Book - Quain's Embryology 1 - Revision history

Z8600021: /* Distinction of the germ */

2012-10-13T12:23:43Z

Distinction of the germ

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	But in birds and reptiles, in which the ova are comparatively large, the greater part of the yolk forming the yellow yolk substance, takes no immediate part in the first formative processes, and these are restricted to the small whitish flat disc, called, the cicatricula in the fowl's egg, which is composed of fine granular protoplasm, occupies a determinate place on the surface of the larger yolk-mass, and, so long as the yolk remains in the ovary, has the germinal vesicle situated in its centre.		But in birds and reptiles, in which the ova are comparatively large, the greater part of the yolk forming the yellow yolk substance, takes no immediate part in the first formative processes, and these are restricted to the small whitish flat disc, called, the cicatricula in the fowl's egg, which is composed of fine granular protoplasm, occupies a determinate place on the surface of the larger yolk-mass, and, so long as the yolk remains in the ovary, has the germinal vesicle situated in its centre.

	Fig. 489. — Ovarian Ovum of a Mammifer.

			'''Fig. 489. Ovarian Ovum of a Mammifer.'''

	a, The entire ovum, viewed under pressure ; the granular cells have been removed from the outer surface, the germinal vesicle is seen in the yolk substance within ; h, the external coat or zona burst by increased pressure, the yolk protoplasm and the germinal vesicle having escaped from within ; c, germinal vesicle more freed from the yolk substance. In all of them the macula is to the part thus distinguishable from the rest of the yolk the name of germ may be given ; and it has also been styled the primary or germinal or formative yolk, and the protoplasm or protoblast, while the remainder of the yolk-substance has been called the nutritive or food yoUc, the secondary yolk or deutoplasm. The oviparous ovum has therefore been named merodlasfic, or partially germinal.		a, The entire ovum, viewed under pressure ; the granular cells have been removed from the outer surface, the germinal vesicle is seen in the yolk substance within ; h, the external coat or zona burst by increased pressure, the yolk protoplasm and the germinal vesicle having escaped from within ; c, germinal vesicle more freed from the yolk substance. In all of them the macula is to the part thus distinguishable from the rest of the yolk the name of germ may be given ; and it has also been styled the primary or germinal or formative yolk, and the protoplasm or protoblast, while the remainder of the yolk-substance has been called the nutritive or food yoUc, the secondary yolk or deutoplasm. The oviparous ovum has therefore been named merodlasfic, or partially germinal.

Z8600021: /* Contractile and other movements in the germ */

2012-10-12T01:19:25Z

Contractile and other movements in the germ

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	==Contractile and other movements in the germ==		==Contractile and other movements in the germ==

	With respect to the last mentioned topic it may be remarked that certain heaving and rotatory motions which have been observed by several embryologists immediately before and during the occui-rence of the cleavage, indicate the play of contractile and it may ~~be°of~~ other forces within the protoplasm ; and these forces have been supposed to have some relation to the nucleus. Recent observations by Flemming in the ovum of Anodonta, of Oellacher in that of the trout, and of Goette in the toad, seem to show that there is some structural condition related to the process of division which may have a connection with its occurence ; for in the eggs of these animals the space within the domain of each segment sphere about to be formed is occupied by fine filaments radiating from the centre towards the circumference, and preceding the formation of the clear nuclear space within. It is probable that the hyaline globules may be the result of the first yolk contraction.		With respect to the last mentioned topic it may be remarked that certain heaving and rotatory motions which have been observed by several embryologists immediately before and during the occui-rence of the cleavage, indicate the play of contractile and it may be of other forces within the protoplasm ; and these forces have been supposed to have some relation to the nucleus. Recent observations by Flemming in the ovum of Anodonta, of Oellacher in that of the trout, and of Goette in the toad, seem to show that there is some structural condition related to the process of division which may have a connection with its occurence ; for in the eggs of these animals the space within the domain of each segment sphere about to be formed is occupied by fine filaments radiating from the centre towards the circumference, and preceding the formation of the clear nuclear space within. It is probable that the hyaline globules may be the result of the first yolk contraction.

