Book - Experimental Embryology (1909) 2
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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
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Chapter II Cell-Division and Growth
IN a future chapter we shall see that there is no necessary connexion between segmentation and differentiation. Nevertheless, since ccl1~division is the first sign, or almost the first sign, that a developing organism gives of its activity; since, moreover, cell-division accompanies the later processes of growth and differentiation, we may briefly discuss what is known of those factors which determine the direction of division in general, and in particular the pattern of segmentation.
We shall first presume that segmenting ova may be grouped under several distinct types, as follows :—
1. Tim radial I_1//2e. Here the first division is meridional, the second meridional and at right angles to the first, the third equatorial—or more often latitudinal—-and at right angles to both the preceding, the fourth meridional and at forty-ﬁve degrees to the first two, the fifth latitudinal. What is characteristic above all of this type is, first, that four surfaces of contact between cells meet in one line; for example, the four surfaees between the first four blastomeres meet in the egg-axis, while each pair of animal cells lies exactly over each pair of vegetative cells after the third division; and secondly, the blastomeres are radially arranged about the axis. This type has been observed in Sponges (Syconl (Sehulze)), in Coelenterates, in Crinoids, Holothurians, and Echinoids (Fig. 12) amongst the Echinoderms, in Eetoproctous Polyzoa, in A111];/ziowz/.e and the
Vertebrates, and in some C1'11stacea—Cc/or/zilzts ((:‘rrobben),’
Luc-g'f'er (Brooks), Cyclops (Hiickcr), L’/‘awe/u'p11.v (Braucr) and some Cirrhipedes. Certain of these cases present special
peculiarities. In Echinoids micromeres are formed at the vegetative pole
by the division of the fourth phase (Fig. 12,
' In 831001: the third cleavage is meridional, the fourth latitudinal. FIG. 12.-Normal development of the sea-urcliin SI;'ong3/Ionenlrolus ”l’i4Il(.\‘. (After Boveri, 1901.)
The animal pole is uppermost in all cases, and in the first two figures the jelly with the canal (micropyle) is shown.
a, primary oocytc, the pigment is uniformly peripheral.
b, ovum after extrusion of polar bodies. The pigment now forms a subequatorial band. The nucleus is ex-axial.
c, d, ﬁrst division (meridional).
e, 8 cells, the pigment almost wholly in the vegetative blastomeres.
f, formation of mesomeres (animal cells) by meridional division: the vegetative cells have divided into macromeres and micromeres.
_r/, blastula. h, mesenehyme hlastula.
1', j, k, imagination of the pigmented cells to form the archenteron of the gastrula. In j the primary mesenchyme is separated into two groups, in each of which, in Ir, a spicule has been secreted. In is the secondary, pigmented mesenchyme is being budded off from the inner end of the archenteron.
In Asteroids and Ophiuroids the division is at first tetrahedral, and to be classed, therefore, with those of the following type; after the second furrow, however, the blastomeres are rearranged, and division theneeforward is radial.
In Vertebiata segmentation is altered in megaleeithal eggs by the amount of yolk present. It becomes meroblastic; still the radial type is preserved, though the sequence of the furrows is often altered, the third, for instance, being frequently meridional, and the fourth latitudinal. Amongst the Ascidians Pg/rosoma has a large-yolked, telolecithal, and radially segmenting egg. In the Placental Mammals the first two (livisions may conform to this type; but segmentation soon becomes irregular. The accumulation of yolk in the Arthropod egg has resulted in a totally different type of meroblastic segmentation. The yolk is here uniformly distributed about the central protoplasm. In the latter is placed the segmentation nucleus, and this central mass divides into a number of cells, which subsequently migrate to the surface and form a blastoderm ; the egg is then centrolecithal (Fig. 1). The stages of the development of this modiﬁcation may be seen in the Crustaeea. In certain forms-——thosc alluded to above (with the exception of the (‘irrhipedes)—— division is holoblastic and radial. In (fam/mru.\', Branc/zipux (Brauer), Pellogastcr (Smith) segmentation is at_first total, but the inner yolk-containing ends of the cells subsequently fuse. In C’1'a7z.r/‘on, ]l[0i/ca, I)aj)/mella, ])a[//erzia, Ore/wstia segmentation is superﬁcial. In Isopods and in Deeapods segmentation is internal. In all cases the result in the end is the same, a peripheral blastoderm, a central yolk. But the blastoderm is not always, though it is often, formed simultaneously over the whole surface. There are cases in which it appears first on the ventral side, and by what may be described as a still more precocious formation of the blastoderm, segmentation may begin at this, the future ventral, point, as in Jlfysis and 0m'scu.9. In these cases the egg is teloleeithal.
In the Insects, Arachnids, Myriapods, and Perz';)atus nomezealandiae, the segmentation is meroblastic and the egg comes to be centrolceithal. In Peripatzw capevms and in some other species it would appear that the yolk has been secondarily lost. II. I CELL-DIVISION 25
2. The‘ second type is the so-called spiral form of cleavage (Fig. 13). This is especially characteristic of the eggs of Polyclads, Nemert-ines, Molluscs (except Cephalopods), Annelids, and Sipunculoids (P//((800/(I-_\'0I1l(I). The peculiarity of this mode of division is that after the ’r'our-celled stage the blastomeres usually known as the 1nacromeres—give elf ‘quartettes’ of micromeres towards the animal pole, the first quartette being given off’ dexiotropically (except in cases oi.’ reversed cleavage),
FIG. 13.— Diagram of a ‘ spi1'a‘-Jy ’ segmenting egg in the 16-cell stage. 2 A-2 D macromeres; 2 «:2 d nueromeres of second quartette; ] n 1, 1 n LL Id 1, 1 cl 2 nncronieres of first quartettc.
the second laeotropically, and so on in regular alternation, until four quartcttes have been produced. The cells of each quartette divide meanwhile in conformity with the same law of alternation of direction of cleavage. The direction of division is thus always oblique to the egg-axis, and this ()l’)ll(lﬂll]_Y can be observed in the division of the first two blastomcres, the result of which is that of the two sister cells A and B A is nearer to the animal pole than B, while in the other pair C is nearer that pole than D; A being to the left of B and C to the left of I) (to an observer standing in the axis with his head to the animal pole), the division is laeotropie. The arrangement of cells approaches the tetrahedral, especially when, as occurs very frequently, A and C are united by a cross, or polar, furrow above, B and D by a polar furrow at the vegetative pole, as in JVc=rci.v, Icﬁnoc/zitou, Limaw, Plano)‘/153, Lejii/Ionolus, Dixcocelis, and others. In Univ, however, it is B and D that are in contact at the animal, A and C at the vegetative, pole. In other cases
(I/yauassu, Capi/cl/a, Umbra!/a, Clmyiirlzz/a, Amp/u'6m'te, A/‘euicolu, for instance) the furrows are parallel, the same two blastomercs, B and D, being in contact at both poles. In Troc/ms both the ‘parallel furrows’ and the ‘ crossed furrows’ conditions are found. A similar disposition is to be observed amongst the mieromeres of the ﬁrst quartette. These mieromeres, also, alternate with the macromeres. Not more than three contact surfaces between blastomeres, therefore, meet in one line.
The eggs of certain Lamellibranchs—-Ykrerio, Cg/clas—i11 which the ‘spiral’ arrangement is obscured by the large size of the D maeromere, and possibly the ova of the Rotifera, are to be referred to this type.
The tetrahedral arrangement of the first four cells is conspicuous in Asteroids and Ophiuroids, where the planes of division of the first two cells are at right angles to one another. Before the next division, however, the cells shift their positions and come to lie in one plane, in which, however, the sister cells are not adjacent, but opposite, to one another.
The eggs of Amp/u'0.mzs sometimes segment spirally (Wilson).
After the completion of the spiral period of division, segmentation beeomes radial and then bilateral.
3. The third type of cleavage is the bilateral. The first two divisions intersect in the axis; the next. may he equatorial, as in Aseidi-ans. In this case the hilaterality becomes evident in the succeeding phase, in which the divisions in two adjacent cells of the animal hemisphere meet the first furrow, while in the other two they meet the second. The bilaterality is marked in the reverse way in the vegetative half of the egg. The egg is thus divided into what will be anterior and posterior, dorsal and ventral, and right and left halves. In future divisions the bilateral symmetry is retained.
The egg of ./Imp/ez'o.avzz.s' may divide in this fashion (Wilson), and this is the normal method, according to Roux, in Ram: esculwz/a.
In the Teleostei and some Ganoids (Lepz'do.w‘ws) the bilaterality becomes evident in the third division, which is parallel to the first, the fourth being parallel to the second division. The egg is in fact iso-bilateral.
The Ctenophore egg also possesses two planes of symmetry, for the third division is meridional and unequal in such a manner that the next stage~—eig11t cells—is composed of two opposite pairs of small and two opposite pairs of large cells.
The mesomeres in the sixteen—celled stage of Ecliinoids are bilaterally arranged.
In the Cephalopoda the egg is large-yolked, and segmentation consequently meroblastic. After the ﬁrsttwo meridional (livisions
V‘ I1.’ I 01' 1’ Flo. 14.—-Tln-ee segmentation stages in the blastoderm of S'I—'pi(I o_)_7i 4-inalis; the segmentation is of the bilateral type. I, left; 2-, right; I— V,
first to ﬁfth cleavages. The top sides of the ﬁg111‘es are anterior. (After Vialleton, from Korschelt and Heider.)
the bilateral disposition sets in, for the furrows of the third phase are unequally inclined to the first furrow in two halves—— the future anterior and posterior halves—oE the egg (Fig. 14). The egg of A.s'aa7'2's megalocep/m/a also exhibits a bilateral cleavage, but not on the plan just described. The iirst division is equatorial. Then the animal cell divides meridionally, and, as it will prove, transversely, the vegetative cell latitudinally.
Before the next division the most vegetative cell (P._,) slips round to what will be the posterior side. All four cells are bilaterally arranged about the plane in which they all lie, and this will become the sagittal plane of the embryo. The anterior and posterior ends, and therewith the right and left sides, are likewise now determined. The bilateral symmetry is preserved in future divisions, at least in the vegetative hemisphere ; in the animal part of the egg the blastomeres of the left side become t.ilted forwards, those of the right side backwards (Fig. 155, p. 255).
4. In the Triclad Turbellarians, in Trematoda and Cestoda, segmentation is irregular, the blastomeres separate from one another and lie amongst the yolk-cells. The same phenomenon may be witnessed in the Salps, and the separation and subsequent reunion of the blastomeres has also been described in Coelenterates and in Asteroids.
Although these types of segmentation are distinct enough from one another, intermediate conditions are readily found. The radial easily passes into the spiral type for example, for in many eggs of the former kind the ‘cross furrows ’ have been observed at either one or both poles, while the animal blastomercs may be rotated slightly on the vegetative, and so lie not over, but in between, them. The radial symmetry again may become bilateral, as when the meridional furrows of the fourth phase, instead of passing through the animal pole, meet the first or second furrow, symmetrically on either side of one of these divisions; this occurs as a variation in I?ana_/'/mew and (normally (Roux)) in Ifamz excz/Zr’):/rt.
In ()phiuroids and Asteroids the tetrahedral arrangement is lost, and the egg segments radially. In Amp/riuamr: all three types occur.
