Book - Developmental Anatomy 1924 - Revision history

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	By		By

	Leslie ~~Bleainerd~~ Arey		Leslie Brainerd Arey (1891-1988)


	Professor Of Anatomy At The Northwestern University Medical School, Chicago .		Professor Of Anatomy At The Northwestern University Medical School, Chicago.


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	Chicago, ill., September, 1924.		Chicago, ill., September, 1924.
	\| [[File:Leslie Arey.jpg\|thumb\|Leslie ~~Bleainerd~~ Arey]]		\| [[File:Leslie Arey.jpg\|thumb\|Leslie Brainerd Arey (1891-1988)]]
	\|}		\|}

Z8600021: /* Titles for Collateral Reading and Reference */

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Titles for Collateral Reading and Reference

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	Duval. Atlas D - Embryologie.		Duval. Atlas D - Embryologie.

	Hertwig. Handbuch der Entwicklungslehre der Wirbeltiere.		Hertwig. [[Book - Text-Book of the Embryology of Man and Mammals\|'''Handbuch der Entwicklungslehre der Wirbeltiere''']].

	Keibel and Mall. Human Embryology.		Keibel and Mall. [[Book - Manual of Human Embryology\|'''Human Embryology''']].

	Kellicott. A Textbook of General Embryology.		Kellicott. [[Book - Outlines of Chordate Development\|'''A Textbook of General Embryology''']].

	Kollmann. Handatlas der Entwicklungsgeschichte des Menschen.		Kollmann. [[Atlas_of_the_Development_of_Man_1\|'''Handatlas der Entwicklungsgeschichte des Menschen''']].

	Lillie. The Development of the Chick.		Lillie. The Development of the Chick.

	Minot. A Laboratory Text-book of Embryology.		Minot. [[Book - A Laboratory Text-Book of Embryology (1903)\|'''A Laboratory Text-book of Embryology''']].

	McMurrich. The Development of the Human Body.		McMurrich. The Development of the Human Body.

	Patten. The Early Embryology of the Chick.		Patten. [[Book - The Early Embryology of the Chick\|'''The Early Embryology of the Chick''']].

	Wilson. The Cell in Development and Inheritance.		Wilson. The Cell in Development and Inheritance.

Z8600021: /* The Vertebrate Groups */

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The Vertebrate Groups

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	There are five vertebrate classes, the higher characterized by the possession of an enveloping embryonic membrane, called the amnion, and another embryonic appendage, known as the allantois:		There are five vertebrate classes, the higher characterized by the possession of an enveloping embryonic membrane, called the amnion, and another embryonic appendage, known as the allantois:

	(R) Anamniota (amnion absent).		(A) '''Anamniota''' (amnion absent).
			: 1. ''Fishes'' - lamprey; sturgeon; shark; bony fishes; lung fish.
			: 2. ''Amphibia'' - salamander; frog; toad; etc.

	~~1. Fishes - lamprey; sturgeon; shark; bony fishes; lung fish.~~		(B) '''Amniota''' (amnion present).
			: 3. ''Reptiles'' - lizard; crocodile; snake; turtle.
	~~2. Amphibia - salamander; frog; toad; etc.~~		: 4. ''Birds.''
			: 5. ''Mammals''. Characterized by hair and mammary glands.
	(B) Amniota (amnion present).		:: (a) Monotremes - duck-bill; primitive mammals that have a cloaca and lay eggs with shells.
			:: (b) Marsupials - oppossum; kangaroo; etc. The young are born immature and are sheltered in an integumentary pouch.
	3. Reptiles - lizard; crocodile; snake; turtle.		:: (c) Placentalia. All other mammals whose young are nourished in the uterus by a placenta.
			::: Ungulate series. Hoofed mammals (cattle; sheep; pig; deer; horse; etc.).
	4. Birds.		::: Unguiculate series. Clawed mammals (mole; bat; rat; rabbit; cat; dog; etc.). The highest order is the Primates (lemur; monkey; ape, man).

	5. Mammals. Characterized by hair and mammary glands, (a) Monotremes - duck-bill; primitive mammals that have a cloaca and lay eggs with shells.

	(C) Marsupials - oppossum; kangaroo; etc. The young are born immature and are sheltered in an integumentary pouch, (r)Placentalia. All other mammals whose young are nourished in the uterus by a placenta.