	Secondary Segmentation.— The segmenting process previously described may be called for it is not yet ascertained in how far the whole of the blastoderm, considered as the organised substratum for the development of the new being, owes its origin directly to the first process of germ segmentation, or to what extent a later process of an analogous kind may contribute to the formation of some of its deeper elements. The most recent observations, such as those of Oellacher and Goette on the egg of the bird, of Ray Lankester on the ova of Cephalopoda, and of Balfour on that of sharks, would tend to support the view that in meroblastic ova at least, the process of segmentation, considered as one of conversion of the yolk into blastodermic elements, is not completed in the first series of such divisions, but continues to take place in a modified form for some time afterwards, thus extending the blastoderm over the surface of the yolk more and more by the addition of newly acquired elements. These elements appear to be formed from nev.^ centres of cell organisation external to the limits of the germinal part of the ovum, by what may be called a process of free cell formation, and to contribute mainly to the production of the deeper part of the blastoderm. (Ray Lankester in Ann. & Mag. of Nat. Hist., 1873, p. 81, and F.M. Balfour in Jom-n. of Microscop. Science, July, 1873 and 1874 ; Goette in Ai-chiv, fiir Mikroskop. Anat., vol. x., 1874.)		==Secondary Segmentation==

			The segmenting process previously described may be called for it is not yet ascertained in how far the whole of the blastoderm, considered as the organised substratum for the development of the new being, owes its origin directly to the first process of germ segmentation, or to what extent a later process of an analogous kind may contribute to the formation of some of its deeper elements. The most recent observations, such as those of Oellacher and Goette on the egg of the bird, of Ray Lankester on the ova of Cephalopoda, and of Balfour on that of sharks, would tend to support the view that in meroblastic ova at least, the process of segmentation, considered as one of conversion of the yolk into blastodermic elements, is not completed in the first series of such divisions, but continues to take place in a modified form for some time afterwards, thus extending the blastoderm over the surface of the yolk more and more by the addition of newly acquired elements. These elements appear to be formed from nev.^ centres of cell organisation external to the limits of the germinal part of the ovum, by what may be called a process of free cell formation, and to contribute mainly to the production of the deeper part of the blastoderm. (Ray Lankester in Ann. & Mag. of Nat. Hist., 1873, p. 81, and F.M. Balfour in Jom-n. of Microscop. Science, July, 1873 and 1874 ; Goette in Ai-chiv, fiir Mikroskop. Anat., vol. x., 1874.)

	==Partial segmentation in unfecundated ova==		==Partial segmentation in unfecundated ova==

Z8600021: /* Segmentation in the bird's ovum */

2012-10-12T01:16:05Z

Segmentation in the bird's ovum

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	Fig. ~~195~~. — Cicatricula of the Bird's Egg.		'''Fig. 495. Cicatricula of the Bird's Egg.''' (Cicatricula - the germinating or formative point in the yolk of an egg)

	A, diagrammatic section tlirough the cicatricula of a newly laid egg ; a, vitelline membrane ; b, segmented germ disc ; c, below this the germ cavity ; d, the yolk cavity within the white yolk ; c, c, the yellow yolk substance.		A, diagrammatic section tlirough the cicatricula of a newly laid egg ; a, vitelline membrane ; b, segmented germ disc ; c, below this the germ cavity ; d, the yolk cavity within the white yolk ; c, c, the yellow yolk substance.

Z8600021: /* Segmentation of the mammals ovum */

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Segmentation of the mammals ovum

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	Fig. 494. — Ovum op the Rabbit from the Uterus. (from Kolliker after Bisclioff).		'''Fig. 494. Ovum op the Rabbit from the Uterus.''' (from Kolliker after Bisclioff).