All three forms may therefore have been derived from one, though what that one was we do not know. In any case, however, one feature is common to them all; in all cases successive divisions are at right angles to one another. This is the law formulated by Sachs long since for the divisions of the cells of plants. It holds good for the segmenting animal ovum, though exceptions may, of course, be found. The alternation of dexiotropic and laeotropic divisions, for instance, in spirally segmenting ova continues for a long period with striking regularity, and it is comparatively rare for a cell to disobey the rule. The rule is, however, no universal law of cell—division. Every embryologist will recollect the continued division of a teloblast in the same direction to form a germ-band, which is such a coilspicuous fact in the development of Molluscs, Annelids, and Arthropods. The four polar nuclei of Insect eggs, lying in one straight line, may also be cited.
The direction of division and the size of the blasfomeres are not, however, the only factors which determine the actual pattern of segmentation. The cells can, and do, shift their positions on one another. This is of common occurrence, and a few examples will suﬂice. The rearrangement of the tetrahedrally disposed cells in Asteroids and Ophiuroids has been ‘noticed already. In many ‘spiral’ ova the micromeres have been observed to rotate on the macromeres, or one quartette to be pushed out of position by the cells of another. In z1.vca;'is the cell P._, slips to one side. 14‘urther, cells change their shape.
Two factors are therefore involved in the production of the pattern of cleavage, the direction of division, and the movements of the cells, and these factors in their turn demand explanation. To these must be added the shape, the size, and the rate of division of the cells.
The two latter depend very largely upon the amount of yolk present in the egg ; yolk-cells are large, the yolk divides slowly, or not at all. This was expressed long sinee by Balfour in the formula, ‘ The Velocity of segn'1enta,tiQn in ‘ FIG. I5.—' Segnientation Oi‘ the
, . Lro0“s one under the influence any Part 01 the ovum 15) roughly of atbeentliijfiigal force (from Korspeaking, proportional to the eon- sc_he1t and Heider, after 0. Hertcentration of the protoplasm there ; ;l(')1(’l'2l'_l111 gzﬁlcolilmllsitvsijl 3131: and the size of the segments is (yolk-syneytiuin) 2 I.-h,bln.stocoel; inversely proportional to the eon- ”” y°lk'““°1°17 "’ y 011"’ centration of the protoplasm.’ ‘ The rule has been vindicated by (). I-[ertwig experimentally. If the egg of the Frog be centri ‘ ("omp. Emb. i. C. 3. 30 CELL-DIVISION AND GROWTH II. I
fugalized with suﬂicient force the yolk is driven still more towards the vegetative pole, while the protoplasm is accumulated in the animal half of the egg. Such eggs segment meroblastically, a cap of cells or blastoderm being formed lying on the surface of a nucleated but undivided yolk. The yolk-nuclei, moreover, are enlarged, as in megalecithal ﬁsh eggs (Fig. 15).
The rule is, of course, only applicable to telolecithal eggs, and for many of these it holds good, notably for Vertebrates. In other classes there are, however, exceptions, which are best known in those \vhose segmentation has been most carefully studied, the ‘ spiral’ eggs of Turbellarians, Annelids, and Molluscs. Large cells, in these ova, often divide more quickly than small ones; the second quartette of micromeres, for instance, is formed before the first quartette divides in 07-epitlula, Uuio, Limaw, Troc/ms, Aplg/sia r/epilans, Discocelis, and the cells of the third quartette before the first products of division have had time to divide again in L[maa', Umbrella, and A/2/ysia limacina. 411 is often formed before the corresponding cells in the other quadrants (in Iﬁzio, for example), but in Crepirlula this is in accordance with the rule, since 4 a, 4 /2, and 40 contain more yolk than 4« cl. In Arem'coZa, though the yolk is uniformly distributed, the cells are still unequal. Other exceptions are to be found in the continued unequal division of teloblasts, in the formation of the micromeres in Echinoids, and in the unequal division of the blastomeres in the third and fourth phases in Ctenophors. According to Ziegler the formation of the micromeres in Ctenophors cannot be due to the presence of yolk, since they are still formed when part of the vegetative hemisphere is removed, as Drieseh and Morgan have also found.
Ziegler indeed puts forward another hypothesis to account for unequal division; he supposes that the centrosomes are heterodynamic. So far there appears to be little evidence in support of this view. It is quite true that in many cases of unequal division the asters—not the centrosomes——-vary in size with the size of the cells. This occurs, for instance, in the division of the first micromeres and of the first somatoblast in Nerei.9, in the formation of the ﬁrst and second quartettes, and in the division of the ﬁrst somatoblast in Uuio, in the division of the cell CD in /laplauc/ma, and in the division of the pole cells of Annelids (Wilson and Vejdovsky). It is doubtful, however, whether it is not the inequality in the cells that is responsible for the inequality of the asters, there being more room in a large cell for the outgrowth of the astral rays. At any rate, ‘there are many cases of unequal clcavage——in polar body formation~——where the asters are of the same size. Until evidence is brought forward of a difference in the size of the centrosomes the hypothesis is no more than a conjecture.
Before quitting this subject we should refer to a rule which Zur Strassen has found to hold good for the rate of segmentation of Ascaris megalocep/zala. The cells do not all divide at the same rate, but in certain groups of cells division is found to occur simultaneously. These cells are related, descended from some one cell, and the more nearly related the cells are, the more nearly together do they divide. Coincidence in time of division depends therefore on the degree of cell-relationship.
The direction of division of the cell depends upon that of the nucleus, since, speaking generally, the division occurs in the equatorial plane of the spindle, or, in other words, the plane of division is at right angles to the direction of elongation of the spindle or separation of the ccntrosomes. The latter again depends on the relation between the nuclear spindle and centresomes on the one hand, and on the other the cytoplasm and its contents, more particularly the yolk. The relation between the (resting) nucleus and the cytoplasm has been expressed by O. Hertwig in the following empirical rule: ‘The nucleus always seeks to place itself in the centre of its sphere of activity.’ The sphere of its activity being not the inert yolk but the cytoplasm, we ﬁnd, in accordance with this rule, that the nucleus places itself in the centre of the egg where the yolk is uniformly distributed (isoleeithal), nearer the animal pole but still in the axis where the yolk is on one side (telolecithal). Examples of the former condition are to be found in Eehinoids (the fertilization nucleus is nearly, but not quite, central) and large-yolked Arthropod ova, of the latter in the eggs of Vertebrates, Molluscs, and many others. The nucleus, however, may wander from this position, as occurs, for instance, in the egg of Eehinoids after the expulsion of the polar bodies and before fertilization. Apart from such exceptions, due very likely to some temporary alteration in the relations of yolk and cytoplasm, the rule is a reliable one.
The relation between the dividing nucleus, the spindle and centrosomes and the cytoplasm has been stated by O. Hertwig in his second empirical rule ‘that the two poles of the division ﬁgure come to lie in the direction of the greatest protoplasmic mass ', by Pﬂiiger in the l'o1-mula, ‘the dividing nucleus elongatcs in the direction of least resistance.’
The objection that has been urged to this latter expression, that in a fluid the pressure is equal in all directions, may be set aside. For though the cytoplasm is fluid it is an extremely viscid ﬂuid, and the presence of the suspended yolk granules
FIG. 16. — Diagram oi the segmentation of the Frog's egg (after 0. llertwig, from Korschclt and Heider). A, first (meridional); B, third (latitudinal) phase of segmentation; p, superﬁcial pigment of animal hemisphere; pr, protoplasm; y, yolk; sp, spindle. must certainly oﬂ'er a greater resistance than the ﬂuid itself, and greater in proportion to their number and size. Pﬂiigcr’s formula, therefore, if not merely a truism, resolves itself into a restatement of Hertwig-’s rule. This rule certainly holds good for a large number of cases, for it explains, for instance, the two ﬁrst meridional divisions of all spherical telolecithal and radially segmenting eggs, the third, latitudinal (in smallyolked eggs 1), and possibly also subsequent meridional and latitudinal divisions (Fig. 16). It will not, however, in the present state of our knowledge, explain the obliquity of the spindles to the egg-axis in spirally dividing ova, nor cases of bilateral division ;
1 ln Sycon the third is meridional, the fourth latitudinal. II. I CELL-DIVISION 33
here, it is evident, other factors must come into play, in the second case probably a bilateral symmetry in the constitution of the cytoplasm. These exceptions may, however, ultimately prove to be special cases of Hertwig’s rule.
A very striking conﬁrmation of the rule is to be found in the division of the egg of Arcane m7_//roreuoxa (Figs. 17, 18). The egg of this worm is ellipsoid. At one end (that turned towards the upper end of the ovary) the polar bodies are extruded, and here the female pronucleus is placed. The spcrmatozoon enters at the opposite end. The line of union of the two pronuclei therefore lies in the long axis of the egg. Nevertheless the fertilization spindle is not formed in the minor axis of the ellipsoid as one might expect. The two pronuclei rotate together through 90°, the spindle is developed, as usual, in a direction at right angles to their line of union, that is to say the axis of the spindle lies in the major axis of the egg, and the rule is conﬁrmed. There is a similar rotation of the fertilization spindle in the egg of another Nematode, Diployaster (Ziegler), and in the Rotifer Airplane/ma the spindle, at ﬁrst oblique, becomes later coincident with the long axis of the ovum (Jennings).
FIG. 17.- Four stages in the fertilization of the egg of A.»-z-(n-is m'_qroveuosa. (After Auerbach, from Korschelt and Heider.)
Fig. 18.- Tl1ree diagrams of the rotation of the fertilization spindle in the egg of Ascaris nigrovenosa. e, s, the directions in which the female and male pronuclei approached one another in A; 1, 2, 3, successive positions of the spindle. (From Korschelt and Heider, after 0. Hertwig.)
Curiously enough, this rotation of the pronuclei does not occur in another ellipsoid egg, that of the Rotifer Callidiua. According to Zelinka, the polar body is formed at one end of the long axis, but the fertilization spindle lies in the minor axis, the ﬁrst division includes the major axis, and the law is disobeyed. After the division, however, the cells rotate, and the plane of contact is then, as in Ascaris m'groveuo.m, transverse.
Again, all polar divisions violate the rule, as also does the ﬁrst division of the fertilized egg of Away-is megalocep/zala, and the division of the cells of the germ-bands of Crustacca parallel to their length (Bergh).
On the other hand, Ilertwig has brought forward experimental evidence in support of his generalization. In the eggs of the Frog the directions of some of the divisions were altered by compression between glass plates. The eggs were just allowed to assume their normal position with the axis vertical. They were then placed between glass plates and compressed.
In the first series of experiments the plates were horizontal. In such eggs the ﬁrst furrow was meridional and vertical, the second meridional and vertical and at right angles to the first. So far, therefore, division was as in the normal egg. In the third division the furrows were, however, not latitudinal and horizontal, but nearly vertical, being parallel to the first furrow above, to the second furrow below. The surface of contact, therefore, formed by the furrows of this phase must pass through a meridional position in the interior of the egg. The fourth furrows are latitudinal. Born has repeated the experiment and confirmed this result (Fig. 19). He adds, however, that the furrows of the third division pass II. I CELL-DIVISION 35
towards the vegetative pole below, or may even remain parallel to the first furrow throughout. The fourth furrows, Born says, are parallel to the second. I have myself observed that this division may be either parallel to the second, or latitudinal, even in different quadrants of the same egg (Fig. 20). It will be observed that the quadrant in which the third furrow is latitudinal is smaller than the others. It is of great interest to observe the striking similarity between the direction of the third and fourth furrows in these eggs and the corresponding divisions in the Teleostean egg where the blastodisc is compressed by the chorion.
