1897 Human Embryology 9

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Minot CS. Human Embryology. (1897) London: The Macmillan Company.

Human Embryology: Introduction | The Uterus | General Outline of Human Development | The Genital Products | History of the Genoblasts and the Theory of Sex | The Germ-Layers | Segmentation | Primitive Streak | Mesoderm and the Coelom | Germ-Layers General Remarks | The Embryo | The Medullary Groove, Notochord and Neurenteric Canals | Coelom Divisions; Mesenchyma Origin | Blood, Blood-Vessels and Heart Origin | Urogenital System Origin | The Archenteron and the Gill Clefts | Germinal Area, the Embryo and its Appendages | The Foetal Appendages | Chorion | Amnion and Proamnion | The Yolk Sack, Allantois and Umbilical Cord | Placenta | The Foetus | Growth and External Development Embryo and Foetus | Mesenchymal Tissues | Skeleton and Limbs | Muscular System | Splanchnocoele and Diaphragm | Urogenital System | Transformations of the Heart and Blood-Vessels | The Epidermal System | Mouth Cavity and Face | The Nervous System | Sense Organs | Entodermal Canal | Figures | References | Embryology History
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Chapter IX. The Primitive Divisions of the Coelom ; Origin of the Mesenchyma

In all true vertebrates the coelom presents the peculiarity of consisting of an upper or dorsal segmented portion and a lower or ventral continuous unsegmented portion. The segmented coelom consists of a series of discrete separate cavities, each of which conmiunicates with the ventral coelom.

Now, in annelids (and their arthropodous descendants) the coelom consists only of separate paired cavities, so that the mesothelium is divided into distinct parts, each inclosing a space; each division is known as a mesomere or mesoblastic somite. Hence we have the morphological question, how has the completely segmented coelom of annelids become transformed, as we must assmne it has, into the partially segmented coelom of vertebrates? The answer is probably given correctly by Hatschek's investigation of the changes of the mesothelium in Amphioxus, 88. 1. In Amphioxus the entire mesoderm becomes segmented ; the ventral cavities of the segments subsequently fuse, while the dorsal parts remain distinct. In the lower vertebrates the segmented coelom appears first and the unsegmented portion later ; whether the latter is temporarily segmented remains for future investigation to determine. In amniota the unsegmented portion of the coelom appears first, as described in Chapter VI. his must be regarded as a secondary modification, probably connected with the evolution of the amnion ; as explained in Chapter XV, the development of the amnion depends upon a precocious and exaggerated development of part of the coelom.

With these general notions in mind, we can better appreciate the early history of the vertebrate coelom. We consider — 1, the primitive segments; 2, the unsegmented coelom; 3, division of the primitive segments; 4, the differentiation of the myotome; 5, origin of the mesenchyma ; G, comparison with Amphioxus.

The Primitive Segments

A segment consists of a pair of cavities symmetrically placed and bounded by mesothelium. The segments are permanent in many invertebrates, but they are greatly modified in all adult vertebrates, and so much modified in the amniota, that they can be said properly to exist only during embryonic stages, although they determine a large part of the adult structure. I have selected the term primitive segments as unlikely to lead to confusion, Imt numerous other names have been proposed ; the one most generally in use in jxroto vertebra (Urwirberjy which was introduced long ago under the erroneous notion that the segments were the direct precursors of the vertebrae, which they are not, properly speaking. The term protovertebrse is, however, more often used in a more restricted seuse, viz., for that part of the primitive segment which is called the myotome in the fullowing pages. Other terms are mesoblastic somites, mennmeres, meiameres, Ursegmente.

  • On the at»ginent8 of the head, see p. 200.

The primitive segments appear very early; the first pair can be recognized in the chick at twenty to twenty-two hours, in the rabbit at the beginning of the eighth day, or even earlier ; in both casea the medullar}' groove is still nowhere closed and the primitive streak is still present. In the anamniota the tirst segments appear at about the same early stage. In the amniote embryo just before the first segment appears the mesoderm forms a continuous sheet; surface views show that it forma two winga, being divided by the mediem down-growth of the medullary groove as stated, p. 149. The mesoderm on each side is considerably thicker alongside the axial line than farther away from it; the distinction is well marked and enables us to distinguish two zones, namely, the thicker segmental zone near the axis, and the thinner but much wider lateral or parietal zone; the segmental zone is the Stammzotie of German writers, the Wirl}elplaffe of Remak, or vertebnil plate ot Balfour. The first noticeable indication of the formation of the primitive segment is a loosening of the cells in the segmental zone along a narrow transverse line; in the chick this occurs about U.l-I mm. in front of the primitive streak, at a time when only a Bhort stretch of tbe headend of the medullary groove is formed. Very Boon there appears a second transverse loosening of the cells and cleavage of the mesodermic segmental zone takes place. According to A. Goette, 75.1, 203, the deavage begins in teleosts and the chick and probably in other vertebrates with a. small depression on the ectodermal side, and this depression gradually deepens to a cleft, which divides the segmental plate completely The disposition of the fissures is such that they include on each side of the axis a cuboidal block of mesoderm and this block with its fellow on the op posite side constitutes the hrst pnm itive segment. Fig 112 The site of the first segment corresponds to the posterior occipital region; the second segment, at least in the chick, is fomied immediately in front of the first; these two segments, according to Chiarugi, 80. 1, 3^9, are the thinl and fourth occipital segments, and together with the first and second segments — subsequently found in front of them — abort in all amniota during early embryonic life, as was discovered by Froriep. According to Julia B. Piatt, 89. 1, 1T», the first division formed in the chick is that between tbe third and fourth occipital segments, smd two segments are subsequently produced in front of this division, while seven are forming l)ehind it. A chick with one segment is showni in Fig. 1 1 2. The medullar}- groove, Rf, is short and broad ; the anterior end of the embryo, ("A/, is already rising above the yolk; the parietal, Pz, and segmental zones, Stz^ are distinct even in the region of the primitive streak, along which the primitive groove, Pf\ is well marked ; the segment is well advanced and another has begun to form in front of it. A chicken embryo with eight segments is shown in Fig. 113, and a rabbit embryo also with eight segments, in Fig. 114; comparison shows many important differences between the two embryos.

