[28]By the term ‘nuclear spindle’ I refer to the peculiar form of a double striated cone assumed by the nucleus just before division, which is no doubt familiar to all my readers. I use the term star for the peculiar stellate figure usually visible at the poles of the nuclear spindle. For a further description of these parts the reader is referred to ChapterIV.[29]For the further details on the nuclear spindlevidethe next Chapter.[30]The best instance of what appears like a polar cell in Arthropoda is a body recently found by Grobben (“Entwicklungsgeschichte d. Moina rectirostris.” Claus’Arbeiten,Vol.II.,Wien,1879) near the surface of the protoplasm at the animal pole of the summer and parthenogenetic eggs ofMoina rectirostris, one of the Cladocera. The body stains deeply with carmine, but differs from normal polar cells in not being separated from the ovum; and its identification as a polar cell must remain doubtful till it has been shewn to originate from the germinal vesicle.[31]“Zur Entwicklung d. Entomostraken.”Niederlandischer Archiv. f. Zoologie,Vol.III.p.62.[32]The instances quoted by Siebold,Parthenogenesis d. Arthropoden, are not quite satisfactory. In Hensen’s case,p.234, impregnation would have been possible if we can suppose the spermatozoa to be capable of passing into the body cavity through the open end of the uninjured oviduct; and though Oellacher’s instances are more valuable, yet sufficient care seems hardly to have been taken, especially when it is not certain for what length of time spermatozoa may be able to live in the oviduct. For Oellacher’s precautions,videZeit. für Wiss. Zool.,Bd.xxii., p.202. A better instance is that of a sow given by Bischoff,Ann. Sci. Nat., series 3,Vol.II., 1844. The unimpregnated eggs were found divided into segments, but the segments did not contain the usual nucleus, and were perhaps nothing else than the parts of an ovum in a state of disruption.[33]Darwin,Cross- and Self-Fertilization of Plants,p.443.[34]Mr J. A. Osborne has recently shewn (Nature, Sept. 4, 1879), that the eggs of a Beetle (Gastrophysa raphani) may occasionally develop, up to a certain point at any rate, without the male influence.[35]Dicyema, which is an apparent exception, has not yet been certainly shewn to develop true ova. If its germs are true ova it forms an exception to the above rule.
[28]By the term ‘nuclear spindle’ I refer to the peculiar form of a double striated cone assumed by the nucleus just before division, which is no doubt familiar to all my readers. I use the term star for the peculiar stellate figure usually visible at the poles of the nuclear spindle. For a further description of these parts the reader is referred to ChapterIV.
[29]For the further details on the nuclear spindlevidethe next Chapter.
[30]The best instance of what appears like a polar cell in Arthropoda is a body recently found by Grobben (“Entwicklungsgeschichte d. Moina rectirostris.” Claus’Arbeiten,Vol.II.,Wien,1879) near the surface of the protoplasm at the animal pole of the summer and parthenogenetic eggs ofMoina rectirostris, one of the Cladocera. The body stains deeply with carmine, but differs from normal polar cells in not being separated from the ovum; and its identification as a polar cell must remain doubtful till it has been shewn to originate from the germinal vesicle.
[31]“Zur Entwicklung d. Entomostraken.”Niederlandischer Archiv. f. Zoologie,Vol.III.p.62.
[32]The instances quoted by Siebold,Parthenogenesis d. Arthropoden, are not quite satisfactory. In Hensen’s case,p.234, impregnation would have been possible if we can suppose the spermatozoa to be capable of passing into the body cavity through the open end of the uninjured oviduct; and though Oellacher’s instances are more valuable, yet sufficient care seems hardly to have been taken, especially when it is not certain for what length of time spermatozoa may be able to live in the oviduct. For Oellacher’s precautions,videZeit. für Wiss. Zool.,Bd.xxii., p.202. A better instance is that of a sow given by Bischoff,Ann. Sci. Nat., series 3,Vol.II., 1844. The unimpregnated eggs were found divided into segments, but the segments did not contain the usual nucleus, and were perhaps nothing else than the parts of an ovum in a state of disruption.
[33]Darwin,Cross- and Self-Fertilization of Plants,p.443.
[34]Mr J. A. Osborne has recently shewn (Nature, Sept. 4, 1879), that the eggs of a Beetle (Gastrophysa raphani) may occasionally develop, up to a certain point at any rate, without the male influence.