FIG. 19.—- Segmentation of the Frog's egg under pressure. The compression is in the direction of tlie axis. A. v1ew of the egg between horizontal plates; the animal part IS
islraxled. II, (I, I), first (1), second (2), third (3), and fourth (4) divisions as seen from the animal pole. (Alter Born, from Korsehelt and lleider )
FIG. 20. — The first four divisions (I, II, III, IV) in a F10g’s egg compressed between horizontal plates in the direction of the axis. The third furrow is more or less meridional and vertical in three quadrants, horizontal in the fourth, and this a smaller quadrant. 'l‘he fourth furrow is meridional in this quadrant, horizontal in the remaining three.
In the second series of experiments made by Hertwig the glass plates were vertical, the eggs, therefore, compressed not, as before, in, but at right angles to, the axis.
The first furrow was meridional, and therefore vertical, and at right angles to the plates. The second was latitudinal and horizontal, and also at right angles to the plates. The furrows of the third phase were parallel to the first, those of the fourth, in the four upper animal cells, parallel to the plates. Born again
F1G.2l. — Segmentation of the Frog's egg under p1'ess1u'e. The pressure is at right angles to the axis.
A, view of the colnpressed egg. The piglnented animal portion is shaded.
If, C, 1), views of the egg from the animal pole after the first (1), the second (2), and the third (3) divisions.
E, 11‘, 0, views of the egg from the compressed side after the first (1), the second (2), the third (3), and the fourth (4) divisions. The ﬁrst furrow may pass as (1') in E.
(From Korschelt and Heider, after Born.)
conﬁrms this account (Fig. 21). The direction of the furrows of the third phase is, however, variable ; it may be not parallel to the first, but perpendicular to it. In this case it may be parallel to the second, or so oblique to it as to become nearly parallel to the glass plates. The direction of the fourth division depends on that of the third, to which it is at right angles. It may, therefore, be either oblique and nearly parallel to the plates, as described by Hertwig, or parallel to the second furrow and perpendicular to the plates. In a third series of experiments I-Iertwig placed the plates obliquely, at 45°. In these eggs the yolk sinks slightly from the upper to the lower side, while the cytoplasm rises in the opposite direction; in other words, a bilateral symmetry is conferred upon the egg by the combined action of pressure and gravity. The plane of this symmetry is midway between and parallel to the plates. The first furrow is at right angles to the plates and to the plane of symmetry.
VVe are indebted to Drieseh for a similar series of experiments on Echinoderm eggs. Drieseh compressed the eggs of 15?:/ri/u/.s~ under a cover-glass supported by a bristle. The direction of the egg—axis with regard to the pressure was not known, but the
FIG. 22.——I9'rIu')ms: segmentation under pressure.
(1, preparation for third division (radial); b, preparation for fourth
division (tangential); 11', after fourth division; 0, another form of the 8-cell stage (third division pa1'allcl to ﬁrst); «I, the same after removal of the pressure. (After Driesch, 1893.) Echinoid egg is nearly isolecithal. VVhen the egg membrane remained intact the ﬁrst two furrows were vertical, that is, in the direction of the pressure, since the slide and cover-glass were horizontal, and at right angles to one another.
The spindles for the next division are again horizontal, and usually tangential, sometimes, however, radial. The eight-celled stage consists, therel'ore,o[’ a ﬂat plate of cells. At the next division the formation of mieromeres —which would ordinarily occur at this moment——is suppressed; the spindles are horizontal and radial, the furrows, therefore, vertical and tangential (Fig. 22 a, /1, /2’).
In certain cases cell-formation is wholly or partially suppressed. When the pressure is less (in those eggs which lie nearer the bristle) the micromeres may be, but generally are not, formed. The spindles are no longer horizontal. Similar results are obtained when the eggs are released from strong compression.
In another experiment the eggs were first deprived of their membranes. The ﬁrst and second furrows are vertical and generally at right angles to one another. Sometimes,-however, 38 CELL-DIVISION AND GROWTII II. I
the second is parallel to the first, or one blastomere may lie apart from the other three. Should the eggs be now released from the pressure, each blastomere becomes rounded off, and——after two more cleavages—-the sixteen-celled stage consists of two plates of eight eells lying over one another. But if the pressure is maintained, the spindles are horizontal and the blastomeres lie all, or nearly all, in one plane (Fig. 22 c, (7).
Fig. 23.--Segnieiitatioii of the egg of E(‘7li1lII8 micI'oh¢bcrcuIulus under pressure. (After Ziegler, 1894.)
(V, 8 cells in one plane; 1;, 16 cells, the last division having been tangential ; c, (I, 16-32 cells: the direction of the spindles in c is shown by the line: it is in the greatest length of each cell; c, 64 cells: a cross signiﬁes a vertical or oblique division, a line a horizontal division.
Ziegler has followed the segmentation of the compressed eggs a step further (Fig. 23). As the figures show, the first two divisions are at right angles to one another, while the furrows of the next two phases are, roughly, parallel to the first and second. In the next division-—sixteen to thirty-two cells—the outer cells divide radially, the inner more or less tangentially, these divisions being, like the previous ones, at right angles to the compressing plates. In the following phase, some cells (those marked with a line) still divide in the same direction as before; but in others (distinguished by a cross) the spindle is perpendicular to the II.
plates and the division horizontal. Ziegler points out that, in the former cases, the cells have greater dimensions in the horizontal plane than in the la.tter. This, however, may be the eﬁeet, not the cause, of the direction of the spindle-axis.
Two other pressure experiments maybe mentioned here. In Nereis Wilson produced a ﬂat plate of eight equal cells by applying pressure in the direction of the axis. The formation of the first quartette of micromeres was thus suppressed. On relieving the pressure eight micromcres were formed. For the Ctenophora (]ierb'z') Ziegler has shown that the normal inequality of the third and fourth divisions is not altered by pressure.‘
The foregoing experiments all agree in demonstrating the perfectly deﬁnite eft'cct produced by pressure upon the segmenting cgg. The nuclear spindles place themselves at right angles to the direction of pressure, the divisions fall at right angles to the compressing plates. This holds good for the ﬁrst three or four divisions, at least, and sometimes for later phases still. In all these cases, therefore, the nuclear spindle elongates in a direction of least resistance, and, in the normal uncompressed egg, we may argue, with Ilertwig, the least resistance is offered by the greatest protoplasmic mass.
Even in the compressed eggs, however, the greatest extension of the protoplasm, or the least extent of the yolk, is a factor which must in some cases come into play. When the egg of the Frog is compressed between vertical plates, the nuclear spindle does not elongate in any direction at right angles to the pressure, but in one only, a horizontal ; and this is the direction of the greatest protoplasmic mass, since the egg-axis is vertical.
Speaking generally, therefore, experiment has upheld Hertwig’s contention that the direction of nuclear division, and therefore of cell-division, is determined by the relation between the nucleus with its centrosomes and the cytoplasm with its yolk.
There are one or two experiments which do not support Hertwig’s view. Boveri stretched the eggs of the sea.-urchin St7'o2z_9;yloceul/'0!/1.9 in the direction of the axis. The fertilization spindle lay in the usual equatorial position, occupied, that is, the minor axis of the ellipsoid.
‘ I have recently had occasion to notice that when the egg of Anfedm is compressed in the direction of the axis the third division is meridional instead of latitudinal.
Again, Roux observed that Frogs’ eggs sucked up into a tube with a narrow bore became elongated either parallel or transverse to the length of the tube, the axis of the egg lying in each case lengthways. In the first case the division was at right angles to, in the second usually parallel to, the tube in accordance with the rule; but exceptionally, in the transversely stretched eggs, the division was not perpendicular to, but coincided with, the extent of the greatest protoplasmic mass.
However important a factor the disposition of the yolk may thus be in deciding the direction of cell-division, it is certainly not the only factor. In the eggs pressed between horizontal plates there are many—an inﬁnite number—of' directions of least resistance. In one of these the segmentation spindle elongates, and at right angles to this the first furrow falls. This is probably determined-——-as it is determined in the normal Frog and Sea-urchin egg-——by the point of entrance of the spermatozoon, or at least by the direction of the sperm-path in the egg. The second division is at right angles to the first, and here the direction may very possibly be decided on lIertwig’s principle. But why, in the next phase, should the furrows be at right angles to the second rather than to the first, for the extent of the protoplasmic mass is as great in each of the four cells, in a direction parallel to the first as to the second furrow? Here, it is clear, some other reason must be found for this succession of divisions at right angles to one another. The cause is probably to be sought for in the direction of division of the centrosomes ; for these divide——frequently soon after the telophase—at right angles to the axis of the previous ﬁgure. VVe thus gain a new expression for Sachs’ Law.
The original direction of divergence of the centrosomes is, however, by no means always the ultimate one, for the growing spindle may be twisted out of its original position. Conklin has made a careful study of this phenomenon in C’)'¢j[Ii(]l(ltl, in which egg he ﬁnds that vortical movements are set up in the cytoplasm by the escape of nuclear sap at the beginning of mitosis. The movements are in opposite directions in sister cells, centre in the spindle poles, and often carry both nucleus and spindle into a fresh place. These currents, which had been noticed previously by other observers (by Mark in Limam and by Iijima in Ale]//zelis), may thus play an important part in the production of the cell pattern. We shall see elsewhere that they, and other protoplasmic movements, are also of the very greatest signiﬁcance in diﬁerentiation.
There remains now to be noticed another principle, which is especially applicable to plant-cells with ﬁxed walls, though it may possibly be used for the phenomena of animal segmentation as well. Berthold has pointed out that when a newly formed cell-wall places itself perpendicular to the previously existing walls it is——at least in a good many instances—simply obeying tlie laws of capillarity, it merely conforms to the principle of least surfaces formulated by Plateau. This principle is as follows: ‘ Homogeneous systems of fluid lamellae so arrange themselves, the individual lamellac adopt a curvature such that the sum of the (external) surfaces of all is under the given conditions a minimum.’
A fluid lamella, of soap solution, for example, placed across the interior of a hollow, rigid cylinder, or parallelepiped, or cube, is, with the ﬁlm coating the internal surface of the vessel -in which it lies, a special case of such a system of lamellae, and, in obedience to the principle, the lamella places itself at right angles to the walls of the cavity and transverse to the long axis.
In the ease of the plant-cell, the cell-plate, formed by solidiﬁcation of the spindle fibres in the equator of the mitotic ﬁgiii'e, represents the soap-lamella, and like the latter in its parallelepiped, the cell-platc, or new cell—wall, places itself perpendicular to the old one, and transverse to its length.
There are very numerous cases in which the law is obeyed, but it is not so in all. Under certain conditions the. lamella should be not at right angles, but oblique to the wall of the chamber across which it is stretched. If, to take a concrete case, the lamella be made to move (by abstracting air) towards one end of its receptacle (a cube or parallelepiped), it will reach a critical position in which the principle of least surface can only be satisﬁed by its occupying an oblique position. The
point at which this occurs is when the lamella is distant 3 from 7r
the end, where a is the length of the side of the cube (short side of the parallelepiped). The lamella new forms the fifth side
to a wedge-sliaped space (quadrant of a cylinder, whose radius
. 4 . . . . is 1- = 9-a), but as more air is abstracted, and it moves still further toward the end, it comes to another critical position when it must lie across one corner, forming so the base of a pyramid, or octant of a sphere. This position is deﬁned by the equation 2-, =11, where 1', is the radius of this sphere. It is impossible, therefore, for a very ﬁat cell, or short cylinder, to be divided in conformity with the principle parallel to its longest side, and yet this occurs, as, for instance, in cambium cells.
It will also be noticed that this principle does not explain why one particular direction is selected when many are apparently equally possible.
We turn now to a consideration of the remaining factor which assists in determining the shape of the cells and so the geometrical pattern of segmentation ; this is the movement of the cells upon one another.
