Book - Quain's Embryology 2 - Revision history

Z8600021: /* The cerebro-spinal nervous centre */

2012-10-13T11:38:17Z

The cerebro-spinal nervous centre

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	'''Fig. 506. Magnified side view of the Head Fig. and Upper Part of the Body of an embryo chick of the fourth day''' (adapted from Remak and Huxley).		'''Fig. 506. Magnified side view of the Head Fig. and Upper Part of the Body of an embryo chick of the fourth day''' (adapted from Remak and Huxley).

	1, ~~cliorda~~ dorsalis ; 2, tlirce of the upper priDiitive cervical vertebne ; C\ one of the vesicles of the prosencephalon, with the nasal fossa below ; C-, vesicle of the thalameacephalon, with thg eye below it ; C'*, the middle cerebral vesicle ;		1, chorda dorsalis ; 2, tlirce of the upper priDiitive cervical vertebne ; C\ one of the vesicles of the prosencephalon, with the nasal fossa below ; C-, vesicle of the thalameacephalon, with thg eye below it ; C'*, the middle cerebral vesicle ;

	C'*, the cerebellum, between which and the cervical vertebra is the medulla oblongata. At the anterior extremity of the chorda dorsalis, where it reaches the ijost-spheuoid, is seen the rectangular bend of the middle of the cranium, which takes i^lace at the sella turcica ; and in front of tliis, towards the eye, the pointed infundibulum ; V, the rudiment of the trigeminus nerve ; VII, the facial ; VIII, the vagus ; IX, the		C'*, the cerebellum, between which and the cervical vertebra is the medulla oblongata. At the anterior extremity of the chorda dorsalis, where it reaches the ijost-spheuoid, is seen the rectangular bend of the middle of the cranium, which takes i^lace at the sella turcica ; and in front of tliis, towards the eye, the pointed infundibulum ; V, the rudiment of the trigeminus nerve ; VII, the facial ; VIII, the vagus ; IX, the hypoglossal ; in front and below these numbers respectively, first, the upper and lower jaw, with the first cleft, which becomes the meatus auditorius externus ; and lower down the second, third, and fourth arches and clefts in succession ; in front of these the aortic bulb attaches the heart ; between VII and VIII, the auditory vesicle.

	hypoglossal ; in front and below these numbers respectively, first, the upper and lower jaw, with the first cleft, which becomes the meatus auditorius externus ; and lower down the second, third, and fourth arches and clefts in succession ; in front of these the aortic bulb attaches the heart ; between VII and VIII, the auditory vesicle.

	===The Nerves===		===The Nerves===

Z8600021: /* Distinction of Embryonic and Peripheral Phenomena */

2012-10-13T11:33:03Z

Distinction of Embryonic and Peripheral Phenomena

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	===Distinction of Embryonic and Peripheral Phenomena===		===Distinction of Embryonic and Peripheral Phenomena===

	From what has gone before it will be seen that the fundamental phenomena of development in the ovum consist essentially in changes which take place in the several layers of the blastoderm. Considered individually and ~~minutel}'~~, they are mainly of the nature of cell multiplication and cell differentiation. Eegarded as a whole they may be placed under two divisions, according as 1st, they have their seat in the parts fi'om which the future embryo is formed, and are therefore intra-emlnjonic, or, 2nd, are extra-emhrnonic, and connected with the production of other parts, having usually a membranous form, which surround the embryo within the ovum, and form principally the amnion, yolk-sac, allantois, and chorion. It is to be remarked, however, that although in the progress of development all these membranes are mainly peripheral or extra-embryonic in their situation, they are not entirely so in their origin, for one of them — the allantois — springs originally from a part within the body of the embryo ; and all of them, in mammals at least, by the original continuity of the blastoderm, are necessarily united at certain places with parts of the embryo. Hence they have been called foetal appendages or foetal membranes.		From what has gone before it will be seen that the fundamental phenomena of development in the ovum consist essentially in changes which take place in the several layers of the blastoderm. Considered individually and minutely, they are mainly of the nature of cell multiplication and cell differentiation. Eegarded as a whole they may be placed under two divisions, according as 1st, they have their seat in the parts fi'om which the future embryo is formed, and are therefore intra-emlnjonic, or, 2nd, are extra-emhrnonic, and connected with the production of other parts, having usually a membranous form, which surround the embryo within the ovum, and form principally the amnion, yolk-sac, allantois, and chorion. It is to be remarked, however, that although in the progress of development all these membranes are mainly peripheral or extra-embryonic in their situation, they are not entirely so in their origin, for one of them — the allantois — springs originally from a part within the body of the embryo ; and all of them, in mammals at least, by the original continuity of the blastoderm, are necessarily united at certain places with parts of the embryo. Hence they have been called foetal appendages or foetal membranes.