[35]Dicyema, which is an apparent exception, has not yet been certainly shewn to develop true ova. If its germs are true ova it forms an exception to the above rule.
The immediate result of the fusion of the male and female pronucleus is the segmentation or division of the ovum usually into two, four, eight, etc. successive parts. The segmentation may be dealt with from two points of view,viz.(1) the nature of the vital phenomena which take place in the ovum during its occurrence, which may be described as the internal phenomena of segmentation. (2) The external characters of the segmentation.
Internal Phenomena of Segmentation.
Numerous descriptions have been given during the last few years of the internal phenomena of segmentation. The most recent contribution on this head is that of Fol (No.87). He appears to have been more successful than other observers in obtaining a complete history of the changes which take place, and it will therefore be convenient to take as type the ovum ofToxopneustes (Echinus) lividus, on which he made his most complete series of observations. The changes which take place may be divided into a series of stages. The ovum immediately after the fusion of the male and female pronucleus contains a central segmentation nucleus.
In the first stage a clear protoplasmic layer derived from the plasma of the cell is formed round the nucleus, from which there start outwards a number of radial striæ, which are rendered conspicuous by the radial arrangement of the yolk granulesbetween them. The nucleus during this process remains perfectly passive.
In the second stage the nucleus becomes less distinct and somewhat elongated, and around it the protoplasmic layer of the earlier stage is arranged in the form of a disc-shaped ring, compared by Fol to Saturn’s ring. The protoplasmic rays still take their origin from the perinuclear protoplasm. This stage has a considerable duration (20 minutes).
In the third stage the protoplasm around the nucleus becomes transported to the two nuclear poles, at each of which it forms a clear mass surrounded by a star-shaped figure formed by radial striæ. The nucleus is hardly visible in the fresh condition, but when brought into view by reagents is found to contain many highly refractive particles, and to be still enveloped in a membrane.
In the fourth stage the nucleus when treated by reagents has assumed the well-known spindle form. The striæ of which it is composed are continuous from one end of the spindle to the other and are thickened at the centre. The central thickenings constitute the so-called nuclear plate. The clear protoplasmic masses and stars are present as before at the apices of the nucleus, and the rays of the latter converge as if they would meet at the centre of the clear masses, but stop short at their periphery. There is no trace of a membrane round either the nuclear spindle or the clear masses; and in the centre of the latter is a collection of granules. The striæ of the polar stars are very fine but distinct.
Between the stage with a completely formed spindle and the previous one the intermediate steps have not been made out for Toxopneustes; but for Heteropods Fol has been able to demonstrate that the striæ of the spindle and their central thickenings are formed, as in the case of the spindle derived from the germinal vesicle,from the metamorphosis of the nuclear reticulum. They commence to be formed at the two poles, and are then (in Heteropods) in immediate contiguity with the striæ of the stars. The striæ gradually grow towards the centre of the nucleus and there meet.
In the fifth stage the central thickenings of the spindle separate into two sets, which travel symmetrically outwardstowards the clear masses, growing in size during the process. They remain however united for a short time by delicate filaments—named by Fol connective filaments—which very soon disappear. The clear masses also increase in size. During this stage the protoplasm of the ovum exhibits active amœboid movements preparatory to division.
In the sixth stage, which commences when the central thickenings of the spindle have reached the clear polar masses, the division of the ovum into two parts is effected by an equatorial constriction at right angles to the long axis of the nucleus. The inner vitelline membrane follows the furrow for a certain distance, but does not divide with the ovum. All connection between the two parts of the spindle becomes lost during this stage, and the thickenings of the fibres of the spindle give rise to a number of spherical vesicular bodies, which pass into the clear masses and become intermingled with the granules which are placed there. The radii of the stars now extend round the whole circumference of each of the clear masses.
In the seventh stage the two clear masses become elongated and travel towards the outer sides of their segments; while the radii connected with them become somewhat bent, as if a certain amount of traction had been exercised on them in the movement of the clear masses. Shortly afterwards the spherical vesicles, each of which appears like a small nucleus and contains a central nucleolus, begin to unite amongst themselves, and to coalesce with the neighbouring granules. Those in each segment finally unite to form a nucleus which absorbs the substance of the clear mass.The new nucleus is therefore partly derived from the division of the old one and partly from the plasma of the cell.The two segments formed by division are at first spherical, but soon become flattened against each other. In each subsequent division of these cells the whole of the above changes are repeated.
