Book - Experimental Embryology (1909) 6
Embryology - 4 Oct 2026    Expand to Translate
|
|---|
| Google Translate - select your language from the list shown below (this will open a new external page) |
|
العربية | català | 中文 | 中國傳統的 | français | Deutsche | עִברִית | हिंदी | bahasa Indonesia | italiano | 日本語 | 한국어 | မြန်မာ | Pilipino | Polskie | português | ਪੰਜਾਬੀ ਦੇ | Română | русский | Español | Swahili | Svensk | ไทย | Türkçe | اردو | ייִדיש | Tiếng Việt These external translations are automated and may not be accurate. (More? About Translations) |
Jenkinson JW. Experimental Embryology. (1909) Claredon Press, Oxford.
- Jenkinson (1909): 1 Introductory | 2 Cell-Division and Growth | 3 External Factors | 4 Internal Factors | 5 Driesch’s Theories - General Conclusions | 6 Appendices
| Online Editor |
|---|
|
| Historic Disclaimer - information about historic embryology pages |
|---|
|
Appendix A
FURTHER REMARKS ON RELATION BETWEEN THE SYMMETRY OF THE EGG, THE SYMMETRY OF SEG MENTATION, AND THE SYMMETRY OF THE EMBRYO IN THE FROG.
IN the measurements, referred to above (pp. 165-8), of the angles between the plane of symmetry of the egg (as determined by the position of the grey crescent), the first furrow and the sagittal plane of the embryo, it was found (1) that there was a certain tendency for the first furrow and the sagittal plane to coincide, since in a. large number of cases small angles preponderated over large ones, the standard deviation of this angle from the mean (which was practically = 0°) being a- = 40-39° i-65 ; (2) that there was a much greater tendency for the plane of symmetry and the sagittal plane to coincide, the standard deviation of the angle between these two planes being o'=29-75° _-J; -63 ; (3) that the first furrow tended either to coincide with or to lie at right angles to the plane of symmetry, the standard deviation about 0° being 18-70° i -60, that about 90° being 23-29° j-_ -86, the value of 0' for all the observations being 47-90° 1- 1-19. The correlation between the first furrow and the sagittal plane was found to be p=-138i -031, that between the plane of symmetry
and the sagittal plane p=-372i -025, that between the plane of symmetry and the first furrow p=-O87 i -032.
These results may be tabulated as follows : rr :0 40-39° + -65. -138 i -031.
tal Plane.
Plane of Symmetry and Sagittal Plane. l 2975 -t '63’
Plane of Symmetry and First Furrow.
First Furrow and Sagit- }
.372 i .025.
} 47.9oi1.19. .os7¢_.o32.
Full details of these results will be found in a paper in Biometrika V. 1906.
For the purpose of making these measurements the eggs were placed in rows parallel to the [mat]; of glass slides, and the angles measured between the various planes and lines ruled across the slide. Such eggs compress one another by their jelly coats; further, the eggs taken ‘from the uterus were placed haphazard on the slides with the axis making any direction with the vertical. The egg takes about half-an-hour to turn into its normal position with the axis vertical, and during this interval gravity may possibly act upon the yolk and protoplasm, of different specific gravities, and impress a plane of bilateral gravitation symmetry upon the egg, as occurs when the egg is permanently inverted (see above, pp. 82-87). This obliquity of the axis may possibly afiect the relations between the planes, and the mutual compression may also be a disturbing factor, since it is known that in compressed eggs the nuclear spindle is perpendicular to the direction of the pressure (pp. 34-36).
These angles have therefore now been measured under four different conditions:
(a) The eggs are close to one another in the rows and the axis is horizontal.‘ (Since the rows are parallel to the length of the slide the pressure, if any, must be in the same direction, while the surfaces of compression or contact are across the slide. The eggs were always so placed that the vegetative poles faced in one direction and the planes of ‘ gravitation symmetry ’ were at right angles to the length of the slide. This holds good of all the following experiments.)
(/3) The eggs close, but the. axis vertical with the white pole below. In these there can be no gravitation plane of symmetry.
(y) The eggs spaced, but the axis horizontal. In these the jellies do not touch.