	The whole surface of the yolk is now divided into cellular compartments. A dark spot below marks the position of a quantity of granular spheres inside the cellular elements of the blastoderm, a, the albuminous layer, now much thinned out and incorporated with the zona ; b, the cells of the outer layer of the blastoderm resulting from segmentation ; o, the spot of granular opaque spheres.		The whole surface of the yolk is now divided into cellular compartments. A dark spot below marks the position of a quantity of granular spheres inside the cellular elements of the blastoderm, a, the albuminous layer, now much thinned out and incorporated with the zona ; b, the cells of the outer layer of the blastoderm resulting from segmentation ; o, the spot of granular opaque spheres.

Z8600021: /* Segmentation of the yolk or germ */

2012-10-12T01:13:08Z

Segmentation of the yolk or germ

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	Fig. 492.— Ova ~~OF THE ILvBBrr uxcEiiGoiNa~~ Segmentation in ~~tueir~~ descent ~~THRouan THE~~ Fallopian Tube. (From Bischoff.) f A, the ovum from the middle part of the tube twelve or fifteen hours after impregnation, the germinal vesicle has disappeared, the yolk is contracted, and two hyaline globules are seen in the cavity between it and the zona ; rotation of the yolk took place in the direction of the arrows ; B, ovum a little more advanced, the first segmentation has taken place, a clear globule or nucleus is seen in both the yolk spheres : spermatozoa adhere everywhere to the zona ; C, an ovum four hours later than that shown in B, the second segmentation has taken place ; D, ovum from the lower part of the tube in which the third stage of segmentation is completed, and eight yolk spheres are formed, the albuminous covering is increased in thickness : diameter of the whole, ~~j^th~~ of an inch.		'''Fig. 492. Ova of the Rabbit undergoing Segmentation in their descent through the Fallopian Tube.''' (From Bischoff.) A, the ovum from the middle part of the tube twelve or fifteen hours after impregnation, the germinal vesicle has disappeared, the yolk is contracted, and two hyaline globules are seen in the cavity between it and the zona ; rotation of the yolk took place in the direction of the arrows ; B, ovum a little more advanced, the first segmentation has taken place, a clear globule or nucleus is seen in both the yolk spheres : spermatozoa adhere everywhere to the zona ; C, an ovum four hours later than that shown in B, the second segmentation has taken place ; D, ovum from the lower part of the tube in which the third stage of segmentation is completed, and eight yolk spheres are formed, the albuminous covering is increased in thickness : diameter of the whole, 1/100th of an inch.


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			'''Fig. 493. Ovum of the Rabbit sixty-eight hours after impregnation''' (Allen Thomson). This ovum is probably in the sixth stage of segmentation. Spermatozoa were observed within the zona, z, the zona ; a, the thick layer of albumen peculiar to the rabbit's ovum at this stage.
	~~Fig. 493.~~ Fig. 493.— Ovum of the Rabbit ~~SIXTY~~-~~EIGHT HOURS AFTER IMPREGNATION~~ (Allen Thomson). This ovum is probably in the sixth stage of segmentation. Spermatozoa were observed within the zona, z, the zona ; a, the thick layer of albumen peculiar to the rabbit's ovum at this stage.


