Z8600021 at 04:31, 31 December 2019

2019-12-31T04:31:06Z

← Older revision		Revision as of 15:31, 31 December 2019
Line 23:		Line 23:
	almost identical with that in Styela, only the peripheral cytoplasm can be distinguished (although it corresponds with the yellowish material in Styela it is grey in Amphioxus). In most other organisms there is no overt sign which distinguishes the regions of different prospective fate, and much ingenuity has had to be used to keep track of the parts of the early egg and discover where they get to and what they develop into. In spite of the technical difficulty of obtaining them, prospective fate maps are the clearest way of summarising the future course of development, and we shall use them again and again to show how the embryological events which look so different in different groups of vertebrates, can really be traced back to the same general scheme.		almost identical with that in Styela, only the peripheral cytoplasm can be distinguished (although it corresponds with the yellowish material in Styela it is grey in Amphioxus). In most other organisms there is no overt sign which distinguishes the regions of different prospective fate, and much ingenuity has had to be used to keep track of the parts of the early egg and discover where they get to and what they develop into. In spite of the technical difficulty of obtaining them, prospective fate maps are the clearest way of summarising the future course of development, and we shall use them again and again to show how the embryological events which look so different in different groups of vertebrates, can really be traced back to the same general scheme.

			[[File:Waddington1956 fig7.1.jpg\|600px]]

	~~FIGURE~~ 7.1		'''Figure 7.1.''' The development of the Ascidian Styela. (a) Before fertilisation, showing the large germinal vesicle(g.v.) and the peripheral yellowish layer (y), and the inner grey yolky cytoplasm. (b) Immediately after fertilisation. The germinal vesicle has ruptured and released a clear grey cytoplasm. The yellow cytoplasm is accumulating at the vegetative pole. (c) The sperm nucleus s is visible in the yellow cytoplasm which lies at the vegetative pole. The clear grey cytoplasm has moved down and lies just above the yellow. (d) Shortly before the first cleavage. The sperm nucleus has moved up and met the female nucleus just below the equator. The yellow and grey cytoplasms have moved up with the sperm nucleus. The yellow crescent is more or less superficial, the grey one extends into the depth of the egg. (e) Two-cell stage. The grey crescent is becoming more diffuse. (f) Eight-cell stage from side. (g) Sixteen-cell stage from animal pole. Note the bilateral symmetry. (h) Early gastrula from vegetative pole, looking into the wide open blastopore b; on the dorsal side of this are the future neural cells #.p and chordal cells c. (i) Longitudinal section through young larva. n.p. neural plate, cnotochord, e ectoderm, end endoderm lining the archenteron, m mesoderm, b blastopore. (In this and Figures f, g and h the yellow crescent material is shaded so that its movements can be followed but it cannot actually be recognised by its colour so clearly as the diagram suggests.) (j) Longitudinal section through a young larva: ¢., notochord; ey., eye spot; end., endoderm; t.e., endoderm of tail; mes., mesenchyme; #., nerve cord; n.v. neural vesicle; pap., adhesive papilla. (After Conklin 1905.)

	The development of the Ascidian Styela. (a) Before fertilisation, showing the large germinal vesicle(g.v.) and the peripheral yellowish layer (y), and the inner grey yolky cytoplasm. (b) Immediately after fertilisation. The germinal vesicle has ruptured and released a clear grey cytoplasm. The yellow cytoplasm is accumulating at the vegetative pole. (c) The sperm nucleus s is visible in the yellow cytoplasm which lies at the vegetative pole. The clear grey cytoplasm has moved down and lies just above the yellow. (d) Shortly before the first cleavage. The sperm nucleus has moved up and met the female nucleus just below the equator. The yellow and grey cytoplasms have moved up with the sperm nucleus. The yellow crescent is more or less superficial, the grey one extends into the depth of the egg. (e) Two-cell stage. The grey crescent is becoming more diffuse. (f) Eight-cell stage from side. (g) Sixteen-cell stage from animal pole. Note the bilateral symmetry. (h) Early gastrula from vegetative pole, looking into the wide open blastopore b; on the dorsal side of this are the future neural cells #.p and chordal cells c. (i) Longitudinal section through young larva. n.p. neural plate, cnotochord, e ectoderm, end endoderm lining the archenteron, m mesoderm, b blastopore. (In this and Figures f, g and h the yellow crescent material is shaded so that its movements can be followed but it cannot actually be recognised by its colour so clearly as the diagram suggests.) (j) Longitudinal section through a young larva: ¢., notochord; ey., eye spot; end., endoderm; t.e., endoderm of tail; mes., mesenchyme; #., nerve cord; n.v. neural vesicle; pap., adhesive papilla. (After Conklin 1905.)