(6) The eggs spaced and the axis vertical. In these, therefore, both the supposedly disturbing factors are removed. The results are given in the following table :—
A B C
First Furrow and Plane of Symmetry Plane of Symmetry Sagittal Plane. and Sagittal Plane. and First Furrow.
(.7) .7 = 38-42 g._ -70. .7 = 31-86: -56. .7 = 41-591-_-84. ,7 = -201;-028. ,7 = -263;:-_-027. ,7 -= -118;-029. (.9) .7 = 33-443-_-56. .7 = 30-17:51. .7 = 39-7l_-1;-61. ,7 = 3523-021. .7 = .27si.o22. ,7 = .o23¢.o24. (-,) .7 —_- 33-49;:-_-96. .7 = 27-53¢-84. .7 = 36-60: 1-108. ,. = .292:-039. —_— -399:-036. p = .075:-043. (a) .7 = 31.45133. .7 —_— 26-80¢-82. .7 = 34-46:1-065. ,7 —_- -364:-033. ,7 = -451 1.035. ,7 -_- -186;-043.
It is evident from this that gravity and ‘ mutual compression ’ (as I will for the moment term it, though it is doubtful whether the pressure has anything at all to do with the result) do affect the
magnitude of the angles between these three planes, for in each case the standard deviation falls, while the correlation coeflicient rises, when they are both removed. It will be observed that, while gravitation (y) has less eflect than compression (3) upon the angles B and C, the reverse is the case with the angle A. We may be able to find a reason for this later on.
There is one point worth noticing. It is quite clear that gravity is not indispensable for the development of a grey crescent and plane of symmetry, though it is true that the position of this plane may be aifected by gravity even in the short interval that elapses before the egg turns over.
The values for the compressed eggs with horizontal axes (or) compare fairly well with those previously obtained, except in the case of the plane of symmetry and the first furrow. In the former series the latter tended either to coincide with or to lie at right angles to the former. In the present series this is not the case. This diflerence is probably to be attributed to the fact that many of the eggs in the first series must have been placed on the slide with the white pole upwards: possibly also the ‘ compression ’ was greater then than now.
It is fortunate that the same data enable us to study exactly the relation between the first furrow and the plane of symmetry on the one hand, and the direction of ‘compression’ and of the gravitation symmetry plane on the other. It must be remembered that these two are at right angles to one another.
Consider first the first furrow.
(a) When the eggs are close but the axis horizontal the first furrow tends to lie at right angles to the slide, that is, in the direction of compression, but at right angles to the gravitation symmetry plane. (a-=38-16 i -69.)
(fl) When the eggs are close but the axis vertical this tendency is not quite so marked. (a'=46-67 i -7' 1.)
(y) When the eggs are spaced and the axis horizontal it is still there, but slight. (o-=49-32 ;l-_ 1-40.)
(6) When the eggs are spaced and the axis vertical the direction of the first furrow is random. (zr=52-76¢ 1-17.)
VVe may conclude, therefore, that the first furrow tends to lie in the direction of the ‘ compression’ and at right angles to the plane of gravitation symmetry. The latter tendency, we know, exists in forcibly inverted eggs, together with a tendency to lie in the plane of symmetry and at 45° to it (above, p. 84). Pressure experiments alo show that division is in the direction of pressure (p. 34 sqq.).
The direction taken up by the plane of symmetry under these different circumstances is-quite distinct from that of the first furrow. It appears to be determined in the first instance by gravitation, as it usually lies in the gravitation symmetry plane. It is not, however, only so determined, for if the eggs (compressed and with axi horizontal) be allowed to develop in the light the plane of symmetry lies either in the gravitation symmetry plane, or in the direction of the incident light (parallel to the length of the slide in the experiment , while in the dark it lies only across the slide. That this secon effect is due to the light and not to the pressure is shown by the fact that it occurs when the eggs are spaced, and that it may be made to vary in position by varying the position of the slide with regard to the light. Light, therefore (ordinary daylight), as well as gravity, can help to determine the -position of the plane of symmetry, and when the latter is excluded it appears that this plane is placed either in or at right angles to the source of light.
Light appears to exert no effect u on the first furrow.
It is now intelligible why, when 1 these factors are operative, the relation between the first furrow and the planes of symmetry of egg and embryo should be disturbed, since, in the conditions of the experiment, those factors which determine the position of the former are at right angles to those on which the direction of the latter depends.