The phenomena which have just been described would appear to occur in the segmentation of ova with remarkable constancy and without any very considerable variations.
The division of the ovum constitutes a special case of cell division, and it is important to determine to what extent the phenomena of ordinary cell division are related to those which take place in the division of the ovum.Without attempting a full discussion of the subject I will confine myself to a few remarks suggested by the observations of Flemming, Peremeschko and Klein. The observations of these authors shew that in the course of the division of nuclei in the salamander, newt, etc. the nuclear reticulum undergoes a series of peculiar changes of form, and after the membrane of the nucleus has vanished divides into two masses. The masses form the basis for the new nuclei, and become reconverted into an ordinary nuclear reticulum after repeating, in the reverse order, the changes of form undergone by the reticulum previous to its division.
It is clear without further explanation that the conversion of the nuclear reticulum of the segmentation nucleus into the striæ of the spindle is a special case of the same phenomenon as that first described by Flemming in the salamander. There are however some considerable differences. In the first place the fibres in the salamander do not, according to Flemming, unite in the middle line, though they appear to do so in the newt. This clearly cannot be regarded as a fact of great importance; nor can the existence of the central thickenings of the striæ (nuclear plate), constant as it is for the division of the nucleus of the ovum, be considered as constituting a fundamental difference between the two cases. More important is the fact that the striæ in the case of the ovum do not appear, at any rate have not been shewn, to form themselves again into a nuclear network.
With reference to the last point it is however to be borne in mind (1) that the gradual travelling outwards of the two halves of the nuclear plate is up to a certain point a repetition, in the reverse order, of the mode of formation of the striæ of the spindle, since the striæ first appeared at the poles and gradually grew towards the middle of the spindle; (2) that there is still considerable doubt as to how the vesicular bodies formed out of the nuclear plate reconstitute themselves into a nucleus.
The layer of clear protoplasm around the nucleus during its division has its homologue in the case of the division of the nuclei of the salamander, and the rays starting from this are also found. Klein has suggested that the extra-nuclear rays of the stars around the poles of the nucleus are derived from a metamorphosis of the extra-nuclear reticulum, which he believes to be continuous with the intra-nuclear reticulum.
The delicate connective filaments usually visible between the two halves of the nuclear plate would seem from Strasburger’s latest observations (No.104) to be derived from the nuclear substance between the striæ of the spindle, and to become eventually reabsorbed into the newly-formed nuclei.
We are it appears to me still in complete ignorance as to the physical causes of segmentation. The view that the nucleus is a single centre of attraction, and that by its division the centre of attraction becomes double and thereby causes division, appears to be quite untenable. The description already given of the phenomena of segmentation is in itself sufficient to refute this view.Nor is it in the least proved by the fact (shewn by Hallez) that the plane of division of the cell always bears a definite relation to the direction of the axis of the nucleus.
The arguments by which Kleinenberg (93) attempted to demonstrate that cell division was a phenomenon caused by alterations in the molecular cohesion of the protoplasm of the ovum still in my opinion hold good, but recent discoveries as to the changes which take place in the nucleus during division probably indicate that the molecular changes which take place in the cohesion of the protoplasm are closely related to, and possibly caused by, those in the nucleus. These alterations of cohesion are produced by a series of molecular changes, the external indications of which are to be found in the visible alterations in the constitution of the body of the cell and of the nucleus prior to division.
Bibliography.
In addition to the papers cited in the last Chapter,vide
(101)W. Flemming.“Beiträge z. Kenntniss d. Zelle u. ihrer Lebenserscheinungen.”Archiv f. mikr. Anat.,Vol.XVI., 1878.(102)E. Klein. “Observations on the glandular epithelium and division of nuclei in the skin of the Newt.”Quart. J. of Micr. Science,Vol.XIX., 1879.(103)Peremeschko.“Ueber d. Theilung d. thierischen Zellen.”Archiv f. mikr. Anat.,Vol.XVI., 1878.(104)E. Strasburger.“Ueber ein z. Demonstration geeignetes Zelltheilungs-Object.”Sitz. d. Jenaischen Gesell. f. Med. u. Naturwiss.,July 18, 1879.
External Features of Segmentation.
Stages of segmentationFig. 38. Various stages in process of segmentation.(After Gegenbaur.)
Fig. 38. Various stages in process of segmentation.(After Gegenbaur.)
