Fowl’s egg, late state of segmentationFig. 45. Surface view of the germinal disc of fowl’s egg during a late stage of the segmentation.c.small central segmentation spheres;b.larger segments outside these;a.large, imperfectly circumscribed, marginal segments;e.margin of germinal disc.
Fig. 45. Surface view of the germinal disc of fowl’s egg during a late stage of the segmentation.
c.small central segmentation spheres;b.larger segments outside these;a.large, imperfectly circumscribed, marginal segments;e.margin of germinal disc.
It is clear from the above that an immense accumulation of food-yolk at the vitelline pole necessarily causes a partial segmentation. It is equally clear that the part of meroblastic ova which does not undergo segmentation is not a new additionabsent in other cases. It is on the contrary to be regarded merely as a part of the ovum in which the yolk-spherules have attained to a very great bulk as compared with the protoplasm; sometimes even to the complete exclusion of the protoplasm.
An ordinary meroblastic ovum consists then of a small disc at the formative pole, known as the germinal disc, composed mainly of protoplasm in which comparatively little food-yolk is present. This graduates into the remainder of the ovum, being separated from it by a more or less sharp line. This remainder of the ovum, which almost always forms the major part, usually consists of numerous yolk-spherules, embedded in a very scanty protoplasmic matrix.
In some cases,e.g.the eggs of Elasmobranchii[46], the protoplasm is present in the form of a delicate network; in other and perhaps the majority of cases, too little protoplasm is present to be detected, or the protoplasm may even be completely absent. In some Osseous Fishes,e.g.Lota, the yolk forms a homogeneous transparent albuminoid substance containing a large globule at the pole furthest removed from the germinal disc. In this case the germinal disc is sharply separated from the yolk. In other Osseous Fishes the separation between the two parts is not so sharp[47]. In these cases we find adjoining the germinal disc a finely granular material containing a large proportion of protoplasm; this graduates into a material with very little protoplasm and numerous yolk-spherules, which is in its turn continuous with an homogeneous albuminoid yolk substance. In Elasmobranchii we find that immediately beneath the germinal disc there is present a finely granular matter, rich in protoplasm, which is continuous with the normal yolk.
The Elasmobranch ovum may conveniently serve as type for the Vertebrata. The ovum is formed of a spherical vitellus without any investing membrane. The germinal disc is recognizable on this as a small yellow spot about 1½ millimetres in diameter. In the germinal disc a furrow appears bisecting the disc, followed by a second furrow at right angles to the first. Thus after the formation of the second furrow the disc is divided into four equal areas. Fresh furrows continue to rise, and eventually a circular furrow, equivalent to the equatorial furrow of the frog’s ovum, makes its appearance, and separates off a number of smaller central segments from peripheral larger segments. In the later stages the smaller segments at first divide more rapidly than the larger, but eventually the latter also divide rapidly, and the germinal disc becomes finally formed of a series of segmentsof a fairly uniform size. So much may be observed in surface views of the segmenting ovum, and it may be noted that there is not much difference to be observed between the segmentation of the germinal disc of the Fowl’s ovum and that of the Elasmobranchii. Indeed the figure of the former (fig. 44) would serve fairly well for the latter. When however we examine the segmenting germinal discs by means of sections, there are some differences between the two types, and several interesting features which deserve to be noticed in the segmentation of the Elasmobranchii. In the first stages the furrows visible on the surface are merely furrows, which do not meet so as to isolate distinct segments; they merely, in fact, form a surface pattern. It is not till after the appearance of the equatorial furrow that the segments begin to be distinctly isolated. In the subsequent stages not only do the segments already existing in the germinal disc increase by division, but fresh segments are continually being formed from the adjacent yolk, and added to those already present in the germinal disc. (Fig. 46.) This fact is one out of many which prove that the germinal disc is merely part of the ovum characterized by the presence of more protoplasm than the remainder which forms the so-called food-yolk. During the latest stages of segmentation there appear in the yolk around the blastoderm a number of nuclei. (Fig. 46,nx´.) These are connected with a special protoplasmic network (already described) which penetrates through the yolk. Towards the end of segmentation, and during the early periods of development which succeed the segmentation, these nuclei become very numerous. (Fig. 47A,n´.) Around many of them a protoplasmic investment is established, and cells are thus formed which eventually enter the blastoderm.