aruiiiotio ( limit t nprn nu-dullnry KTuore: Kf, rlKnnbuldnl i ' Kilullarr Eruove: iV. tfiiiiluVn struak: J(ir. t niFdullUT RrooTc: Af. airm wUiu-lila ; ,. nnaoi: TV. hnit HpanH'nt; Hh. hlDd-bntia: Hh, midbrain. Irnni Kulliker.

Fig. -BalibilEUiihrj'o with F.ielit Sfgments, vli. Fow-hraln; ah. nMIc vpi.lcU-a; a. heart; a/, amniotic (oM: inA. nild-braLn; hh, hlncl-braln: pi, parirtal xnnv; tti. wgmental khii- : ap, area pcUuclda ; r/. rdge of ojiea inMlullBr]' gnxivi'; hh-, {nimltlve aeg».»>•. __ ..^...^.^ .«j Qf h^^; ^^ heart;

Fig. — Tnuwverg* Section o( a Piistiu. Embryo Hllli (ourteen^SeKinentB, through

IB Fourth Swroent. nut.

Ihe Centre ot

[tultar; BToove: EC. eclodt

thelium; Cw. cavity of BenmeDt; Enl. d(nii; itA, tiotochord. Aflor C. Babl.

flJk. pericardial oaTity; [

From KOillbrr,

The examination of transverse sections shows that the primitive segments in all anamniote vertebrates are hollow and bound by mesothelium on all sides. The relations can be understood readily in elasmobranchs. Fig. 115 is from a Pristiurus embryo and shows the cavity of the segment very clearly ; the embryo is much more separated from the yolk than is the case with amniote embryos at a corresponding stage, consequently the lateral or parietal zone of meso<lerm lies nearly vertical, instead of resting horizontally, as it does upon the yolk of amniota ; in the parietal zone there is as yet no cavity (coelom) ; the ventral or unsegmented coelom arises later. It is probable that the segmental cavities spread down into the parietal zone, and that their ventral (/. e. lower or so-called parietal) ends fuse together and form one large main body cavity. This probability, as C. Rabl has said, 88.2, rests upon the analogy with the ascertained process in Araphioxus, and i\\x>n the fact that the segmental cavities appear first, and expand outward or away from the axis. Whether, however, they do actually give rise to the main coelom by their partial lateral or ventral fusion or not, there are no observations at present to decide.

Of the development of the segments in the primitwe vertebrates (marsipobranchs, ganoids, and amphibians) , there is not much known, though there are many scattered observ^ations recorded. There appears, however, to be a distinct thickened zone of mesoblast on each side of the axis, and from this zone the segments are developed as pairs of culK>idal blocks of mesothelium ; the central cavity of the segment is very small ; its mesothelium is thick. The main coelom is at first a fissure farther away from the axis, and it has not yet been sliown that there is from the start a communication lx>tween the segmental and the main coelom, although the mesoblast is continuous. In Petromyzon, if I understand Goette aright, the mesoderm is at first solid; GcH^tto states, 90.1, 48, that in the cervical region the main lx>dy cavity appears first, but in the rump the primitive segments acquire their cavities first ; that is, while the mesoilerm of the i^irietal zone is still solid. This is inip<jrtant as foreshadowing the precocious development of the cervical coelom (cavity of the amnio-cardial vesicles) in amniota. In s(H*tions the primitive segments in Petromyzon and Amphibia are triangular, filling out the space Ix^tween the me<lullary canal and the adjacent ectoderm and entcxlerm on (Vich side. In Bonibinator, A. Goette, 75.1, 'JOtj, and Petromyzon, A. Gtx^tte, 80.1, the first segment api)ears near the middle of the embryo, and new segments are added in front to make the cervical region and a much larger number progressively backward to fonn the rump of the adult.

In S<iur<)})si(1n transverse sections of the primitive segments, when they are first formed, show no cavity, nor does any appear until considerably later. The development begins with the (iifferentiation of the segmental zone (Remak's Cnrtrbelplafte, Balfour's vertebral plate), which is accomplished by the thickening of the mesoderm near the axis of the embryo. The process is intimately associated with the upward movement of the medullary plate to form the medullarj- groove, His, 68. 1, 81, and Goette, 75. 1 ; the space between the ectoderm and entoden.i is enlarged by this movement, and is always nearly filled by the mesoderm ; consequently the segmental zone appears triangular in cross-sections, the base of the triangle being i^ainst the wall of the medullary groove, its two sides again&t the ectoderm and entoderm respectivelv, and its apex merging into the lateral mesoderm, which is very much thinner than the segmental plate. The changes just described show a very exact adjustment of the growth of the mesoderm to changes in the outer germ-layers. Such adjustments occur throughout all embryological developments, and are, I think, due to methods of growth rather than to simple mechanical conditions. His has attributed, 68.1, 81, 'XS, special influence to the attachments of the mesoderm to the other layers, and the consequent strain up<m the segmental plate as the medulla rises ; but the enlargement of the plate depends upon the multiplication of the cells, and wo cannot assume that the strain causes cell proliferation. At most, one might say that the strain determines the shape ot the segmental plate. But is it not more natural to assume that the cells of the mesodenn simply spread out until they till the available space?

