1897 Human Embryology 6

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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 VI. The Mesoderm and the Coelom

The morphology of the mesoderm is one of the most vexed questions of the day. Scarcely an embryologist can be found who has not published opinions on this question considerably at variance with the opinions of others. It has been maintained that the mesoderm arises from the ectoderm ; that it arises from the entoderm, or from both ; from neither, but from two special segmentation spheres ; that it has a double origin, part coming from the blastoderm, part from the yolk, and even that there is no mesoderm.

We now know positively that in all vertebrates there is a distinct and unmistakable mesoderm, which spreads out from the primitive streak in all directions and has distinctive histological characteristics.

Two large and complex cavities appear in this mesoderm, one on each side of the median axial line. The mesodermic cells which bound these two cavities assume an epithelial arrangement, and are designated as the mesothelium; the cavities constitute the coelom or primitive body cavity ; the mesothelium at various points throws off cells which compose the mesenchyma (embryonic connective tissue). We have, accordingly, three distinct phases to study, viz. : 1, the origin of the mesoderm ; 2, formation of the coelom and mesothelium ; 3, the origin of the mesenchyma. Finally, we must review the principal theories in regard to the morphological significance of the mesoderm.

I. Origin of the Mesoderm

Mesoderm of Elasmobranchs.— In the cartilaginous fishes the mesoderm arises from the entoderm close to the ectental line. The observations of Balfour in his monograph, 78.3 (see also his works, I., 246-268), established the fact that the mesoderm appears after the two primary layers and is connected with the entoderm. This fact has since been abundantly confirmed, see KoUraann, 86.2, Swaen, 87.1, Riickert, 86.1, 87.1, Rabl, 89.2, D. Schwarz, 89. 1, et al. These later observations, particularly those of Riickert and Rabl, have settled the exact point, or rather area, of entoderm which is mesoblastogenic. Unfortunately Rabl overlooked the phenomena of concrescence, and consequently reached conclusions as to the development of the mesoderm which T feel no hesitation in pronouncing erroneous. The mesoderm is differentiated along the embryonic rim before concrescence takes place; hence, when concrescence is partly completed, there is an axial stretch of mesoderm, and from the hind end of this the mesoderm stretches out toward each side along the embrj^onic rim in connection with the entoderm, us has been described in Chapter V. We can distinguish the axial mesoderm from the lateral mesoderm ; but later on, when concrescence has progressed farther, there is no hiteral mesoderm, for it has become axial. Rabl, however, failed to study the later stages, and so came to consider that this temporary condition of the mesoderm signitie<l a double origin ; accordingly, he distinguishes between the "gastral" (axial) and " peristomial" (lateral) mesoderm, and makes the unsuccessful attempt to show that the '* gastral" and " peristomial" mesoderms are of essentially different origin in all vertebrates. Had Rabl understood concrescence he would certainly have not fallen into these errors. There is no positive evidence that there is an evagination of the entoderm as the Hertwigs maintain can be shown in the amphibians — see below. On the contrar}^ the cells grow forth from the entoderm so as to constitute a sheet between the primary germ-layers. Soon the connection with the entoderm is permanently severed.

  • This chapter has already been published in the American Naturalist, Oct., 1890, but as here reprinted has been extensively altered.

The fact that the mesoilerm appears first in the embryonic rim rendei-s it easy to make sure of its springing from the entoderm. Later, when concrescence moves the rim into the axial line, all three germ-layers are united in the primitive axis, and it becomes more difficult to decide which of the layers the mesoderm is specially connected with. To conclude : in elasmobranchs the mesoderm arises over a limited area of the entoderm near the ectental line; it separates from the entoderm apparently by a pr(x»ess of delamination, but the exact means of separation have yet to be investigated ; it remains for a while connected with the entxxlerm along the embryonic axis; after its separation from the entoderm the mesoderm expands by proliferation of its own cells and receives no accretions from the yolk, so far as at present known.

Mesoderm of Teleosts

So far as the published accoimts go the middle layer of bony fishes arises as maintained by Balfour, ** Comp. Embryol.," II., 74, from the entoderm. Such appears to be the significance of Ryder's observ^ations, 84.1, 41, of A. Goette's, 73.1, E. Zielger's, Agassiz and Whitman's, 84.1, and of others. For a good description, together with citations of conflicting authorities, see M. Kowalewski, 86.1, 4G<,»-474. Apparently the blastodermic rim is turned under, and the turned-under t)ortion yields the entoderm, and is intimatelv connected with the sheet of mesodermal cells, very much rs in sharks ; the mesoderm is several layers thick and sti*et<*hes in under the ect<Hlennal blastodenn, gradually thinning out; the cells of the middle layer are at first closely compacted.