Section of Pristiurus embryoFig. 46. Section through germinal disc of a Pristiurus embryo during the segmentation.n.nucleus;nx.nucleus modified prior to division;nx´.modified nucleus of the yolk;f.furrow appearing in the yolk adjacent to the germinal disc.
Fig. 46. Section through germinal disc of a Pristiurus embryo during the segmentation.
n.nucleus;nx.nucleus modified prior to division;nx´.modified nucleus of the yolk;f.furrow appearing in the yolk adjacent to the germinal disc.
The result of segmentation is the formation of a lens-shaped mass of cells lying in a depression on the yolk. In this a cavity appears, the homologue of the segmentation cavity already spoken of. It lies at first inthe midst of the cells of the blastoderm, but very soon its floor of cells vanishes, and it lies between the yolk and the blastoderm. (Fig. 47A.) Its subsequent history is given in a future Chapter.
Segmentation proceeds in Osseous Fishes in nearly the same manner as in Elasmobranchii. In some cases the germinal disc is small as compared with the yolk, in other cases it is almost as large as the yolk. The only points which deserve special notice are the following: (1) Nuclei, precisely similar to those in the Elasmobranch yolk, appear in the protoplasmic matter around the germinal disc; (2) After the deposition of the ova there is present in some forms a network of protoplasm extending from the germinal disc through the yolk[48]. At impregnation this withdraws itself from the yolk. It is to be compared to the protoplasmic network of the Elasmobranch ovum.
Two sections of a Pristiurus embryoFig. 47. Two longitudinal sections of the blastoderm of a Pristiurus embryo at stages prior to the formation of the medullary groove.ep.epiblast;ll.lower layer cells;m.mesoblast;hy.hypoblast;sc.segmentation cavity;es.embryo swelling;n´. nuclei of yolk;er.embryonic rim.
Fig. 47. Two longitudinal sections of the blastoderm of a Pristiurus embryo at stages prior to the formation of the medullary groove.
ep.epiblast;ll.lower layer cells;m.mesoblast;hy.hypoblast;sc.segmentation cavity;es.embryo swelling;n´. nuclei of yolk;er.embryonic rim.
There are two types of meroblastic ova. In one of these (Aves, Elasmobranchii) the germinal disc is formed in the ovarian ovum. In the second type the germinal disc is formed after impregnation by a concentration of the protoplasm at one pole. This concentration is analogous to what has already been described for Anodon and other Molluscan ova (p.100).
The ova of some Teleostei are intermediate between the two types.
The ovum of the wood-louse, Oniscus murarius[49], may be taken as an example of the second type of meroblastic ovum. In this egg development commences by the appearance of a small clear mass with numerous transparent vesicles. This mass is the protoplasm which has becomeseparated from the yolk. It undergoes segmentation in a perfectly normal fashion. Examples of other cases of this kind have been described by Van Beneden and Bessels[50]in Anchorella, and in Hessia by Van Beneden[51]. It appears from their researches that the protoplasm collects itself together, first of all in the interior of the egg, and then travels to the surface. It arrives at the surface after having already divided into two or more segments, which then rapidly divide in the usual manner to form the blastoderm.
There are some grounds for thinking that the cases of partial segmentation in the Arthropoda are not really quite comparable with those in other groups, but more probably fall under the next type of segmentation to be described. The grounds for this view are mentioned in connection with the next type.
In most if not all meroblastic ova there appear during and after segmentation a number of nuclei in the yolk adjoining the blastoderm, around which cells become differentiated. (Figs. 46and47.) These cells join the part of the blastoderm formed by the normal segmentation of the germinal disc. Such nuclei are formed in all craniate meroblastic ova[52]. In Cephalopods they have been found by Lankester, and in Oniscus by Bobretzky. They have been by some authors supposed to originate from the nuclei of the blastoderm, and by others spontaneously in the yolk.