	'''Fig. 499. Ovum of the Rabbit from the Uterus''' (from Kolliker after Bischoff). The ovum was about one seventh of an inch in diameter ; a, the remains of the zona ~~peUucida~~ or external membrane ; b, the vesicular blastoderm ; c, the germinal area ; d, the outer limb of the double layer.		'''Fig. 499. Ovum of the Rabbit from the Uterus''' (from Kolliker after Bischoff). The ovum was about one seventh of an inch in diameter ; a, the remains of the zona pllucida or external membrane ; b, the vesicular blastoderm ; c, the germinal area ; d, the outer limb of the double layer.

Z8600021: /* The Limbs */

2012-10-13T11:29:35Z

The Limbs

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	===The Limbs===		===The Limbs===

	The limbs do not commence till after the rudiments of all the organs already		The limbs do not commence till after the rudiments of all the organs already referred to have made their appearance. They are to be regarded as out-growths from the lateral part of the trunk, and take their origin by a sort of budding out or extension of the elements composing the wall of the trunk in two determinate places, which are nearly the same in all vertebrate animals, and receiving prolongations of the bones, muscles, nerves, and blood-vessels corresponding to a certain number of the vertebral somatomes in the situation of the anterior and posterior limbs respectively.


	referred to have made their appearance. They are to be regarded as out-growths from the lateral part of the trunk, and take their origin by a sort of budding out or extension of the elements composing the wall of the trunk in two determinate places, which are nearly the same in all vertebrate animals, and receiving prolongations of the bones, muscles, nerves, and blood-vessels corresponding to a certain number of the vertebral somatomes in the situation of the anterior and posterior limbs respectively.


	{{Quain footer}}		{{Quain footer}}

Z8600021: /* Reproductive and urinary organs */

2012-10-13T11:29:17Z

Reproductive and urinary organs

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	===Reproductive and urinary organs===		===Reproductive and urinary organs===

	As completing the present short notice of the development of the rudiments of the principal organs of the embryo, there may, lastly, be mentioned the temporary organs named the Wolffian todies, with their ducts and associated parts, which are the precursors of, and are very constantly and intimately associated with, the first origin and subsequent ~~ca^oIution~~ of the reproductive and urinary organs. These bodies, when fully formed, constitute a pair of symmetrical organs which occupy nearly the whole extent of the abdominal cavity, and consist mainly of ~~shor't~~ transverse tubes presenting a glomerular vasctilar structure very similar to that which exists in the glandular structure of the permanent kidneys. They have thus been named the ~~iJrimordkd~~ kidneys (see fig. 513, W, p. 702).		As completing the present short notice of the development of the rudiments of the principal organs of the embryo, there may, lastly, be mentioned the temporary organs named the Wolffian todies, with their ducts and associated parts, which are the precursors of, and are very constantly and intimately associated with, the first origin and subsequent evoIution of the reproductive and urinary organs. These bodies, when fully formed, constitute a pair of symmetrical organs which occupy nearly the whole extent of the abdominal cavity, and consist mainly of short transverse tubes presenting a glomerular vasctilar structure very similar to that which exists in the glandular structure of the permanent kidneys. They have thus been named the primordial kidneys (see fig. 513, W, p. 702).


	The Wolffian bodies arise mainly in connection with the central portion of the mesoblast, and as the permanent kidneys and their ducts, the testicles and ovaries, and the respective male and female passages are in their origin all intimately connected with the Wolffian bodies, we may look upon the urinary organs and the internal reproductive organs as equally products of the middle layer. The external sexual organs are integumental in their origin, and may be considered as arising in the epiblast and mesoblast jointly.		The Wolffian bodies arise mainly in connection with the central portion of the mesoblast, and as the permanent kidneys and their ducts, the testicles and ovaries, and the respective male and female passages are in their origin all intimately connected with the Wolffian bodies, we may look upon the urinary organs and the internal reproductive organs as equally products of the middle layer. The external sexual organs are integumental in their origin, and may be considered as arising in the epiblast and mesoblast jointly.