Mesoderm of Amphibia

Here it seems also clearly established that the mesoderm arises from the entoderm, principally along and alongside the me<lian line, as a sheet of cells with no cavity (coelom), included between th(»m; along the centre of the primitive axis and at the blast(>jK)ric margin the connection between the mesoderm and ent^xlenii is both evident and intimate; see Bellonci, 84.1, Tav. II., for figures showing this |K)int in the axolotl, and O. Schultze, 88.1, for similar figures of Rana fusca. These facts have been recorded by bo many observers that there can be little doubt or none of their entire accuracy ; eee the description and cuts, ante, p. 13U. It may lie considered as stil) uncertain whether the sheet of meaoderm receives accretions at itt) distal ed^ from the yolk cells (entodermic) upon which it rests. There usually is no sharp limit between the two, and therefore we must consider it probable that at first the mesoderm receives additions from the yolk ; later on it is found divided from the vitelline cells, and after it has split off it probably grows independently. The gniwthof the mesoderm at first from the yolk has been found in Pelromyzon by A. E. Shipley, 88.1,177. 178 {of "Studies"), although in later stages the mesoderm 19 severed from the yolk.

In later stages the mesoderm is wanting in the median line, and thus constitutes two masses or two lateral sheets. This bilnteral division is effected by the development of the mechdlarj- groove and notochord, as described in Chapter VIII. The mesodermic connee. tion with the entoderm is retained, but is double owing to the division. Along the median dorsal line of the nrcheuteron runs the strip of entoderm which forms the notochord ; on each side of this strip runs the lino of t^nnection between entodenii and mesoderm. The study of this ttecondary condition has led many authors into the error of ascribing a double origin to the amphibian mes<Mlerni, and inferentially to the vertebrate mesoderm in general. This brings us to the consideration of O. Hertwig's views, which form one of the chief sui>{>orts of the " Coelomtheorie" of the brothers Hertwig. For further discussion of this thoorj- see p. 155.

(). Hertwig, 82.1, 83.1, studiwl stages in which the notochord bad api>ean'd, and at this time, &tO. Scbnltze, 88.1, has shown, the luimitive relations of the lay"■ ers no longer e-Yist, but Hertwig

regarded the Hei*i>ndary arnmgements in question as pi'imaiy. Ho found no niesixierai in the axial line nln>v6 tho notochord; at the edge of the notochord, where it joins the undiffeivntiated epithelial entoderm of the archenteron, there is on each aide a groove which in cross-sections appears as a notch, Fig. 8S; the notch is of variable depth, is stinietiincs absent, and is a temporary feature. In the neighborhood of the furrow alongside the notochord, the mesoderm is Still iutimatelv Connected with the entoderm. "Thiwe relations are bfheve<l I by Hertwig to indicate that tho moso<lerm arises as two masses, which is not the case, and that each mass is really a diverticulum of the archenteron, the furrow being the month of the diverticular cavity. Hertwig's figures, 82.1, Taf. XIII.. XIV., offer the plainest representations of the mesoderm in Triton as jtaired diverticula; but these figures * are evidently diagrammatic, and they must be termed inaccurate, I think, in the very respects which are essential to Hertwig's theory. This appears from the investigations of Goette, 75.1, Bellonci, 84.1, Bambeke, 68.1, O. Schultze, 88.1, and others. Compare also K. Lampert, 83.1. The reader may compare, for instance, Hertwig's Fig. 10, /.c, Taf. XIII., with Bellonci's Fig. 11, /.c, Tav. III. O. Schultze's detailed criticism, /.c, 344-340, of Hertwig's account seems to me entirely justified, and I accordingly accept it as a complete disproof. This criticism shows that Hertwig's conception is based upon insufficient and erroneous observations ; insufficient because he did not investigate the early condition of the mesoderm, and failed to recognize the fugitive and unessential character of the parachordal grooves ; erroneous because the cavity in the mesoderm does not really communicate with that of the archenteron. There are other errors which Schultze points out and which are important. Robinson and Assheton, 91.1, 495, have also failed to verify Hertwig's statements.