Some of the earliest observations on these nuclei were made by Lankester[53]in the Cephalopods. He found that they appeared first in a ring-like series round the edge of the blastoderm, and subsequently all over the yolk in a layer a little below the surface. He observed their development in the living ovum and found that they “commenced as minute points, gradually increasing in size like other free-formed nuclei.” A cell area subsequently forms around them.
By E. van Beneden[54]they were observed in a Teleostean ovum to appear nearly simultaneously in considerable numbers in the granular matter beneath the blastoderm. Van Beneden concludes from the simultaneous appearance of these bodies that they develop autogenously. Kupffer at an earlier period arrived at a similar conclusion. My own observations on these nuclei in Elasmobranchii on the whole support the conclusions to be derived from Lankester’s, Kupffer’s and Van Beneden’s observations. As mentioned above, the nuclei in Elasmobranchii do not appear simultaneously, butincrease in number as development proceeds; and it is possible that Van Beneden may be mistaken on this point. No evidence came before me of derivation from pre-existing nuclei in the blastoderm. My observations prove however that the nuclei increase by division. This is shewn by the fact that I have found them with the spindle modification (fig. 46,nx´), and that in most cases they usually exhibit the form of a number of aggregated vesicles[55], which is a character of nuclei which have just undergone division. It should be mentioned however that I failed to find a spindle modification of the nuclei in the later stages. Against these observations must be set those of Bobretzky, according to which the nuclei in Oniscus are really the nuclei of cells which have migrated from the blastoderm. Bobretzky’s observations do not however appear to be very conclusive.
It must be admitted that the general evidence at our command appears to indicate that the nuclei of the yolk in meroblastic ova originatespontaneously. There is however a difficulty in accepting this conclusion in the fact that all the other nuclei of the embryo are descendants of the first segmentation nucleus; and for this reason it still appears to me possible that the nuclei of the yolk will be found to originate from the continued division of one primitive nucleus, itself derived from the segmentation nucleus.
The existence of these nuclei in the yolk and the formation of a distinct cell body around them is a strong piece of evidence in favour of the view above maintained, (which is not universally accepted,) that the part of the ovum of meroblastic ova which does not segment is of the same nature as that which does segment, and differs only in being relatively deficient in active protoplasm.
The following forms have meroblastic ova of the first type: the Cephalopoda,Pyrosoma, Elasmobranchii, Teleostei, Reptilia, Aves, Ornithodelphia (?). The second type of meroblastic segmentation occurs in many Crustacea, (parasitic Copepoda, IsopodaMysis, etc.). It is also stated to be found inScorpio.
The ova of the majority of groups in the animal kingdom segment according to one of the types which have just been described. These types are not sharply separated, but form an unbroken series, commencing with the ovum which segments uniformly, and ending with the meroblastic ovum.
It is convenient to distinguish the ova which segment uniformly by some term; and I should propose for this the term alecithal[56], as implying that they are without food-yolk, or that what little food-yolk there is, is distributed uniformly.
The ova in which the yolk is especially concentrated at one pole I should propose to calltelolecithal. They constitute together a group with an unequal or partial segmentation.
The telolecithal ova may be defined in the following way: ova in which the food-yolk is not distributed uniformly, but is concentrated at one pole of the ovum. When only a moderate quantity of food-yolk is present the pole at which it is concentrated merely segments more slowly than the opposite pole; but when food-yolk is present in very large quantity the part of the ovum in which it is located is incapable of segmentation, and forms a special appendage known as the yolk-sack.
There is a third group of ova including a series of types of segmentation nearly parallel to the telolecithal group. This group takes its start from the alecithal ovum as do the telolecithal ova, and equally with these includes a series of varieties of segmentation running parallel to the regular and unequal types of segmentation which directly result from the presence of a greater or smaller quantity of food-yolk. The food-yolk is however placed, not at one pole, butat the centre of the ovum. This group of ova I propose to name centrolecithal. It is especially characteristic of the Arthropoda, if not entirely confined to that group.
Centrolecithal ova.As might be anticipated on the analogy of the types of segmentation already described, the concentration of the food-yolk at the centre of the ovum does not always take place before segmentation, but is sometimes deferred till even the later stages of this process.
