Book - Text-Book of the Embryology of Man and Mammals 8
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Hertwig O. Text-book of the embryology of man and mammals. (1892) Translated 1901 by Mark EL. from 3rd German Edition. S. Sonnenschein, London.
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Development of the Primitive Segments
THE more one pursues the development of Vertebrates into later stages, the more numerous become the changes which simultaneously appear in the different regions of the embryonic body. We cannot here undertake to describe step by step the processes which are simultaneously accomplished, for by that method the presentation would become fragmentary and the comprehension of the separate processes would be made more difficult ; but it is necessary, in the interest of a didactic method, to select from all the manifold phenomena a single process of the development, and to follow it up until it has come to a preliminary termination.
After the formation of the middle germ-layer two important processes take place in the embryonic fundament. One process leads to a division of the middle germ-layers into the two lateral plates and into two series of cuboidal bodies, which are situated at the right and the left of the chorda, and which, under an erroneous interpretation, were formerly called prolovertebrce, but for which one should now substitute exclusively the more accurate name primitive segments [mesoblastic somites]. The other process, which occurs at about the same time, at least in the case of the higher Vertebrates, leads to the origin of those cells from which the sustentative substances and the blood of Vertebrates are derived.
Fig. 103. Amphioxus embryo with five pairs of primitive segments in optical section, after HATSCHEK.
A Seen from the side. B Seen from the dorsum. In figure B are indicated the openings of the cavities of the primitive segments into the intestinal cavity, which can be seen by deeper focussing. V, Anterior, &, posterior end ; a/.-, outer, ik, inner, mk, middle germ-layer ; dh, intestinal cavity ; n, neural tube ; en, neurenteric canal ; us 1 , first primitive segment ; ush, cavity of primitive segment ; ud, ccelenteron.
In this chapter we shall take into consideration the formation of the primitive segments first in the eggs of Amphioxus and the Amphibians, and then in those of Fishes, Birds, and Mammals.
In Amphioxus the formation of the primitive segments is more nearly simultaneous with the development of the middle germlayer than in the remaining Vertebrates. As soon as the two crelomic sacs begin to grow out from the ccelenteron at the front end of the embryo, there begins a division of them into two rows of small sacs lying one behind the other (fig. 103 A, I>, us), and this division proceeds from in front backwards. Here again we have to do with a process of folding, which repeats itself many times in the same manner.
The wall of the groove-like crelomic evagination, composed of cylindrical cells, becomes, at a little distance from its head-end, folded transversely to the longitudinal axis of the embryo ; this fold grows from above and from the side downwards into the body- cavity ; in the same manner a second transverse fold is soon formed on either side of the body at a little distance behind the first ; behind the second a third, a fourth, and so on, at the same rate as that at which the embryonal body elongates and the fundament of the middle germ - layer increases by the progress of the evagination toward the blastopore.
In the embryo represented in fig. 103 five sacs may be counted on either side of the body. The evagination is taking place at the region marked mk ; it advances still farther toward the blastopore and gives rise to a considerable series of primitive segments, the number of which in a larva only twenty-four hours old has already increased to about seventeen pairs. The primitive segments exhibit at first an opening, by means of which their cavities (usK) are in communication with the intestinal cavity. But these openings soon begin to be closed in succession, by their margins growing toward each other and then coalescing; this takes place in the same sequence as that in which the detachment of the parts takes place, from before backwards. At the same time the primitive segments (fig. 104) gradually spread out both dorsally and ventrally, while their cells increase in number and become altered in form. They grow upward more and more at the side of the neural tube, which has meanwhile detached itself completely from its matrix, the outer germ-layer.
Fig. 104. Cross section through the middle of the body of an Amphioxus embryo with 11 primitive segments, after HATSCHEK.
ak, Outer, ik, inner germ- layer ; mi 1 , parietal, ink'-, visceral lamella of the middle germ-layer ; us, primitive segment ; n, neural tube ; ch, chorda ; //<, body-cavity; dh, intestinal cavity.
Fig. 105 Two cross sections through a Triton embryo.
A, Cross section through the region of the trunk in which the neural tube is not yet closed an the primitive segments begin to be constricted off from the lateral plates.
B, Cross section through the region of the trunk in which the neural tube is closed and the primitive segments have been formed.
mf, Medullary folds ; mp, medullary plate ; n, neural tube ; ch, chorda ; ak, outer, ik; inner germ-layer ; mfc 1 , parietal, ink' 2 , visceral middle layer ; dh, intestinal cavity ; Ih, body-cavity ush, cavity of primitive segment ; dz, yolk-cells.
Toward the ventral side they insert themselves between the secondary intestine and the outer germ-layer.