	==Segmentation of the mammals ovum==		==Segmentation of the mammals ovum==

Z8600021: /* Disappearance of the germinal vesicle */

2012-10-12T01:10:08Z

Disappearance of the germinal vesicle

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	The most marked change in the interior of the ovum which is known to accompany its maturation and escape from the ovary is the disappearance of the germinal vesicle, — a phenomenon which occurs in all vertebrates, and in a large proportion of, but probably not all invertebrate animals. This change is iindependent of fecundation. The details of the process have not been traced in mammals, but from various observations in birds and batrachia, and more especially from the recent minute researches of Oellacher in fishes, it follows that the disappearance of the vesicle really depends on its extrusion from the substance of the yolk in which it was imbedded, and is attended with the bursting or breaking down of its delicate outer membrane ; so that when the vesicle is thrust out on the surface of the yolk, and opened out, its fluid contents must be effused in the space intervening between the vitelline membrane and the surface of the yolk. The actual expulsion of the vesicle in the trout's ovum is attributed by Oellacher to the contractions of the yolk protoplasm, and the expulsion of the vesicle in this animal takes place previous to the rupture of its membrane and dispersion of its contents. (Archiv. f. Mikroscop. Anat. vol. viii, p. 24.)		The most marked change in the interior of the ovum which is known to accompany its maturation and escape from the ovary is the disappearance of the germinal vesicle, — a phenomenon which occurs in all vertebrates, and in a large proportion of, but probably not all invertebrate animals. This change is iindependent of fecundation. The details of the process have not been traced in mammals, but from various observations in birds and batrachia, and more especially from the recent minute researches of Oellacher in fishes, it follows that the disappearance of the vesicle really depends on its extrusion from the substance of the yolk in which it was imbedded, and is attended with the bursting or breaking down of its delicate outer membrane ; so that when the vesicle is thrust out on the surface of the yolk, and opened out, its fluid contents must be effused in the space intervening between the vitelline membrane and the surface of the yolk. The actual expulsion of the vesicle in the trout's ovum is attributed by Oellacher to the contractions of the yolk protoplasm, and the expulsion of the vesicle in this animal takes place previous to the rupture of its membrane and dispersion of its contents. (Archiv. f. Mikroscop. Anat. vol. viii, p. 24.)

	~~Fig. 490. — Mature Ovarian Ovum of the guinea pig (from BischofF. ) ^so~~

	The zona ~~pelhicida~~ is hidden by the adherent ~~''A~~ , »		'''Fig. 490. Mature Ovarian Ovum of the guinea pig''' (from Bischoff).

			The zona pellicida is hidden by the adherent cells of the membrana granulosa, which have assumed a pediculated form next its surface. The finely granular yolk substance fills the cavity of the zona. The germinal vesicle has disappeared.


	~~cells of the membrana granulosa, which have assumed a pediciilated form next its surface. The finely granular yolk substance fills the cavity of the zona. The germinal vesicle has disappeared.~~

	The time at which the disappearance takes place seems to be subject to some variation. Most frequently it is close upon the time of the escape of the ovum from the Graafian follicle ; but sometimes it is several hours later, and in other instances it seems to occur previously ; and, indeed, in many cases preparatory changes in the position, form, and consistence of the vesicle have been observed while the ovum was still within the ovary.		The time at which the disappearance takes place seems to be subject to some variation. Most frequently it is close upon the time of the escape of the ovum from the Graafian follicle ; but sometimes it is several hours later, and in other instances it seems to occur previously ; and, indeed, in many cases preparatory changes in the position, form, and consistence of the vesicle have been observed while the ovum was still within the ovary.
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	As the mammiferous ovum leaves the ovary it has still adhering to its outer surface one or two layers of the cells belonging to that part of the tunica granulosa with which it was surrounded in the Graafian follicle. These cells assume towards the period of maturation more or less of a pediculated form (see fig. 490), but after one or two days they gradually fall away from the surface of the zona, and leave that membrane free in the Fallopian tube.		As the mammiferous ovum leaves the ovary it has still adhering to its outer surface one or two layers of the cells belonging to that part of the tunica granulosa with which it was surrounded in the Graafian follicle. These cells assume towards the period of maturation more or less of a pediculated form (see fig. 490), but after one or two days they gradually fall away from the surface of the zona, and leave that membrane free in the Fallopian tube.


	==Fecundation==		==Fecundation==

Z8600021: /* Fecundation */

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Fecundation

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	Fig. 491. — Ovum of the Rabbit from the Fallopian Tube, twelve hours after Impregnation (from Bischoff).		'''Fig. 491. Ovum of the Rabbit from the Fallopian Tube, twelve hours after Impregnation''' (from Bischoff).