			[[File:Waddington1956 fig7.2.jpg\|600px]]

			'''Figure 7.2.''' Presumptive areas in the 8-cell stage of an ascidian, seen from the right side. White, ectoderm; a, mesoderm (muscle); b, endoderm; c, notochord; 1, spinal cord; tg, brain. (After Vandebroek 1938, Reverberi 1948 and Ortolani.)



	~~FIGURE~~ 7.2

	Presumptive areas in the 8-cell stage of an ascidian, seen from the right side. White, ectoderm; a, mesoderm (muscle); b, endoderm; c, notochord; 1, spinal cord; tg, brain. (After Vandebroek 1938, Reverberi 1948 and Ortolani.)


Line 48:		Line 43:

	The shape of the blastopore alters somewhat as the gastrulation proceeds, but there is no need to repeat here all the details given by Conklin in his classical account of Amphioxus. At first the endoderm plate is triangular, with a wide straight dorsal lip and two lateral boundaries which converge towards the ventral lip. Although the sinking in of the endoderm begins ventrally, it proceeds fastest on the dorsal side, and it is the dorsal lip which is the most sharply inflected. In Styela this lip is originally made up of about six transverse rows of cells, each row containing rather more than a dozen cells, all of which have been derived from the light-grey chorda-neural crescent. The lower three rows lie inside the lip, and come into the roof of the primitive gut; these give rise to the chorda, while the remaining three rows stay on the outer surface and become the neural plate. During the gastrulation, the arrangement of the cells in the rows is profoundly altered; they slip between one another, so that the chorda and neural plate, whose material was originally arranged in transverse rows, become longitudinal strands running forward from the blastopore lip. The effect of this is that the cells of the mesodermal crescent on the other side of the egg have to move over towards the dorsal side to fill the gap which would otherwise be left; and as the stretching of the dorsal organs continues, the primitive gut becomes a tube with a strip of chorda along its most dorsal part, a strip of mesoderm on each side of that, and the main hollow of the tube lined with endoderm.		The shape of the blastopore alters somewhat as the gastrulation proceeds, but there is no need to repeat here all the details given by Conklin in his classical account of Amphioxus. At first the endoderm plate is triangular, with a wide straight dorsal lip and two lateral boundaries which converge towards the ventral lip. Although the sinking in of the endoderm begins ventrally, it proceeds fastest on the dorsal side, and it is the dorsal lip which is the most sharply inflected. In Styela this lip is originally made up of about six transverse rows of cells, each row containing rather more than a dozen cells, all of which have been derived from the light-grey chorda-neural crescent. The lower three rows lie inside the lip, and come into the roof of the primitive gut; these give rise to the chorda, while the remaining three rows stay on the outer surface and become the neural plate. During the gastrulation, the arrangement of the cells in the rows is profoundly altered; they slip between one another, so that the chorda and neural plate, whose material was originally arranged in transverse rows, become longitudinal strands running forward from the blastopore lip. The effect of this is that the cells of the mesodermal crescent on the other side of the egg have to move over towards the dorsal side to fill the gap which would otherwise be left; and as the stretching of the dorsal organs continues, the primitive gut becomes a tube with a strip of chorda along its most dorsal part, a strip of mesoderm on each side of that, and the main hollow of the tube lined with endoderm.