	~~Fig. 511. — Human Embryo op about FOUR WEEKS (froiu Kolliker after A.Thomson), f/, the anterior limb rising as a .semicircular plate from the lateral ridge. (The figure is elsewhere described).~~


			'''Fig. 511. Human Embryo op about four weeks''' (from Kolliker after A.Thomson), f/, the anterior limb rising as a .semicircular plate from the lateral ridge. (The figure is elsewhere described).

	===The Limbs===		===The Limbs===

Z8600021: /* Alimentary Canal */

2012-10-13T11:27:23Z

Alimentary Canal

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	===Alimentary Canal===		===Alimentary Canal===


	The formation of the rudiment of the alimentary canal or primitive intestine takes place below or within the boat-shaped part of the embryo previously described by the folding in, soonest at the cephalic and later at the caudal extremities, and subsequently along the sides, first of the hypoblast, from which the epithelial lining only of the intestine is formed, and afterwards of certain parts of the splanchnoplenre section of the mesoblast which give rise by their meeting in the middle to the mesentery, and furnish in their extension over the intestinal tube the muscular and peritoneal coats and the connective-tissue and vascular elements of the gut.		The formation of the rudiment of the alimentary canal or primitive intestine takes place below or within the boat-shaped part of the embryo previously described by the folding in, soonest at the cephalic and later at the caudal extremities, and subsequently along the sides, first of the hypoblast, from which the epithelial lining only of the intestine is formed, and afterwards of certain parts of the splanchnoplenre section of the mesoblast which give rise by their meeting in the middle to the mesentery, and furnish in their extension over the intestinal tube the muscular and peritoneal coats and the connective-tissue and vascular elements of the gut.


	The primitive alimentary canal is thus constituted in its early form by an anterior and posterior ca3cal tube, of which the anterior is the first produced, — both of them closed at the extremity by the reflected • layers of the blastoderm, — and by a wide middle part between the tubular portions, which at first has the form of a groove or gutter running under the vertebral axis of the embryo, and completely open below into the cavity of the yolk-sac. As development proceeds, the intestinal folds involve m.ore and more of this central open part and convert it into the tubular form ; and the opening into the yolk is thus gradually narrowed, while the reflected part of the blastodermic layers which pass between the yolk-sac and the intestine becomes lengthened out so as to take the form of an elongated duct known as the ductus vikUoinfest inalis (see figs. 510 and 512).		The primitive alimentary canal is thus constituted in its early form by an anterior and posterior ca3cal tube, of which the anterior is the first produced, — both of them closed at the extremity by the reflected • layers of the blastoderm, — and by a wide middle part between the tubular portions, which at first has the form of a groove or gutter running under the vertebral axis of the embryo, and completely open below into the cavity of the yolk-sac. As development proceeds, the intestinal folds involve m.ore and more of this central open part and convert it into the tubular form ; and the opening into the yolk is thus gradually narrowed, while the reflected part of the blastodermic layers which pass between the yolk-sac and the intestine becomes lengthened out so as to take the form of an elongated duct known as the ductus vikUoinfest inalis (see figs. 510 and 512).
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	Fig. 510. — Diagrammatic Section through the Ovum of a Mammal in the long ~~AXIS~~ of ~~THE~~ Embryo.		'''Fig. 510. Diagrammatic Section through the Ovum of a Mammal in the long axis of the Embryo.'''

	e, the cranio-vertebral axis ; ~~/, /~~, the cepbalic and cauflal portions of the primitive alimentary canal ; a, the amnion ; a', the point of reflection into the false amnion ; v, yolk sac, communicating with the middle part of the intestine by v i, the vitello-~~intestinai~~ duct ; M, the allantois. The ovum is surrounded externally by the villous chorion.		e, the cranio-vertebral axis ; , the cepbalic and cauflal portions of the primitive alimentary canal ; a, the amnion ; a', the point of reflection into the false amnion ; v, yolk sac, communicating with the middle part of the intestine by v i, the vitello-intestinal duct ; M, the allantois. The ovum is surrounded externally by the villous chorion.


