‘-3 Such relatively huge Zygotes are of interest as being in bulk the largest single cells known in either the Animal or the Vegetable Kingdom. The adjoining list based upon data obtained by various observers

	~~1 A general account of the processes of gametogenesis and fertilization has already been given in Vol. I. and they are not further dealt with in this volume. I~~

	2 Assuming that the Zygote of Aeyyormis bore the same ratio in size to its protective envelopes as does that of the Ostrich it would measure about 160 mm. in diameter.		* 1 A general account of the processes of gametogenesis and fertilization has already been given in Vol. I. and they are not further dealt with in this volume. I

			* 2 Assuming that the Zygote of Aeyyormis bore the same ratio in size to its protective envelopes as does that of the Ostrich it would measure about 160 mm. in diameter.

	~~VOL. II B 2 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.~~

	~~interest as being in bulk the largest single cells known in either the~~

	~~Animal or the Vegetable Kingdom. The adjoining list based upon data obtained by various observers~~

	-—M‘Intosh and Masterman, Bashford Dean, Boulenger, Budgett, Bles, Semon, Salensky and others~—-brings out in more detail the differences in the size of the Zygote amongst the lower Vertebrates.		-—M‘Intosh and Masterman, Bashford Dean, Boulenger, Budgett, Bles, Semon, Salensky and others~—-brings out in more detail the differences in the size of the Zygote amongst the lower Vertebrates.

	~~APPROXIMATE DIAMETER 01*‘ EGG~~ (~~lN M1LLlMETRES~~) ~~OF VARIOUS FISHES AND AMPHIBIANS~~		===Approximate Diameter Of Egg (in millimetres) Of Various Fishes And Amphibians===

	Amph/io:1:u.s' -1. Botlms ma.'r'i'nm.s 1. Petro1nyr:on 1. Pleummectes platessa. 1-8. Bdellosfnmu. 14 -— 29 x 7 - 10-5. P. 7m7('rocephalw.s 1-3. P'I"ist't'm‘u,s melnxnustomus 16. P. limamla -75. A(:¢mth'ias 35 - 40. Salsa, 'vu.l_qm"z's 1-2.		Amph/io:1:u.s' -1. Botlms ma.'r'i'nm.s 1. Petro1nyr:on 1. Pleummectes platessa. 1-8. Bdellosfnmu. 14 -— 29 x 7 - 10-5. P. 7m7('rocephalw.s 1-3. P'I"ist't'm‘u,s melnxnustomus 16. P. limamla -75. A(:¢mth'ias 35 - 40. Salsa, 'vu.l_qm"z's 1-2.
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	the yolk is com- y . l		the yolk is com- y . l

	, _ . n, lint-leIi.~. 1;, pigmi-iii. pa-I'al':1V61y 3131311 111 'l‘lu~ yolk gi‘:inule.s' :m- lll\|ll(':I.lI-Il in lmth li;_:un_-s by dark dots. amount, as in Amphilomus (Fig. 1, A), it may be distributed nearly equally throughout the egg substanoe; in other words there is an 3 approach		, _ . n, lint-leIi.~. 1;, pigmi-iii. pa-I'al':1V61y 3131311 111 'l‘lu~ yolk gi‘:inule.s' :m- lll\|ll(':I.lI-Il in lmth li;_:un_-s by dark dots. amount, as in Amphilomus (Fig. 1, A), it may be distributed nearly equally throughout the egg substanoe; in other words there is an 3 approach to the isolecithal condition: but as a rule in the Vertebrate the yolk is large in amount and is concentrated towards the lower or abapical pole of the egg, the protoplasm towards the upper or apical pole being comparatively poor in yolk (Telolecithal condition).


	~~Fin. 1. 4 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.~~

	to the isolecithal condition: but as a rule in the Vertebrate the yolk is large in amount and is concentrated towards the lower or abapical pole of the egg, the protoplasm towards the upper or apical pole being comparatively poor in yolk (Telolecithal condition).