Wo find in Amphibia at a certain stage the axial (Rabl's gastrales) and lateral (Rabl's peristomales) mesoderm. The former is in the region of the completed concrescence, the latter round the edge of the anus of Rusconi. The former is connected with the entoderm alone, the latter with the ectoderm also, since the entoderm is connected with the ectoderm around the unconcresced blastoporic rim. The connection with the ectoderm renders it possible that the middle layer receives cell ^ from the outer layer, but there is no direct proof of this. When the concrescence is completed the mesoderm is said to sever, in the posterior axial region, its connection with the entoderm, but to retain awhile its connection with the outer germ layer. The same phenomenon recurs in the amniota. It cannot, however, be taken to signify that the middle layer originates from the ectoderm, since at an earlier stage it is clearly entodermal.

Mesoderm of Sauropsida

We may consider reptiles and birds together, since the early history of the middle layer is very similar in the two classes.

In reptiles, so far as our present unsatisfactory knowledge enables us to judge, the mesoderm arises by delamination from the entoderm, but remains connected with it along the axial line; in front (i.e., in the head-process) it is connected with the entoderm only, but posteriorly it is fused with the tissue of the primitive streak, and thereby is indirectly connected with the ectoderm. After its delamination the mes()derm expands independently of the other germ-layers except, perhaps, along the axis. That the relations are like those in birds has been shown clearly by Strahl, 83.1, and also by Weldon, 83.1, whose Figure 1 is reproduced, ante, Fig. 71. The intimate connection of the mesoderm with the entoderm at the blastodermic rim before concrescence is sufficiently established by Kollmann, 84.3, 403-400, though his conception that this part of the mesoderm is a separate structure, which he terms akroblast, renders it difficult to f<jllo\v certain parts of his description. C. K. Hofmann may also be cited, though his accomit (Bronn's "Thiereich, Reptilien/' p. 1881) is of doubtful accuracy in several respects. L. Will, 89. 1, 1127, finds that in the gecko the mesoderm is united with the entoderm in the head-process, but omits to describe its exact connection with the primitive streak ; the stages showing the origin of the mesoderm he does not mention. The processes involved will undoubtedly be understood as soon as the concrescence of the axis has been worked out — a fundamental question, which as yet not a single investigator has heeded.

  • Some of them are reproduceil in Hertwig's "Lehrbuch der Entwickelimgsgeschichte," 6te8 Capitel.

In birds the exclusively entodermic origin of the mesoderm is, in my opinion, conclusively demonstrated by the researches of Duval, 84.1, 104-117; the entoderm gradually thickens by migrations of its cells over a considerable axial area; the upper stratimi of this thickened area separates off as the mesoderm except that in the axial line it retains its connection with the entoderm; when concrescence take* place the two layers form the primitive axis. In the region of the primitive streak there is a single large mass of cells, Fig. 71, Pr, which is continuous with all three germ-layers. Now if the homology maintained in the previous chapter be correct between the primitive streak and the anus of Rusconi, then the cells of the streak are also ent<Klermal, and the middle germ-layer is connected in both a.xial regions directly only with the entoderm. After the mesoderm has separated from the entoderm except in the median line it may continue to receive accretions from the entoderm in the median line, but, as far as known, makes no peripheral additions except from its own growth. So far as heretofore observed the mesoderm receives no cells from the ectoderm.

Mesoderm of Mammals

In this class, according to the best recent investigations, the mesoderm appears to have a distinctly twofold origin. According to Bonnet, o4. 1, 100, part of the mesoderm grows out from Hensen's knot at a time when the knot is a thickening of the outer layer and has not yet acquired any connection with the inner layer ; another portion is produced peripherally. Fig. 84, mes, by delamination from the inner layer; the two anlages grow toward one another and unite into one continuous mesoderm, in wiiich all trtwre of the primitive double origin is obliterated. KoUiker has recorded the outgrowth of the mesoderm from Hensen's knot in the rabbit, and his statement has been confirmed by Fr. Carius, 88. 1, 17. In later stages we find the relations of the layers similar to those in Sauropsida, there being a head-process with the mesoderm connected axially with the inner layer, and a primitive streak, with which the mesoderm fuses; the inner layer of the blastodermic vesicle is connected with the front part of the streak. This stage is cjuite well known, cf, Heape, 83.1, on the mole. Bonnet on the sheep, 84.1, Kolliker on the rabbit ("Grundriss"), Selenka on the opossum, 86.1, Lieberkiihn, 82.1, and others, especially the very careful descriptions of the rabbit's layers by C. Rabl, 89.2.

At present it seems to me impossible to offer any satisfactory interpretation of the ol:)serv'ed double origin of the mammalian mesoderm. The relations of the me^nlerm to the primitive axis (head-process) and primitive streak are identical with those in birds and reptiles.