It still remains for us to inquire into the internal causes of the direction of these planes in the egg. Roux, as has been pointed out, has asserted that the grey crescent appears on the opposite side of the egg to that on which the spermatozoon has entered (pp. 80, 165), and further that the point of entry of the sperm also determines the meridian of the first furrow, since this either includes the sperm-path, or is parallel to it, or, when it is crooked, includes or is parallel to the inner portion or ‘ copulation ’ path, which is taken to represent the line of approximation of the two pronuclei; the outer part being simply the ‘ penetration’ path. Roux also arbitrarily selected a fertilization meridian (meridian of the sperm-entry), and showed that this became the ventral side (opposite the grey crescent) later on, as well as the ineridian of the first furrow (p. 248).
I have been able to accurately investigate—by means of sections-—the relation between the fertilization meridian, first furrow, and sperm-path in a number of eggs in which the direction of the symmetry plane had been previously determined, and the results of the measurements of these angles are given here. The eggs fall into two series, those which were compressed and had their axes horizontal (a), and those which were spaced and had their axes vertical, the white pole being below (6). In (a) the gravitation symmetry plane and the direction of compression were at right angles to one another, as before.
8 a Meridian of sperm entry a- = 21-02° 1-_ 1-63. o- = 31-04°: 1-34. and first furrow. p = -435 3 -074. 9 =-613 i -038.
Meridian of sperm entry 0' = 25-67° i 1-35. 0 = 41-01° 3-_ 1-78. and symmetry plane. p = -302 i -083. / P = -006 1 -061.
SP§;§§f:;l3(g;§‘}ff1,§§;“ } . .. .—. 17.94° : 1.15. o‘ = 21.47° 1 -93.
From this it is clear that there is a very close relation indeed between the point of entry of the spermatozoon and the direction of the first furrow, especially when the disturbing efiects of pressure and gravity are removed. There is, however, little relation between the sperm meridian and the plane of symmetry even under the most favourable circumstances, and when the conditiofis are not favourable the correlation is negligible. There is however (in the 6 series) a considerable correlation (p = -479 i '070) between the sperm-pat/l and the plane of symmetry. It should be remembered, however, that all these eggs were exposed to the light. From what we know of the eifect of this agent upon the direction of the symmetry plane, it would not perhaps be too hold a hazard to surmise that in darkness there would be a correlation between the sperm entrance and the plane of symmetr .
Eiien after the removal of this disturbance there remain factors which interfere with the completeness of the correlation between these planes; these must probably be looked for in the incomplete radial symmetry of certain eggs—due possibly to pressure in the uterus—and to the slight squeezings and distortions the eggs may be subjected to when they are being taken from the Frog.
It will be seen that the relation between the sperm-path and first furrow is closer than that between the latter and the sperm entrance. This is because though the furrow may be placed to one side of the entrance point, it may still be parallel to the path , or, if not to the ‘penetration ’ path then to the inner or ‘copulation ’ path, as observed by Roux. This ‘ copulation’ path is usually observed when the penetration path is turned away from the first furrow, that i, when it has not been directed towards the egg-axis.
The same data give the position of the point or of entrance with regard to the direction of ‘pressure ’ and ‘gravitation symmetry’. In the (a) series the sperm tends to enter in the direction of ‘pressure’, that is, on that side of the egg on which it is in contact with its neighbours. Hardly a single spermatozoon enters on that side of the egg on which the white pole had been turned up, and very few on the opposite side.
It is scarcely possible to suppose that either the compression of the egg or the gravitation plane brings the spermatozoa round to the side of compression, but it may be imagined that either by capillarity or by some chemotactic stimulus the spermatozoa are especially attracted to the point where the rapidly swelling coats of adjacent eggs come into contact, and that therefore fertilization is principally effected upon this side. This explains why the first furrow lies so often in this direction. The pressure may of course afiect the position of the planes in the egg later on.
When the eggs are spaced the sperm enters on any side at random. The deviation of the sperm entrance from the egg-axis (the angle between sperm-entrance radius and egg-axis) varies in the two series of observations. When the eggs are spaced and the axes vertical, the sperm enters mainly near the equator, never near the animal pole; when the eggs are compressed and the axis horizontal, usually at about 45° from the axis, though it may enter near the pole or near the equator. This difierence obviously depends on the diiference in the initial position of the eggs on the slide. The deviation has apparently very little effect on any of the planes we have been considering.