	This segregation of the dead yolk and the living protoplasm towards opposite poles of the egg is well seen in the relatively huge egg of the bird where the protoplasm is concentrated in a germinal disc containing practically no yolk and forming a cap at the apical pole of an enormous mass of yolk practically free from protoplasm.		This segregation of the dead yolk and the living protoplasm towards opposite poles of the egg is well seen in the relatively huge egg of the bird where the protoplasm is concentrated in a germinal disc containing practically no yolk and forming a cap at the apical pole of an enormous mass of yolk practically free from protoplasm.
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	It may be objected that the pigment is produced before the egg is laid (eg. the (lommon Frog) and therefore before it is exposed to the action of light, but as a matter of fact the body-wall of the adult is by no means opaque to light rays and even while still in the ovary the eggs are exposed to the inﬂuence of faint light. If we may take it, as seems probable, that the influence of natural selection has gradually developed in such cases the particular type of sensitiveness to light which leads to the formation of melanin, on account of its protective value, then there is nothing surprising in the developing of this sensitiveness at earlier and earlier periods until at last it has resulted in the pigmentation of the still intra-ovarian egg in response to the feeble light rays which penetrate the body-wall.		It may be objected that the pigment is produced before the egg is laid (eg. the (lommon Frog) and therefore before it is exposed to the action of light, but as a matter of fact the body-wall of the adult is by no means opaque to light rays and even while still in the ovary the eggs are exposed to the inﬂuence of faint light. If we may take it, as seems probable, that the influence of natural selection has gradually developed in such cases the particular type of sensitiveness to light which leads to the formation of melanin, on account of its protective value, then there is nothing surprising in the developing of this sensitiveness at earlier and earlier periods until at last it has resulted in the pigmentation of the still intra-ovarian egg in response to the feeble light rays which penetrate the body-wall.

	The other possible explanation of this precocious pigment formation is that the production of the pigment though originally taking place as a direct reaction to light in the laid egg, has become so engrained in the constitution of the species that it now comes about even in the absence of the original stimulus. The objection to this explanation is that it postulates the inheritance of an “acquired ~~I SEGMENTATION 5~~		The other possible explanation of this precocious pigment formation is that the production of the pigment though originally taking place as a direct reaction to light in the laid egg, has become so engrained in the constitution of the species that it now comes about even in the absence of the original stimulus. The objection to this explanation is that it postulates the inheritance of an “acquired character,” and that is unfortunately not justified by our knowledge so far as it goes at present.

	character,” and that is unfortunately not justified by our knowledge so far as it goes at present.

	~~SEGMENTATION~~		==Segmentation==

	The first important steps in the evolution of the unicellular Zygote into the multicellular adult are seen in the» process of Segmentation which is, in fact, a process of mitotic cell division showing special peculiarities in difl'eren_t groups of the Vertebrata. During this process there appear in succession on the surface of the egg grooves which gradually deepen and eventually divide the egg incompletely or completely into distinct segments or Blastomeres. Before entering into the details of this process it will be convenient to describe it in outline and deﬁne the various technical terms used in its description.		The first important steps in the evolution of the unicellular Zygote into the multicellular adult are seen in the» process of Segmentation which is, in fact, a process of mitotic cell division showing special peculiarities in difl'eren_t groups of the Vertebrata. During this process there appear in succession on the surface of the egg grooves which gradually deepen and eventually divide the egg incompletely or completely into distinct segments or Blastomeres. Before entering into the details of this process it will be convenient to describe it in outline and deﬁne the various technical terms used in its description.
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	The ﬁrst meridional furrow gradually deepens so that the egg becomes completely divided into two blastomeres or segments each representing a hemisphere of the Zygote. A second meridional furrow (/3) now appears in a plane perpendicular to that of the ﬁrst and by the deepening of this the egg becomes divided into four equal blastomeres. s		The ﬁrst meridional furrow gradually deepens so that the egg becomes completely divided into two blastomeres or segments each representing a hemisphere of the Zygote. A second meridional furrow (/3) now appears in a plane perpendicular to that of the ﬁrst and by the deepening of this the egg becomes divided into four equal blastomeres. s