The Vertebrate Type of Origin of the Mesoderm

The preceding paragraphs show that in all classes of vertebrates the origin of the mesoderm is essentially the same, except that in some mammals it begins in two regions of the entoderm almost simultaneously. The relations in the mammals we do not understand. In the non-mammalian vertebrates the mesodenn first appears as a thickening of the entoderm over a not inconsiderable area around the concrescing blastodermic rim, and it becomes separated from the entoderm by the gradual parting of the upper cells to form the true mesoderm from the lower cells or i)ermanent entoderm ; this delamination does not take place next the blastodermic rim (or — after concrescence — in the axial line) ; hence in the region of the primitive axis the three layers may be connected for a time ; further, as the tissue of the primitive streak is at first connected with the ectoderm, the mesoilerm is thereby indirectly continuous with the outer germ-layer during veiy' early stages. It is important to note that the mesoderm arises over a considerable area during the same period ; that its formation may be more or less advanced before concrescence of the rim ; and that after concrescence it stretches across the axis of the embryo between the ectoderm and entoderm, thus becoming a continuous sheet or layer. This fact that the mesoderm is a single anlage needfe to be specially emphasized. So far as known to me there is not a single vertebrate which has been shown to lack this stage, but on the con'trary its occurrence is established for all classes and by so many observers that we may well assert that there are few facts in embryology better established. Later the mesoderm becomes divided in the axial line,* and consideration of this secondary condition has led to several theories of the mesoderm, which would hardly have been brought forward had their authors not neglected to take into account the earlier condition of the middle layer. Some of these theories are discussed below.

After its delamination the mesoderm is a distinct layer and grows independently, receiving no accretions from the other layers except in the axial line, where it receives cells from the entoderm and in the region of the primitive streak. The edge of the expanding sheet of mesoderm is free, as has been pointed out in the previous chapter, resting upon the yolk but not fused with it. It is therefore, it seems to me, impossible to admit that there is a peripheral ingrowth of tissues arising from the yolk and entering the mesoderm to form the blood, etc. Compare below. Theories of the Mesoderm^ p. 153.

The primitive mesodermic cells are embryonic in character; that is, they have a large, usually nucleolated, nucleus, and very little protoplasm (Minot, 125). They are connected together by fine threads, and may lie some distance apart, then presenting an obvious resemblance to the mesenchynia of later stages. The cells become more closely compacted as development progresses, and when the coelom appears they take on a distinctly epithelial arrangement to make the mesothelium. The cells frequently contain yolk grains — in the case of Amphibia numerous and large ones. In l3irds the yolk grains are few, but are easily observed, Fig. SI ; in mammals they are almost entirely absent.

  • Mitsukiiri. 91.1, has atteniptofl to dt»ny the views T have advance<i, because in turtles the mesoderm is divided, as known by his own observations. He has overlooked the fact that his observations refer to the secondary sta^e only when the nie<1iillary groove and notochord are present, and that they have no bearing on the question of the earlier and primitive condition.

Expansion of the Mesoderm

After the mesoderm is once formed na a distinct layer without connection with the primitive layers except in the axial line, it expands independently — that is, by the proliferation of its own cells. During its early expansion the mesoderm assumes in all amuiota n definite series of characteristic outlines. It is at fii-st pear-sHaped, Fig. 89, A, the anterior end being ix>inted ; it extends a short distance oidy in front of the primitive streak, and is widest a little distance behind the area pellucida, ap. The same stt^e is found in mammals, see KoUiker, ("Grundriss," p. !i;Sand Fig. Tl.) The condition in the chick at about the twentieth hour <if incubation is iiidicate<l by Fig. 80, B, drawn on the same scale as A, and nt the cUipm.- of the first day by Fig. !)0. In tHo last mentioned figure it will bi! notice<l that the uieaoflerm is exiMinding unequidly in front, having sent out two lateral wings whicli leave a median space iH^tween thcni without mesoderm. These wings continue their growth and finally nnvt in fnnit, so that in the anterior jiart of the area (jeHuciila there is a small tract without any mesoderm, although there is mesuUrm all around it: this tract is thf^ proa nut ion, of which a fuller history is given in Chapter XV. The expansion docs not bike place by anymeatis with thti exact regidarityindicated by Figs, 89, flO, hut, on the contrary, in birds, as shown by Zuinstein, 87. 1, the outline of tint middle layer is always irregular and more or less RSjnnmetrical. Although thei-e are not yet many observations availablo as to the iiutlineof the gri>wing mesodcnn, yet it is pn)bablo that the pn-i-oding descriptinn is essentially coiTect, not merely for bij-ds but fur all anniiota. It is certainly so for the rabbit. Van feeneden et Julin, 84. 1.