Examples of a regular segmentation in centrolecithal ova are afforded by Palæmon (Bobretzky) and Penæus (Hæckel). A type of unequal segmentation like that of the Frog occurs inGammarus locusta(Beneden and Bessels), where however the formation of a central yolk mass does not appear to take placetill rather late in the segmentation. More irregular examples of unequal segmentation are also afforded by other Crustaceans,e.g.various members of the genusChondracanthus(Beneden and Bessels) and by Myriapods. In all these cases segmentation ends in the formation of a layer of cells enclosing a central mass of food-yolk.
Segmentation of a Crustacean ovumFig. 48. Segmentation of a Crustacean ovum (Penæus).(After Hæckel.)The sections illustrate the type of segmentation in which the yolk is aggregated at the centre of the ovum.yk.central yolk mass.1 and 2. Surface view and section of the stage with four segments. In 2 it is seen that the furrows visible on the surface do not penetrate to the centre of the ovum.3 and 4. Surface view and section of ovum near the end of segmentation. The central yolk mass is very clearly seen in 4.
Fig. 48. Segmentation of a Crustacean ovum (Penæus).(After Hæckel.)
The sections illustrate the type of segmentation in which the yolk is aggregated at the centre of the ovum.
yk.central yolk mass.
1 and 2. Surface view and section of the stage with four segments. In 2 it is seen that the furrows visible on the surface do not penetrate to the centre of the ovum.3 and 4. Surface view and section of ovum near the end of segmentation. The central yolk mass is very clearly seen in 4.
The peculiarity of the centrolecithal ova with regular or unequal segmentation is that (owing to the presence of the yolk in the interior) the furrows which appear on the surface are not continued to the centre of the egg. The spheres which are thus distinct on the surface are really united internally.Fig. 48, copied from Hæckel, shews this in a diagrammatic way.
Many ova, which in the later stages of segmentation exhibit the characteristics of true centrolecithal ova, in the early stages actually pass through nearly the same phases as holoblastic ova.Thus inEupagurus prideauxii[57](fig. 49), and probably in the majority of Decapods, the egg is divided successively into two, four and eight distinct segments, and it is not till after the fourth phase of the segmentation that the spheres fuse in the centre of the egg. Such ova belong to a type which is really intermediate between the ordinary type of segmentation and that with a central yolk mass. Eupagurus presents one striking peculiarity,viz.that the nucleus divides into two, four and eight nuclei, each surrounded by a delicate layer of protoplasm prolonged into a reticulum, before the ovum itself commences to become segmented. The ovum before segmentation is therefore in the condition of a syncytium.
Segmentation of Eupagurus PrideauxiiFig. 49. Transverse section through four stages in the segmentation of Eupagurus Prideauxii.(After P. Mayer.)
Fig. 49. Transverse section through four stages in the segmentation of Eupagurus Prideauxii.(After P. Mayer.)
The segmentation of Asellus aquaticus[58]is very similar to that of Eupagurus, etc. but the ovum at the very first divides into as many segments (viz.eight) as there are nuclei.
In Gammarus locusta the resemblance to ordinary unequal segmentation is very striking, and it is not till a considerable number of segments have been formed that a central yolk mass appears.
In all the above types, as segmentation proceeds, the protoplasm becomes more and more concentrated at the surface, and finally a superficial layer of flat blastoderm cells is completely segmented off from the yolk below (fig. 49D).
In cases like those of Penæus, Eupagurus, etc., the yolk in the interior is at first nearly homogeneous, but at a later period it generally becomes divided up partially or completely into a number of distinct spheres, which may have nuclei and therefore have the value of cells. In many cases nuclei have however not been demonstrated in these yolk spheres, though probably present; yet, till they have been demonstrated, some doubt must remain on the nature of these yolk spheres. It is probable thatnotall the nuclei which result from the division of the first segmentation nucleus become concerned in the formation of the superficial blastoderm, but that some remain in the interior of the ovum to become the nuclei of the yolk spheres.