The first sign of the mesomeies is the assumption by the cells, that are to form them, of a more distinctly epithelial arrangement, the cells radiating in all directions, but there remains in the centime a core or nucleus {Uririrbe kerii) of cells.


ll'i, C, which have

smidl bodies witli

anastomosing proto b™^- plasmatic pnKxjsses;

Bpa^ 1" n J3 iinjxiHsiblc, at

. un. -TradsrcrMi Si'Oloii llimnBl" « R-o-ntly FcinnBil Prlml- least at present, tO

state whether this epithelial tit^suc is an ingrowth fi-om all sides or only from one or tw<i, but I am strongly inclined to think that it is ]»rol«ibly pai-t of that side of the primitive segment which is next the medtillary canal. The cross-seclioiis further show that the mes<nnfie is nmre (i>mplex than is apjian'nt in surface views, in that it consists of a wider triangular part, Fig. I Hi. .1///, next the medullary grcxne, and a narrower liiteriil portion, ..V, next the parietal zone; it is to the foniier thiit the term ()rotovertebra fcf?) is rewtricte*! by most writers, while the latter is termed the iiifenHPih'iife ifU iiKtSfS, as projMised by Balfour; both are parts of the primitive segment. The appearance of an epithelial arrangement of the cells is confined to the " proto vertebra" senstt strictu. The square blocks seen in surface views correspond to the protovertebrae only and not to the whole segment.

The most exact observations on the primitive segments of mammals known to me are those of Heape, 86. 2, on the mole, of R. Bonnet, 89. 1, on the sheep, and of J. Kollmann, 91.1, on the human embryo. The vertebral plate thickens as the medullary plate rises and becomes triangular in cross-section ; the mesodermal cells, which up to this point have been of the anastomosing type, become elongated and radiating, and gradually assume an epithelioid character, which becomes most distinct on the ectodermal side; the cells gradually withdraw from the centre of the segment, leaving a cavity.* The cells of the segment multiply rapidly, most of the divisions taking place in radial, but some in tangential, planes. The segments have the triangular form already noticed in other classes. The cells have branching prolongations, which extend out to the primar}' germ-layers, and are especially marked on the ectodermal side. In the sheep the cavities of the first four segments, and of them only (Bonnet, /.c*., 50), extend through the lateral portions of the segments and communicate with the main coelom ; these four segments Bonnet assigns to the occipital region. A similar series of communications have been recordeil for the chick by S. Dexter, 90. 1.

The Ventral or Unsegmented Coelom

This portion of the coelom, which persists in the adult, gives rise to the pericardial, pleural, and abdominal cavities, which are morphologically parts of one continuous cavity, the ventral coelom. Many terms are in use to designate the ventral coelom; by English embryologists it is wsadlly plenrO'peritoneal space or cavity; or often simpl}' body-cavity {Leibeshohle^ cavite somatique) ; by German writers it is sometimes termed lateral coelom, sometimes the Parietnllwhley although the latter tenn is properly used only for the pleuro-pericardial division of it. Hatschek has proposed sp1(mchnoca'Jt\ which is adopteil in this work.

The splanchnoca»le appears in all cases in the parietal zone of the mesoblast as a narrow fissure, the method of origin of which has already been described, p. 151. The fissure rapidly widens and extends toward the axis until it almost reaches the primitive segments and also spreads out laterally and into the so-called extra-embryonic region of the amniota, but there is for a considerable period a circular area inclosing the region of the embryo like a ring, in which the meso])last contains no coelom ; this mesodermic ring is known as the vascular area (area vasculosa, frefasshof) , and has for its special function the production of the first Wood-vessels and blood-corpuscles ; see Chapter X. In later stages the coelom extends into the vascular area.

The splanchnocoele is developed earlier, and acquires a greater distention at first in the future cervical region. A. Goette, 90. 1, 4S-40, states that in Petromyzon it precedes in the region of the heart the ap})earance of the segmental cavity. In the Amphibia the precocity of the cervical coelom appears also, and it ia perhaps true of other ananmiota. The development of the main coelom is still more hastened in all the amniota, heing in them intimately associated with the development of the amnion. In the chick this is very well marked, because as probably in all sauropsida the splancbnoocele enlarges so rapidly in the cervical region that, even while the number of primitive segments ia very small, we can recognize a vesicular space in the mesoderm on either side of the head of the embryo; for these spaces, which are the ParietaViohlen of German embryologists, I propose the nam© of amnio-cardial vesicles. They are ^own in Fig. 117, Ves. Their rapid expansion soon brings them into contact, and then into fusion with ono another under the neck of the embryo. The heart is lodged in thia cavity, of which the lateral increase protlnces the so-called bead-fold of the amnion. It is on account of this double destiny that the name amnio-cardial vesicles is proposed. The relations, which are traced out through the later stages in Chapter XV., may be more fully understood from Fig, 117, which is a cross-section of a chick through the heart region at an older stage than we are now considering. The posterior limit of the amnio-cardial coelom is marked by the course of the omphalomesaraic veins, which arise later and establish the communication between the area vasculosa and the venous end of the embryonic heart. The topographical relations are described in Chapter XIII., on the germinal area. In mammals the same peculiarity of the precocious dilatation of the amnio-cardial coelom probably recurs, but has not yet been properly investigated. In the sheep (Bonnet, 89. 1), the amnion appears extraordinarily early, and, as it must be preceded by the formation of the coelom, we find in tlio sheep a huge ring of splanchnoccele around the embryo whilo it is still in the primitive streak stage.