	Ungulate series. Hoofed mammals (cattle; sheep; pig; deer; horse; etc.).

	Unguiculate series. Clawed mammals (mole; bat; rat; rabbit; cat; dog; etc.). The highest order is the Primates (lemur; monkey; ape, man).

	====The Vertebrate Body Plan====		====The Vertebrate Body Plan====

Z8600021: /* The Vertebrate Groups */

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The Vertebrate Groups

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	2. Amphibia - salamander; frog; toad; etc.		2. Amphibia - salamander; frog; toad; etc.

	{B) Amniota (amnion present).		(B) Amniota (amnion present).

	3. Reptiles - lizard; crocodile; snake; turtle.		3. Reptiles - lizard; crocodile; snake; turtle.
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	4. Birds.		4. Birds.

	5. ~~hlammals~~. Characterized by hair and mammary glands, (a) Monotremes - duck-bill; primitive mammals that have a		5. Mammals. Characterized by hair and mammary glands, (a) Monotremes - duck-bill; primitive mammals that have a cloaca and lay eggs with shells.
	cloaca and lay eggs with shells.


	(C) Marsupials - oppossum; kangaroo; etc. The young are born immature and are sheltered in an integumentary pouch, (r)Placentalia. All other mammals whose young are nourished in the uterus by a placenta.		(C) Marsupials - oppossum; kangaroo; etc. The young are born immature and are sheltered in an integumentary pouch, (r)Placentalia. All other mammals whose young are nourished in the uterus by a placenta.
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	Unguiculate series. Clawed mammals (mole; bat; rat; rabbit; cat; dog; etc.). The highest order is the Primates (lemur; monkey; ape, man).		Unguiculate series. Clawed mammals (mole; bat; rat; rabbit; cat; dog; etc.). The highest order is the Primates (lemur; monkey; ape, man).

	The Vertebrate Body Plan~~. -~~ All vertebrate animals are constructed in accordance with a common body plan. The distinctive characteristics of the vertebrate type include: .		====The Vertebrate Body Plan====
			All vertebrate animals are constructed in accordance with a common body plan. The distinctive characteristics of the vertebrate type include: .
	~~1. A tubular central nervous system, dorsally placed (Fig. 4).~~

	~~2. A notochord, between the neural tube and gut (Fig. 4). This cellular \|3rimitive-axis is replaced, wholly or in part, by the vertebral column.~~

	~~3. A pharynx, which develops paired pouches and clefts that determine the positions of important nerves, muscles and blood vessels (Fig. 91).~~

	4. The position of the mouth. Unlike the condition in many invertebrates, it is not surrounded by a circumoral ring of nervous tissue which connects a dorsal - brain - with a ventral chain of ganglia.

	~~5. The limbs. Two pairs, with an internal skeleton (Fig. 227).~~

	6. A coelom, which is divided into a dorsal, segmental part (cavities of the somites), and a ventral, unsegmented part, partitioned by the septum transversum (diaphragm) into thoracic and abdominal portions
	~~(Fig. 4 â€¢~~ .

			# A tubular central nervous system, dorsally placed (Fig. 4).
			# A notochord, between the neural tube and gut (Fig. 4). This cellular \|3rimitive-axis is replaced, wholly or in part, by the vertebral column.
			# A pharynx, which develops paired pouches and clefts that determine the positions of important nerves, muscles and blood vessels (Fig. 91).
			# The position of the mouth. Unlike the condition in many invertebrates, it is not surrounded by a circumoral ring of nervous tissue which connects a dorsal - brain - with a ventral chain of ganglia.
			# The limbs, Two pairs, with an internal skeleton (Fig. 227).
			# A coelom, which is divided into a dorsal, segmental part (cavities of the somites), and a ventral, unsegmented part, partitioned by the septum transversum (diaphragm) into thoracic and abdominal portions (Fig. 4).

	==Titles for Collateral Reading and Reference==		==Titles for Collateral Reading and Reference==

Z8600021: /* Fundamental Conceptions */

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Fundamental Conceptions

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	===Fundamental Conceptions===		===Fundamental Conceptions===
			====The Anlage====
			This German word, which lacks an entirely satisfactory English equivalent, is a term applied to the first discernible cell, or aggregation of cells, which is destined to form any distinct jiart or organ of the embryo. In the broad sense, the fertilized ovum is the anlage of the entire adult organism; furthermore, in the early cleavage stages of certain embryos it is possible to recognize single cells or cell groups from which definite structures will indubitably arise. The term anlage, however, is more commonly applied to the primordia that differentiate from the various germ layers. Thus the epithelial thickening over the optic vesicle is the anlage of the lens.