Segmentation in CheliferFig. 50. Segmentation and formation of the blastoderm in Chelifer.(After Metschnikoff.)In A the ovum is divided into a number of separate segments. In B a number of small cells have appeared (bl) which form a blastoderm enveloping the large yolk spheres. In C the blastoderm has become divided into two layers.
Fig. 50. Segmentation and formation of the blastoderm in Chelifer.(After Metschnikoff.)
In A the ovum is divided into a number of separate segments. In B a number of small cells have appeared (bl) which form a blastoderm enveloping the large yolk spheres. In C the blastoderm has become divided into two layers.
InMyriapods(Chilognatha) a peculiar form of segmentation has beenobserved by Metschnikoff[59]. The ovum commences by undergoing a perfectly normal, though rather irregular total segmentation. But after the process of division has reached a certain point, scattered masses of very small cells make their appearance on the surface of the large spheres. These small cells have probably arisen in a manner analogous to that which characterizes the formation of the superficial cells of the blastoderm in the types of centrolecithal ova already described. They rapidly increase in number and eventually form a continuous blastoderm; while the original large segments remain in the centre as the yolk mass. In the interesting ArachnidChelifersegmentation takes place in nearly the same manner as in Myriapods (fig. 50).
Stages in segmentation of Tetranychus Telarius eggFig. 51. Four successive stages in the segmentation of the egg of Tetranychus Telarius.(After Claparède.)
Fig. 51. Four successive stages in the segmentation of the egg of Tetranychus Telarius.(After Claparède.)
It is clear that it is not possible in centrolecithal ova to have any type of segmentation exactly comparable with that of meroblastic ova. There are however some types which fill the place of the meroblastic ova in the present group,in as much as they are characterised by the presence of a large bulk of food-yolk which either does not segment, or does not do so till a very late stage in the development. The essential character of this type of segmentation consists in the division of the germinal vesicle in the interior, or at the surface of the ovum into two, four, etc. nuclei (fig. 51). These nuclei are each of them surrounded by a specially concentrated layer of protoplasm (fig. 51) which iscontinuous with a general protoplasmic reticulum passing through the ovum [not shewn infig. 51]. The yolk is contained in the meshes of this reticulum in the manner already described for other ova.
The ovum, like that of Eupagurus before segmentation, is now a syncytium. Eventually the nuclei, having increased by division and become very numerous, travel, unless previously situated there, to the surface of the ovum. They then either simultaneously or in succession become, together with protoplasm around them, segmented off from the yolk, and give rise to a peripheral blastoderm enclosing a central yolk mass. In the latter however many of the nuclei usually remain, and it also very often undergoes a secondary segmentation into a number of yolk spheres.
The eggs of Insects afford numerous examples of this mode of segmentation, of which the egg of Porthesia[60]may be taken as type. After impregnation it consists of a central mass of yolk which passes without a sharp line of demarcation into a peripheral layer of more transparent (protoplasmic) material. In the earliest stage observed by Bobretzky there were two bodies in the interior of the egg, each consisting of a nucleus enclosed in a thin protoplasmic layer with stellate prolongations. This stage corresponds with the division into two, but though the nucleus divides, the preponderating amount of yolk prevents the egg from segmenting at the same time. By a continuous division of the nuclei there becomes scattered through the interior of the ovum a series of bodies, each formed of nucleus and a thin layer of protoplasm with reticulate processes. After a certain stage some of these bodies pass to the surface, simultaneously (in Porthesia) or in some cases successively. At the surface the protoplasm round each nucleus contracts itself into a rounded cell body, distinctly cut off from the adjacent yolk.
The cells so formed give rise to a superficial blastoderm of a single layer of cells. Many of the nucleated bodies remain in the yolk, and after a certain time, which varies in different forms, the yolk becomes segmented up into a number of rounded or polygonal bodies, in the interior of each of which one of theabove nuclei with its protoplasm is present. This process, known as the secondary segmentation of the yolk, is really part of the true segmentation, and the bodies to which it gives rise are true cells.