In the simplest known type of segmentation the ovum first of all divides into two, then four, eight, sixteen, thirty-two, sixty-four, etc. cells (fig. 38). These cells so long as they are fairly large are usually known as segments or spheres. At the close of sucha simple segmentation the ovum becomes converted into a sphere composed of segments of a uniform size. These segments usually form a wall (fig. 39, E), one row of cells thick, round a central cavity, which is known as the segmentation cavity or cavity of Von Baer. Such a sphere is known as a blastosphere. The central cavity usually appears very early in the segmentation, in many cases when only four segments are present (fig. 39, B).
The Segmentation of AmphioxuxFig. 39. The Segmentation of Amphioxux.(Copied from Kowalevsky.)sg.segmentation cavity. A. Stage with two equal segments. B. Stage with four equal segments. C. Stage after the four segments have become divided by an equatorial furrow into eight equal segments. D. Stage in which a single layer of cells encloses a central segmentation cavity. E. Somewhat older stage in optical section.
Fig. 39. The Segmentation of Amphioxux.(Copied from Kowalevsky.)
sg.segmentation cavity. A. Stage with two equal segments. B. Stage with four equal segments. C. Stage after the four segments have become divided by an equatorial furrow into eight equal segments. D. Stage in which a single layer of cells encloses a central segmentation cavity. E. Somewhat older stage in optical section.
In other instances, which however are rarer than those in which a segmentation cavity is present, there is no trace of a central cavity, and the sphere at the close of segmentation is quite solid. In such instances the solid sphere is known as a morula. It is found in some Sponges, many Cœlenterata, some Nemertines, etc., and in Mammals; in which group the segmentation is not however quite regular. All intermediate conditions between a large segmentation cavity, and a very minute central cavity which may be surrounded by more than a single row of cells have been described.
The segmentation cavity has occasionally, as in Sycandra, the Ctenophora and Amphioxus, the form of an axial perforation of the ovum open at both extremities.
When the process of regular segmentation is examined somewhat more in detail it is found to follow as a rule a rather definite rhythm. The ovum is first divided in a plane which may be called vertical, into two equal parts (fig. 39, A). This division is followed by a second, also in a vertical plane, but at right angles to the first plane, and by it each of the previous segments is halved (fig. 39, B.) In the third segmentation the plane of division is horizontal or equatorial and divides each of the four segments into two halves, making eight segments in all (fig. 39, C). In the fourth period the segmentation takes place in two vertical planes each at an angle of 45° with one of the previous vertical planes. All the segments are thus again divided into two equal parts. In the fifth period there are two equatorial planes one on each side of the original equatorial plane, and thirty-two spheres are present at the close of this period. Sixty-four segments are formed at the sixth period, but beyond the fourth and fifth periods the original regularity is not usually preserved.
In many instances the type of segmentation just described cannot be distinctly recognized. All that can be noticed is that at each fresh segmentation every segment becomes divided into two equal parts. It is not absolutely certain that there is not always some slight inequality in the segments formed, by which, what are known as the animal and vegetative poles of the ovum, can very early be distinguished. A regular segmentation is found in species in most groups of the animal kingdom. It is very common in Sponges and Cœlenterates. Though less common so far as is known amongst the Vermes, it is yet found in many of the lower types,viz.Nematoidea, Gordiacea, Trematoda, Nemertea (apparently as a rule),Sagitta,Chætonotus, some Gephyrea (Phoronis); though not usual it occurs amongst Chætopoda,e.g. Serpula. It is the usual type of segmentation amongst the Echinodermata. Amongst the Crustacea it appears (for the earlier phases of segmentation at any rate) not infrequently amongst the lower forms, and even occurs amongst the Amphipoda (Phronima). It is however very rare amongst the Tracheata,Poduraaffording the one example of it known to me. It is almost as rare amongst Mollusca as amongst the Tracheata, but occurs inChitonand is nearly approached in some Nudibranchiata. In Vertebrata it is most nearly approached inAmphioxus[36].