	The next furrow to appear may be one running round the equator of the egg (equatorial). In eggs, however, which are not absolutely isolecithal-—~and this holds for all the lower Vertebrates-~~— 6 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.~~		The next furrow to appear may be one running round the equator of the egg (equatorial). In eggs, however, which are not absolutely isolecithal-—~and this holds for all the lower Vertebrates-

	the third furrow appears, not at the equator but at a level nearer the apical pole, and is termed a latitudinal furrow, corresponding as it does with a parallel of latitude upon a terrestrial globe. The distance of this furrow from the equator, its degree of latitude so to speak, is roughly proportional to the degree of telolecithality of the particular egg, suggesting that the volume of living protoplasm may be roughly equivalent in amount upon the two sides of the division plane to which this furrow gives rise.		the third furrow appears, not at the equator but at a level nearer the apical pole, and is termed a latitudinal furrow, corresponding as it does with a parallel of latitude upon a terrestrial globe. The distance of this furrow from the equator, its degree of latitude so to speak, is roughly proportional to the degree of telolecithality of the particular egg, suggesting that the volume of living protoplasm may be roughly equivalent in amount upon the two sides of the division plane to which this furrow gives rise.

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	At somewhere about this period there begins a new type of mitotic division in which the individual blastomere becomes split in a direction parallel to a plane tangential to its outer surface, so that it divides into an outer blastomere visible in surface view and an inner one concealed in the interior of the egg.		At somewhere about this period there begins a new type of mitotic division in which the individual blastomere becomes split in a direction parallel to a plane tangential to its outer surface, so that it divides into an outer blastomere visible in surface view and an inner one concealed in the interior of the egg.

	With the further progress of segmentation the blastomeres divide over and over again, so that eventually the egg is converted into a very large number of small cellular elements. The rapidity with which the cells divide bears a rough inverse relation to the richness ‘of their contents in yolk. Dead inert yolk tends to cause the cell to lag behind in the process of division, and the result of this in telolecithal eggs is that the difference in size between inieromeres _and macromeres becomes more and more marked as segmentation goes on—-—the lower and more richly yolked segments ~~1 SEGMENTATION 7~~		With the further progress of segmentation the blastomeres divide over and over again, so that eventually the egg is converted into a very large number of small cellular elements. The rapidity with which the cells divide bears a rough inverse relation to the richness ‘of their contents in yolk. Dead inert yolk tends to cause the cell to lag behind in the process of division, and the result of this in telolecithal eggs is that the difference in size between inieromeres _and macromeres becomes more and more marked as segmentation goes on—-—the lower and more richly yolked segments tending to lag, in their mitotic division, more and more behind the less yolky upper elements. This inequality is found at its maximum in the large eggs of Elasmobranchs, Reptiles, and Birds, where the main mass of the egg has its proportion of protoplasm reduced so nearly to vanishing point that it does not divide at all. It is only a small portion of the egg in the neighbourhood of the apical pole that is rich enough in protoplasm to carry out the process of segmentation into separate cells. This is known as the germinal disc or, later on, when it has segmented into a mass of cells, blastoderm. An egg of such a type, showing partial or incomplete segmentation, is termed meroblastic in contrast with the more primitive holoblastic type in which the egg segments as a whole.

	tending to lag, in their mitotic division, more and more behind the less yolky upper elements. This inequality is found at its maximum in the large eggs of Elasmobranchs, Reptiles, and Birds, where the main mass of the egg has its proportion of protoplasm reduced so nearly to vanishing point that it does not divide at all. It is only a small portion of the egg in the neighbourhood of the apical pole that is rich enough in protoplasm to carry out the process of segmentation into separate cells. This is known as the germinal disc or, later on, when it has segmented into a mass of cells, blastoderm. An egg of such a type, showing partial or incomplete segmentation, is termed meroblastic in contrast with the more primitive holoblastic type in which the egg segments as a whole.