Other examples of this type may be cited. In Aphis[61]Metschnikoff shewed that the first segmentation nucleus divides into two, each of which takes up a position in the clearer peripheral protoplasmic layer of the egg (fig. 52, 1 and 2). Following upon further division the nuclei enveloped in a continuous layer of protoplasm arrange themselves in a regular manner, and form a syncytium, which becomes segmented into definite cells (fig. 52, 3 and 4). The existence of a special clear superficial layer of protoplasm has been questioned by Brandt.
Segmentation of Aphis RosaeFig. 52. Segmentation of Aphis Rosae.(Copied from Metschnikoff.)In all the stages there is seen to be a central yolk mass surrounded by a layer of protoplasm.In this protoplasm two nuclei have appeared in 1, four nuclei in 2. In 3 the nuclei have arranged themselves regularly, and in 4 the protoplasm has become divided into a number of columnar cells corresponding to the nuclei.w.pole of the blastoderm which has no share in forming the embryo.
Fig. 52. Segmentation of Aphis Rosae.(Copied from Metschnikoff.)
In all the stages there is seen to be a central yolk mass surrounded by a layer of protoplasm.In this protoplasm two nuclei have appeared in 1, four nuclei in 2. In 3 the nuclei have arranged themselves regularly, and in 4 the protoplasm has become divided into a number of columnar cells corresponding to the nuclei.
w.pole of the blastoderm which has no share in forming the embryo.
InTetranychus telarius, one of the mites, Claparède found on the surface of the ovum a nucleus surrounded by granular protoplasm (fig. 51); which is no doubt the first segmentation nucleus. By a series of divisions, all on the surface, a layer of cells becomes formed round a central yolk mass. The result here is the same as in Insects, but the nucleus with its granular protoplasm is from the first superficial. In other cases, such as that of the common fly[62], a layer of protoplasm is stated to appear investing the yolk; and in this there arise simultaneously (?) a number of nuclei at regular intervals, around each of which the protoplasm separates itself to form a distinct cell. Closely allied is the type observed by Kowalevsky in Apis. Development here commences by the appearance of a number of protoplasmicprominences, each forming a cell provided with a nucleus, the nuclei having no doubt been formed by previous division in the interior of the ovum. They appear at the edge of the yolk, and are separated from one another by short intervals. Shortly after their appearance a second batch of similar bodies appears, filling up the interspaces between the first-formed prominences. In the fresh-water Gammarus fluviatilis the protoplasm is stated first of all to collect at the centre of the ovum, where no doubt the segmentation nucleus divides. Subsequently cells appear at numerous points on the surface, and by repeated division constitute an uniform blastoderm investing the central yolk mass. This mode of formation of the blastoderm is closely allied to that observed by Kowalevsky in Apis.
Between ova with a segmentation like that of Insects, and those with a segmentation like that of Penæus, there is more than one intermediate form. The Eupagurus type, with the division of the first nucleus into eight nuclei before the division of the ovum, must be regarded in this light; but the most instructive example of such a transitional type of segmentation is that afforded by Spiders[63].
segmentation of Philodromus LimbatusFig. 53. Three stages in the segmentation of Philodromus Limbatus.(After Hub. Ludwig.)
Fig. 53. Three stages in the segmentation of Philodromus Limbatus.(After Hub. Ludwig.)
The first phenomenon which can be observed after impregnation is the conglomeration of the yolk spheres into cylindrical columns, which finally assume a radiating form diverging from the centre of the egg. In the centre of the radiate figure is a protoplasmic mass, probably containing a nucleus, which sendsout protoplasmic filaments through the columns (fig. 53A). After a certain period of repose the figure becomes divided into two rosette-like masses, which remain united for some time by a protoplasmic thread: this thread is finally ruptured (fig. 53B). The whole egg does not in this process divide into two segments, but merely the radiate figure, which is enclosed in a finely granular material. The two rosettes next become simultaneously divided, giving rise to four rosettes (fig. 53C): and the whole process is repeated with the same rhythm as in a regular segmentation till there are formed thirty-two rosettes in all (fig. 54A). The rosettes by this time have become simple columns, which by mutual pressure arrange themselves radiately around the centre of the egg, which however they do not quite reach.