Most of the eggs which have a perfectly regular segmentation are of a very insignificant size and rarely contain much food-yolk:in the vast majority of eggs there is present however a considerable bulk of food material usually in the form of highly refracting yolk-spherules. These yolk-spherules lie embedded in the protoplasm of the ovum, but are in most instances not distributed uniformly, being less closely packed and smaller at one pole of the ovum than elsewhere. Where the yolk-spherules are fewest the active protoplasm is necessarily most concentrated, and we can lay down as a general law[37]that the velocity of segmentation in any part of the ovum is roughly speaking proportional to the concentration of the protoplasm there; and that the size of the segments is inversely proportional to the concentration of the protoplasm. Thus the segments produced from that part of an egg where the yolk-spherules are most bulky, and where therefore the protoplasm is least concentrated, are larger than the remaining segments, and their formation proceeds more slowly.
Though where much food-yolk is present it is generally distributed unequally, yet there are many cases in which it is not possible to notice this very distinctly. In most of these cases the segmentation is all the same unequal, and it is probable that they form apparent rather than real exceptions to the law laid down above. Although before segmentation the protoplasm may be uniformly distributed, yet in many instances,e.g.Mollusca, Vermes, etc., during or at the commencement of segmentation the protoplasm becomes aggregated at one pole, and one of the segments formed consists of clear protoplasm, all the food-yolk being contained in the other and larger segment.
Unequal Segmentation.The type of segmentation I now proceed to describe has been called by Haeckel (No.105) ‘unequal segmentation’, a term which may conveniently be adopted. I commence by describing it as it occurs in the well-known and typical instance of the Frog[38].
The ripe ovum of the common Frog and of most other tailless Amphibians presents the following structure. One half appears black and the other white. The former I shall call the upperpole, the latter the lower. The ovum is composed of protoplasm containing in suspension numerous yolk-spherules. The largest of these are situated at the lower pole, the smaller ones at the upper pole, and the smallest of all in the peripheral layer of the upper pole, in which also pigment is scattered and causes the black colour visible from the surface.
Segmentation of Common FrogFig. 40. Segmentation of Common Frog. Rana Temporaria.(Copied from Ecker.)The numbers above the figures refer to the number of segments at the stage figured.
Fig. 40. Segmentation of Common Frog. Rana Temporaria.(Copied from Ecker.)
The numbers above the figures refer to the number of segments at the stage figured.
The first formed furrow is a vertical furrow. It commences in the upper half of the ovum, through which it extends rapidly, and then more slowly through the lower. As soon as the first furrow has extended through the egg, and the two halves have become separated from each other, a second vertical furrow appears at right angles to the first and behaves in the same way (fig. 40, 4).
Section through Frog’s ovumFig. 41. Section through Frog’s ovum at the close of segmentation.sg.segmentation cavity.ll.large yolk-containing cells.ep.small cells at formative pole (epiblast).
Fig. 41. Section through Frog’s ovum at the close of segmentation.
sg.segmentation cavity.ll.large yolk-containing cells.ep.small cells at formative pole (epiblast).
The next furrow is equatorial or horizontal (fig. 40, 8). It does not ariseat the true equator of the egg, but much nearer to its upper pole. It extends rapidly round the egg and divides each of the four previous segments into two parts, one larger and one smaller. Thus at the end of this stage there are present four small and four large segments. At the meeting point of these asmall cavity appears, which is the segmentation cavity, already described for uniformly segmenting eggs. It increases in size in subsequent stages, its roof being formed of the smaller cells and its floor of the larger. The appearance of the equatorial furrow is followed by a period of repose, after which two rapidly succeeding vertical furrows are formed in the upper pole, dividing each of the four segments of which this is composed into two. After a short period these furrows extend to the lower pole, and when completed 16 segments are present—eight larger and eight smaller (fig. 40, 16). A pause now ensues, after which the eight upper segments become divided by an equatorial furrow, and somewhat later a similar furrow divides the eight lower segments. At the end of this stage there are therefore present 16 smaller and 16 larger segments (fig. 40, 32). After 64 segments have been formed by vertical furrows which arise symmetrically in the two poles (fig. 40, 64), two equatorial furrows appear in the upper pole before a fresh furrow arises in the lower; so that there are 128 segments in the upper half, and only 32 in the lower. The regularity is quite lost in subsequent stages, but the upper pole continues to undergo a more rapid segmentation than the lower. While the segments have been increasing in number the segmentation cavity has been rapidly growing in size; and at the close of segmentation the egg forms a sphere, containing an excentric cavity, and composed of two unequal parts (fig. 41). The upper part, which forms the roof of the segmentation cavity, is formed of smaller cells: the lower of larger yolk-containing cells.