	The blastomeres into which the egg divides being composed of protoplasm——a somewhat viscous ﬂuid——-tend under the physical laws of surface tension to assume a spherical shape except when ﬂattened by pressure against their neighbours. There thus exist normally chinks between the blastomeres ﬁlled with wateryﬂuid.		The blastomeres into which the egg divides being composed of protoplasm——a somewhat viscous ﬂuid——-tend under the physical laws of surface tension to assume a spherical shape except when ﬂattened by pressure against their neighbours. There thus exist normally chinks between the blastomeres ﬁlled with wateryﬂuid.
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	The details of the segmentation process may now be followed out as it occurs in the various types of lower Vertebrates.		The details of the segmentation process may now be followed out as it occurs in the various types of lower Vertebrates.

	~~AMPHIOXUS~~		===Amphioxus===

	Amphioxus is, of all the lower Vertebrates, that in which developmental processes are least interfered with by the presence of yolk, and for this reason the phenomena shown during its segmentation must form the basis for the comparative study of the corresponding phenomena in the Vertebrata in general.		Amphioxus is, of all the lower Vertebrates, that in which developmental processes are least interfered with by the presence of yolk, and for this reason the phenomena shown during its segmentation must form the basis for the comparative study of the corresponding phenomena in the Vertebrata in general.
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	F" end until linal .‘ ‘it forms a wide Ill(:1‘.i(.llull:.l.l vulle_y encircling the entire egg (}'_4"ir_f. 3, This valley g1‘aLl11ally deepens dividing the egg into two halves. Finally after about 5 minutes from the commencetm-nt of the process the protoplasmic bridge connecting		F" end until linal .‘ ‘it forms a wide Ill(:1‘.i(.llull:.l.l vulle_y encircling the entire egg (}'_4"ir_f. 3, This valley g1‘aLl11ally deepens dividing the egg into two halves. Finally after about 5 minutes from the commencetm-nt of the process the protoplasmic bridge connecting the two lmlves srmps across and the egg is now completely divided

	the two lmlves srmps across and the egg is now completely divided

	Fro. 3.»--Illustrating the process ofsegI'n'1cntat.i«)n1 o: (_After Hnt.~'«_-.h(-.k, 188].)		Fro. 3.»--Illustrating the process ofsegI'n'1cntat.i«)n1 o: (_After Hnt.~'«_-.h(-.k, 188].)

Z8600021: /* Chapter I - Segmentation, Gastrulation, and the Formation of the Germ Layers */

2018-06-16T00:11:52Z

Chapter I - Segmentation, Gastrulation, and the Formation of the Germ Layers

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	=~~=CHAPTER~~ I - Segmentation, Gastrulation, and the Formation of the Germ Layers==		=Chapter I - Segmentation, Gastrulation, and the Formation of the Germ Layers=

	THE Vertebrate begins its individual existence in the form of a single		THE Vertebrate begins its individual existence in the form of a single
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	Ziegler. Entwiukelungsgcsch. (1. niedercn Wirbeltiere, Jena, 1902.		Ziegler. Entwiukelungsgcsch. (1. niedercn Wirbeltiere, Jena, 1902.

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Z8600021: Created page with "==CHAPTER I - Segmentation, Gastrulation, and the Formation of the Germ Layers== THE Vertebrate begins its individual existence in the form of a single cell, the Zygote or fe..."

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Created page with "==CHAPTER I - Segmentation, Gastrulation, and the Formation of the Germ Layers== THE Vertebrate begins its individual existence in the form of a single cell, the Zygote or fe..."

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