Finally, let us try and gain some conception of the mechanism by which the direction of the furrow depends on the point of sperm entry. It is apparently quite simple, for the sperm-path is directed usually towards the axis, the sperm nucleus travels along that path to meet the female nucleus, which is also in the axis, the centrosome of the sperm divides at right angles to that path, the fertilization spindle is developed between the diverging centrosomes and cell-division takes place in the equator of the spindle ; the first furrow includes therefore the sperm-path. Should, however, the ‘penetration ’ path not be exactly radial, for whatever reason, the sperm nucleus turns aside to meet the female pronucleus, there is a ‘ copulation’, as distinct from a ‘ penetration’ path, the centrosome divides at right angles to the former, and this, then, is included in or parallel to the plane of the furrow. In those cases in which the sperm-path is parallel to the furrow it is always quite close to it, and we may suppose perhaps that the first division "has not been quite equal. (The division of the centrosomes has not, I believe, been observed in the Frog, and the foregoing description has been taken from the Axolotl. In this genus the definitive centrosome is formed from the sperm nucleus, when the latter has already penetrated some little way into the egg.) .
The causes of the formation of the grey crescent which marks the symmetry plane are not so clear.
Roux describes it as being due to the immigration of superficial pigment. Now we have strong reason for believing that
both the entrance-funnel——produced when the spermatozoon first
touches the egg-—and the sperm-sphere are local aggregations of
watery substance. The accumulation of what appears to be a
more watery substance about the middle piece which has been
observed in the Axolotl,appears also to occur in the Frog: at least
the same formation of large clear vacuoles in the sperm-sphere may
be seen in the latter as in the former. Should this be actually so,
we may suppose that the streaming movement centred in the
entrance-funnel and sperm-sphere is responsible for drawing away
the pigment from a certain region of the surface; hence the grey
crescent. The sperm-sphere is on the inner side of the sperm
nucleus: hence the grey crescent would appear on that side of
the egg which is opposite to the entrance of the spermatozoon,
should no disturbance of the streaming movement have taken
place, and, since the sperm-path is radial, would be symmetrically
disposed with regard to it. In this case, fertilization meridian,
sperm-path, grey crescent and plane of symmetry, first furrow,
and, later on, sagittal plane, would all coincide. There is, as
we have seen, a very fair correlation between the sperm-entrance
and the first furrow, and again between the sperm-path and the
grey crescent. But should some other streaming movement of
the cytoplasm be set up by the gravitation of the heavy yolk
particles, or by pressure, or by light, then the relation between
the two processes, the division of the centrosome which determines the direction of the first furrow, on the one hand, and
on the other, the streaming movement towards the sperm-sphere
which determines the position of the grey crescent, would be
disturbed, and while the entrance point of the sperm might still
continue to determine, though not so completely, the position of
the furrow, it might come to be without relation to the symmetry
of the egg and of the embryo; and this is what is actually
observed.
Though it is diflicult to assign the exact cause of each and every deviation from the rule, this much is certain, that however they may coincide in ‘typical’ development (I use R0ux’s expression), the factors which determine cell-division, and those which determine differentiation, may be influenced by different external causes in widely diifering ways, and are therefore presumably distinct. Nor does this artificial separation of the two processes in any wise prejudice the complete normality of the development of the embryo".
Lillie has shown (Jozmz. Esp. Z002. iii. 1906) that in the egg of
C’/Iaetopterus there are granules of difierent kinds which pass, in
segmentation, into definite cells. By means of the centrifuge
some of these--the endoplasmic—-may be driven to one side of
the egg, but in whatever position these organ-forming granules
may be thus artificially placed, the cleavage has the same relation
to the egg axis (as determined by the polar bodies) as in the
normal egg. The factors of cell-division are thus separable from
those of differentiation.
To the cases quoted in the summary on pp. 245, 246 might be added the various instances in which an egg may be made, by heat or pressure or shaking, or in artificial parthenogenesis, to segment abnormally and yet give rise to a normal larva.