			====The Law of Genetic Restriction====
			As development advances, there is a constantly increasing restriction in the kind of differentiation open to the various parts. Each emerging tissue or organ is more rigidly bound to its particular type of differentiation than was the generalized material from which it came. A line of specialization, once begun, cannot be abandoned for another type. The parent tissue, likewise, is limited by losing the capacity for duplicating anlages already formed. Thus, the primitive thyroid can never become anything but a thyroid, whereas the gut that formed it also buds off, at other levels, the lungs, liver, and pancreas. Yet if the embryonic thyroid were destroyed, the pharynx would never replace it. From mesenchyme arise connective tissue, blood cells, and smooth muscle; when once the specialization begins, there can be no retraction or transformation to another type.

	~~The Anlage.~~ - ~~This German word~~, ~~which lacks an entirely satisfactory English equivalent~~, is a ~~term applied to~~ the ~~first discernible cell, or aggregation~~ of ~~cells~~, which ~~is destined~~ to ~~form any distinct jiart or organ of~~ the ~~embryo~~. ~~In the broad sense~~, ~~the fertilized ovum is the anlage of the entire adult organism; furthermore~~, ~~in the early cleavage stages of certain embryos it is possible to recognize single cells or cell groups from~~ which ~~definite structures will indubitably arise~~. The ~~term anlage~~, ~~however~~, is ~~more commonly applied to the primordia that differentiate~~ from ~~the various germ layers. Thus the epithelial thickening over the optic vesicle is the anlage~~ of the ~~lens~~.		====Continuity of the Germ Plasm====
			According to this important conception of Weismann, the body-protoplasm, or soma, and the reproductive-protoplasm differ fundamentally. The germinal material is a legacy that has existed since the beginning of life, from which representative portions are passed on intact from one generation to the next. Around this germ plasm there develops in each successive generation a shortlived body, or soma, which serves as a vehicle for insuring its transmission and perpetuation. The reason, therefore, why offspring resembles parent is because each develops from portions of the same stuff.

			====The Law of Biogenesis====
	The Law of Genetic Restriction. - As development advances, there is a constantly increasing restriction in the kind of differentiation open to the various parts. Each emerging tissue or organ is more rigidly bound to its particular type of differentiation than was the generalized material from which it came. A line of specialization, once begun, cannot be abandoned for another type. The parent tissue, likewise, is limited by losing the capacity for duplicating anlages already formed. Thus, the primitive thyroid can never become anything but a thyroid, whereas the gut that formed it also buds off, at other levels, the lungs, liver, and pancreas. Yet if the embryonic thyroid were destroyed, the pharynx would never replace it. From mesenchyme arise connective tissue, blood cells, and smooth muscle; when once the specialization begins, there can be no retraction or transformation to another type.		Of great theoretical interest is the fact, constantly observed in studying, embryos, that the individual in its development repeats hastily and incompletely the evolutionary history of its own species. This law of recapitulation was first stated clearly by Muller in 1863, and was termed by Haeckel the law of biogenesis. In accordance with it, the fertilized ovum is compared to a unicellular organism like the Ameba: the blastula is supposed to represent an adult Volvox type; the gastrula, a simple sponge; the segmented embryo, a worm-like stage ; and the embryo with gill slits may be regarded as a fishlike stage. Moreover, the blood of the human embryo in development passes through stages in which its corpuscles resemble in structure those of the fish and reptile; the heart is at first tubular, like that of the fish, and the arrangement of blood vessels is equally primitive; the kidney of the embryo is like that of the amphibian, as are also the genital ducts. Many other examples of this law may readily be observed.


	Continuity of the Germ Plasm. - According to this important conception of Weismann, the body-protoplasm, or soma, and the reproductive-protoplasm differ fundamentally. The germinal material is a legacy that has existed since the beginning of life, from which representative portions are passed on intact from one generation to the next. Around this germ plasm there develops in each successive generation a shortlived body, or soma, which serves as a vehicle for insuring its transmission and perpetuation. The reason, therefore, why offspring resembles parent is because each develops from portions of the same stuff.