That such movement does occur we have already seen; the question which immediately suggests itself is whether in taking
up their new positions the cells obey the laws of capillarity as enunciated for systems of fluid lamellac such as soap-bubbles by Plateau in his principle of least surfaces.
This principle, as we have seen, demands that the sum of the external surfaces should be, under the conditions, a minimum, or, expressed in physical rather than in geometrical language, that the total surface energy should be minimal. In accordance with this doctrine of minimal surface energy a drop of fluid floating in a fluid medium assumes, as need hardly be said, the form of a sphere. In a system of drops contact surfaces will be formed between the drops, provided that each possesses a coating film which has a positive energy with the media it separates; a film, that is, of such a nature that the total surface energy would be diminished by apposition, without, however, involving the disappearance of the separating ﬁlm and fusion of the drops. In other words, the ﬁlm must be insoluble in both the external and the internal media. A simple example of this is afforded by the behaviour of the spheres of jelly covering the eggs of the Frog, when taken from water and floated between chloroform and benzole. Two or more such drops of jelly cohere by their coating ﬁlms, and form systems of lamellae —the films, that is, at the external surfaces and between the opposed surfaces of the drops———in which the principle of least surfaces is obeyed. Soapbubbles form similar systems. But where this condition is not fulﬁlled, as in oil-drops ﬂoated, for instance, between alcohol and water, the drops either unite or separate, each retaining its spherical form.
The geometrical analysis of such systems given by Plateau is as follows. In a system of two bubbles the curvature of the
surface of contact is given by the equation r = £7, where 7' is the radius of that surface, p, p’ the radii of the larger and smaller bubbles. Since the pressure varies inversely with the radius, the surface of contact is convex towards the larger bubble. When p = p’, 7- = a, and this surface is plane. Since there is equilibrium the external surfaces of the bubbles and their common surface meet at angles of 120°.
In a system of three bubbles there are three contact surfaces; these meet in one line and make angles of 120° with one another. When there are four bubbles, however, the four con tact surfaces cannot meet in one line except for an inappreciable instant; they immediately shift their positions in such a way that two opposite bubbles meet and separate the other two from one another. There are thus ﬁve surfaces of contact, and these make angles of 120° with one another as before. This is the arrangement when four bubbles—whether equal or unequal is no matter—are placed side by side in the same plane. When, however, one bubble is placed in a different plane to the remaining three, four surfaces are formed and disposed in such a manner that the four lines, each formed by the intersection of three of these surfaces, meet in one point, making with one another angles of 109° 28’ 16", the angles at the centre of a tetrahedron. In short, the four are now tetrahedral] y arranged. The systems of drops of jelly alluded to above arrange themselves as do soap-bubbles under similar circumstances. What holds good of four holds good of an assemblage of any number of bubbles. The size of the bubbles is a matter of indifference, except to the curvature of the surfaces of contact, and, to a certain extent, to the arrangement. Thus, if four equal bubbles be placed in a plane, they will form together ﬁve surfaces of contact, one of which will be between two opposite bubbles. If these two be now diminished, or the opposite two enlarged, the surface of contact will be between the opposite pair of larger bubbles. On the other hand, it is possible to bring smaller opposite bubbles into contact, while the larger ones remain apart. Again, on four bubbles lying in one plane, four small ones may be superimposed in such a fashion that while two lie at either end of the surface of contact, the other two lie over between the two opposite large bubbles below. If now the two latter small bubbles be enlarged, they will displace the other two until all four come to lie not over but between the
FIG. 24.—Diugra1ns of systems of soap-bubbles.
A-0, four small bubbles superimposed on four large ones. In A and B the bubbles are not compressed ; in C the lower bubbles have been circumscribed by a. cylindrical vessel. In B the upper bubbles are small enough to show the surfaces of contact between each and the two adjacent large bubbles below. These surfaces are invisible in A and C.
D is a system of eight bubbles in one plane, four forming a cross in the centre.
In all figures notice the ﬁfth contact surface or ‘polar furrow ’.
bubbles below, the usual arrangement when four are superimposed on four (Fig. 24 A—C).
The ﬁnal disposition must depend, therefore, not merely on the principles of least surfaces, but also, provided that the conditions of that principle are fulfilled, on the sizes and initial arrangement of the bubbles.
It will hardly need pointing out that very many ova adopt the form which presents the least external surface, that of a sphere, when placed in a fluid medium, and it is also a familiar fact that after the first (and subsequent) divisions the blastemeres are ﬂattened against one another (Cytarme, to use Roux’s term), and that whether they are compressed by an egg membrane or not (examples of the second alternative are to be found in Unio, .D1'eis3eu8z'a, Umbrella, C'7'q2i(I'/(la, /lp/yxia limecimz, /late;-iae), though the surface of contact is not always curved when the cells are unequal. The two cells, however, often become rounded of and partially separated from one another prior to the next division. Such a separation (Cytochorismus) has also been observed by Roux in the ease of cells of the Frog’s egg, which, having been isolated in albumen or salt solution, have subsequently reunited.
That the cells flatten against instead of repelling one another, as free oil-drops would do, suggests that they, like soap-bubbles, are provided with an insoluble coating-ﬁlm, while their subsequent separation may be provisionally explained by supposing that this coating-film becomes temporarily dissolved under the action of some substance formed in the cell. This idea is borne out by a striking experiment of Herbst’s, who found that in sea-water deprived of its calcium the blastomeres of the seaurchin egg came apart and resumed their spherical shape. At the same time the surface membrane underwent a visible alteration, becoming radially striated. It seems reasonable to conclude that there is a membrane by which contact is normally effected, and that this is soluble in sea-water devoid of calcium. On the addition of calcium the cells eohere again.
It may be mentioned that when systems of drops of jelly, ﬂoating in a medium of oil and united by their coating-ﬁlms of water, are removed to alcohol, in which both oil and water are soluble, the ﬁlms disappear and the drops separate.
In the next stage (four cells) the type of segmentation in which the laws of capillarity are most strictly obeyed is obviously that which we have distinguished above as the spiral or tetrahedral type, and Robert has been able to show that successful imitations of the four-, eight-, twelve-, and sixteen-celled stages of the egg of 2’7'oc/we may be made with soap-bubbles.
Four equal bubbles were placed in a porcelain cup, which held them together in the same way that the actual cells are held together by the vitelline membrane. Five surfaces of contact were formed, that between two opposite bubbles representing the cross furrow or polar furrow in the egg. In the fl’/'oc/ms egg, however, the polar furrows need not be parallel at the animal and vegetative pole; they may be at right angles to one another, and this tetrahedral arrangement of crossed polar furrows may be imitated by lifting up one of the bubbles and bringing it into contact with its opposite, one pair of bubbles being new in contact below, the other pair above. This arrangement is, however, unstable whilc the four bubbles remain in one plane, the two bubbles soon coming into contact both above and below. When the bubbles are not conﬁned within a cup the instability of the ‘ crossed-furrow ’ condition is extreme.
By reducing the volume of the bubbles that are in contact the other two may be brought together; as the polar furrow changes positions there is at least a temporary condition when they are crossed.
As we have already pointed out, both conditions—the ‘ parallel furrows’ and the ‘ crossed furrows ’—-are met with in the eggs at the four-celled stage of Molluscs, Annelids, and marine Turbellarians. Whether both opposite pairs or only one opposite pair of blastomeres are in contact does not, however, appear to depend upon whether the vitelline membrane is close to and compresses the egg or not. In most cases of crossed furrows the membrane ﬁts, it is true, quite closely (Nereis, Io/moo/titou, Porlar/cc, Lcpidouotus, Jjiscocelis, P/(yea, and possibly Li)/may and Planorbis, if there is in these two, as in P/13/ea, a very fine membrane between the albumen and the ovum); so also, speaking generally, where the furrows are parallel the membrane is absent (Umbrella, ./ljzlysia, Dreisseueia, Crepirlu/a), but in Am];/aitrile and C/gmzenella it is lightly applied to the egg.
It is remarkable that when the furrows are crossed, it is the A and C cells which meet at the animal pole, the B and D cells at the vegetative (except only in U/tio), and this must depend on other properties of the cells than their surface tensions. But it may be very plausibly suggested that the explanation of the fact that it is the cells B and D which meet to make the ‘parallel’ furrows is to be looked for simply in the large size of D.
Robert has indeed shown that by simply altering the sizes of the bubbles the conditions observed in the four-celled stage of other types—Nereis, A/-euicola, Uuio, zlplysia, I)isc-ocelz'.s°-may be faithfully copied.
It only remains to be added that the contact surfaces of the cells, like those of the bubbles, make angles of 120° with one another.
Robert has also imitated the eig-ht—eelled stage (the four micromeres alternating with the four maeromercs), the stage of twelve cells (division of the micromeres), and that of sixteen cells (second quartettc formed). The bubbles of the second quartette may be made to slide in between the maeromeres and so rotate the whole first quartette, as happens in the egg. The division of the micromeres in the egg results in the arrangement of four cells crosswise in the centre, four others occupying the spaces between the arms of the cross. The bubbles behave in the same manner.
In the eigl1t—celled stage the micromeres alternate with the macrorneres. In the case of the bubbles this is not necessarily so; the two sets of bubbles may be superposed if the ‘polar furrow’ in one tier is at right angles to that in the other, or if, as pointed out above, the upper bubbles are small. Otherwise superposition is a very unstable condition.
It would appear then that many of the patterns exhibited by eggs with a spiral cleavage are explicable by reference to the laws of surface tension. The principle of least surfaces may be extended to other cases. The first four blastomeres of Ophiuroids and Asteroids form a perfect tetrahedron, though this arrangement is subsequently discarded for one which could not be imitated with soap-bubbles (we may notice in passing that in the ﬁrst case the egg is tightly invested by its membrane, in the second it is perfectly free). In zlscarzlv megalocep/aala the four cells come to lie, as do four bubbles, in one plane, and polar furrows have been seen in many eggs which belong to another type of segmentation (in Coelenterates (llyrlractiuia), Sponges (Spongillu), Crustacea (Brauc/z2'pu.v, Luci/‘er, 0rc/Iestia), Vertebrates (Petra/1z_yzo:2, Rana), Ascidians, and Am/2/u'oama).
The principle of least surfaces——not more than three surfaces meeting in a line, not more than four lines meeting in a point— is, however, not of itself suflicient to explain the whole of the phenomena even in this most favourable tetrahedral type; other factors must intervene, just as other factors intervene in a mass of soap-bubbles—their size and initial arrangement~— in the determination of the actual pattern. These other factors are the direction of cell-, that is of nuclear, division, and the magnitude of the cells; and these, as we have seen, in turn depend upon the relation between the nucleus and the cytoplasm with its included yolk. Thus it is the direction of the spindles which determines whether the mieromeres of the first quartette shall be given oﬁ laeotropieally or dexiotropically ; the direction of division, oblique to the egg-axis, again determines that the mieromeres shall alternate with the macromeres and not be superimposed upon them ,- the size of the cells and the direction
FIG. 25.-— Mitotic division with elongation of the cell-body in a protozoon,Acanthor_1/stis aculeuta. (After Schaudinn, from Korsehelt and Heider.)
of division may determine the position of the polar furrow, while the rate of division will also not be without effect, since the whole arrangement at any stage depends in part on the disposition at the stage before.