II. Formation of the Coelom and Mesothelium

Early in the course nf development theii' apix'ars in the mesiMlerm a complex series of caviti<-s, which very mum l»ec.ime UTiitcHl s... as to form two large cavities, one <in eacli siih', which together constitute the co'hiiti or embrj-onic Uniy cavity. In the adult mannna! the coelom is represented by the iK-ricai-dial, pleural," and alxiomina! cavities: the coelom alsii includes the cavities of the musc-nlar segmenta (protovertebrjp) and tilso certain tubular iMirts of the urogenital sj-8tem. But although its subsequent changes are complex, the cielom amsiatB at an early stage of a pair of tisaures in the mesoderm. As the ctelomatic cavities appear the cells bounding them take on a distinctly epithelial character. the mesodermic epithelium bounding the ctelom is t<?rme<l the mesothelium, and it is probable — if we judge from our present imperfect knowledge— that the entire mesodenn is in all vertebrates first converted into mesothelium, before undergoing differeiitiation.

Only one precise account of the mode of development of the coelom in mammals is known to me, namely, that of Bonnet, 84.1, 202, for the sheep at about thirteen days. Around the embryo at some distance from the axis there appear a series of irregular fissures of rounded or elongated form, which may in part open on the mesodermic surface ; gradually the fissures enlarge and fuse, at the same time becoming more closely bounded by the mesodermic cells ; thus there arises a continuous cavity in the mesodenn which is for a time crossed by cells and cell processes ; similar connections between the two leaves of the mesoderm while the coelom is forming and their subsequent rupture have been noticed in Amphibia by B. Solger, 86.1, 383, in Elasmobranchs by E. Ziegler, 88.1, 38.3, and 1 find similar phases with great distinctness in the chick; meanwhile the cells, which are loosely put together, fomiac<»mpact layer of epithelium hounding the cavity, whicli wo ciin now designate as the icflom or primitive oody cavity. By similar pnx-esses the coelom gi-ows toward the axial region, but never penetrates it, the primitive sti-eak and head-process never developing a median co.lom.

Albrccht Budge, 87. 1 , ha.s made a very exact study of the arranment of the fissures in tlit me-(Klenii of the chick by means of injections of Prussian bhit * The fissures fonii a net W( irk of (hanntlo and by their fusion pro<lmt, the ctelomatic cavities. The ch nintla appear first around the ixnpher^ v£ the area vascnlosa, and thtnce their development progresses centrifugallv,bnt most rapidly towanl the head; the channels fu.'ie first around the head to make the amnio-cardial cttlom (P<iri<-t(tlliiilile of His) ; now appeal's a circidar sinus just inside the vena temiinalis; the coelom gniws back through the embrj^o and forms the Ixxly cavity of the rump; alongside the rump, as shown in Fig. HI, Fig. oi.— TLe im^s. appears a network of channels, J^^S'^'th'i^ssiuni which soon fuse to create the coelom under the lateral aumiotic fold, and this unites with the coelom of the rump, forming the completed coelom continuous with that of the pericardium. The network of channols Budge regards as primary lymph spaces. Compare Chapter XIX.

Whether in all vertebrates the coelom results from the fusion of numerous small spaces or not, is not yet determined by actual otoervation. It is probable that it does so, and we may, therefore, say that the vertebrate coelom is what Huxleyterms a schizocoele, i.e., a cavity produced by splitting the mesoderm, compare p. 155. I consider it also probable that the coelom always begins to appear at a httle distance from the axis of the embrj-o and spreads both centripetally and centrifugally.

Additional and important points in the earliest history of the coelom are treated in Chapter IX. We must add here that the coelomatic fissure divides the mesoderm on each side into an upper or outer leaf (Hautfaserblatt) and a lower or inner leaf (Darmfaserblatt). Fig, !)2. The upper leaf may be called the somatic mesoderm, Soin, the lower leaf the splanchnic mesoderm, Spl, as proposed by Balfour. The upper leaf lies close against the ectoderm; the two layers together form the somatopleure or body-wall. The lower leaf lies close against the entoderm ; these two layers together form the splanchnopleure or wall of the alimentarj- tract. Both the somatic leaf of mesoderm and the splanchnic consist at first solely of mesothelium, but very soon each cimtains meseiichyma also ; the latter arises from the mesothelium ; axially the two layers become continuous both with one another and with the axial mesoderm.