	~~The Law of Biogenesis -~~ Of great theoretical interest is the fact, constantly observed in studying, embryos, that the individual in its development repeats hastily and incompletely the evolutionary history of its own species. This law of recapitulation was first stated clearly by Muller in 1863, and was termed by Haeckel the law of biogenesis. In accordance with it, the fertilized ovum is compared to a unicellular organism like the Ameba: the blastula is supposed to represent an adult Volvox type; the gastrula, a simple sponge; the segmented embryo, a worm-like stage ; and the embryo with gill slits may be regarded as a fishlike stage. Moreover, the blood of the human embryo in development passes through stages in which its corpuscles resemble in structure those of the fish and reptile; the heart is at first tubular, like that of the fish, and the arrangement of blood vessels is equally primitive; the kidney of the embryo is like that of the amphibian, as are also the genital ducts. Many other examples of this law may readily be observed.

Z8600021: /* Primitive Segments */

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Primitive Segments

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	===Primitive Segments===		===Primitive Segments - Metamerism===
	[[File:Arey1924 fig004.jpg\|thumb\|Fig. 4. Diagrammatic transverse section of a vertebrate embryo (Minot-Prentiss).]]		[[File:Arey1924 fig004.jpg\|thumb\|Fig. 4. Diagrammatic transverse section of a vertebrate embryo (Minot-Prentiss).]]

	~~Metamerism. -~~ A prominent feature of vertebrate embryos are the primitive segments, or metameres (Fig. 59). These segments are homologous to the serial divisions of an adult earth-worm - s body, divisions which, in the earth worm, are identical in structure, each containing a ganglion of the nerve cord, a muscle segment, or myotome, and pairs of blood vessels and nerves. In vertebrate embryos, the block like primitive segments lie next the neural tube and are known as mesodermal segments, or somites (Fig. 4). Each pair gives rise to a vertebra, to two myotomes, or muscle segments, and to paired vessels; each set of mesodermal segments is supplied by a pair of spinal nerves: consequently, the adult vertebrate body is segmented like that of the earth worm. As a worm grows by the formation of new segments at its tail-end, so the metameres of the vertebrate embryo begin to form in the head and are added tailward. There is this difference between the segments of the worm and the vertebrate embryo; the segmentation of the worm is complete, while that of the vertebrate is incomplete ventrally.		A prominent feature of vertebrate embryos are the primitive segments, or metameres (Fig. 59). These segments are homologous to the serial divisions of an adult earth-worm's body, divisions which, in the earth worm, are identical in structure, each containing a ganglion of the nerve cord, a muscle segment, or myotome, and pairs of blood vessels and nerves. In vertebrate embryos, the block like primitive segments lie next the neural tube and are known as mesodermal segments, or somites (Fig. 4). Each pair gives rise to a vertebra, to two myotomes, or muscle segments, and to paired vessels; each set of mesodermal segments is supplied by a pair of spinal nerves: consequently, the adult vertebrate body is segmented like that of the earth worm. As a worm grows by the formation of new segments at its tail-end, so the metameres of the vertebrate embryo begin to form in the head and are added tailward. There is this difference between the segments of the worm and the vertebrate embryo; the segmentation of the worm is complete, while that of the vertebrate is incomplete ventrally.


	Somatopleure and Splanchnopleure. In early embryos the mesoderm splits into two layers, the somatic (dorsal) and splanchnic (ventral) mesoderm (Fig. 4). The ectoderm and somatic mesoderm constitute the ~~lu)dy~~ wall, which is termed the somatopleure. In the same way, the entoderm and splanchnic mesoderm combine as the splanchnopleure; it forms the mesenteries and the walls of the gut, heart, and lungs.		===Somatopleure and Splanchnopleure===
			In early embryos the mesoderm splits into two layers, the somatic (dorsal) and splanchnic (ventral) mesoderm (Fig. 4). The ectoderm and somatic mesoderm constitute the body wall, which is termed the somatopleure. In the same way, the entoderm and splanchnic mesoderm combine as the splanchnopleure; it forms the mesenteries and the walls of the gut, heart, and lungs.