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	As the parts constituting the face are at first entirely absent, there is necessarily no cavity corresponding to the mouth, but as the structures which give rise to the jaws and face come to be developed below the cranium, the buccal and nasal cavities are generally deepened by the increasing projection of these parts, and the mouth at last communicates with the forepart of the primitive alimentary canal by an opening formed into it at the fiiuces. The mouth, therefore, derives its lining from the epiblast and forms no part of the original hypoblastic inflection Avhich gives rise to the pharyngeal cavity.		As the parts constituting the face are at first entirely absent, there is necessarily no cavity corresponding to the mouth, but as the structures which give rise to the jaws and face come to be developed below the cranium, the buccal and nasal cavities are generally deepened by the increasing projection of these parts, and the mouth at last communicates with the forepart of the primitive alimentary canal by an opening formed into it at the fiiuces. The mouth, therefore, derives its lining from the epiblast and forms no part of the original hypoblastic inflection Avhich gives rise to the pharyngeal cavity.


	The posterior opening of the alimentary canal is formed at a considerably later period than that of the fauces. When first produced by the solution of continuity in the posterior reflection of the blastodermic layers, it represents in mammals as well as in birds a cloaca, or part of the primitive intestine common to the alimentary canal and the genitourinary passages.		The posterior opening of the alimentary canal is formed at a considerably later period than that of the fauces. When first produced by the solution of continuity in the posterior reflection of the blastodermic layers, it represents in mammals as well as in birds a cloaca, or part of the primitive intestine common to the alimentary canal and the genitourinary passages.

Z8600021: /* Vascular system */

2012-10-13T11:25:49Z

Vascular system

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	Fig. 508. — Outlines of the anterior ~~HALF OF THE EmbRYO ChICK VIEWED FROM BELOW~~, ~~SHOWING THE HeART IN ITS EARLIER STAGES OF FORMATION~~ (after Remak).		'''Fig. 508. Outlines of the anterior half of the embryo chick viewed from below, showing the heart in its earlier stages of formation''' (after Remak).

	A, Embryo of about 28 to SO hours ; B, of about 36 to 40 hours ; a, anterior 'cerebral vesicle ; h, proto-vertebral segments ; c, cephalic fold ; 1, 1, primitive omf)halo-mesenteric veins entering the heart posteriorly ; 2, their union n the auricle of the heart ; 3, the middle l^art of the tube corresponding to the ventricle ; 4 (in B) the arterial bulb.		A, Embryo of about 28 to SO hours ; B, of about 36 to 40 hours ; a, anterior 'cerebral vesicle ; h, proto-vertebral segments ; c, cephalic fold ; 1, 1, primitive omf)halo-mesenteric veins entering the heart posteriorly ; 2, their union n the auricle of the heart ; 3, the middle l^art of the tube corresponding to the ventricle ; 4 (in B) the arterial bulb.
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	Fig. 509. — Diagrammatic Outlines op the Heaut and Peimitive Vessels op the Embryo Chick as seen from below and enlarged.		'''Fig. 509. Diagrammatic Outlines op the Heaut and Peimitive Vessels op the Embryo Chick as seen from below and enlarged.'''

	a, soon after the first establishment of the circulation ; B, c, at a somewhat later period ; 1, 1, the veins retiiruing from the vascular area ; 2, 3, 4, the heart, now iu the form of a notched tulje ; 5, 5, (upper) the two primitive aortic arches ; 5, 5, (lower) the primitive double aorta ; A, the single or united aorta ; 5', 5', the continuation of the double aortaj beyond the origin of the large omphalo-mesenteric arteries, 6, 6.		a, soon after the first establishment of the circulation ; B, c, at a somewhat later period ; 1, 1, the veins retiiruing from the vascular area ; 2, 3, 4, the heart, now iu the form of a notched tulje ; 5, 5, (upper) the two primitive aortic arches ; 5, 5, (lower) the primitive double aorta ; A, the single or united aorta ; 5', 5', the continuation of the double aortaj beyond the origin of the large omphalo-mesenteric arteries, 6, 6.