The mode of segmentation of the Frog’s ovum is typical for unequally segmenting ova, and it deserves to be noticed that as regards the first three or more furrows the segmentation occurs with the same rhythm in the unequally segmenting ova as in those which have an uniform segmentation. There appear two vertical furrows followed by an equatorial furrow. The general laws which were stated with reference to thevelocityof segmentation and the size of the resulting segments are well exemplified in the case of the Frog’s ovum.
The majority of the smaller segments in the segmented Frog’s ovum are destined to form into the epiblast, and the larger segments become hypoblast and mesoblast.
With a few exceptions (the Rabbit, Lymnæus, etc.) the majority of the smaller segments always become epiblast and of the larger segments hypoblast.
The Frog’s ovum serves as a good medium type for unequally segmenting ova. There are many cases however in which a regular segmentation is far more closely approached, and others in which it is less so.
One familiar instance in which a regular segmentation is nearly approached is afforded by the Rabbit’s ovum, which has indeed usually been regarded as offering an example of a regular segmentation.
The ovum of the Rabbit[39]becomes first divided into two sub-equal spheres. The larger and more transparent of the two may, from its eventual fate, be called the epiblastic sphere, and the other the hypoblastic. The two spheres are divided into four, and then by an equatorial furrow into eight—four epiblastic and four hypoblastic. One of the latter assumes a central position. The four epiblastic spheres now divide before the four hypoblastic. There is thus introduced a stage with twelve spheres. It is followed by one with sixteen, and that by one with twenty-four. During the stages with sixteen spheres and onwards the epiblastic spheres gradually envelop the hypoblastic, which remain exposed on the surface at one point only. There is no segmentation cavity.
In Pedicellina, one of the entoproctous Polyzoa, there is a sub-regular segmentation, where however the two primary spheres can be distinguished much in the same way as in the case of the Rabbit.
A very characteristic type of unequal segmentation is that presented by the majority of Gasteropods and Pteropods and probably also of some Lamellibranchiata. It is also found in some Turbellarians, in Bonellia, some Annelids, etc. In many instances it offers a good example of the type where in the course of segmentation the protoplasm becomes aggregated at one pole of the ovum, or of its segments, to become separated off as a clear sphere.
The first four segments formed by two vertical furrows atright angles are equal, but from these there are budded off four smaller segments, which in subsequent stages divide rapidly, receiving however, a continual accession of segments budded off from the larger spheres. The four larger spheres remain conspicuous till near the close of the segmentation. The process of budding, by which the smaller spheres become separated from the larger, consists in a larger sphere throwing out a prominence, which then becomes constricted off from it.
In the extreme forms of this unequal segmentation we find at the end of the second cleavage two larger spheres filled with yolk material and two smaller clear spheres; and in the later stages, though the large spheres continue to bud off small spheres, only the two smaller ones undergo a regular segmentation, and eventually completely envelop the former. Such a case as this has been described in Aplysia by Lankester[40].
The types I have described serve to exemplify unequal segmentation. The Rabbit’s ovum stands at one end of the series, that of Aplysia at the other; and the Frog’s ovum between the two.
Great variations are presented by the ova with unequal segmentation as to the presence of a segmentation cavity. In some instances,e.g.the Frog, such a cavity is well developed. In other cases it is small,e.g.most Mollusca, while not unfrequently it is altogether absent.
Before leaving this important type of segmentation, it will be well to enter with slightly greater detail into some of the more typical as well as some of the special forms which it presents.
As an example of the typical Molluscan type the normal Heteropod segmentation, accurately described by Fol[41], may be selected.
The ovum divides into two and then four equal segments in the usual vertical planes. Each segment has a protoplasmic and a vitelline pole. The protoplasmic pole is turned towards the polar bodies. In the third segmentation, which takes place along an equatorial plane, four small protoplasmic cells or segments are segmented or rather budded off from the four large segments, so that there are four small segments in one plane and four large below these. In the fourth segmentation the four large segments alone are active and give rise to four small and four large cells; so that there are formed in all eight small and four large cells. The four small cells of thethird generation next divide, forming in all twelve small cells and four large. The small cells of the fourth generation then divide, and subsequently the four large cells give rise to four new small ones, so that there are twenty small cells and four large. The small cells form a cap embracing the upper pole of the large segments. It may be noted that from the third stage onwards the cells increase in arithmetical progression—a characteristic feature of the typical gasteropod segmentation.