Via. aa.— Section of Btlc mesoderm ; l^i, spluichDit cells; ild, uiHlulUry graavti; Aftar W. Walcleyer.

The morphology of the coelom is so important that it is difficult to understand why so many investigators have slurred over the question of its embryonic development. Exsict observations on its first appearance and on the first stages of its expanMion in various types are u^jently needed, and would certainly do more than anyming else to throw light on the still oliscure problem of the origin of the mesoderm.

The hiniogenesis of the mesothelium varies somewhat in the different tj-pes. Primitively (marsiohranchs, amphibians), thec^lla are rounded in form, contain considerable yolk, and aro at first loosely aggregated, compare Fig. 88, When the coelom appeam the cells become more closely appressed and so gradually assume more and more the cliaracteristics of a cuboidal epithelium. In the amniota, on the other, hand, the mesodennal cells contain very little yolk, Fig. 81, in which the yolk grains are shown as bUick dots; the cells are connected by their processes; as the coelom develops the processes are shortened, and the cells bei^ome more closel}' packed, and thus gradually arrange themselvos into a cuboidal mesotholium.

III. Origin of the Mesenchyma

The genesis of the mesench^^ma is ti*eated in Chapter IX., as it cannot be understood without a knowleilge of the development of the primitive segments. I will, therefore, merely stat« here the general methods of its prcxluction in order to render intelligible the following discussion of the theories of the mesixlerm.

By mesenchyma we understand the whole of the mesoderm of the embryo, except the mesothelial lining of the cctlom. So far as at present demonstrated it arises solely from the mesothelium. Single cells leave the mesothelium on the side away from the coelom ; these cells remain connected with one another and with the mesothelial cells by protoplasmatic pnK'csses, but they do not lie close together as in an epithelium, so there is a considerable though variable amount of intercellular space. By the migration of the cells and their multiplication a large amount of mesodermic tissue is produced, which fills up all the r(K>ni between the mesothelium and the two primary germ-layers. At first no definite distinction between the mesothelium and the mesenchyma can be est^iblished, but ultimately they become and remain distinct tissues, with divergent histories.

IV. Theories of the Mesoderm

From the time of Von Baer's " Entwickelungsgeschichte," of which the first part appeare<l in 1S:>8, until 1S08, when \V. His' great monograph on the chick, 68.1, was jmblished, embryologists recognized the three layers, and regardcnl the mesoderm as a natural unit. His led the way to our present conception by a little-kno\\ii article, 66. 1, on the membranes and cavities of the Ixxly, and his monograph, 68.1, above mentioned fully established the necessity of recognizing two main groups of mesodermic tissues; accordingly he dividwl the mesoderm into two parts, the (nvhibUist and partihlast^ corresponding respectively essentially t<j mesothelimn and mt^senchyma. Under archiblast His included not only the mesothelial tissues pro]>er, but also the smooth or organic musculature; luider j)aral)last the mesenchymic tissue except the smooth muscle. the tenns used corresponded to his theory of the origin of the two parts of tin* mesoderm, for he Ijelieved that the archiblast arose in the axial region and was containeil in the embryo from the start, while the parablast arose peripherally and grew in toward the embryo, a conception which was jjerhaps suggested In' the appearance of thci ])lood -vessels, first, outside the embryo proper. Seeking still farther for the source of the supposed i)t»ripli(Tal para])last he l)elieveil he had found it in the germinal wall. The study of the relations of the' wall in the chick induced him to think that the elements of the white yolk he came parablast cells; moreover, the study of the hen's ovary led him to the conclusion that the white yolk was developed from the granulosa cells, and that these cells arise from leucocytes. He thus traced back the parablastic cells to maternal leucocytes. As subsequent chapters will show more fully, the granulosa cells are not leucocytes ; in Chapter III. it has already been shown that the granulosa cells do not enter the ovum ; the white yolk grains never become cells, for it has been proved that all nuclei of the segmenting ovum come from previous nuclei and lie in protoplasm, not in the yolk grains ; and, finally, it has been shown in this chapter that the mesoderm arises as a whole, not from double sources. Professor His' views as to the origin of the parablast must be given up, but this is no reason for overlooking, as certain writers have done, the fundamental significance of the distinction drawn between the two primary groups of mesodermic tissues. Subsecjuent research has made only one important change necessary — namely, the transfers of smooth musculature from one group to the other. In view of this change, of the fact that parablast has been used with various other meanings, and of the unaptness of His' names — since we renounce the theory they correspond to — it will be well to use exclusively the newer terms, mesothelirun and mesenchyma.