There is one other point that is worthy of notice. The mitotic spindle possesses considerable rigidity, and is able as it elongates to materially alter the shape of the cell. This may be seen in many cases in Annelid, Mollusean and other eggs—the division of the first mieromeres in Nereis is an instance—and in the Protozoa (Fig. 25). Another interesting case is the Rotifer Asplcmc/ma, where, preparatory to the fourth division, the shortest axis of the cells——in which the spindles are placed-—becomes by the elongation of the spindles the longest. This alteration of shape is itself an important factor in deciding the positions to be taken up by the daughter cells. II. I CELL-DIVIS ION 49
In the other types——radial and bilateral——thc principle of least surfaces is obviously disobeyed, for here four or more surfaces meet in one line and at angles other than 120°.
Roux (1897) has, however, shown that if a certain condition be imposed on the system of lamellac, ﬁgures may be produced which very closely resemble the patterns presented by radially and bilaterally segmenting ova. This indispensable condition is that the system shall be surrounded by a rigid boundary, as the eggs themselves are by a membrane. Roux’s system was made by dividing into two, four, and eight a drop of paraflin oil suspended in a closely ﬁtting cylindrical vessel between alcohol and water. To this medium was added calcium acetate to prevent the drops reuniting. The drop was divided with a glass rod.
Fm. 2(5.~ltoux's oil-drops. A and B, the drop divided equally; U and D, unequally. Each of the two equal drops divided equally in E, unequally in 1". (From Korschelt and Heider, after Roux.)
When the two drops formed by the first division were equal the surface of contact was ﬂat, when unequal convex towards the larger one, in accordance with the rule (Fig. 26 A—l)).
When the second was also equal, four drops were formed with four surfaces of contact meeting in one line, or enclosing between them a small ‘segmentation’ cavity. If the division of the two equal drops was unequal, and the smaller cells adjacent, they pushed into the larger ones; the result, in fact, was the same as would have been produced by an equal following on an unequal division, the four surfaces meeting in one line as before (Fig. 26 E, F). The appearance presented is like a side view of a radially
Fm. 27.—-Arrangement of four oil-drops produced by unequal division of two equal drops, the small and large drops alternating. The first division is shown by I: the second (II) may pass as in a or in b, but the result is always as in c, the two large drops meeting in a polar furrow and excluding the small drops from the centre; the system is symmetrical (iso~bilateral) about the dotted lines in c. (After Roux, from Korschelt and Heider.)
segmenting egg after the third division. When, however, the smaller drops were 11ot adjacent, but opposite, five surfaces of con ” I 3 /. tact were formed,
a polar furrow appearing between
A A the two larger and
I L 1 joining the centres \ /7 W of mass of the two smaller drops,
I I whether these are unequal or not.
C’ . . The direction 1n
ﬁ which the division of the drops is per” V 3’ formed isirrelevant; the ﬁnal result is 4% always the same.
/ Should two adjacent
_lfIG. 2S.-— A_and B are diag1':1111s of an oil—drop drops be equal, the divided into four and eight to explain Roux’s 1 f _ - - 1 notation. C is a ﬁgure of the oil-drop divided into P0 M m row 15 Sh]
eight equal parts. (From Korsclielt and Heider.) formed by the union of those two which have together the larger mass (Fig. 27).
The length of the polar furrow varies directly with the size of the drops which unite to form it; its direction makes an angle with the plane separating the first two, which varies
FIG. 29. ~ Arrangenicnts of six oil-drops. In all cases A = B = a = b. In A, rt’ = a", b’ = b". In B, a’ > a", b’ > b”. In C, a’ < a”, b’ < b”. I, ﬁrst furrow; II, second furrow. (From Korsclielt and Heidcr, after Roux.)
FIG. 30. - Various arrangements of eight oil-clrops, all bilaterally syiiiinetrical about the ﬁrst furrow (1). In all cases the ﬁrst division has been equal. In A and B the second division (II) has also been equal, but in C 0, b are smaller than A, B. In A, a”, la”, A”, B" < rs’, b’, A’, B’. In B and C, a”, b" < a’, b’, but A", B” = A’, B’; hence a”, I)" < A", B". (From Korschelt and lleider, after Roux.)
_Ei_G. 31. —~ Arrangement of six (A) and eight (13) oil-drops, after iineqnal division of four equal drops (A = B = a = 1;), the smu.llcr and linger drops regularly alternating. (From Korschelt and Hcider, after Roux.)
inversely with its length, so that when all the drops are equal the cross furrow lies in the same plane with the first division, and so disappears.
By another division it is possible to make a ring of eight drops whose surfaces of contact all meet in one line, or in a ‘segmentation’ cavity (Fig. 28). To realize this condition, however, it is necessary that the division should be equal, and its direction accurately radial. If unequal, the larger drop invariably passes towards or wholly into the inside. If oblique or tangential the inner drop passes into the segmentation cavity (Fig. 32).
FIG. 3'Z.——'l‘hree stages in the passage of a large drop (a") into the centre of the system. The ﬁist stage extremely unstable. (From Korschelt and Heider, after Roux.)
Unequal division of all four equa.l drops produces very interesting patterns, some of which recall the appearance of bilaterally segmenting ova, when the divisions are corresponding-ly unequal on each side of the ﬁrst or second division (Figs. 29, 30), while others resemble certain phases of ‘ spiral’ division when small and large cells regularly alternate (Fig. 3]). It is a rule for the smaller of the two drops to go to the periphery, while the larger assumes an oblong or wedge shape, passing towards the centre if it does not slip entirely inside. The latter occurs with clean oil, when the large drop is flanked by small ones on both sides.
It is also possible to divide four equal drops horizontally into two tiers. The upper drops, however-—-unless absolutely undistnrbed~quiekly come to alternate with the lower.
In these systems of drops the ﬁnal arrangement is due to, ﬁrst, the principle of least surfaces; secondly, the circumscribing boundary; thirdly, the size of the drops; and fourthly, in some cases, the direction in which they are divided.
It only remains for us to consider, with Roux, to what extent the cells of a radially segmenting egg, such as that of Rana fusca, are governed by the same influences as determine the pattern of the drops.
The resemblances, it will be conceded, are often very close. There are also important differences. The polar furrow, which is often present in the Frog’s egg, is not necessarily between the cells with the greatest mass. Again if, in the four-celled stage, with no polar furrow, one of the cells be diminished by puncture, a polar furrow does not always appear, as it would with oil-drops, nor, if it does, is it always formed by the union of the larger cells. 01', if when a polar furrow is present between the larger cells, one of these is diminished by puncture until it, together with its opposite, is less than the other two, the polar furrow nevertheless retains its position.
In the sixteen-celled stage the animal cells together form a ring of eight around the axis. The cells are not necessarily equal, and a small cell may be compressed by, instead of compressing, adjacent large ones, while they, not it, move away to the periphery.
Other differences are that large cells bulge into small, that cells are elongated tangentially instead of radially, that there are amoeboid processes at the inner ends of the cells, and intercellular spaces between them.
Further, Roux has examined the behaviour of the isolated cells of the Frog's egg in the morula stage. The cells were separated in a medium of albumen, or salt-solution, or a mixture of the two. They first approach and then ﬂatten against one another (Cytarme), as do the blastomeres in the egg, completely or incompletely. The contact surface is generally symmetrical to the line joining the centres of mass of the two cells ; it may be concave towards either the small or the large cell. More than two cells may unite to form rows or heaps. The angles made by the surfaces of contact may be 120°, or have other values. Four surfaces may meet in one line ,- at other times the arrangement is tetrahedral. In a 1-25 Z solution of salt the cells are elongated, and united end to end in long branching strings. The pigment, diffused through the cell, later returns to its original position at the surface, or usually to the middle of the free surface of each.
The cells may also move over one another (Cytolisthcsis) by sliding or rotation, or both. Even two cells will glide on one another, as two soap-bubbles will not. In complexes two threesurface lines may unite to form one four-surface line, a behaviour the very opposite of that exhibited by soap-bubbles.
It appears, then, that in the living egg of the Frog (and other radial and bilateral types) there are factors which overcompensate, to use Roux’s expression, the purely physical factors by which the behaviour of the oil-drops is governed. These organic factors are that division is slow, and begins on the outside ; that the direction of division—determined by the yolk—is persistent; that the cell contents are neither perfectly ﬂuid nor perfectly structureless ; that the cells being different, their surface tensions may be of dilferent magnitudes, and the whole system, therefore, not homogeneous; and that the cells possess a more or less solid rind or membrane, the rind which becomes wrinkled transversely to the furrow when the cell divides.
It would seem that this rind is an important factor, for if Roux’s experiments be repeated with drops of albumen suspended between xylol and oil of cloves, to which a little alcohol has been added, it will be seen that each drop gets a su1:erficial membrane, and that by these membranes adjacent drops adhere. In fact, such drops behave more like the cells of the egg‘ than do the oildrops. Thus, a small cell goes towards the inside, or the outside, according" to the way in which the division is made, and, after a horizontal division of four equal cells, the upper remain superimposed upon the lower.
At the same time, it is apparently because the cells have this surface ﬁlm, which the oil-drops have not, that they are able to flatten against one another as soap-bubbles do; while, on the other hand, it is because the ﬁlm is solid that the cells are unable to move upon one another and adopt the geometrical arrangement seen in systems of soap-bubbles.
There is still another kind of cell-movement to which brief reference must here be made, since it is found in one type of segmentation at least. In the segmenting eggs of some Platyhelmia (Triclads), Ascidians (Salps), Echinoderms (Asteroids), and Coelenterates (0ceam'a), the blastomeres have been seen to completely separate from one another, afterwards reuniting. Roux has observed a similar reunion of the artiﬁcially isolated cells of the Frog’s egg. This Cytotropism, as Roux calls it (Cytotaxis would be a preferable term), is noticed when the slide is kept perfectly horizontal and streaming movements of the medium (albumen) are rigidly excluded. The cells become rounded, and then approach one another in, more or less, a straight line, oscillating slightly backwards and forwards. The cells must not be too far apart, not further than a radius of small, or less of large cells. Groups of two or more cells behave in the same way.
The movement may be simply a surface-tension phenomenon, or, as Roux suggests, more complex, of the nature of a response to a mutual ehcmotactic stimulus.
These various kinds of cell-motion are also an important feature in such processes of differentiation as the union of cells to form muscles, tendon, epithelia, and so forth.
A review of all the facts thus leads us to conclude that while some of the phenomena of segmentation-—-the ﬂattening of cells against one another, the pattern made by the cells in cleavage, especially of the spiral type—are largely referable to the action of the purely physical laws of surface tension, there are many cases, the radial and bilateral types, and the radial and bilateral periods of spirally segmenting eggs, in which the operation of these laws is restricted and conﬁned by other causes. But in any case those laws can only co-operate with other factors, which are to be looked for in the rate and direction of division, and in the magnitude of the cells, factors which themselves are dependent on the relation between the cell and its nucleus.
Before concluding this section we have to call attention to some experiments which may possibly throw some light on an event of fairly frequent occurrence in ontogeny—the division of the nucleus without the division of the cell,‘ as in the formation of coenocytia such as striated muscle ﬁbres and the trophoblast of the placenta ; or the fusion of distinct cells into a syncytium, as in the trophoblast again; or the secondary union of yolk-cells.
In the Alcyonaria the nucleus may divide three, four, or ﬁve times before the egg simultaneously breaks up into eight, sixteen, or thirty-two cells. See especially E. B. Wilson, ‘On the development of Renilla,’ Phil. Trans. Roy. Soc, clxxiv, 1883.
Driesch has observed that in the egg of Ea/Linus cell-division may be wholly or partially suppressed by pressure, and also by diluting the sea-water. Nuclear division continues (Fig. 33).