In the later stages of segmentation the large cells cease to give rise to smaller ones in the same manner as before. One of them divides first into two unequal parts, of which the smaller becomes pushed in towards the centre of the egg. The larger cell then divides again into two, and the three cells so formed occupy the centre of a shallow depression. The remaining larger cells divide in the same way, and give rise to smaller cells which line a pit which becomes formed on one side of the ovum. The original smaller cells continue in the meantime to divide and so form a layer enclosing the larger, leaving exposed however the opening of the pit lined by the latest products of the larger cells.
The eggs of Anodon and Unio serve as excellent examples of the type in which the ovum has a uniform structure before the commencement of segmentation, but in which a separation into a protoplasmic and a nutritive portion becomes obvious during segmentation.
In Anodon[42]the egg is at first uniformly granular, but after impregnation it throws out on one side a protuberance nearly free from granules (fig. 42, 1)
Segmentation of Anodon piscinalisFig. 42. Segmentation of Anodon piscinalis.(Copied from Flemming.)r.polar cells.v.vitelline sphere. 1. Commencing division into two segments; one mainly formed of protoplasm, the other of yolk. 2. Stage with four segments. 3. Formation of blastosphere, and segmentation cavity. 4. Definite segmentation of the yolk sphere.
Fig. 42. Segmentation of Anodon piscinalis.(Copied from Flemming.)r.polar cells.v.vitelline sphere. 1. Commencing division into two segments; one mainly formed of protoplasm, the other of yolk. 2. Stage with four segments. 3. Formation of blastosphere, and segmentation cavity. 4. Definite segmentation of the yolk sphere.
In the case of this clear protuberance and of the similar protuberances which follow it, the protoplasm is not at first quite free from food-yolk, but only becomes so on being separated from the yolk-containing part of the ovum. We must therefore suppose that the production of the clear segments is in part at least due to the yolk-spherules becoming used up to form protoplasm. Such a formation of protoplasm from yolk-spherules has been clearly shewn to occur in other types by Bobretzky and Fol.
The protuberance soon becomes separated off from the larger part of the egg as a small segment composed of clear protoplasm. From the larger segment filled with food-yolk, a second small clear segment is next budded off, and simultaneously (fig. 42, 2) the original small segment divides into two. Thus there are formed four segments, one large and three small; the large segment as before being filled with food-yolk. The continuation of a similar process of budding off and segmentation eventually results in the formation of a considerable number of small and of one large segment (fig. 42, 3). Between this large and the small segments is a segmentation cavity.
Eventually the large yolk segment, which has hitherto merely budded off a series of small segments free from yolk, itself divides into two similar parts. This process is then repeated (fig. 42, 4) and there is at last formed a number of yolk segments filled with yolk spheres, which occupy the place of the original large yolk segment. Between these yolk segments and the small segments is placed the segmentation cavity.
The segmentation of the ovum of Euaxes[43]resembles that of Unio in the budding off of clear segments from those filled with yolk, but presents many interesting individualities.
A very peculiar modification of the ordinary Gasteropod segmentation is that described by Bobretzky for Nassa mutabilis[44].
The ovum contains a large amount of food-yolk, and the protoplasm is aggregated at the formative pole, adjoining which are placed the polar bodies. An equatorial and a vertical furrow (fig. 43A), the former near the upper pole, appear simultaneously, and divide the ovum into three segments, two small, each with a protoplasmic pole, and one large entirely formed of yolk material. One of the two small segments next completely fuses with the large segment (fig. 43B), and after the fusion is complete, a triple segmentation of the large segment takes place as at the first division, and at the same time the single small segment divides into two. In this way four partially protoplasmic segments and one yolk segment are formed (fig. 43C). One of the small segments again fuses with the large segment, so that the number of segments becomes again reduced to four, three small and one large. The protoplasmic ends of these segments are turned towards each other, and where they meet four very small cells become budded off, one from each segment (fig. 43D). Four small cells are again budded off twice in succession, while the original small cells remain passive, so that there come to be twelve small and four large cells. In later stages the four first-formed small cells give rise to still smaller cells and then the next-formed do the same. The large cells continue also to give rise to small ones, and finally, by a continuous process of division, and fresh budding of small cells from large cells, a cap of small cells becomes formed covering the four large cells which have in the meantime pressed themselves together (fig. 43E). A segmentation cavity of not inconsiderable dimensions becomes established between this cap of small cells and the large cells.