The parablast theory has been defended by His, 76.2, and modified by him, 82.1, At present he holds to the distinction originally drawn, but is inclined to withdraw his hypothesis of the origin of the parablast. A number of writers have agreed with His as to the separate peripheral development of the mesenchyma (parablast). Among these may bo mentioned Rauber, 77.1, 83.4, and several authors who have deiilt with the development of the blood, see Chapter X. The most important of the disciples of His is KoUmann, who, in a series of articles, 84.1, 3, 85.1, », has maintained the double origin of the mesoderm. Of these pa|)ers the most important is that on the "Rand^vulst," or germinal- wall, of the structure of which in the chick it gives an excellent description. Kollmann regards the germinal-wall not as part of the entoderm, but as a distinct organ composed of segmentation spheres, and destined to produce bloodvessels with blood, and probably also connective tissue ; this peripheral anlage (Randkeim) ho designates as acroblast^ and the single cells derived from it he names poreiiten. Waldeyer, 83.1, has accepted the parablast theorj^, but with a modification by which he seeks to reconcile conflicting observ^ations. His article is written with characteristic clearness and exhaustive masterv of the literature, and will h& found especially valuable by those who wish to pursue this subject farther. Waldeyer distinguishes between the primary and secondary segmentation ; the former producing the ectoderm, entoderm, and arcliiblastic mesoderm, the latter occurring later and giving rise to the parablast. According to Waldeyer this remnant of the ovum (which in holoblastic ova consists of cells, in meroblastic ova of egg protoplasm) has its cell division (segmentation) retarded, and the cells thus tardily ])roduced immigrate into and l>etween the germ-layers already developed.

The opposition to the parablast theory is the sum of numerous observations, which, as pointed out in the previous part of this chapter, prove — it seems to me — that the mesoderm arises in all vertebrates (except mammals?) as a unit, and subsequently separates into mesothelium and mesenchyma. The leading opponent of the separate origin of the parablast is KoUiker in both his text-books ("* Entwickelungsgeschichte," etc., and '* Grimdriss") , and in separate articles, see especially, 84.2, 4, and his criticism, 86.3, of Kolhnann. I agree with KoUiker that it has l)een sufficiently demonstrateil that the " akroblast'* belongs to the entoderm, and after the delamination of the mesoderm is transformed into the epithelium of the yolk-sac; for a conchisiv^e demonstration that this is so in reptiles, see H. Strahl, 87.1.

The coelom theory of the brothers Hertwig includes a fundamental modification of the i)arabla8t th(Kjry. The main features of the coelom theor}^ are not original with the Hertwigs, but may be found in previous writers ; nevertheless they were the first to present the theory in a complete formula and with a backing of fiicts, both new and collateil, from otlu»rs so extensive as to compel attention. In justice to E. Ray Lankestor it must \xi state<l that he is really the author of the coelom theory, having in 1877 (77.1) published the hjTXjthesis that the ca'lom is derivc^il from the archenteron, and that the mesoderm of vertebrates represents solid entodennal diverticula. It is unfortunate that the Hertwigs have not made due acknowledgment of what they owed to Lankestor and others. They made a series of investigations on the germ-layers of various representatives of the animal kingdom, and presented their general results in a comprehensive article (O. and R. Hertwig, 81.1), and (.). Hertwig has again expounded the theory in his text-lxK)k of embryology. The coelom theory consists of two j)arts : 1, the coelom is formed by diverticula of the archenteron and its lining, the mesothelium, is part of the entoderm; "2, the mesencli\'ina consists of cells thrown off by the other germ-layers and is essentially distinct from the mesothelium. The value of this theory lav in the clearness of its formulation, thus facilitating discussion, and also in its bringing out the difference more clearly between the epithelial and the non -epithelial portions of the mesodenn. As we have seen, there is no evidence of a character to render even probable that the mesoderm of vertebrates represents archentcTic diverticula, and the whole niescnlenn aj)pear8 as a single germ-layer, which is subsequently differentiated into mesenchyma and mesotheliiun. Hence, both essential parts of the coelom theory are inap])licabl(» to vertebrates, at least in the present state of our knowledge. For further disc»ussion of the difficulties of the Hertwigs' theory, see Rabl, 89.2, 1 08-20-2, also Alex. Goette, 30.1, 18, as well as]). 14«;. The Hertwigs recognized the significance of the parablast and added the imixirtant rectification, which Flemmings' observations, 78.2, had already rendered necessarj', of separating the sm<M)th nniscles from the striated skeletal muscles — a separation the propriety of which was wnmgly questional by Balfour, "Comp. Eni])rvol. 11.," :).")H. By this aclvance the two groups of mes<xlermal tissues In^came properly delimited.