Morgan has found that the egg of another sea-urchin (Arbacia) will not segment in a 2 Z solution of salt in sea-water; on replacing the eggs in sea—water, however, the nucleus divides with great rapidity several times, and this is followed by celldivision. So Loch notices that the eggs when treated in this way, and brought back to their normal medium, divide simul taneously into four. The egg
es of the fish Cteuolabm.s (accord O“ o«° ing to the same author) behaves in a similar fashion when first f I deprived of, and then restored
,, 1, to oxygen. . FIG. 33.—Echinus: suppression of lmf’ again’ has Seen the re cell-division by 1)1'essure, I), and by union Of sister cells and nuclei
heat, a. Nuclear division continues. ' . ' . (After Driesch, 1893.) in the eggs of Avbacza leleased from pressure.
Three distinct agencics——mechanical pressure, increase of osmotic pressure, and decrease of osmotic prcssure—arc all capable of effecting this interesting change in the usual relations of cell and nucleus. We can only guess at the real cause, and surmise that it will be found in an alteration of internal and external surface tensions.
No'rE.—For a complete bibliography of segmentation the well-known textbooks of 0. Hertwig and Korsehelt and Heider must be consulted. The literature of ‘spiral’ segmentation is given by Robert (quoted below).
F. M. BALI-‘OUR. Comparative Embryology, London, 1885.
G. BERTHOLD. Studien ﬁber Protoplasmameehanik. VII. Theilungsrichtungen und Thcilungsfolge, Leipzig, 1886.
G. BORN. Ucber Druckversuche an Froscheiern, Anat. Anz. viii, 1893.
T. BOVERI. Die Entwiekelung von Asca;-is megalorephala mit beson— derer Riicksieht auf die Kernverhaltnisse, Fesfschr. Xupﬂkr, Jena, 1899.
E. G. CONKLIN. P1-otoplasniic movement as a factor of differentiation, Woods Hell Biol. Lech, 1898. II. I CELL-DIVISION 57
H. DRIESCH. Entwicklungsmechanische Studien, IV, Zeilschr. wiss. Zool. lv, 1893.
H. DRIESCH. Entwicklungsinechanische Studien, VIII, Mitt. Zool. Stat. Neapel, xi, 1895.
A. FISCIIEL. Zur Entwicklungsgeschichte der Echinodermen. I. Zur Mechanik der Zelltheilung. II. Versuche mit vitaler Fiirbung, Arch. Ent. Mech. xxii, 1906.
J. H. GEROULD. Studies on the Embryology of the Sipunculidae, Mark Anniversary Volume, New York, 1903. _
A. GRAF. Eine 1-iickgitngig gemachte Furchung, Zool. Anz. xvii, 1894.
C. HERBST. Ueber dais Auseinandergehen von Furchungs- und Gewebezellen in ka.lkfrciem Medium, Arch. Ent. M¢'c7z. ix, 1900.
O. HERTWIG. Die Zelle und die Gewebe, Jena, 1893.
O. HERTWIG. Ueber den Worth der ersten Furchungszellen fiir die Organbildung des Embryo, Arch. nu'l.-r. Anal. xlii, 1893.
O. HERTWIG. Ueber einige mu befruehteten Froschci durch Centrifugalkraft hervorgerufene Mcclnmomorphosen, S.-B. Kimigl. prcuss. All-ad. 11733., Berlin, 1897.
J. LOEB. Investigations in physiological morphology, Joum. Morph. vii, 1892.
J. LOEB. Untersuchungen ﬁber die physiologischen Wirkungen des Sauerstoffnmngels, I7l:2ger‘s Arch. lxii, 1896.
J. LOEB. Ueber Kerntheilung ohne Zellt-heilung, Arch. Ent. Mech. ii, 1896.
T. H. MORGAN. The action of salt solutions on the unfertilized and fertilized eggs of Arbacia and of other aniimtls, Arch. Ent. Mech. viii, 1899.
E. PFLi'IGER. Ueber die Einwirkung (ler Schwerkra.i't und andere Bedingungen nuf die Richtung der Zclltheilung, 1_’ﬂc'«'ger‘s Arch. xxxiv, 1884.
J. PLATEAU. Statique des liquides, Paris, 1873.
A. RAUBER. Der karyokiuetische Process bei erhohtem und vermin(lertexn Atmosphiirendruck, Vcrs. Deutsch. Naimf. u. Aerzte, Magdaburg, 1884.
A. ROBERT. Rec-herchcs sur le développement des Troques, Arch. Zool. E.1'p. et G6». (3), x, 1902.
W. Roux. Ueber die Zeit der Bestimmung der Hauptrichtungen des Froschembryo, Leipzig, 1883, also Ges. Abh. 16.
W. Roux. Ueber den ‘Cytotropismus’ der Furchungszellen des Grasfrosches (Rmmfusca), Arch. Ent. Mech. i, 1894.
W. ROUX. Ueber die Selbstordnung (Cytotztxis) sich ‘beriihrendcr' Furchungszellen des Froscheies durch Zellonzusammenfiigung, Zellentrennung und Zellengleiten, Arch. Ent. Mech. iii, 1896.
W. Roux. Ueber die Bedeutung ‘geringei-' Verschiedenheiten der relativen Grﬁsse der Furchungszellen fur den Clim-akter des Furchungsschemas, Arch. Ent. Mech. iv, 1897. 58 CELL-DIVISION AND GROWTH II. 2
G. SMITH. Fauna und Flora. des Golfes von Neapel: Rhizocephala,
Berlin, 1906. O. ZUR STRASSEN. Embryonalentwicklung des Ascarisnmgalorephala,
Arch. Em‘. Jlfcah. iii, 1896. E. B. VVILSON. (_1leava.ge and mosaic work, An-7:. Ivlul. lilo:-72. iii, 1896. H. E. ZIEGLER. Ueber Furchung unter Pressung, Vcrh. Aunt. Gesell.
viii. 1894. H. E. ZIEGLER. Untersuehungen ﬁber die eisten Entwicklungsvor gétnge der Nematoden, Zeifschr. W1'.s-s. Zool. lx, 1895. H. E. ZIEGLER. Experimentelle Studien ﬁber die Zelltheilung, Arch. Elli. llferh. vii, 1898.
Following Davenport we deﬁne growth as increase in size or volume. Since, therefore, growth is increase in all three dimensions of space, it is most accurately measured not by increase in some one dimension—such as stature——but by increase of mass or weight.
Growth depends upon the intake of food and the absorption of water and exhibits itself in the form of increase in the amount of living matter or of secretions of watery or other substances, organic or inorganic, intra-cellular or extra-cellular, such as ehondrin, fat, muein, cellulose, calcium phosphate, and the like.
That growth depends———in later stages at least—upon the intake of food is obvious. That it is due to the absorption of water has been demonstrated effectively by Davenport for the tadpoles of Amphibia (zlmtlys/oma, Rana, I311/2)). The method employed was to weigh known numbers of the tadpoles at diﬂ:'erent ages, desieeate and weigh again. The results of the investigation are shown in the accompanying ﬁgure (Fig. 34-), from which it will be seen that the percentage of water rises with remarkable rapidity———from 56% to 96% during the first fortnight after hatching. After that point the amount of water present slightly but steadily declines.
The same result is brought out by an analysis of the terminal buds and successive internodes of plants. It is found in I/ele7'oceutrou (Kraus) that the percentage of water rises rapidly from the terminal bud to the first internode, more slowly from the first to the second internode, and then remains constant.
It would thus appear that during the period of most rapid growth, growth is eﬁfected by imbibition of water rather than by assimilation, since the weight of dry substance in the tadpole during this period does not increase at all.
In later development the proportion of water slowly falls. This may be seen not only in Davcnport’s table of the growth of Frogs but in the data furnished by Potts for the Chick and by
FIG. 34.— Curve showing change in percentage of water in Frog tadpoles from the first to the eighty-fourth day after hatching. Abscissae, days; ordinates, percentages. (After Davenport, from Korschelt and IIe1der.)
Fehling for the human embryo. These data. are given in the accompanying tables (Tables I, II). The percentage of water, at first high, slowly falls in both cases; conversely, the percentage of other substances increases.
‘ These results indicate that during later development growth is largely effected by excessive assimilation or by storing up formed substance’ (Davenport).
There are other external agencies by which growth may be affected in various ways—-such as heat, light, and atmospheric pressure. These will be discussed in another chapter. For the present let us conﬁne our attention to certain features which are characteristic of growth in general, of the growth of the animal organism under normal conditions. These are the changes that take place during growth in the rate of growth itself, in the variability of the organism and in the magnitude of the correlations between its various parts.
Showing the percentage of water in Chick embryos at various stages up to hatching. ( From Davenport, 1899 (2), after Potts.) ’1_‘he table also shows the hourly and daily percentage increments of weight.
Absolute Hourly Daily
H f . . P r entage
48 0-06 83 54 0-20 0-14 38-3 919-2 90 58 0-33 0-13 16-0 384-0 88 91 1-20 0-87 7-9 189-6 83 96 1-30 0-10 1-7 40-8 68 124 2-03 0-73 2 0 48-0 69 264 6-72 4-69 1 6 38-4 59 TABLE II
Showing the percentage of water in the Human embryo at various stages up to birth. (From Davenport, 1899 (2), after Fehling.) The table also shows the weekly percentage increments of weight.
Age in Absolute weight \Vec-kly per- Percentage
weeks. in grammes. Increase‘ ccntnge increment. of water.
6 0-975 97-5 17 36-5 35-525 331-2 91-8 22 100-0 63 5 34-8 92-0 24 242-0 142-0 71-0 89-9 26 569-0 327-0 67 6 86-4 30 924-0 355-0 15-6 83-7 35 928-0 4 0 0-1 82 9 39 1640-0 712-0 19-2 74-2
VVe follow Minot and Preyer in measuring the rate of growth by the percentage increments of weight (or of other measurements where weight is not available) during a given interval of time ; that is to say, by expressing the increase in weight during a given period as a. percentage of the weight at the beginning (or end) of that period. The change of rate, if any, is found by taking such percentage increments for successive equal increments of time.
As a first example let us consider the data furnished by Minot II. 9. GROWTH 61
himself for the rate of growth, after birth, of guinea.-pigs (Table III, Fig. 35).
Showing the change of rate of growth in male and female Guinea.-pigs. as measured by daily percentage increments of weight. (From Minot, 1891.)
Average daily per cent. Average daily per cent.
Ago in Age in
d“Y‘- Ma1§§°'°m§'§§$1cs. “‘°““‘s- Ma1§§.°r°m°§§:.a1..s. 1-3 0.0 2.1 8 0.05 0.2 4-6 5.6 5.5 9 0.3 0.2 7-9 5.5 5.4 10 0.1 0.1
10-12 4.7 4.7 11 0.04 0.1
13-15 5.0 5.0 12 0.1 0.05
16-18 4.1 4.3 13 -0.2 0.3
19-21 3.9 3.5 14 0.5 -0.03
22-24 3.1 1.7 15 0.2 0.00
25-27 2.3 1.9 16 0.07 0.2
23-30 2.3 2-6 17 -0.1 -0.02
31-33 1.9 1.3 13 -0.05 -0.2
34-36 1.7 1-6 19-21 0.006 -0.1
37-39 1.9 1.3 22-24 0.02 -0.05
40-50 1.2 1.1
55-65 1.3 1.3
70-80 1.2 0.3
35-95 0.9 0.9
100-110 0.7 0.3 115-125 0.6 0.5 130-140 0.1 0.2 145-155 0 4 -0.03 160-170 0.3 0.5 175-135 0.2 0.2 190-200 0.2 0.2 205-215 0.4 0.3
15Il| I7 D E II N 75 I50
FIG. 3-'3.—Curve showing the daily percentage increments in weight of female Guine-.1-pigs. (From Mxnot, 1907 )
An inspection of tlie accompanying table and ﬁgure in which Minot’s results are reproduced will show at once that there is in both sexes, almost from the moment of birth, a. decline in the growth-rate. The decline is not, however, uniform. The rate falls rapidly between about the ﬁfth day (when it is from 5% to 6%) and the ﬁftieth, from the ﬁftieth day onwards more slowly, becoming eventually very small, zero or even negative. The younger the animal, therefore, the faster it grows; the more developed it is the more slowly it grows. The rate of growth in fact varies inversely with the degree of diﬁerentiation. A mammal, therefore, which is born in a less developed condition than is the guinea-pig ought to grow at first more rapidly still. The rabbit is such an animal, and Minot has been able to show that on the fourth day after birth the young rabbit adds 17 % to its weight. The curve also shows the same rapid decline in the growth-rate as was observed in the guinea-pig, followed by a period of gentle decrease.