  • Bonnet's fljfiiiv. I.e. Arch. Anat. un<l physiol. IHW, Taf. v. Fijf. 5. -(- On the splauchniKHjele of the head, tsee also \i. 199.

The splanchnocoele of the body proper — that is, of the region behind the neck and heart — appeai-s after the primitive segments, even in the sheep, in which the extra-embryonic coelom is so very early developed. Moreover, in the body the main coelom expands more slowly than in the neck. The expansion takes place at lirst only in the part of the meso<lerm next the primitive segments, Fig. . Already the thickening of the segmental plate {Urivirhdplatte), which accompanied the uprising of the medullary plate, has marked out partially the region of the embryo from that of the yolk, and now the distention of the splauchnocoele increases and finally completes the demarcation of the embryonic region from the extra-embryonic. The splanchnocoele extends in all amniota only part way through the mesoderm, until quite late in development, so that at a gradually increasing distance from the embryo there is a layer of mesoderm without any cavity, and the cells of which preserve the mesench}Tnal type. This undivided mesoderm develops the first blood and blood-vessels. After the first vessels of the area have appeared the splanchnoccele spreads out orer them, so that the first vessels lie then belotv the coelom — i. ^., in the splanchnopleure.

As the splanchnoccele develops, the mesodermal cells assume gradually a more and more distinctly epithelial character, so that the main coelom becomes bounded by mesothelium, as described in Chapter VI., and the somatic leaf of the mesoderm is differentiated from the splanchnic ; toward the axis of the embryo the two leaves pass into one another, and also at the distal edge of the coelom the two leaves pass without any distinct limit into the uncleft mesoderm of the area v^isculosa.

In conclusion, I wish to emphasize the fact that the splanchnoccele (pleuroperitoneal cavity) is almost, if not quite, from the start divided into a precociously enlarged cervical portion (amnio-cardial vesicles, Parietalhohle) and a rump portion (abdominal cavity) ; the boundary between the two portions is marked by the omphalomesaraic veins, which run from the area vasculosa into the embryo proper at nearly right angles to the embryonic axis. This primitive disrK:)sition is of fundamental morphological significance.

Coelom. of the Head

No thorough investigation of the history of the early stages of the mesoderm in the head has yet been made for any vertebrate. Until tliis is done wo cannot hojx) to understand the morphology of the head, because the progress of research has denionstrated more and more clearly that the head is made up of series. of greatly modified segments, but the number and metamorphoses of the head segments can be determined only by knowing the entire history of the mesod(»nn. Balfour was the first, 78.3, to demonstrate the existence of the coelom in the head, and to partially work out its subdivisions. The subject wjis further advanced by A. Mihies Marshall, 81.2, Van Wijhe, 82.1, A. Dohm, 90.2, and others. Marshall and Van Wijhe's results have been subjected by Gegenbaur, 88.1, ;J-8, to criticism, which seems to me by no means fortunate. Gegenbfiur's conclusion, that the number of cephalic segments gives no trustwr)rthy indication of the ancestral history I must entirely dissent from, since I believe that the number of mesodermic segments in the head of the embryos of the lower vertebrates is the onlij trustworthy clew to the morphogeny of the head, which we can seek at j)resent.

A. Dohm, 90. 1, 335, made the discover}' of a large number of segments in the head of vertebrate embryo, having observeil seventeen or eightecai in the head of Torpedo marmorat^i of *3 mm. Killian, 91.1, confirms and rectifies (/. c, p. 103) Dohm's observ^ations, and describes seventeen to eighteen segments, Fig. 1 1 K, in the head of Ehi-smo bra nchs, as follows : Oral zone with two segments; mandibular zone with three; spiracular zone with three, corresponding to the first gill cleft ; hyoid zone with four, in the region of the second gill cleft; glossopharyngeal zone with two; occipital zone with four.

Eillian observed these segments in Balfour's stages F and J, of Torpedo ocellata.

In later stages Van Wijhe, 82.1, whose results have been verified, found only nine segments. The number is presumably reduced cliiefiy by abortion, but partl^j also by fusion. Van Wijhe's segments are as follows : The first or prsB-oral is identical with Balfour's prsB-mandibular cavity; and it is identified by Killian with his oral zone ; it is possible that the first segment of the oral zone is identical with the " new" head cavity described by Julia B. Piatt, 91.2; Van Wijhe's first segment is small and acquires its cavity late, being solid after the remaining eight myotomes have developed their cavities; it is connected by a short band of cells across the ;' '^isf ^^phaiic median line with its fellow of the disappears; the first segment produces four muscles, the rectus superior, intemus, and inferior, and the obliquus superior. The second or mandibular segment (Balfour's mandibular cavity) corresponds with Eillian's mandibular zone; its cavity disappears in Balfour's stage O ; it produces the muscles of mastication ; according to Killian, it is produced by the fusion of three segments. The third segment seems also to be the product of the fusion of three primitive segments of Killian's spiracular zone; its cavity has a communication through the hyoid arch with the ventral coelom (pericardial cavity). The fourth segment corresponds in position over the second or hyoid gill cleft with the three segments of Killian's hyoid zone (Dohm's eleventh to thirteenth segments). The fifth segment corresponds to the two segments of Killian's glossopharyngeal zone. Killian's four occipital segments all persist independently of one another to constitute Van Wijhe's sixth to ninth segments, which I think are to be further identified with the four temporarily present hypoglossal or occipital segments which Froriep has discovered, 86.1, in amniote embryos. Van Wijhe regarded nine as the total maximum number of segments in the vertebrate head, and sought, 89.2, to identify nine corresponding segments in Amphioxus.