	A few ~~gvaimlar~~ cells adhere to the outer surface of the zona, in which and in the zona itself spermatozoa are seen ; «, zona ; b, two hyaline globules witliin the cavity left by the shrinking of the yolk.		A few granular cells adhere to the outer surface of the zona, in which and in the zona itself spermatozoa are seen ; a, zona ; b, two hyaline globules witliin the cavity left by the shrinking of the yolk.

Z8600021: /* The mature ovarian ovum */

2012-10-12T01:07:18Z

The mature ovarian ovum

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	Distinction of the germ.— In the ovum of the mammal the whole yolk-substance is so uniformly of the same appearance and structure throughout that, but for the presence of the germinal vesicle, little distinction can be perceived between one part and another ; and, further, the greater part of the yolk is immediately engaged in the first or preliminary changes which precede the occurrence of embryonic development. The whole yolk-mass, or its greater part, is therefore directly formative or germinal, or, as it has been said, the ovum is hoJoblastic.		==Distinction of the germ==

			In the ovum of the mammal the whole yolk-substance is so uniformly of the same appearance and structure throughout that, but for the presence of the germinal vesicle, little distinction can be perceived between one part and another ; and, further, the greater part of the yolk is immediately engaged in the first or preliminary changes which precede the occurrence of embryonic development. The whole yolk-mass, or its greater part, is therefore directly formative or germinal, or, as it has been said, the ovum is hoJoblastic.

Z8600021: /* The mature ovarian ovum */

2012-10-12T01:06:11Z

The mature ovarian ovum

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	==The mature ovarian ovum==		==The mature ovarian ovum==

	The unfecundated ovarian ovum which is approaching maturity, and is about to be discharged from the Graafian follicle and pass into the Fallopian tube, is composed of the following parts, viz. : # The firm, almost homogeneous external vitelline membrane, which is termed ''zona pellucida'' in mammals		The unfecundated ovarian ovum which is approaching maturity, and is about to be discharged from the Graafian follicle and pass into the Fallopian tube, is composed of the following parts, viz. :
			# The firm, almost homogeneous external vitelline membrane, which is termed ''zona pellucida'' in mammals
	# The yolk substance or vitellus, a mass of soft or semifluid protoplasmic matter, involving numerous granules and oil globules, and containing embedded in it, near the surface		# The yolk substance or vitellus, a mass of soft or semifluid protoplasmic matter, involving numerous granules and oil globules, and containing embedded in it, near the surface
	# The ''germinal vesicle'', consisting of a spheroidal delicate enclosing membrane with protoplasmic fluid and fine granules within, and containing in its earlier states if not to the last		# The ''germinal vesicle'', consisting of a spheroidal delicate enclosing membrane with protoplasmic fluid and fine granules within, and containing in its earlier states if not to the last

Z8600021: /* The mature ovarian ovum */

2012-10-12T01:05:45Z

The mature ovarian ovum

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	==The mature ovarian ovum==		==The mature ovarian ovum==

	The unfecundated ovarian ovum which is approaching maturity, and is about to be discharged from the Graafian follicle and pass into the Fallopian tube, is composed of the following parts, viz. : ~~— 1.~~ The firm, almost homogeneous external vitelline membrane, which is termed zona ~~pellncida~~ in mammals ~~; 2.~~ The yolk substance or vitellus, a mass of soft or semifluid protoplasmic matter, involving numerous granules and oil globules, and containing embedded in it, near the surface ~~; 3.~~ The germinal vesicle, consisting of a spheroidal delicate enclosing membrane with protoplasmic fluid and fine granules within, and containing in its earlier states if not to the last ~~; 4.~~ The germinal ~~sjwt~~ or macula.		The unfecundated ovarian ovum which is approaching maturity, and is about to be discharged from the Graafian follicle and pass into the Fallopian tube, is composed of the following parts, viz. : # The firm, almost homogeneous external vitelline membrane, which is termed ''zona pellucida'' in mammals
			# The yolk substance or vitellus, a mass of soft or semifluid protoplasmic matter, involving numerous granules and oil globules, and containing embedded in it, near the surface
			# The ''germinal vesicle'', consisting of a spheroidal delicate enclosing membrane with protoplasmic fluid and fine granules within, and containing in its earlier states if not to the last
			# The ''germinal spot'' or ''macula''.