	Coelom~~. -~~The space between the somatopleure and splanchnopleure is the ~~ccclom~~, or body cavity. At the first splitting of the mesoderm, isolated clefts are produced. These unite on each side and eventually form one cavity - the coelom. With the extension of the mesoderm, the ~~cot'lom~~ surrounds the heart and gut ventrally (Fig. 4). Later, it is subdivided into the pericardial ~~caznty~~ about the heart, the pleural cavity of the thorax, and the peritoneal cavity of the abdominal region. The ~~ci)ithelia~~ lining the several body cavities are termed mesothelia.		===Coelom===
			The space between the somatopleure and splanchnopleure is the coelom, or body cavity. At the first splitting of the mesoderm, isolated clefts are produced. These unite on each side and eventually form one cavity - the coelom. With the extension of the mesoderm, the coelom surrounds the heart and gut ventrally (Fig. 4). Later, it is subdivided into the pericardial cavity about the heart, the pleural cavity of the thorax, and the peritoneal cavity of the abdominal region. The epithelia lining the several body cavities are termed mesothelia.


	The Nephrotome~~. -~~ The bridge of cells connecting the primitive segment with the unsegmented somatic and splanchnic layers is the nephrotome, or intermediate cell mass (Fig. 4). From these will develop the urogenital glands and ducts.		===The Nephrotome===
			The bridge of cells connecting the primitive segment with the unsegmented somatic and splanchnic layers is the nephrotome, or intermediate cell mass (Fig. 4). From these will develop the urogenital glands and ducts.

			===Developmental Processes===
	Developmental Processes~~. -~~ The developing embryo exhibits a ~~])rogressively comjilex~~ structure, the various steps in the production of which occur in orderly sequence. There may be recognized in development a number of component mechanical processes which are used repeatedly by the embryo. The general and fundamental process conditioning ilifferentiation is cell multiplication, and the subsequent growth of the daughter cells. The more important of the specific developmental ~~])rocesses~~ are the following: ( i) cell migration; (2) localized growth, resulting in eidargements and constrictions; (3) cell aggregation, forming (a) cords, (b) sheets, [c] masses; (4) delamination, that is, the splitting of single sheets into separate layers; (5) folds, including circumscribed folds which produce ia) evaginations, or out-pocketings, (b) invaginations, or in-pocketings.		The developing embryo exhibits a progressively complex structure, the various steps in the production of which occur in orderly sequence. There may be recognized in development a number of component mechanical processes which are used repeatedly by the embryo. The general and fundamental process conditioning ilifferentiation is cell multiplication, and the subsequent growth of the daughter cells. The more important of the specific developmental processes are the following: ( 1) cell migration; (2) localized growth, resulting in eidargements and constrictions; (3) cell aggregation, forming (a) cords, (b) sheets, [c] masses; (4) delamination, that is, the splitting of single sheets into separate layers; (5) folds, including circumscribed folds which produce (a) evaginations, or out-pocketings, (b) invaginations, or in-pocketings.

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Derivatives of the Germ Layers

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	{\|		{\|
			\|-bgcolor="CEDFF2"
	! width=30%\|Ectoderm		! width=30%\|Ectoderm
	! width=30%\|Mesoderm		! width=30%\|Mesoderm

Z8600021: /* Mitosis */

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Mitosis

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	[[File:Arey1924 fig002.jpg\|thumb\|Fig. 2. Diagrams of the phases of mitosis (Schafer).]]		[[File:Arey1924 fig002.jpg\|thumb\|Fig. 2. Diagrams of the phases of mitosis (Schafer).]]
	====Mitosis====		====Mitosis====
	In the reproduction of typically active somatic cells and in all germ cells, complicated changes take place in the nucleus. These changes give rise to thread-like structures, hence the process is termed mitosis (thread) in distinction to amitosis (no ~~threadk~~ Mitosis is divided for convenience into four phases (Fig. 2) :		In the reproduction of typically active somatic cells and in all germ cells, complicated changes take place in the nucleus. These changes give rise to thread-like structures, hence the process is termed mitosis (thread) in distinction to amitosis (no thread). Mitosis is divided for convenience into four phases (Fig. 2) :