Surface view of Philodromus LimbatusFig. 54. Surface view and optical section of a late stage in the segmentation of Philodromus Limbatus(Koch). (After Hub. Ludwig.)bl.blastoderm;yk.yolk spheres.
Fig. 54. Surface view and optical section of a late stage in the segmentation of Philodromus Limbatus(Koch). (After Hub. Ludwig.)
bl.blastoderm;yk.yolk spheres.
When only two rosettes are present the protoplasm with its nucleus occupies a central position in each rosette, but gradually, in the course of the subsequent subdivisions, it travels towards the periphery, and finally occupies, when the stage with thirty-two rosettes is reached, a peripheral position. The peripheral protoplasm next becomes separated off as a nucleated layer (fig. 54B). It forms the proper blastoderm, and in it the nuclei rapidly multiply and finally around each an hexagonal or polygonal area of protoplasm is marked off; and a blastoderm, formed of a single layer of flattened cells, is thus constituted. The columns within the blastoderm now form (fig. 54B) more or less distinct masses, which are stated by Ludwig to be without protoplasm.
From observations of my own I am inclined to differ from Ludwig as to the nature of the parts within the blastoderm. My observations have been made onAgelena labyrinthicaand commence at the close of the segmentation. At this time I find a superficial layer of flattened cells, and within these a number of large polyhedral yolk cells. In many, and I believe all, of the yolk cells there is a nucleus surrounded by protoplasm. It is generally placed at one side and not in the centre of a yolk cell, and the nuclei are so often double that I have no doubt they are rapidly undergoing division. It appears to me probable that, at the time when the superficial layer of protoplasm is segmented off from the yolk below, the nuclei undergo division, and that a nucleus with surrounding protoplasm is left with each yolk column. For further detailsvideChapter on Arachnida.
Although by the close of the segmentation the protoplasm has travelled to a superficial position, it may be noted that at first it forms a small mass in the centre of the egg, and only eventually assumes its peripheral situation. It is moreover clear that in the Spider’s ovum there is, so to speak, an attempt at a complete segmentation, which however only results in an arrangement of the constituents of the ovum in masses round each nucleus, and not in a true division of the ovum into distinct segments.
It seems very probable that Ludwig’s observations on the segmentation of Spiders only hold good for species with comparatively small ova.
In connection with the segmentation of the Insects’ ovum and allied types it should be mentioned that Bobretzky, to whose observations we are largely indebted for our knowledge of this subject, holds somewhat different views from those adopted in the text. He regards the nuclei surrounded by protoplasm, which are produced by the division of the primitive segmentation nucleus, as so many distinct cells. These cells are supposed to move about freely in the yolk, which acts as a kind of intercellular medium. This view does not commend itself to me. It is opposed to my own observations on similar nuclei in the Spiders. It does not fit in with our knowledge of the nature of the ovum, and it cannot be reconciled with the segmentation of such types as Spiders or even Eupagurus, with which the segmentation in Insects is undoubtedly closely related.
The majority if not all the cases in which a central yolk mass is formed occur in the Arthropoda, in which group centrolecithal ova are undoubtedly in a majority. In Alcyonium palmatum the segmentation appears however to resemble that of many insects.
One or two peculiar varieties in the segmentation of ova of this type may be spoken of here. The first one I shall mention is detailed in the important paper of E. Van Beneden and Bessels which I have already so often had occasion to quote: it is characteristic of the eggs of most of thespecies of Chondracanthus, a genus of parasitic Crustaceans. The ovum divides in the usual way but somewhat irregularly into 2, 4, 8 segments which meet in a central yolk mass; but after the third division instead of each segment dividing into two equal parts it dividesat onceinto four, and the division into four having started, reappears at every successive division. Thus the number of the segments at successive periods is 2, 4, 8, 32, 128, etc. In another peculiar case, an instance of which[64]is afforded byAsellus aquaticus, after each of the earlier segmentations all the segments fuse and become indistinguishable, but at the succeeding segmentation double the number of segments appears.