Fig. 118. —Head of an Embryq^of Torpedo Ucel lata, in Balfour's S oephalic pranitiTe ments; or first circle below I and U indicates the optic vcasicic. After Killian.

That a series of coelomatic cavities exist in the head of the amphibian embryo was, if I am not mistaken, first observed by Scott and Osborn, 79. 1. Houssay, 90. 1, has sought to identify the number of cephalic myotomes in the axolotl. He accepts the idea of the exact correspondence between the branchial pouches and the myotomes in segmental order; and as he maintains that there is a gill pouch, which corresponds to the auditory nerve and aborts during embryonic life, and further regards the nose, hypophysis, and mouth each as representing a separate segment, he finds that there must be at least eleven segments in the head of the axolotl, as follows: 1, nose; 2, hypophysis; 3, mouth; 4, "event;" 5, hyo-mandibular ; G, hyoid; 7, ear; 8, first branchial; 0, second; 10, third; 11, fourth branchial; for each of these he assumes a separate myotome. He has actually obser\'ed, 90. 1, the nine somites corresponding to those described by Van Wijhe (see alxwe), and further claims to have found evidence that the second and third of these are both really double, thus identifying eleven mesomeres, which, he says, 91.1, 5>^y appear in the following order :

IniHisition 1 2 3 4 5 6 7 8 9 10 11

In time 1 2 11 10 6 3 4 5 7 8 9

Or iu groups 1' 2 3' 2" 1' 1" 2" 3' 4" 5' 6"

Van Bemmelen, 89.1, 254, in a superb reconstruction of the head of a snake embryo, shows three myotomes l)elonging to the eyelmll, but gives no information concerning them, and represents no other myotomes in the head until the hypoglossal region with its four myotomes is reached. A. Oppel, 90.1, describes the cephalic segments in Anguis embryos; he has recorded the presence of Van Wijhe's first to third and sixth to ninth segments.

The splanchnocoelo of the head becomes the pericardial cavity of the adult; its mesothelium, where it covers the heart, gives rise to the cardiac muscle, and it is supposed to extend between the gill pouches to produce the muscles of the branchial arches. Along the level of the branchial pouches the splanchnocoele becomes in part divided, as fii-st showni by Balfour, 78.3, into a scries of separate cavities by the outgi'owth of the giU pouches and the union of the entcxlerm of each pouch with the ectoderm. Each of these cavities has an elongated form fuid communicates on the dorsal side with a myotome, and on the ventral side with the i)ericanlial cavity (Van Wijhe, 82. 1, Van Bemmelen, 90. 1). We may distinguish, therefore, the mandibular coelom, the hyoi<l coelom, and the branchial ci^tlom (one cavity in each gill arch). The connection of the cavities of the arches with l)oth the myotomes and i)ericardial cavity is apparently lost, but as to the 8ei)iiration there are no definite observations. The actual cavities in the arches are soon obliterated, but their mesothelial walls i^ei-sist and produce the branchial muscles ; compare Chapter XXI.

Division of the Primitive Segments

The primitive segments very early divide, each into two parts — the myotome (protovertebra of authors) next the metlullary canal, and the smaller nephn)i(>t)u' {inienuediate masH^ Kolliker's Mittelplatte) ; next the lateral plates or mesothelium of the splanchnoccele. Fig. 110. The division is evidently indicated as soon as the primitive segments are formed, the thicker proximal end being destined for the myotome, the thinner distal end for the nephrotome ; the latter originally unites the myotome with the lateral plates, hence its name of '* intermediate cell mass ; as its principal function is to develop the nephridia it may be more conveniently named the vephrotome, as proposed bv Ruckert, 88.1.

The nephrotome has to separate from the splanchnocoelic mesothelium (lateral plates) on the one side and the myotome upon the other Unfortunately this double separation has been as yet very inadequately studied, except in the case of elasmobranchs, where the development of the nephridia has been carefully investigated; for details compare Chapter XI. In Bombinator a groove appears on the ectodermal side and gradually deepens until it separate the myotome from the rest of the mesoderm ; this groove does not pass through in the shortest direction, but extends obliquely upward, A. Goette, 75.1,213. The nephrotome loses its connection with the myotome relatively early, but retains, at least in some segments, the connection with the lateral plates for some time longer in most elasmobranchs and amphibians throughout life, but in anmiota only during embryonic stages. The exact histological changes by which the nephrotome serv^es its double connections are still unknown. A. Goette, 90. 1, 4i), states that in Petromyzon the isolation of the nephrotome takes place in the front end of the body when the mesoderm has a well-developed coelom, but in the rear part while the mesoderm has no coelom either in the vertebral or lateral plates.