Z8600021: /* The cerebro-spinal nervous centre */

2012-10-13T11:24:13Z

The cerebro-spinal nervous centre

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	The brain and spinal cord have at first together the form of an elongated tube, of which the primary wall is of nearly equal thickness throughout. The cjdindrical portion in the region of the protovertebraa forms the spinal cord. In the dilated cephalic portion, constituting the rudimentary brain, there is from a very early period a partial division into three portions by slight intervening constrictions of the wall of the medullary tube. These constitute the three jn-imarij enrpphalic resides, and give rise in the next stage of development to the five fundamental portions of the brain usually recognised by embryologists and comparative anatomists, viz., forebrain, interbrain, midbrain, hindbrain and aftcrbrain. The general cavity enclosed by the inflection and union of the medullary plates constitutes the mesial ventricles of the brain and the canal of the spinal cord.		The brain and spinal cord have at first together the form of an elongated tube, of which the primary wall is of nearly equal thickness throughout. The cjdindrical portion in the region of the protovertebraa forms the spinal cord. In the dilated cephalic portion, constituting the rudimentary brain, there is from a very early period a partial division into three portions by slight intervening constrictions of the wall of the medullary tube. These constitute the three jn-imarij enrpphalic resides, and give rise in the next stage of development to the five fundamental portions of the brain usually recognised by embryologists and comparative anatomists, viz., forebrain, interbrain, midbrain, hindbrain and aftcrbrain. The general cavity enclosed by the inflection and union of the medullary plates constitutes the mesial ventricles of the brain and the canal of the spinal cord.

	~~Fig. 506. — Magnified side view of the Head Fig. AND Upper Part op the Body op an EnDRro-CHicK OF THE FouRTH Day (adapted from Remak and Huxley).~~

	1, cliorda dorsalis ; 2, tlirce of the upper priDiitive cervical vertebne ; C\ one of the vesicles of the prosencephalon, with the nasal fossa below ; C-, vesicle of the thalameacephalon, with thg eye below it ; C'*, the middle cerebi-al vesicle ;

	C'*, the cerebellum, between which and the ~~cer- C^~[- ^""^^ ^ ] JL'^>C<$J^^!-— ' TI vical~~ vertebra is the medulla oblongata. At the anterior extremity of the chorda dorsalis, where it reaches the ijost-spheuoid, is seen the rectangular bend of the middle of the cranium, which takes i^lace at the sella turcica ; and in front of tliis, towards the eye, the pointed infundibulum ; V, the rudiment of the trigeminus nerve ; VII, the facial ; VIII, the vagus ; IX, the
			'''Fig. 506. Magnified side view of the Head Fig. and Upper Part of the Body of an embryo chick of the fourth day''' (adapted from Remak and Huxley).

			1, cliorda dorsalis ; 2, tlirce of the upper priDiitive cervical vertebne ; C\ one of the vesicles of the prosencephalon, with the nasal fossa below ; C-, vesicle of the thalameacephalon, with thg eye below it ; C'*, the middle cerebral vesicle ;

			C'*, the cerebellum, between which and the cervical vertebra is the medulla oblongata. At the anterior extremity of the chorda dorsalis, where it reaches the ijost-spheuoid, is seen the rectangular bend of the middle of the cranium, which takes i^lace at the sella turcica ; and in front of tliis, towards the eye, the pointed infundibulum ; V, the rudiment of the trigeminus nerve ; VII, the facial ; VIII, the vagus ; IX, the

	hypoglossal ; in front and below these numbers respectively, first, the upper and lower jaw, with the first cleft, which becomes the meatus auditorius externus ; and lower down the second, third, and fourth arches and clefts in succession ; in front of these the aortic bulb attaches the heart ; between VII and VIII, the auditory vesicle.		hypoglossal ; in front and below these numbers respectively, first, the upper and lower jaw, with the first cleft, which becomes the meatus auditorius externus ; and lower down the second, third, and fourth arches and clefts in succession ; in front of these the aortic bulb attaches the heart ; between VII and VIII, the auditory vesicle.

Z8600021: /* Inflection of the Walls of the Body of the Embryo */

2012-10-13T11:22:24Z

Inflection of the Walls of the Body of the Embryo

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	The first rudiments of the embryo, as before described, lie prone and flat on the surface of the yolk, consisting almost entirely in thickenings, with some incurvations, of certain parts of the blastoderm. In the formation of these parts the two upper layers, epiblasfc and mesoblast, are alone concerned, and the hypoblast takes no part in them, but passes thin and flat across the space occupied by the embryonic rudiments.		The first rudiments of the embryo, as before described, lie prone and flat on the surface of the yolk, consisting almost entirely in thickenings, with some incurvations, of certain parts of the blastoderm. In the formation of these parts the two upper layers, epiblasfc and mesoblast, are alone concerned, and the hypoblast takes no part in them, but passes thin and flat across the space occupied by the embryonic rudiments.