Accurate observations on the prenatal rate of growth of these two mammals are lacking, but Henscn’s few observations (quoted by Preyer) on the weight of guinea-pig embryos show that the daily percentage increase descends from 220% on the twenty-first day to 116% on the twenty-ninth day, to 33% on the forty-third (lay, and again to 6% on the sixty-fourth day, that is just after birth, the moment at which Minot’s observations begin. Again, Minot has found, as a result of the investigation of the weight
Showing the decrease in the rate of growth of the Human embryo before birth. Percentage increments calculated from the ﬁgures given by
Hecker, Toldt, and Hennig. (From Preyer.)
Average monthly percentage increments of Month. Weight. Length.
(Hecken) (Toldt.) (Hennig.) 1 _ _ __ 2 — 133-3 433 0 3 — 100-0 110-0 4 418-2 71-4 92-8 5 398-2 66-7 69-8 6 123-2 50-0 28-2 7 92-2 16-7 14-2 8 28 8 14-3 11-9 9 25-6 12-5 6-3 10 — 11-1 4 3 II. 2 GROWTH 63
of spirit specimens of rabbit embryos that the mean daily percentage increment is 704 between the ninth and ﬁfteenth days, but between the ﬁfteenth and twentieth days only 212.
The postnatal decline in the growth-rate is therefore only a continuation of a process which has been going on for some time, perhaps from the first moment at which growth began.
The human being forms no exception to this rule. Data of the growth of the human embryo before birth are somewhat meagre, but an inspection of the tables will show that whatever the discrepancies may be between the results obtained by Fehlin g (Table II) and Hecker (Table IV), they agree in this, that the growth-rate falls with great rapidity between the fourth and the sixth months,thereafter more slowly till the end of pregnancy. This is graphically represented in the curve (Fig. 36). It will be observed from the table (Table IV) that the rate of increase of stature also declines, but less abruptly. This is a poi11t to which we shall return. For the study of the postnatal growth of man very numerous data have FIG’ 36' —' Curve Sh°‘”il‘<‘=’
_ monthly prenatal percentage 1nbecn collected byvarious observers. cmments in Man. (From Minot, Measurements of the bed y weight 1907-) have been made on Belgians by Quetelet, on Boston school children by Bowditch, on the school children of Worcester, Mass., and Oakland, Mass., by Boas, and on English of the artisan and the well-to—do classes by Roberts. It is unnecessary to reproduce all these data here, for they all show the same decline in the growth Monnvso-zaascrogm 64 CELL-DIVISION AND GROWTH II. 2
rate, but Quetelet’s measurements for males, being the completest series, are given in the accompanying ﬁgure (Fig. 37). The ﬁgure shows that at the end of the ﬁrst year after birth the per.
FIG. 37.——Curve showing the yearly percentage increments in weight of Boys. (From Minot, 1907.)
centage increment is as high as 200% (or nearly), but that then this increment drops to just over 20% at the end of the second year. From this point the decline is slow but sure, until at the thirtieth year the annual percentage increase is only 0-1 %. The change of rate of growth in females is practically the same as in males. The monthly percentage increment immediately before birth is about 20% according to Miihlmann’ s curve (Fig. 36) ; this represents an annual percentage increment of, say, 250 %, and the annual increase at the end of the first year is about 200 %. The postnatal deerease of growth is, therefore, as in other mammals, a continuation of the prenatal change. Further, there are two points at which the rate diminishes with great rapidity-—between the fourth and sixth months of pregnancy and between the ﬁrst and second years after birth. It would be of the greatest interest to discover the causes of these sudden decreases. Elsewhere the II. 2 GROWTH 65
diminution is gradual. A point of importance is that in both years there is a slight temporary rise in the growth-rate about the time of puberty (see the curve, Fig. 37'). This has been noticed by all observers, but the actual time of its occurrence ditfers in diEercnt cases ; the rise is invariably earlier in females than in males. A comparison of the growth of the three mammals considered is interesting. A Guinea-pig reaches 775 grammes in 43.2 days. A Rabbit ,, 2,500 ,, 395 ,, A Man ,, 63,000 ,, 9,428 ,, or the average daily increment is for a Guinea-pig L82 grammes. Rabbit 6-30 ,, Man 669 ,,
Hence ‘men are larger than rabbits because they grow longer, but rabbits are larger than guinea—pigs because they grow faster’. Minot, however, distinguishes between the ‘rapidity’ of growth, the average actual increment, and the ‘ rate’ of growth, the percentage increment. The average percentage increments for these mammals are
The rate is, therefore, much slower in man than in the otln r two. These percentages Minot calls the coefficients of growth. Together with the duration of growth they determine the ultimate size of the organisms.
The progressive loss of growth-power Minot speaks of as ‘senescence’, and compares to the loss of the power of celldivision in the ‘senile decay’ of Protozoa. The same author has also brought forward evidence to show that during differentiation there is an increase in the amount of cytoplasm in the cell, a decrease in the size of the nucleus, and a decrease in the ‘mitotic index’, that is in the proportion, in any tissue, of dividing cells. During segmentation, of course, the reverse of this is taking place, since cell-division is rapid and the protoplasm per cell is being constantly diminished until a fixed ratio between nucleus and cytoplasm is reached (Boveri) (see below, p. 268). Minot suggests that ‘ senescence ’ and differentiation alike depend on an increase in the protoplasm.
FIG. 38.— Curves showing the change with age of the rate of growth In the larva of the sen.-urchin Strongylocentrotus (from Vernon's data), and the pond-snail Limnaea (from Semper's ﬁgures). The nbscissae are days, the ordinates percentage increments.
The decline of the growth-rate may also be seen in Pott's weighings of the Chick embryo before hatching (Table I) and Minot’ s Weights of young chickens. It appears from these that the daily percentage increment is 919 % at the beginning of the third day of incubation, 189% at the end of the fourth day; at this point there is a sudden drop to 40 Z, which is still the rate of growth after eleven days of incubation; eight days after hatching the ra.te is 9% in the male, not quite 9% in the female, and then comes a period of more or less gradual decline, until when the chicken is 342 days old it is able to add less than 0-5 % to its weight per diem.
Sempei-’s observations on the pond-snail, Limizaea, and Vernon’s on the sea-urchin, St1'0ngyloce7m'ot1m, are other examples which
FIG. 39.—Dai1y percentage increments of weight in tadpoles: the continuous line (a) gives the whole weight, the broken line (b) the dry weight. (After Davenport, 1899.)
may be mentioned. The results of these autl101's are shown in the accompanying charts (Fig. 38). Their measurements are of lengths, not of weights.
So far we have found no exception to the law of the decline in the rate of growth as development proceeds. Davenport's measurements of tadpoles will not, however, conform to the generalization. As the ﬁgure shows (Fig. 39), the daily percentage increments, whether of the whole weight or of the weight of dry substance only, ﬁrst rise abruptly, then descend and then rise again. An explanation of this anomaly may possibly be found in the fact that Davenport/s measurements are 68 CELL-DIVISION AND GROWTH II. 2
taken during that early period when growth is due in the main to absorption of water, the other measurements (as may be seen from Tables I, II) during the later period when the percentage of water has already begun to decline and growth is effected by other means. '
It is_, of course, a commonplace of embryology that the growth of all the organs of the body does not occur at the same rate.
FIG. 40. — Curves showing the alteration during the ﬁrst twenty years
of life of the rate of growth as measured by weight, stature, and chestgirth in the human being (males). (Constructed from Quetelc-t’s data.) The abscissae are years, the ordinates percentage increments. (The percentage increment of weight for the ﬁrst year could not be included in the figure. It is given in Fig. 37.) There are nevertheless few cases in which the exact difference in rate has been ascertained. From those few cases, however, it appears that the individual parts, though they do not grow with equal rapidity, still obey the same law as the whole.
Thus human stature exhibits the same loss of growth-power as is shown by the weight of the whole body, with this difference, however, that the rate is not so high in early stages, the descent in later stages less abrupt. This will be seen in Table IV, in which such ﬁgures as are obtainable for the prenatal growth-rate are given, and in Fig. 40, in which the curve of change of growth-rate in human stature has been constructed from Q.uetelet’s data (male Belgians). The percentage increment in the first year is only 39-6 as against 190-3 for weight, in the second year 13-3 us against 22-2 for weight. Thenceforward the rate slowly declines, until at the fortieth year it is zero, and after the ﬁftieth year increasingly negative. The rate of increase of stature is always slightly less than that of weight. Q.uetelet’s ﬁgures do not show the rise in rate about the time of puberty, but this phenomenon is apparent in the data furnished by Bowditch, Boas, and Roberts (see Fig. 4-2). The change in the growth-rate is practically the same in women as in men. As with weight, the rise of rate at the time of puberty is earlier.
FIG. 41. — Curves showing the alteration during the first twenty years of life of the rate of growth of stature, length of head, length of vertebral column, and length of leg in the human being (males). (Constructed from Quetelet’s data.) Ordinates, percentage increments; abseissae, years.
The decline in the growth-rate of chest-girth is shown in the same ﬁgure (Fig. 40). It will be noticed at once that in this case the drop in the first year is very great indeed, from nearly 50 % to nearly 5 %, and that the rate is only diminished by another 24 ‘Z in the next nineteen years. The weight will depend upon the volume and the volume on both stature and girth; in. fact a rough weight-curve might be constructed from the measurements of stature and girth. It is evident that the sudden loss in the rate of total growth after the first year is due to the very rapid decrease in the percentage increment of girth.
It maybe mentioned that other measurements of girth—girth by the sternum, the waist, the hips, the neck, the biceps, the thigh——show the same exceedingly abrupt decrease, almost to the minimum rate, between the ﬁrst and second years.
In other cases——distance between the eyes, width of mouth, length of hand, length of foot, arm-length, leg-length, length and breadth of head, distance from the crown of the head to the first vertebra, length of the vertebral column—the change is more gradual ,- the rate of change, however, diifers in different cases. As an instance of this let us consider the measurements—— from the crown to the ﬁrst vertebra, the length of the vertebral column, and the leg-length——whieh together make up the total stature. The growth-curves of these three and of the whole stature are presented in the ﬁgure (Fig. 41), from which it will be seen that the growth of the leg is faster than that of the vertebral column (until the eighteenth year), and this than that of the head. Increase in stature takes place at nearly the same rate as that of the vertebral column , but is on the whole a little faster.