	Prophase		Prophase
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	# Astral rays appear in the cytoplasm about each centriole. They radiate from it, and the threads of the central or achromatic spindle are formed between the two asters, thus constituting the amphiaster (II).		# Astral rays appear in the cytoplasm about each centriole. They radiate from it, and the threads of the central or achromatic spindle are formed between the two asters, thus constituting the amphiaster (II).
	# The nuclear membrane and nucleolus disappear, the karyoplasm and cytoplasm becoming confluent.		# The nuclear membrane and nucleolus disappear, the karyoplasm and cytoplasm becoming confluent.
	# During the above changes the chromatic network of the resting nucleus resolves itself into a skein, or spireme, which soon shortens and breaks up into distinct, heavily-staining bodies, the chromosomes (u, uI). The definite number of chromosomes is always found in the cells of a given species, the chromosomes may be block-shaped, rod- shaped, or bent in the form of a U or V.		# During the above changes the chromatic network of the resting nucleus resolves itself into a skein, or spireme, which soon shortens and breaks up into distinct, heavily-staining bodies, the chromosomes (II, III). The definite number of chromosomes is always found in the cells of a given species, the chromosomes may be block-shaped, rod- shaped, or bent in the form of a II or V.
	# The chromosomes arrange themselves in the equatorial plane of the central spindle (IV). If U- or V-shaped, the angle of each is directed toward a common center. The amphiaster and the chromosomes together constitute a mitotic figure, and at the end of the prophase this is called a monaster.		# The chromosomes arrange themselves in the equatorial plane of the central spindle (IV). If U- or V-shaped, the angle of each is directed toward a common center. The amphiaster and the chromosomes together constitute a mitotic figure, and at the end of the prophase this is called a monaster.

Z8600021: /* General Features Of Development */

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General Features Of Development

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	Cell Division. - All cells arise from pre-existing cells by division. There are two methods of cell division - amitosis and mitosis.		Cell Division - All cells arise from pre-existing cells by division. There are two methods of cell division - amitosis and mitosis.


	Amitosis. - Cells may divide directly by the simple fission of their nuclei and cytoplasm. This rather infrequent process is called amitosis. Amitosis is said by many to occur only in specialized or moribund cells. It is the type of cell division demonstrable in the epithelium of the bladder.		Amitosis - Cells may divide directly by the simple fission of their nuclei and cytoplasm. This rather infrequent process is called amitosis. Amitosis is said by many to occur only in specialized or moribund cells. It is the type of cell division demonstrable in the epithelium of the bladder.

	[[File:Arey1924 fig002.jpg\|thumb\|Fig. 2. Diagrams of the phases of mitosis (Schafer).]]		[[File:Arey1924 fig002.jpg\|thumb\|Fig. 2. Diagrams of the phases of mitosis (Schafer).]]
	====Mitosis====		====Mitosis====
	In the reproduction of typically active somatic cells and in all germ cells, complicated changes take place in the nucleus. These changes give rise to thread-like structures, hence the process is termed mitosis (thread) in distinction to amitosis (no threadk Mitosis is divided for convenience into four phases (Fig. 2) :		In the reproduction of typically active somatic cells and in all germ cells, complicated changes take place in the nucleus. These changes give rise to thread-like structures, hence the process is termed mitosis (thread) in distinction to amitosis (no threadk Mitosis is divided for convenience into four phases (Fig. 2) :
	~~Prophase. - 1. The centrosome divides and the two minute bodies resulting from the division move apart, ultimately occup^dng positions at opposite poles of the nucleus (I-uI).~~

			Prophase
			# The centrosome divides and the two minute bodies resulting from the division move apart, ultimately occupying positions at opposite poles of the nucleus (I-III).
			# Astral rays appear in the cytoplasm about each centriole. They radiate from it, and the threads of the central or achromatic spindle are formed between the two asters, thus constituting the amphiaster (II).
			# The nuclear membrane and nucleolus disappear, the karyoplasm and cytoplasm becoming confluent.
			# During the above changes the chromatic network of the resting nucleus resolves itself into a skein, or spireme, which soon shortens and breaks up into distinct, heavily-staining bodies, the chromosomes (u, uI). The definite number of chromosomes is always found in the cells of a given species, the chromosomes may be block-shaped, rod- shaped, or bent in the form of a U or V.
			# The chromosomes arrange themselves in the equatorial plane of the central spindle (IV). If U- or V-shaped, the angle of each is directed toward a common center. The amphiaster and the chromosomes together constitute a mitotic figure, and at the end of the prophase this is called a monaster.