Finally, it might be further mentioned here that at a still later stage, as is to be seen on the right side of fig. 104, the dorsal portions of the primitive segment are constricted off from the ventral. The former lose their lumina and furnish the transversely striped musculature of the body, but from the cavities of the latter originates the real unsegmented body-cavity, since the partitions which at first separate them become thinner, break through, and finally disappear.
Similar processes take place in a somewhat modified manner in the case of the remaining Vertebrates.
In the Tritons the middle germ- layer (fig. 105 A) becomes thickened on both sides of the chorda (ch) and of the fundament of the central nervous system (H), which is not yet closed into a tube, and at the same time there appears a cavity (ush) in its thickened part, caused by the separation of the visceral and parietal lamellse. The thickening is not produced by an increase in the number of the layers of cells, but simply by the fact that the cells increase in height and grow out into long cylinders, which are arranged around the cavity like an epithelium. We distinguish these thickened parts of the middle germ-layer, which lie on either side of the chorda and the nervous system, as the primitive-segment plates, from the lateral parts, or the lateral plates. In the territory of the latter the cells are lower, and ordinarily there is 110 distinctly marked cavity between visceral and parietal layer.
Whereas in Amphioxus the process of forming somites extends itself over the whole of the middle germ-layer, in the case of the Amphibians, and likewise all the remaining Vertebrates, it affects only the part which is next to the chorda and the neural tube, leaving the lateral plates, on the contrary, untouched. The segmentation begins at the headend, and proceeds slowly toward the blastopore ; it is accomplished by folding and constricting off. The epithelial lamella next to the neural tube and the chorda, being composed of cylindrical cells, is raised up into small transverse folds, which, separated from each other by intervals of uniform size, grow into the cavity of the primitivesegment plate, and give rise to small sacs lying one behind the other (fig. 106).
Fig. 106. Frontal section through the clorsum of an embryo Triton with fully developed primitive segments.
One sees on both sides of the chorda (ch) the primitive segments (us) with their cavities (s/i).
Soon afterwards each little sac is constricted off from the lateral plates (fig. 105 A and B). Consequently one now meets, both in transverse and frontal sections at the right and left of chorda and neural tube, cubical sacs the walls of which are formed of cylindrical cells ; these sacs are everywhere surrounded by a fissurelike space, and they enclose a small cavity (the primitive-segment cavity), which is a derivative of the body-cavity. From the front layer of the fold is produced the posterior wall of the newly formed segment, from its posterior layer the front wall of the remnant of the primitive-segment plate, or of the sac which is next to be constricted off, Of the Vertebrates which are developed out of meroblastic eggs, the Selachians appear to exhibit most clearly the original mode of the formation of primitive segments. A distinct body-cavity is formed on either side of the trunk by the separation of the parietal and visceral lamellae of the middle germ-layer (fig. 110). The dorsal portion of the cavity, which flanks the neural tube, acquires thickened walls (mp), and corresponds to the part previously designated as the primitive-segment plate, which at the same time with the appearance of the body-cavity begins to be divided into primitive segments. In the anterior part of the body a series of transverse lines of separation become visible (fig. 195 ?>ip 1 }, the number of which is continually increased toward the hind end of the body. For a long time the cavities of the primitive segments, which are separated from one another by these transverse furrows, remain in communication ventrally with the common body-cavity by means of narrow openings. One may therefore describe this state of affairs by saying that the body-cavity is provided toward the back of the embryo with a series of small sac-like evaginations, which lie close together one after the other. Afterwards the primitive segments are entirely constricted off from the body-cavity, and then their thickened walls come into close contact, and thus cause the disappearance of the cavities of the segments (fig. Ill tup).
Whereas in the Selachians it is still evident that the formation of the primitive segments depends upon folding and constricting off, the process is obscured even to obliteration in the case of Reptiles, Birds, and Mammals; this is referable simply to the fact that the two larnellse of the middle germ-layer remain for a long time firmly pressed together, only subsequently beginning to separate, and that they are composed of several layers of small cells. The process of Balding and constricting off appears here as a splitting up of a solid cell-plate into small cubical blocks.
The part of the middle germ-layer that is next to the chorda and neural tube appears in a cross section of a Chick embryo (fig. 107) as a compact mass (Pv) consisting of many superposed small cells, which, as far as it is not divided up into separate blocks, is designated as primitive-segment plate or protovertebral plate. In fio-. 107 it is still connected at the side by means of a thin isthmus of cells with the lateral plates, in whose territory the middle germlayers are thinner and separated from each other by a fissure.
In observing the blastogerm from the surface the region of the primitive-segment plates, as is to be seen in the posterior part of a nine- days-old Babbit embryo (fig. 108). appears darker than the region of the lateral plate; so that the two are distinguished from each other ; one is stem-zone (stz), the other parietal zone (p~).