C. Rabl, 89.2, has directed especial attention to the fact that in elasmobranchs there is a special outgrowth of the wall of the primitive segments on the side nearest the chorda and from the point where the nephrotome joins the myotome, Fig. 122. This outgrowth* is the beginning of the inesench3nna, and recurs, of course, segmentally, so that the term sclerotome may l>e applied to it, but all trace of segmental division is very soon lost, nor does the segmental origin of the axial niosenchyma, which is developed from these outgrowths, determine the subsequent morphological differentiation, so far iis yet knoA\Ti. Rabl likens this outgrowth to an evagination, and points out that the cavity of the nephrotome presents a slight diverticulum at first, where the outgrowth takes place. He compcires this evagination with the evagination at a corresponding point in Amphioxus, which has been described by Hatschek, 88.1, and is said to grow up between the myotome and the medulla; in Amphioxus, however, the cells retain an epithelial character, while in the vertebrate they are mesenchjTnal ; but as no strict line can be drawn between these two types of tissue, the histological difference cannot be held to invalidate the homology drawn by Rabl.

The cavity of the primitive segment varies greatly in the various classes of vertebrates. In the primitive forms, Petromyzon, Amphibians, etc., the myotomic portion is wedge-shapeil, api)earing triangular in cross-section, and considerably wider than the cavity of the nephrotome. In elasmobranchs, ct\ C. Rabl, 89.2, Taf. X., Figs. 1-0, a similar diffei'ence exists at first, but very soon the two walls of the myotome come close together, Fig. 122, obliterating the cavity; the nephrotomic portion, on the contrary, widens meanwhile. In Lepidosteus the medullary and entodermal sides of the myotome are represented as several layers of cells thick by Balfour and Parker, 82.1, PL 23, Figs 2vS, 29, so that the myotome appears partly filled with cells belonging, however, to its inferior wall. We ave in this case perhaps a transition to the amniote structure, in which the encroachment of cells is so great that no distinct cavity can be recognized in the myotome. Fig. 116; and since the nephrotomic cavity appears very late, it results that in the amniota there is no distinct cavity whatsoever in the primitive segments, though there is a cavity later in both the myotome and nephrotome.

  • Coinparo Rubl, I.e., "Moriih. Jh. ,xv. , Tuf. x., Fi>?. 4, »k.

The primitive aortae lie close below the myotomes on each side, Figs. 119, 122, 161, 105; a glance at any of these will show the reader that the mesoderm derived from the myotome from the very first comes into contact with and soon envelops the medullary tube, Mdy the notochord, Cfe, and the aorta, Ao, and also reaches over part of the entodermal wall of the archenteron.

Shape of the Myotome

As described above, the myotome, when first formed and even before it is separated from the nephrotome, appears more or less nearly square in surface views and triangular in cross-section. Very s(X)n it enlarges in Amphibia and amniota, so as to appear square in section also, Fig. 119. The cavity in Amphibia is very distinct and the epithelial character of the walls well marked ; but in all amniota, so far i\s known, the cavity at this stage is still obliterated by the core of cells (Remak's Unrirbelkern), By the assumption of the cuboidal shape the myotome becomes more sharply marked off from the intermediate mass or nephrotome, and as the lateral or main coelom has been exiwinding during the same period, there is established a space alxDve the n(^phrotome and between the myotome and the lateral plates. It is in this space that the primitive longitudinal duct of the urogenital system, Fig. 116, Tr.(/., is situatetl as H(x>n as dev(»loped — a fact which led many writers U) attribute the origin of the duct to a differentiation of the intermediate cell mass.

Differentiation of the Myotome

We can distinguish three steps in the differentiation: 1, ])rcxluction of mesenchyma fivm the inner wall of the myotome. Fig. 119; 'i, production of the true nuiscle i)late. Fig. 120; li, conversion of tho outer wall into niesenchyma to form the dermal layer. Fig. 121.

The poduction of mesenchyma from the inner wall begins very early, and is marked by a l(>os(»ning and moving apjirt of the mesothelial cells imtil the entire inner wall, at least in amniota, is converted into tissue of the mesenchymal type, Fig. 119, mes. Owing to the moving apart of the cells the tissue occui)ies a large space and fills up the myotomic cavity. While the metamorphosis is going on the cells multiply ra])idly. The c<^urse of this change of the inner wall has been carefully studied by W. Hea])e, 86.2, in the mole, by R. Bonnet, 89.1, 45-5."), in the sh(»e]), and by Erik Miiller, 88.1, in the chick. Miiller has further demonstrated that the muscular envelope of the aorta comes from the mesenchyma produced by the iuTier myotomic wall. In elasmobranchs, according to C. Rjibl,

2, the greater part of the inner wall of the myotome very early shows the differentiation of muscle fibres, the cells retaining the mesothelial type. Fig. 122, and the mesenchyma is produced only from that part of the inner wall, which is nearest the nt^phrotome {Mitfelplaffe of Remak) ; in elasniobrancliR. therefore, the mesenchyma appears more as an outgrowth from one [viint — a fact which leads Rabl to a signiiicant comparison with Amphioxua, as stated above. In amniota the persistence of the outgrowth is indicated 1^ the fact that the metamorphosis of the mesothelium of the inner wall begins near the uephrotome; it spreads, however, rapidly, so that nearly the entire wall undergoes the transformation. Myownobser* vations are incomplete, but they indicate that in amniota the differentiation of myotomic muscles invariably follows later. Where the inner wall jo'ns tl e outer the cells reta'n the mesothelial arrangement for a very cont. derable per od (see F gs 11 1 and 121).