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	Distinction of the germ.— In the ovum of the mammal the whole yolk-substance is so uniformly of the same appearance and structure throughout that, but for the presence of the germinal vesicle, little distinction can be perceived between one part and another ; and, further, the greater part of the yolk is ~~immediate!)'~~ engaged in the first or preliminary changes which precede the occurrence of embryonic development. The whole yolk-mass, or its greater part, is therefore directly formative or germinal, or, as it has been said, the ovum is ~~hoJohJastk~~.		Distinction of the germ.— In the ovum of the mammal the whole yolk-substance is so uniformly of the same appearance and structure throughout that, but for the presence of the germinal vesicle, little distinction can be perceived between one part and another ; and, further, the greater part of the yolk is immediately engaged in the first or preliminary changes which precede the occurrence of embryonic development. The whole yolk-mass, or its greater part, is therefore directly formative or germinal, or, as it has been said, the ovum is hoJoblastic.


	But in birds and reptiles, in which the ova are comparatively large, the greater part of the yolk forming the yellow yolk substance, takes no immediate part in the first formative processes, and these are restricted to the small whitish flat disc, called, the cicatricula in the fowl's ~~Qgg~~, which is composed of fine granular protoplasm, occupies a determinate place on the surface of the larger yolk-mass, and, so long as the yolk remains in the ovary, has the germinal vesicle situated in its centre.		But in birds and reptiles, in which the ova are comparatively large, the greater part of the yolk forming the yellow yolk substance, takes no immediate part in the first formative processes, and these are restricted to the small whitish flat disc, called, the cicatricula in the fowl's egg, which is composed of fine granular protoplasm, occupies a determinate place on the surface of the larger yolk-mass, and, so long as the yolk remains in the ovary, has the germinal vesicle situated in its centre.

	Fig. 489. — Ovarian Ovum of a ~~MajiFig~~. ~~489. MIFEK. -f~~		Fig. 489. — Ovarian Ovum of a Mammifer.

	a, The entire ovum, viewed under pressure ; the granular cells have been removed from the outer surface, the germinal vesicle is seen in the yolk substance within ; h, the external coat or zona burst by increased pressure, the yolk protoplasm and the germinal vesicle having escaped from within ; c, germinal vesicle more freed from the yolk substance. In all of them the macula is to the part thus distinguishable from the rest of the yolk the name of germ may be given ; and it has also been styled the primary or germinal or formative yolk, and the protoplasm or protoblast, while the remainder of the yolk-substance has been called the nutritive or food yoUc, the secondary yolk or deutoplasm. The oviparous ovum has therefore been named merodlasfic, or partially germinal.		a, The entire ovum, viewed under pressure ; the granular cells have been removed from the outer surface, the germinal vesicle is seen in the yolk substance within ; h, the external coat or zona burst by increased pressure, the yolk protoplasm and the germinal vesicle having escaped from within ; c, germinal vesicle more freed from the yolk substance. In all of them the macula is to the part thus distinguishable from the rest of the yolk the name of germ may be given ; and it has also been styled the primary or germinal or formative yolk, and the protoplasm or protoblast, while the remainder of the yolk-substance has been called the nutritive or food yoUc, the secondary yolk or deutoplasm. The oviparous ovum has therefore been named merodlasfic, or partially germinal.