The development of the primitive segments is observable in the Chick at the beginning of the second day of incubation, in the Babbit at about the eighth day. Clear transverse streaks appear in the stem-zone at some distance from the primitive streak, about in the middle of the embryonic fundament, both on the right and the left of the chorda and neural tube (fig. 108). They correspond to transverse fissures, by means of which the primitive- segment plates are divided into the small and solid cubical primitive segments (uw). In the nine-days-old Rabbit embryo represented in fig. 108 these plates are resolved in front into eight pairs of primitive segments (uw), whereas in the hind end of the embryonic area they still have the form of a continuous mass of cells, the stem-zone (stz), which in surface-views appears darker than its surroundings.
In a somewhat more advanced stage the primitive segment, which probably secretes at the same time fluid, develops in its interior, as in the case of the Amphibia and Selachii, a cavity, around which the cells group themselves in a radial manner. This cavity, too, is at first in communication laterally with the fissure of the body-cavity, until the primitive segment has been fully constricted off.
In Vertebrates, besides the trunk-region, a part of the head-region of the embryo is also affected by this process of segmentation which we have been considering. We must therefore speak in the one case of head-segments, and in the other of trunksegments. Up to the present time the number and condition of the head-segments have been made out (by BALFOUR, MILNES MARSHALL, and VAN WIJHE) most accurately for the Selachians. In this instance there are nine pairs of hollow head-segments. In the higher Vertebrates such segments, although fewer in number, have also been described ; however, the less sharply differentiated structures of the latter demand still further investigation.
Fig. 108. Rabbit embryo of the ninth day, seen from the dorsal side, after KOLLIKER. Magnified 21 diameters.
The stem-zone (sfc) and the parietal zone (pz) are to be distinguished. In the former 8 pairs of primitive segments have been established at the side of the chorda and neural tube.
op, Area pellucida ; ;;/', medullary groove ; r/<, fore brain ; ab, eye-vesicle ; n/J/, mid brain ; hh, hind brain ; uw, primitive segment ; stz, stein-zone ; pz, parietal zone ; /<-, heart ; pk, pericardial part of the body-cavity ; rd, margin of the entrance to the head-gut (voi-dci-e Damijifortc), seen through the overlying structures ; uf, amniotic fold ; vo, vena omphalomesenterica.
But, in any event, the accurate study of the earliest embryonic segmentation of the body into a large number of metameres yields this result of the highest importance for the general morphology of the Vertebrate body, that the head not less than the trunk represents a segmented, portion of the body and has in no wise been produced from a single primitive segment.
- In Vertebrates the middle germ-layers immediately after their origin are differentiated into several fundaments by processes of folding and constricting off.
- The process of differentiation in the middle germ-layer exhibits two modifications.
- In Amphioxus the middle germ-layers are, at the time of their first appearance, completely separated into primitive segments lying one behind the other. It is only later that each primitive segment is divided into a dorsal portion (the real primitive segment) and a ventral portion. The dorsal portion, or primitive segment proper, furnishes the transversely striped musculature of the trunk. The ventral segments form the body-cavity, which is at first segmented, but afterwards with the disappearance of the partitions becomes a single cavity.
- In all other Vertebrates the fundaments of the middle germ-layers are divided first into a dorsal and a ventral region into the primitive-segment plates and the lateral plates. The lateral plate remains unsegmented. The body-cavity, which becomes visible in it by separation of the parietal and the visceral lamella? of the middle layer, is from the beginning on each side of the body a single space. The primitive-segment plate alone is divided into successive primitive segments.
- The segmentation of the middle germ-layers also extends over the future head-region of the embryo. One therefore distinguishes (a) Head-segments, the number of which amounts to nine ; (b) Trunk-segments , the number of which is constantly being increased during the development of the posterior trunk region.
Text-Book of the Embryology of Man and Mammals: Description of the Sexual Products | The Phenomena of the Maturation of the Egg and the Process of Fertilisation | The Process of Cleavage | General Discussion of the Principles of Development | The Development of the Two Primary Germ-Layers | The Development of the Two Middle Germ-Layers | History of the Germ-Layer Theory | Development of the Primitive Segments | Development of Connective Substance and Blood | Establishment of the External Form of the Body | The Foetal Membranes of Reptiles and Birds | The Foetal Membranes of Mammals | The Foetal Membranes of Man | The Organs of the Inner Germ-Layer - The Alimentary Tube with its Appended Organs | The Organs of the Outer Germ-Layer | The Development of the Nervous System | The Development of the Sensory Organs | The Development of the Skin and its Accessory Organs | The Organs of the Intermediate Layer or Mesenchyme | The Development of the Blood-vessel System | The Development of the Skeleton
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