The muscle plate p oper ar ses from cells of the inner wall next the myotomic cavity or we may say — s nee the cavity is obliterated

— fr tl e cell oan>st tl t 11 The cell become elongated

p(r llel tl tleln^,t I 1 \ f tl e c b tin. I'^o, j/- the

I I ilso lo gate tl I x. t tl be miug ovuJ, and

HS si o n tl e t mir tl 1 t tl e cl ik, larger than

tl e le b( tl t tl 1 Id r J, e h a »■«, and of the

outer n t II f Tl 1 f tl e n er wall, mes, is

the s le oto of t (I rm. rt r t consists of mesench -m 1 11 II art o« t rt*l separated fr the parts of the myotome which are still mesothelial. Whitu the muscle-plate is forming the meseuchyma merges with it, but gradually it oecomes sharply marked off from the muscle cells. The nmscle-plate is continuous at its edge with the outer wall, Cm, and retains the continuity for a very long period. The muscle-plate and outer mesothelium now form a single and highly characteristic structure, familiar to all embryologists ; the structure is a double plate, which takes an oblique position in the embryo; as seen in cross-sections the double plate descends from near the dorsal border of the medullar}' tube downward and outward toward the somatopleure.

The next change Is the production of mesenchjTaa from the outer wall; the cells of the mesothelium move asunder until they come to lie quite far apart, Fig. 131, Cii, forming from the start a much looser tissue than did the mesenchyma from the inner wall; but at this stage, Fig. Til, the inner mesenchyma, mp.t, is spreading around the medullary- canal, and as it spreads assumi^ also a looser texture. The mesothelium, iiisth, still persists Jiround the four mai^ns of the double plate, apparently as an organ to produce cells to lie added on the one hand to the muscle platt' pro]>er, 3fn, on the other to the cutis (dermal mesenchymii), Cn. In sections the mesothelium usually makes a U-ahaped figure, which in highly characteristic of iill vertebrate embryos.

Fig .— Trauwenie Swtton thn>UKh th'8sT«i1y Hours, itti, mh'. Mpwnuh;iii(i of ini

In the primitive vertebrates as exemplified by Petromyzon (Goette, 1, Taf. VI., Figs. 00-03), the flatteiietl myotome consists of two closely appressed epithelial plates with a narrow fissure between them and passing over at their edges into one another ; the upper edge of the myotome is nearly on a level with the dorsal margin of the medulla ; the myotome inclines obliquely outwanl and downward and has its lower edge on the level of the archenteric cavity ; the outer layer of epithelium is the thinner, while the inner layer is considerably thickened ; as the myotome develops farther this difference between the two layers increases.

The amphibian myotomes resemble verj- closely those of Petromyzon, but soon come to differ from them by the midtiplication of cells of the inner layer (A. Goetto, 75.1, m\ Figs. 138-140), which becomes several c(Jls thick and loses at the same time its distinctly epithelial character in the inner i)art of the layer, though it ret^iins it in the outer i>art, there remaining, on the side nearest the entoderm, a single row of cells in epithelial form, so that we have here a condition established secondarily which in the amniota exists almost from the start — namely, a core of looser cells filling the myotomic cavity, but belonging to the ent<xlermal side; it is at this stage that in Bombinator the myotome separates from the remaining mesoderm. In later stages the amphibian myotome gives off from prol)ably all parts of its wall cells to form part of the mesen(*h}nna. while the cells which remain form the definite muscle-plate.

Origin of the Mesenchyma

The first author to trace the origin of the mesenchyma to the primitive mosothclium was Alexander Goette, who fully demonstrated the fact in his great work on the '* Unke, "75.1. Goette designates the m(»senchyma as Bihlu nqagetrehey and seems to me to have hoen the fii*st to fully recognize the morphological significance of the tissue. But his work has not hitherto received its deserved attention. Scattered through numerous special pajwrs are isolated observations which might be profitably collated, and which suffice to show that the mesenc^hyma arises from the mesothelium. In spite of this the brothers Hertwig advanceil, 81. 1, as stated previously, p. 155, the theory that the two mesodermal tissues are of different origin — a theory which we now know to be false is, indeed, was proved by Goette six years before the Hertwigs' theory. That all parts of th(} niescHlenn have a common origin was the view of the older enibrvologists, and, in fact, the differentiation of the middle layer was in the main correctly given by Remak, 50. 1. The unity of the mesoderm has always lx>en maiiitaine<l by Kolliker in his text-lxM^ks and ai*ticlt»s, one of which, 84.4, contains a series of well-founded criticisms of othiT views and a sufficient defence of his own. RcK*ently tin* origin of the mesenchyma has been specially investigated by H. Ziegler, C. 88.1,Rabl, 89.2, and Van Wijhe, 89. 1, in elasmobranchs, and by R. Bonnet, 89. 1, in the sheep.

The mesenchyma rises from cells thrown off from the mesothelium. the entire mesothelium participates in this ])rocess, but not to an eciual degrees nor at the same time throughout its whole extent. The first i)art to prcKluce the mesenchymal cells in elasmobranchs is the splanchnic leaf nt the jK)int where the nephrotome unites with the myotome; at this i)oint, as stated above, there are traces of an evagination. A little later, Fig. 132, the outer wall of the myotome throwB off cells throughout its whole extent, and at the same time a much less active emigration is going on from the nephrotome, while it is not until much later that the walls of the splanchnoccele contrihute to the mesenchyua. Whether the mesoderm of the area vasculosa, in which there is at first no coelom, contributes directly to the mesenchyma is uncertain; it certainlj' produces (see Chapter X.) the blood-vessels, and whether the vessels ought to be considered as meseuchjina or as a -Gl distinct tissue is still under debate. An excellent diagram illustrating the mesothelial sources of the mesenchyma is given by H. Ziegler, 88.1. Taf. XIII.. Fig. 1. For amphibians we have Goette's detailed ■ account; the mesenchyma arises from all parts of the luesothelium, the cells moving off from their epithelial union but remaining connected together by short thick processes, wliich are never numerous, though variable in numlxjr ; the cells all contain a great deal of deutoplasm; aa development prc^^i^sses the yolk grains disappear, the cells become entirely protoplasmatic, and the number of intercellular processes increases, the processes at the same time becoming finer and longer. There are regional distinctions in the density nf the tissue, which are constant. The tissue increases by additions fi-om the mesothelium during a certain period, and continuously by the proliferation of its own