	As the elongation proceeds, the blastopore narrows like the mouth of a laundry bag when the string is pulled tight. Before it is completely closed in Amphioxus the ectoderm just above the ventral lip grows up in a curved ridge, the sides of which rapidly come together, covering the still-open blastopore from sight, and progress forwards above the neural plate, like two flaps being drawn together by a zip-fastener up the mid-dorsal line. Underneath these flaps, the neural plate continues to get longer and narrower, and its centre sinks down to form a trough or groove; in the most anterior end, indeed, it rolls up completely so as to produce a neural tube, such as we shall find in the higher vertebrates. Meanwhile important changes are beginning in the walls of the archenteron. These can be most simply described by saying that the endoderm, which in a cross section still makes up only the ventral and lateral walls of the gut tube, now starts to grow inwards from each side towards the dorsal midline. In doing so, it first undercuts the mesodermal strips, pushing them outwards as grooves which eventually become completely cut off from the gut as hollow sacks; and after pushing off the mesoderm, it then grows under the chorda, excluding it also from the walls of the gut, which thus becomes completely lined by endoderm. It should be noted that although this is a convenient method of describing what happens, we do not actually know that it is primarily the endoderm which pushes off the mesoderm, and not the latter which actively withdraws itself; such questions could only be definitely answered by an experimental analysis which has not yet been made. It may well be the mesoderm which plays the active part in its removal from the gut wall, since soon after this it undergoes a change for which the endoderm can scarcely be made responsible; the long hollow sacks of mesoderm become nipped off into a series of short, square chambers, which lie in orderly pairs on each side of the chorda; these are the somites.		As the elongation proceeds, the blastopore narrows like the mouth of a laundry bag when the string is pulled tight. Before it is completely closed in Amphioxus the ectoderm just above the ventral lip grows up in a curved ridge, the sides of which rapidly come together, covering the still-open blastopore from sight, and progress forwards above the neural plate, like two flaps being drawn together by a zip-fastener up the mid-dorsal line. Underneath these flaps, the neural plate continues to get longer and narrower, and its centre sinks down to form a trough or groove; in the most anterior end, indeed, it rolls up completely so as to produce a neural tube, such as we shall find in the higher vertebrates. Meanwhile important changes are beginning in the walls of the archenteron. These can be most simply described by saying that the endoderm, which in a cross section still makes up only the ventral and lateral walls of the gut tube, now starts to grow inwards from each side towards the dorsal midline. In doing so, it first undercuts the mesodermal strips, pushing them outwards as grooves which eventually become completely cut off from the gut as hollow sacks; and after pushing off the mesoderm, it then grows under the chorda, excluding it also from the walls of the gut, which thus becomes completely lined by endoderm. It should be noted that although this is a convenient method of describing what happens, we do not actually know that it is primarily the endoderm which pushes off the mesoderm, and not the latter which actively withdraws itself; such questions could only be definitely answered by an experimental analysis which has not yet been made. It may well be the mesoderm which plays the active part in its removal from the gut wall, since soon after this it undergoes a change for which the endoderm can scarcely be made responsible; the long hollow sacks of mesoderm become nipped off into a series of short, square chambers, which lie in orderly pairs on each side of the chorda; these are the somites.


			[[File:Waddington1956 fig7.3.jpg\|600px]]

	~~FIGURE~~ 7.3 Development of Amphioxus.		'''Figure 7.3.''' Development of Amphioxus.

	(a) Section through egg one hour after fertilisation, showing the conjugation of the pronuclei, a polar body, and the grey crescent material on the left (posterior) side.		(a) Section through egg one hour after fertilisation, showing the conjugation of the pronuclei, a polar body, and the grey crescent material on the left (posterior) side.
Line 98:		Line 95:


			[[File:Waddington1956 fig7.4.jpg\|600px]]

			'''Figure 7.4.''' Isolation and grafting of blastomeres in ascidians. (a) Separation of the 8-cell stage of an ascidian into four pairs of cells. The two posterior animal cells give ectoderm, and the two posterior vegetative no brain or palps, and the vegetative anteriors endoderm and notochord but no neural tube. (b) If the anterior vegetative cells are removed, the rest does not develop the brain and palps (although the anterior animal cells from which they should form are still present), nor neural tube. (c) Ifthe anterior animal cells are removed, and the brain and palps are again absent. (4) The anterior animal couple are removed and replaced by a posterior animal couple (shaded). Still the brain and palps are absent. (After Reverberi 1948.)

	~~Ficure~~ 7.4 Isolation and grafting of blastomeres in ascidians. (a) Separation of the 8-cell stage of an ascidian into four pairs of cells. The two posterior animal cells give ectoderm, and the two posterior vegetative no brain or palps, and the vegetative anteriors endoderm and notochord but no neural tube. (b) If the anterior vegetative cells are removed, the rest does not develop the brain and palps (although the anterior animal cells from which they should form are still present), nor neural tube. (c) Ifthe anterior animal cells are removed, and the brain and palps are again absent. (4) The anterior animal couple are removed and replaced by a posterior animal couple (shaded). Still the brain and palps are absent. (After Reverberi 1948.)

Z8600021 at 02:56, 23 November 2019

2019-11-23T02:56:49Z

Show changes

Z8600021: Created page with "{{Header}} {{Ref-Waddington1956}} {{Waddington1956 TOC}} {{Waddington1956 TOC}} {{Footer}} Category:Historic EmbryologyCategory:1950'sCategory:Textbook"

2019-11-23T01:43:30Z

Created page with "{{Header}} {{Ref-Waddington1956}} {{Waddington1956 TOC}} {{Waddington1956 TOC}} {{Footer}} Category:Historic Embryology Category:1950's Category:Textbook"

New page

{{Header}}
{{Ref-Waddington1956}}
{{Waddington1956 TOC}}

{{Waddington1956 TOC}}
{{Footer}}
[[Category:Historic Embryology]][[Category:1950's]][[Category:Textbook]]

Waddington1956 7 - Revision history

Z8600021 at 04:31, 31 December 2019

Z8600021 at 02:56, 23 November 2019

Z8600021: Created page with "{{Header}} {{Ref-Waddington1956}} {{Waddington1956 TOC}} {{Waddington1956 TOC}} {{Footer}} Category:Historic EmbryologyCategory:1950'sCategory:Textbook"