	In the further progress of development a great change of form is now produced by the downward inflection of the whole three layers of the blastoderm, in consequence of which the embryo rises, as it were, out of the plane of the rest of this membrane, and begins to be notched off" from its peripheral parts. The first of these folds, termed crplmlic, (fig. 504) takes place at the extremity of the embryo which contains the rudiment of cranium representing the head, and precedes by a considerable interval the other folds. A similar downward fold subsequently follows at the caudal extremity, and there is also between the cephalic and caudal folds a simultaneous depression of the layers of the blastoderm in lateral folds, so that the embryo takes in some measure the form of an inverted boat, with its keel upwards, and its hollow side opening* towards the yolk cavity, and the fore part being, as it were, partially covered in by the deck of the cephalic fold. Thus are produced the downward ventral or visceral plates which form the side walls of the head and trunk ; and at a later period, by the increased constriction or convergence of the folds round the place of communication between the embryo and the peripheral parts of the blastoderm, there is formed the umbilicus (see figs. 510 and 512).		In the further progress of development a great change of form is now produced by the downward inflection of the whole three layers of the blastoderm, in consequence of which the embryo rises, as it were, out of the plane of the rest of this membrane, and begins to be notched off" from its peripheral parts. The first of these folds, termed crplmlic, (fig. 504) takes place at the extremity of the embryo which contains the rudiment of cranium representing the head, and precedes by a considerable interval the other folds. A similar downward fold subsequently follows at the caudal extremity, and there is also between the cephalic and caudal folds a simultaneous depression of the layers of the blastoderm in lateral folds, so that the embryo takes in some measure the form of an inverted boat, with its keel upwards, and its hollow side opening* towards the yolk cavity, and the fore part being, as it were, partially covered in by the deck of the cephalic fold. Thus are produced the downward ventral or visceral plates which form the side walls of the head and trunk ; and at a later period, by the increased constriction or convergence of the folds round the place of communication between the embryo and the peripheral parts of the blastoderm, there is formed the umbilicus (see figs. 510 and 512).
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	Fig. 505. ~~— Transyersk~~ Section through the ~~EMBRro-CnicK~~ before and some time after ~~THE CliOSURE OF THE MEDULLARY CanAL~~, ~~TO SHOW THE UPWARD AND DOWNWARD INFLECTIONS OF THE~~ Blastoderm (after llemak).		'''Fig. 505. Transverse Section through the Embryo ChicK before and some time after the closure of the Medullary Canal, to show the upward and downward inflection of the Blastoderm''' (after llemak).

	A, At the end of the first day. 1, notochord ; 2, primitive groove in the medullary canal ; 3, edge of the dorsal lamina ; 4, corneous layer or epiblast ; 5, mesoblast divided in its inner part ; 6, hypoljlast or epithelial layer ; 7, section of protovertebral plate.		A, At the end of the first day. 1, notochord ; 2, primitive groove in the medullary canal ; 3, edge of the dorsal lamina ; 4, corneous layer or epiblast ; 5, mesoblast divided in its inner part ; 6, hypoljlast or epithelial layer ; 7, section of protovertebral plate.

	B. On the ~~thii-d~~ day in the lumbar region. 1, notochord in its sheath ; 2, medullary canal now closed in ; 3, section of the medullary substance of the spinal cord ; 4, corneous layer ; 5, somatopleure of the mesoblast ; 5', splanchnopleure (one figure is placed in the pleuroperitoueal cavity); 6, hyjioblast layer in the intestine and spreading over the yolk ; 4x5, part of the fold of the amnion formed by epiblast and somatopleure.		B. On the third day in the lumbar region. 1, notochord in its sheath ; 2, medullary canal now closed in ; 3, section of the medullary substance of the spinal cord ; 4, corneous layer ; 5, somatopleure of the mesoblast ; 5', splanchnopleure (one figure is placed in the pleuroperitoueal cavity); 6, hyjioblast layer in the intestine and spreading over the yolk ; 4x5, part of the fold of the amnion formed by epiblast and somatopleure.