with Forty. CClls. GoCttC also, 75.1, 4!)T-4!t8,

'wTMfSu'h iissprts that after the circulation

luyRmuvr; »i. oniainiOTitteRuiKli'on: J/tf. begins leUCOCyteS IcaVC the blood vessels and are transformed into AKeTr ^^fyp Bildiing>tffencb.tzellt'n: he does not seem to me to offer sufficient pioof to justify this assertion. Goette attributes. I.e., 4'Xi, the moving ing ajtart of the cells, not, as seems to me most reasonable, to their own growth, but to the accumulation of intercellular fluid, which he assumes to l>e producetl by transfusion from the archenteron. In maininalH and birds the niiuiuer in which the myotome contributes to the mesenchyma is now prettv- thoroughly imderstood, but the share taken by the nephrotome and lateral plates has still to 1k! ascertained. In Ixith classes the metamor{}ho3is of the outer wall occui-s much later than that of the inner wall, which very early becomes considerably thickened hy the multiplication of its cells. Heape, 86.2, deecribeB the process in the mole nearly in the following words: The myotomes at Heape'a stage H commence first in the anterior r^ion, and gradually jissuming the same relations posteriorly, to divide into two portions, an outer arched epithelial portion and a thicker inner portion composed of anastomosing cells of distinctly mesencbj'mal type, which give rise to the axial mesenchyma, and participate in the formation of the definite muscle-plate. The myotomic cavity is very marked. In the next stage (J) the anterior myotomes exhibit still further changes ; the inner layer has grown very considerably, and the row of its cells next the cavity are .more closely packed and so have assumed the epithelial form, while the remainder nf the layer presen'es the anastomosing character of the cells ; the inner layer of the myotome is therefore separated into its two parts: the epithelial part becomes continuous with the outer layer, and the two epithelia together constitute the so-called double muscleplate. Although arising from separate segments the axial meseuI'hyma loses almost immethately every trace of segmental arrangement, and there is no real proof that its segmental origin has direct intluonce uiH>n the segmental arrangement of the vertebral and other structures differentiated later from the mesenchyma. Ultimately, as in other vertebrates, the entire outer layer is converted into mesenchyma. which forms the dennal layer, R. Bonnet, 89.1, 5i.

Comparison with Amphioxus

Hatschek's observations, 88.1, on the differentiation of the mesoderm of Amphioxus showthat there are many striking resemblances with the history of the vertebrate mesoderm as given alxive. The mesoderm consists at first of a series of paired mesothehal sacs; the ventral portions of the sacs fuse into a continuous splancbnocixle ; in a larva several weeks old the inner wall of the dorsal segments is a thick epithelium, which produces the muscles on the inner or entodermal side of the caHtj* of the segment {myocoele of Hatschek); the mesothelium becomes a thin pavement epithelium. After about three months of pelj^c-life, the larva changes into Amphioxus and takes to the sand. At this time the lower edge of the segment is foxind to have formed a diverticulum, which stretches upward beween the muscles on the one side and the meiiuUa on the other. The segments have also extended into the dorsal and ventral fins and have there formed cavities. These relations are illustrated by the accompanying diagram. Fig. I:i3, after Hatschek. The points of special interest to us are four: 1, the formation of the splanchnocoele by the fusion of segmental cavities; 2, the development of the muscles exclusively from the inner layer of the secondary segments ; 3, the absence of differentiation in the outer layer of the segment ; 4, the outgrowth of mesothelium passing upward between the muscular layers and the axial structures, medulla, and notochord. It is probable that all these four peculiarities recur in the true vertebrates, though masked principally by the fact that the outer layer of the segment and the epiaxial diverticulum both lose, the former gradually, the latter almost from the start, all trace of epithelial structure, and become converted into mesenchyma. Of course the assumption that the vertebrate splanchnocoele arises in the same way as in Amphioxus, is at present entirely hypothetical.

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Human Embryology: Introduction | The Uterus | General Outline of Human Development | The Genital Products | History of the Genoblasts and the Theory of Sex | The Germ-Layers | Segmentation | Primitive Streak | Mesoderm and the Coelom | Germ-Layers General Remarks | The Embryo | The Medullary Groove, Notochord and Neurenteric Canals | Coelom Divisions; Mesenchyma Origin | Blood, Blood-Vessels and Heart Origin | Urogenital System Origin | The Archenteron and the Gill Clefts | Germinal Area, the Embryo and its Appendages | The Foetal Appendages | Chorion | Amnion and Proamnion | The Yolk Sack, Allantois and Umbilical Cord | Placenta | The Foetus | Growth and External Development Embryo and Foetus | Mesenchymal Tissues | Skeleton and Limbs | Muscular System | Splanchnocoele and Diaphragm | Urogenital System | Transformations of the Heart and Blood-Vessels | The Epidermal System | Mouth Cavity and Face | The Nervous System | Sense Organs | Entodermal Canal | Figures | References | Embryology History

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