Although, as has been already stated, it does not seem possible to have a true meroblastic segmentation in centrolecithal ova, it does nevertheless appear probable that the apparent cases of a meroblastic segmentation in the Arthropoda are derivatives of this type of segmentation. The manner in which the one type might pass into the other may perhaps be explained by the segmentation inAsellus aquaticus[65]. In this ovum large segments are at first formed around a central yolk mass, in the peculiar manner mentioned in the previous paragraph, but at the close of the first period of segmentation minute cells, which eventually form a superficial blastoderm, are produced from the yolk cells. They do not however appear at once round the whole periphery of the egg, but at first only on the ventral surface and later on the dorsal surface. If the amount of food-yolk in the egg were to increase so as to render the formation of the yolk cells impossible, and at the same time the formation of the blastodermic cells were to take place at the commencement, instead of towards the close of the segmentation, a mass of protoplasm with a nucleus might first appear at the surface on the future ventral side of the egg, then divide in the usual way for meroblastic ova, and give rise to a layer of cells gradually extending round to the dorsal surface. A meroblastic segmentation might perhaps be even more easily derived from the type found in Insects. It is probable that the cases of Scorpio, Mysis, Oniscus, the parasitic Isopoda, and some parasitic Copepoda belong to this category; and it may be noticed that in these cases the blastopore would be situated on the dorsal and not on the ventral side of the ovum. The morphological importance of this latter fact will appear in the sequel.
The results arrived at in the present section may be shortly restated in the following way.
(1) A comparatively small number of ova contain very little or no food-yolk embedded in their protoplasm; and have what food-yolk may be present distributed uniformly. In such ova the segmentation is regular. They may be described as alecithal ova.
(2) The distribution of food-yolk in the protoplasm of the ovum exercises an important influence on the segmentation.
The rapidity with which any part of an ovum segments variesceteris paribuswith the relative amount of protoplasm it contains; and the size of the segments formed varies inversely to the relative amount of protoplasm. When the proportion of protoplasm in any part of an ovum becomes extremely small, segmentation does not occur in that part.
Ova with food-yolk may be divided into two great groups according to the eventual arrangement of the food-yolk in the protoplasm. In one of these, the food-yolk when present is concentrated at the vegetative pole of the ovum. In the other group it is concentrated at the centre of the ovum. Ova belonging to the former group are known as telolecithal ova, those to the latter as centrolecithal.
In each group more than one type may be distinguished. In the first group these types are (1) unequal segmentation, (2) partial segmentation. The features of these three types have been already so fully explained that I need not repeat them here.
In the second group there are three distinct types, (1) equal segmentation, (2) unequal segmentation. These two being externally similar to the similarly named types in the first group. (3) Superficial segmentation. This is unlike anything which is present in the first group, and is characterized by the appearance of a superficial layer of cells round a central yolk mass. These cells may either appear simultaneously or successively, and their nuclei are derived from the segmentation within the ovum of the first segmentation nucleus.
The types of ova in relation to the characters of the segmentation may be tabulated in the following way:
Segmentation.
(1) alecithal ova
regular
(2) telolecithal ova
(a) unequal
(b) partial
(3) centrolecithal ova
(a) regular (with segments united in central yolk mass)
(b) unequal (with segments united in central yolk mass)
(c) superficial.
Although the various types of segmentation which have been described present very different aspects, they must nevertheless be looked on as manifestations of the same inherited tendency to division, which differ only according to the conditions under which the tendency displays itself.
This tendency is probably to be regarded as the embryological repetition of that phase in the evolution of the Metazoa, which constituted the transition from the protozoon to the metazoon condition.
From the facts narrated in this chapter the reader will have gathered that similarity or dissimilarity of segmentation is no safe guide to affinities. In many cases, it is true, a special type of segmentation may characterize a whole group; but in other cases very closely allied animals present the greatest differences with respect to their segmentation; as for instance the different species of the genus Gammarus. The character of the segmentation has great influence on the early phenomena of development, though naturally none on the adult form.
External Features of Segmentation.
(105)E. Haeckel.“Die Gastrula u. Eifurchung.”Jenaische Zeitschrift,Vol.IX.1877.(106)Fr. Leydig.“Die Dotterfurchung nach ihrem Vorkommen in d. Thierwelt u. n. ihrer Bedeutung.”Oken Isis.1848.