	~~2. Astral rays appear in~~ the ~~cytoplasm about each centriole~~. ~~They radiate from it~~, ~~and the threads~~ of the ~~central or achromatic spindle are formed~~ between the ~~two asters, thus constituting~~ the ~~amphiastcr (u)~~.		Metaphase - The longitudinal splitting of the chromosomes into exactly similar halves constitutes the metaphasc (IV). The aim of mitosis is thus accomplished, an accurate division of the chromatin between the nuclei of the daughter cells.


	3. The ~~nuclear membrane~~ and ~~nucleolus disappear~~, the ~~karyoplasm and cytoplasm becoming confluent~~.		Anaphase - The two groups of daughter chromosomes separate and move up along the central spindle fibers, each toward one of the two asters. Hence this is called the diaster stage (V, VI). Each centriole may divide in preparation for the next diviSion of the daughter cells.

			Telophase - i. The daughter chromosomes resolve themselves into a reticulum and daughter nuclei are formed (Vu, VuI).

	4. During the above changes the chromatic network of the resting nucleus resolves itself into a skein, or spireme, which soon shortens and l;)reaks up into distinct, heavily-staining bodies, the chromosomes (u, uI). x\ definite number of chromosomes is always found in the cells of a given species, the chromosomes may be block-shaped, rod- shaped, or bent in the form of a U or V.

			2. The cytoplasm divides in a plane perpendicular to the axis of the mitotic spindle (VIII). Two complete daughter cells have thus arisen from the mother cell.
	5. The chromosomes arrange themselves in the equatorial plane of the central spindle (IV). If U- or V-shaped, the angle of each is directed toward a common center. The amphiaster and the chromosomes together constitute a mitotic figure, and at the end of the prophase this is called a monaster.


	Metaphase. - The longitudinal splitting of the chromosomes into exactly similar halves constitutes the metaphasc (IV). The aim of mitosis is thus accomplished, an accurate division of the chromatin between the nuclei of the daughter cells.


	Anaphase. - The two groups of daughter chromosomes separate and move up along the central spindle fibers, each toward one of the two asters. Hence this is called the diaster stage (V, VI). Each centriole may divide in preparation for the next diviSion of the daughter cells.

	~~Telophase. - i. The daughter chromosomes resolve themselves into a reticulum and daughter nuclei are formed (Vu, VuI).~~


	2. The cytoplasm divides in a plane perpendicular to the axis of the mitotic spindle (~~VuI~~). Two complete daughter cells have thus arisen from the mother cell.

	The number of chromosomes is constant in the cells of a given species. The smallest assortment, two, occurs in Ascaris megalocephala univaleus, a round worm parasitic in the intestine of the horse. The largest number known is found in the brine shrimp, Artemia, where 1 68 have been counted. The chromosome enumeration for the human cell has been variously stated but the results of Winiwarter (1912), Grosser (1921 ), and Painter (1923) now agree on a relatively high number, which Painter establishes as 48 for whites and negroes of both sexes.


			The number of chromosomes is constant in the cells of a given species. The smallest assortment, two, occurs in Ascaris megalocephala univaleus, a round worm parasitic in the intestine of the horse. The largest number known is found in the brine shrimp, Artemia, where 168 have been counted. The chromosome enumeration for the human cell has been variously stated but the results of Winiwarter (1912), Grosser (1921), and Painter (1923) now agree on a relatively high number, which Painter establishes as 48 for whites and negroes of both sexes.

	===The Germ Layers===		===The Germ Layers===

Z8600021: /* Modern Embryology */

2016-10-24T01:14:58Z

Modern Embryology

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	====Modern Embryology====		====Modern Embryology====
	As an organized and definite science, began with Balfour (1874), who reviewed, digested, and made accessible the earlier scattered facts. Throughout this period, the experimental method of investigation has been used increasingly; without it many structural and physiological aspects of development would remain unsolved.		As an organized and definite science, began with [[:File:Francis Balfour.jpg\|Balfour (1874)]], who reviewed, digested, and made accessible the earlier scattered facts. Throughout this period, the experimental method of investigation has been used increasingly; without it many structural and physiological aspects of development would remain unsolved.

	===General Features Of Development===		===General Features Of Development===

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	{\|		{\|
			\|-bgcolor="CEDFF2"
	! width=30%\|Ectoderm		! width=30%\|Ectoderm
	! width=30%\|Mesoderm		! width=30%\|Mesoderm