Segmentation of Nassa mutabilisFig. 43. Segmentation of Nassa mutabilis(from Bobretzky). A. Upper half divided into two segments. B. One of these has fused with the large lower segment. C. Four small and one large segment, one of the former fusing with the large segment. D. Each of the four segments has given rise to a small segment. E. Small segments have increased to thirty-six.
Fig. 43. Segmentation of Nassa mutabilis(from Bobretzky). A. Upper half divided into two segments. B. One of these has fused with the large lower segment. C. Four small and one large segment, one of the former fusing with the large segment. D. Each of the four segments has given rise to a small segment. E. Small segments have increased to thirty-six.
Many eggs, such as those of the Myriapods[45], present an irregular segmentation; but the segmentation is hardly unequal in the sense in which I have been using the term. Such cases should perhaps be placed in the first rather than in the present category.
The type of unequal segmentation is on the whole the most widely distributed in the animal kingdom. There is hardly a group without examples of it.
It occurs in Porifera, Hydrozoa, Actinozoa and Ctenophora. Amongst the Ctenophora this segmentation is of the most typical kind. Four equal segments are first formed in the two first periods. In the third period a circumferential furrow separates four smaller from four larger segments.
This type is also widely distributed amongst the unsegmented (Gephyrea, Turbellaria), as well as the segmented Vermes, and is typical for the Rotifera. It appears to be very rare in Echinoderms (Echinaster Sarsii). It is not uncommon in early stages of the segmentation of the lower Crustacea.
For Mollusca (except Cephalopoda) it is typical. Amongst the Ascidia it occurs in several forms (Salpa,Molgula) and amongst the Craniata it is typical in the Cyclostomata, Amphibia, and some Ganoids,e.g. Accipenser.
Partial segmentation.The next type of segmentation we have to deal with has long been recognized as partial segmentation. It is a type in which only part of the ovum, called the germinal disc, undergoes segmentation, the remainder usually forming an appendage of the embryo known as the yolk-sack. Ova belonging to the two groups already dealt with are frequently classed together as holoblastic ova, in opposition to ova of the present group in which the segmentation is only partial, and which are therefore called meroblastic. For embryological purposes this is in many ways a very convenient classification, but ova belonging to the present group are in reality separated by no sharp line from those belonging to the group just described.
Early stages of the segmentation in a fowl’s eggFig. 44. Surface views of the early stages of the segmentation in a fowl’s egg.(After Coste.)a.edge of germinal disc.b.vertical furrow.c.small central segment.d.larger peripheral segment.
Fig. 44. Surface views of the early stages of the segmentation in a fowl’s egg.(After Coste.)
a.edge of germinal disc.b.vertical furrow.c.small central segment.d.larger peripheral segment.
The origin and nature of meroblastic ova will best be understood by taking an ovum with an unequal segmentation, such as that of the frog, and considering what would take place in accordance with the laws already laid down, supposing the amount of food-yolk at the vitelline pole to be enormously increased. What would happen may be conveniently illustrated byfig. 44, representing the segmentation of a fowl’s egg. There would first obviously appear a vertical furrow at the formative or protoplasmic pole. (Fig. 44A,b.) This would gradually advance round the ovum and commence to divide it into two halves. Before the furrow had however proceeded very far itwould come to the vitelline part of the ovum; here, according to the law previously enunciated, it would travel very slowly, and if the amount of the food-yolk was practically infinite as compared with the protoplasm, it would absolutely cease to advance. A second vertical furrow would soon be formed, crossing the first at right angles, and like it not advancing beyond the edge of the germinal disc. (Fig. 44B.)
The next furrow should be an equatorial one (as a matter of fact in the fowl’s ovum an equatorial furrow is not formed till after two more vertical furrows have appeared). The equatorial furrow would however, in accordance with the analogy of the frog,not be formed at the equator, but very close to the formative pole. It would therefore separate off as a distinct segment (fig. 44C,c), a small central,i.e.polar, portion of each of the imperfect segments formed by the previous vertical furrows. By a continuation of the process of segmentation, with the same alternation of vertical and equatorial furrows as in the frog, a cap or disc of small segments would obviously be formed at the protoplasmic pole of the ovum, outside which would be a number of deep radiating grooves (fig. 45), formed by the vertical furrows, the advance of which round the ovum has come to an end owing to the too great proportion of yolk spheres at the vitelline pole.