	The fundamental steps, therefore, in the development of the vertebrate embryo result in the formation in the axial part, or head and trunk of the body, of two cavities, of which one is situated above and the othor below the notochordal axis ; the upper constituting the cranio-vertebral canal, and containing the rudiment of the cerebrospinal nervous centre ; the lower ~~forminc!^~~ the walls of the body ~~whicli~~ enclose the great nutritive viscera in the thoracic, abdominal and pelvic divisions of the trunk ; — along with ~~whichmay~~ be associated the parts which form the face and jaws, and Avhich enclose the cavities of the nose, month, and pharynx, incUiding in their substance the hyoid bone and its accompanying branchial arches.		The fundamental steps, therefore, in the development of the vertebrate embryo result in the formation in the axial part, or head and trunk of the body, of two cavities, of which one is situated above and the othor below the notochordal axis ; the upper constituting the cranio-vertebral canal, and containing the rudiment of the cerebrospinal nervous centre ; the lower forming the walls of the body which enclose the great nutritive viscera in the thoracic, abdominal and pelvic divisions of the trunk ; — along with which may be associated the parts which form the face and jaws, and Avhich enclose the cavities of the nose, month, and pharynx, incUiding in their substance the hyoid bone and its accompanying branchial arches.

	===The cerebro-spinal nervous centre===		===The cerebro-spinal nervous centre===

Z8600021: /* Protovertebrae */

2012-10-13T11:20:01Z

Protovertebrae

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	===Protovertebrae===		===Protovertebrae===

	On either side of this axial cord a thick mass or plate of mesoblast is collected along its whole length, and very soon there appear several transverse clefts in these plates forming the commencement of protovertebral segmentation. The first formed of these divisions is near the anterior or uppermost of the future cervical vertebra3, and they rapidly extend backwards in the posterior or lower cervical and dorsal region (fig. 501, jw, and fig. 503, uic). The divisions becoming more distinct, separate small quadrilateral masses, which have received the name of protovertebra, by which it is meant to indicate that they are not the same with the permanent vertebral pieces of the skeleton, but rather correspond to embryonic somatomes, or mciameric segments, corresponding closely in number with the permanent vertebral divisions, but including the rudiments of other parts, such as those of the spinal nerves, along with those of the ~~vertebra3~~.		On either side of this axial cord a thick mass or plate of mesoblast is collected along its whole length, and very soon there appear several transverse clefts in these plates forming the commencement of protovertebral segmentation. The first formed of these divisions is near the anterior or uppermost of the future cervical vertebra3, and they rapidly extend backwards in the posterior or lower cervical and dorsal region (fig. 501, jw, and fig. 503, uic). The divisions becoming more distinct, separate small quadrilateral masses, which have received the name of protovertebra, by which it is meant to indicate that they are not the same with the permanent vertebral pieces of the skeleton, but rather correspond to embryonic somatomes, or mciameric segments, corresponding closely in number with the permanent vertebral divisions, but including the rudiments of other parts, such as those of the spinal nerves, along with those of the vertebrae.


	The basis of the cranium, into which the notochord extends, does not at first present any transverse division similar to that of the vertebral portion of the axis, and the notochord itself is at first without segmentation, and forms therefore a simple and entire cylinder.		The basis of the cranium, into which the notochord extends, does not at first present any transverse division similar to that of the vertebral portion of the axis, and the notochord itself is at first without segmentation, and forms therefore a simple and entire cylinder.

	Pleural Cleavage of the Lateral Parts of the Mesoblast.* — Together with the formation of the protovertebral plates and their transverse segmentation, another important change begins in the lateral part of the mesoblast external to these plates, which consists in its cleavage into an upper or outer and a lower or inner lamina, and the consequent formation between them of an interval or space (figs. 503, sji, and 50-1, i^p). The two laminae thus separated constitute respectively the somatopleure and s})lanchnopleure portions of the mesoblast, and the space between them is the commencement of the pleuroperitoneal cavity, which afterwards forms by its partition within the embryo the pleurae, pericardium, and peritoneum, and which beyond the embryo extends into the space between the amnion and the other developed membranes of the ovum.