There are few cases—besides man—in which we possess information as to the growth of the parts. In the sea-urchin, S57'07I_q,I/[Odell/7'0tl(8, Vernon has shown that the growth-rate of the oral and aboral arms of the Pluteus diminishes rapidly from the third to the fifth days, more slowly from the ﬁfth to the eighth days. After this the rate becomes negative, as the skeleton of the Pluteus is used up by the developing urchin. The curves of change of rate of growth-—as constructed from Vernon’s ﬁgures—-are shown in the chart (Fig. 38).
In Oarcirzm mamas a gradual decrease in the growth-rate of the frontal breadth can be ascertained from Weldon’s data.
We have next to consider another feature of growth, the alteration of variability. The facts at our command are derived from a study of Echinoid larvae (Vernon), Duck embryos (Fisehcl), Guinea-pigs (Minot), the Periwinkle (Bumpus), the Crab, C'arcz'mts (Weldon), and the human being (Bowditch, Pearson, Robeits, and Boas). Vernon has shown that in the Pluteus of Stronyy/ocentrolus the variability of the body-length increases regularly up to the fifth day, and then decreases regularly again to the sixteenth day. So Fischel’s measurements of Duck embryos seem to establish a greater variability in younger than in older stages. This is true of the whole length, the head (as far back as the first somitc), the hand and trunk together, and the total length exclusive of the primitive streak. The data are, however, too few to be treated statistically; the variability can only be roughly estimated from the extent of the limits within which the part varies at each stage.
Minot, who expresses the variability of guinea-pigs by the difference between the mean weight and the mean weight of the individuals above, and of those below, the mean, likewise ﬁnds that the range in variation diminishes with age, and further that, in the case of the males, there is a pcriod—-from about the fourth to the ninth months—when the variability is very much less than at any other time. No such sudden fall is observed in the female, only a steady diminution.
A more satisfactory calculation of the altera.tion of variability may be made from the measurements taken by Bumpus of the ‘ventricosity’ (ratio of breadth to length) of the shell of the Periwinkle, Lit/orina lit/area. The series of observations is very large, and includes both British and American forms. In the accompanying table (Table V) the coeflicients of variability (the standard deviation expressed as a percentage of the mean) are given for each age, as determined by length of shell, for the English and American periwinkles separately and also for the complete series. It will be seen that the variability increases slightly and then diminishes again. This is the case also in the American examples, where the fall at the end of growth is greater still, but in the British specimens there is only a slight fall, at 20-21 mm., followed by a considerable rise. The possible signiﬁcance of this diiference in the behaviour of the same species on the two sides of the Atlantic we shall discuss
in a moment. TABLE V Showing the alteration in the variability of the ventrieosity of the shell of the Periwinkle (Liltorina littorea.) during growth.
Coefficient of variability (E; x 100).
Length in mm. British. American. All. -15 2-77 3-27 3-25
16-17 2-94 3-41 3-34 18-19 3-02 3-39 3-35 20-21 2-93 3-03 3-13
22 3-25 2-84 3-02
In the meantime let us consider another case, the Crab, Cr/rcimc.v mocnas, the variability of the frontal breadth of which was examined by the late Professor VVelden. Weldon found that the variability, as measured by the quartile error , first increased and then suddenly diminished with age (as determined by carapaee length). If the variability is measured by the coefficient of variability (easily calculated from VVel(lon’s data) the result is the same. This will appear from the table
(Table VI). TABLE VI
Showing the change in the variability of the frontal breaulth with age in Curr-inns momms. (After Weldon.)
Cara ace Len th Q. 0'
E, ,,,m_ 3 51- x 100. 7-5 9-42 1-64 8-5 9-83 1-76 9-5 9-51 1-73 10-5 9-58 1-78 11-5 10-25 1-93 12-5 10-79 2-06 13-5 10-09 1-95
For the calculation of the variability at different ages in man data have been provided by Roberts, Bowditch, and Boas. Some II. 2 GROWTH 73
of these results are collected in the following table (Table VII), from which it may be gathered that the variability diminishes at first, then rises until it attains a maximum at about the time of puberty, and then diminishes again, reaching ﬁnally a value which is lower than the original. The values for the coeﬂicient obtained by the diifercnt investigators are fairly similar, and agree very well with those first given by Pearson (for male new—born infants, weight 15-66, stature 6-50; for male adults, weight 10-83, stature 3-66). It may be seen from the values for the newborn that the variability has already undergone adiminution before the age at which the other observations begin.
Showing the change in the value of the coeﬁieient of variability in the male Human being during growth.
Coeffioieiit of variability (1% X 100).
Vtleight. Stature. Years. Boston Worcester, English Boston Toronto \Vorecstor, (Bowditch). Mass. Artisans (Bowditch \. (Boas). Mass. (Boas). (Roberts). (Boas) 4 14-00 5 11-56 11-48 4-76 4-82 6 10-28 12-04 10-08 4-60 4-34 5-40 7 11-08 11-87 10-29 4-42 4-35 4-24 8 9-92 11-83 10-78 4-49 4-58 4-32 9 11-04 12-29 10-85 4-40 4-41 4-30 10 11-60 12-92 11-06 4-55 4-68 4-44 11 11-76 14-45 11-90 4-70 4-53 4-51 12 13-72 15-56 11-48 4-90 4-85 4-49 13 13-60 18-07 11-76 5-47 5-36 5-21 14 16-80 16-80 12-74 5-79 5-64 5-43 15 15-32 18-28 14-00 5-57 5-71 5-19 16 13-28 13-95 12-95 4-50 3-92 17 12-96 11-23 11-55 4-55 3-32 18 10-40 12-18 3 69 19 10-29 20 9-03 10-50 10-92 12-04
Further, the variability does not merely diminish as the animal grows older. Its diminution accompanies the diminution in the rate of growth, and when—as at the time of puberty in man—that rate increases, the variability increases too.
The variability of such parts as have been examined for the purpose alters in the same way as that of the whole body. Besides weight and stature Boas has recorded measurements of height sitting, head-length, head-width, length of fore-arm, and hand-width.
Though the evidence, it must be admitted, is scanty, it is none the less a remarkable fact that in all the cases we have examined the variability, whether of the whole organism or of its parts, decreases with the decrease in the rate of growth. We seem to be in the presence of a phenomenon of general occurrence, though what the signiﬁcance of the phenomenon is is not at present clear.
As is well known, Weldon has argued that the decline in the variability of the older crabs is due to a selective death-rate, an argument which is supported by the same author’s observations on the snail Clausilia, since in this form the variability of the adult was found to be the same as the variability of the same individuals when young, but less than the variability of the general population of young. It is possible that the marked decrease in the variability of the American as compared with the British periwinkles may also be attributable to the same cause, since this animal has only recently been introduced into America, and may, therefore, be subjected to a more severe struggle for existence in its new environment.
It is doubtful, however, whether this explanation will ﬁt all cases.
Vernon has suggested that at periods of rapid growth the effect produced upon the organism by a change in its environment must be much greater than at other times, and, since he has further shown that one of the effects of an adverse change of circumstances is an increased variability, he argues that an increase in variability would naturally accompany a high growthrate.
Lastly, Boas points out that the rate of growth is itself a variable magnitude, and this ‘ variation in period’ may, with other causes, be a factor in producing the actual variation at each stage. Should that be so, the variability would necessarily increase and diminish with an increasing and diminishing growthrate, since those that are above the mean would tend to remove themselves further from the mean than those that are below could approach it, and the more so the faster they were growing, and conversely.
We have ﬁnally to consider very brieﬂy what little is known of the alteration with growth in the value of the correlation between various organs. Such data as we have indicate that, like variability, this value rises and falls with the growth-rate.
Boas has ascertained the correlation coefﬁcicnt (p) in man between weight, stature, height sitting, length and width of head at diiferent ages. Some of these results are tabulated below (Table VIII) ; from this table it will be evident that the value of p decreases, increases, and decreases again. The values are for girls, and the period of increase is earlier than that found for boys. In the chart (Fig. 42) are given the successive values of growth-rate (stature), variability (height), and correlation coefficient (height sitting and head-length) for boys; the three magnitudes rise at about the time of puberty, and subsequently decline together. Boas urges that if the actual variability is in part the eﬁ’cct of variation in period, this eifect will be greater during periods of rapid development. It follows from this that if the various organs of the body are equally affected by a change in the growth-rate, correlations would be closer during periods of rapid growth than at other periods.
Values of the correlation coefficient, p, during growth for four diﬂbrent correlations. Girls, Worcester, Mass. (Boas).
Years. Stature and Stature and Stature and Stature and Weight. Height sitting. Length of Head. Width of Head. 7 -73 -74 -30 -21 8 -76 -79 -36 -15 9 -80 -82 -35 -16 10 ~83 -83 -37 -16 ll -81 -84 -37 -25 12 -77 -82 -38 -27 13 -73 -83 .38 -37 14 -67 -82 -30 -25 15 -6!’) -79 -26 -22 16 -60 -74 -25 -10
It will be noticed that the value of p for the different organs is different, being greater between axial organs—stature and height sitting, stature and length of head-—than between longitudinal and transverse parts, such as stature and width of head. The correlation between stature and weight is liigli.
To whatever cause it may be due this diminution of correlation with age is of the greatest interest, since it points to an increasing power of self-diﬁerentiation in the parts of the body. From other sources also there is evidence of a. progressive loss of totipotentiality of the parts, of an inereasing- independence of the parts, of a tendency to be increasingly governed in their development by factors that reside wholly within themselves. But this evidence must be discussed elsewhere.
FIG.42.- Figure to show how the rate of growth (percentage increments of stature), the variability (of stature) and the correlation coefficient (between height sitting and length of head) rise together at the time of puberty in man and then fall together. (Constructed from the tables of Boas.)
F. BOAS. The growth of’ Toronto children, U.S.A. Report of the Commissioner of Ed'ucatz'on, ii, 1897.
F. BoAs and C. WISSLER. Statistics of growth, Unitecl Slates Education Commission, i, 1904.
H. P. BOWDITCII. The growth of children, Massachusetts Sfale Board of Iloalih, 1877.
II. C. BUMPUS. The variations and mutations of the introduced Littorina., Zool. Bull. i, 1898.
C. B. DAVENPORT. The role of water in growth, Proc. Boslou Soc. Nat. Hist. xxviii, 1899 (1).
C. B. DAVENPORT. Experimental morphology, New York, 1899 (2).
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G. KRAUS. Ueber die Wa.sse1-vertheilung in der Pﬂmize, Fests-rhr. Fem‘ Iuuulertjiilu-. Best. Natmf. Ges., Halle, 1879.
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K. PEARSON. Data. for the 1)l‘Ol)1el1l of evolution in man. Ill. On the magnitude of certain coeﬂicients of correlation in mam. 1’rac. Roy. Soc. lxvi, 1900.
W. PREYER. Spezielle Physiologie (les Embryo. VIII. Dns embryona.le Wachsthum. Leipzig, 1885.
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K. SEMPER. Animal Life, 5th ed., London, 1906.
H. M. VERNON. '1‘he effect of environment on the development of Echinoderm larvae : an experimental enquiry into the causes of variation, Phil. Trams. Roy. Soc. elxxxvi, B, 1895.
II. M. VERNON. Variation, London, 1898.
W. F. R. WELDON. An attempt to measure the death-ra.te due to the selective destruction of (larcinus mamas with respect to a, pu.rticul:|.r dimension, 1’roc. Roy. Soc. lvii, 1894-5.
W. I‘‘. R. WELDON. A first study of na.tura,l selection in (L'luu.x-iliu Imm'nalu, Biometrilca, i, 1901-2.
- Jenkinson (1909): 1 Introductory | 2 Cell-Division and Growth | 3 External Factors | 4 Internal Factors | 5 Driesch’s Theories - General Conclusions | 6 Appendices
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