	Fig. 504.— Diagrammatic longitudinal section through the Axis of an Embryo (from Foster and Balfour)		===Pleural Cleavage of the Lateral Parts of the Mesoblast===

			Together with the formation of the protovertebral plates and their transverse segmentation, another important change begins in the lateral part of the mesoblast external to these plates, which consists in its cleavage into an upper or outer and a lower or inner lamina, and the consequent formation between them of an interval or space (figs. 503, sji, and 50-1, i^p). The two laminae thus separated constitute respectively the somatopleure and splanchnopleure portions of the mesoblast, and the space between them is the commencement of the pleuroperitoneal cavity, which afterwards forms by its partition within the embryo the pleurae, pericardium, and peritoneum, and which beyond the embryo extends into the space between the amnion and the other developed membranes of the ovum.



			'''Fig. 504. Diagrammatic longitudinal section through the Axis of an Embryo''' (from Foster and Balfour)

	The section is supposed to be made at a time when the head-fold has commenced, but the tail-fold has not yet appeared. A , epiblast ; B, mesoblast ; C, hypoblast ; FSo, folil of the somatopleure ; FSp, fold of the splauchuopleure ; Am, commencing (head) fold of the amnion : NC, neural canal, closed in front, but still open behind ; Oh, notochord, — in front of it, uncleft mesoblast in the base of the cranium ; D, the commencing foregut, or alimentary canal ; Ht, heart ; pp, pleuro-peritoneal cavity.		The section is supposed to be made at a time when the head-fold has commenced, but the tail-fold has not yet appeared. A , epiblast ; B, mesoblast ; C, hypoblast ; FSo, folil of the somatopleure ; FSp, fold of the splauchuopleure ; Am, commencing (head) fold of the amnion : NC, neural canal, closed in front, but still open behind ; Oh, notochord, — in front of it, uncleft mesoblast in the base of the cranium ; D, the commencing foregut, or alimentary canal ; Ht, heart ; pp, pleuro-peritoneal cavity.


	* "Pleural" is here used in the sense " parietaL		* "Pleural" is here used in the sense " parietaL


	===Inflection of the Walls of the Body of the Embryo===		===Inflection of the Walls of the Body of the Embryo===

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Difference in Animals

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	The foregoing description applies to the ~~sj-mmetrical~~ position and central mode of development of the blastoderm which belong to the ova of reptiles, buxls and mammals : but it is right to state that in the lower vertebrata, or in amphibia and in osseous and cartilaginous fishes, there are several remarkable differences. Among these may be particularlj' noticed the non-symmetrical development of the blastodenn, and the excentric position of the commencement of the embryo : the involution of the epiblast at the aperture of the blastodenn termed ' anus " by Rusconi. or blastopore, at which the cells of the epiblast become continuous with the larger deeper cells from which the mesoblast and hypoblast originate. (See F. M. Balfour, " On the Development of the Elasmobranch Fishes,"" and " Comparison of the Development of Vertebrates,"" in the Quart. Joum. of Jlicroscop. Science for Oct. 1874. and July, 1875 ; E. Eay Lankester. '• On the Primitive Cell Layers of the Embryo." Sec, in the Ann. and Jlag. of Nat. Hist, for 1871.		The foregoing description applies to the symmetrical position and central mode of development of the blastoderm which belong to the ova of reptiles, buxls and mammals : but it is right to state that in the lower vertebrata, or in amphibia and in osseous and cartilaginous fishes, there are several remarkable differences. Among these may be particularlj' noticed the non-symmetrical development of the blastodenn, and the excentric position of the commencement of the embryo : the involution of the epiblast at the aperture of the blastodenn termed ' anus " by Rusconi. or blastopore, at which the cells of the epiblast become continuous with the larger deeper cells from which the mesoblast and hypoblast originate. (See F. M. Balfour, " On the Development of the Elasmobranch Fishes,"" and " Comparison of the Development of Vertebrates,"" in the Quart. Joum. of Jlicroscop. Science for Oct. 1874. and July, 1875 ; E. Eay Lankester. '• On the Primitive Cell Layers of the Embryo." Sec, in the Ann. and Jlag. of Nat. Hist, for 1871.

	===The Blastoderm of Mammals===		===The Blastoderm of Mammals===