(\ RnbV.H theortjof the mesoderm is based, it seems to me, wholly ujKm his failure to miderstand the law of concrescenee. That the mesoclerni appears (perhaps in all vertebrates) while concrescence is going on 18 well ascertained; consequently, there is an axial meeoeiTtt (Rabl's " gastrales mesoderm") where concrescence has taken place, and a lateral mesoderm (Rabl's " peristomales mesoderm") in the part of the blastodermic rim which has not concresced. Until Rabl proves that his " peristomales" mesoderm does not become axial mesoderm in later stages his theory can havenostanding. Davidoff, 90.1, 'Ji;J, makes the best criticism of Rabl's theory which I have seen. Rabl's memoir brings out one pointof very great importance for the elucidation of the early stages of vertebrates — namely, that the "peristomal" mesoderm, in other words, that of the blastodermic rim in setachians. and of the lips of the anus of Eusconi in amphibians, is represented in the amniota by the mesoderm of the primitive streak. If this interpretation, which is much strengthened by L. WilPs researches on the gecko, 89. 1 , be verified, then the primitive streak is the homolugue in amniota of the anus of Riisconi, and is the region where concrescence is incftmplote; the hea<l -process is then the part where concrescence is finished ; this concords with the obseri-ed facttlmt the head-process grows at the expense of the primitive streak, as it would do if concrescence c<mtinuetl.

Alexander Ooette's theory, 90.1, 2-1-33, is that the walls of the archenteron in Amphioxus and the true vertebrates comprise a dorsal region which develops the notochord and mesoderm, and a ventral region which develops the digestive tract. Owing to the great amount of yolk in true vertebrates the dorsal region is spread so as to lie upon the yolk, hence it is sej^arated from the yolk or entoderm by delamination instead of forming a true evagination ad in Amphioxus. It occurs to me that Goette's theory may be perhaps verified with the modification that the notochordal canal corresponds to his dorsal region, the yolk cavity to his ventral r^:ion of the archenteron.

Hatschek\t germ-band theory uSers, to my mind, the best-founded explanation of the vertebrate mesoderm, because it connects it with the mode of development of the middle layer m the annelids and other invertebrates. To understand the theory we must first consider the formation of the mesoderm m Amphioxus.

The ovum of Amphioxus is discharged from the body and impregnated cxtemal Ij it is about 0.105 mm. in diameter, and as it contains only a unmll amount SErt. of J oik, undergoes a holoblastic segmentation, which results in a well-marked

  • "^ E.1. blastnlu stage. Fig, CO, followed by a
  • gaftrula stage. The gastnila elongates,

FiQ 13 Trann en«, Beii <m lit aa ^^o blastoixire remaining open at the poa Amphi Tusc r (1 w( '^ '"fV tcnor extremity. Diifcrentiations now

iiotwfinrd ^n( " nUKi*!'!?^' n.1 tukc place by wliich the ofto<l(;rin fonns

BHat«.iitk^ the axial anlage of the nervous system and the entoderm produces the notoclionl

and the me'*oiknii— the three processes going on sinmltiuieously.

The iccomi»dnMng Fig 9) lepresents a cross-sectiim of a larva.

The ectoderm, Ec, everywhere bounds the ftection; on the dorsal side a portion of the ectoderm has been separated off to form the medullary plate, Mdf above which is a small cavity. The cavity. In, ot the archenteron is irregular, but sj-mmetrical in outline ; the entoderm bounding it can be separated into four part«; 1, the lower portion, which forms the permanent entoderm, Ent; -i, the upper median portion, which becomes the notochord, Ch, compare Chapter VII, ; 3, 4th, two lateral portions constituting the diverticula, Ms; each diverticulum is a separate pouch, and as the development prc^reeses thoe are formed a series of pairs of pouches, stretching on either side along fhe notochord ; later the pouches separate alt4%e£er from the archenteron, each becoming a closed sack; the first pair o£ pouches, however, retain their connection for a consiclerable period with the archenteron and l]a\ e been describeil by older writers as glandular oi^ans. The de%elopnitnt of the i>ouches is, with the exception noted, mo«t ad\an(<Hl anteri<irl\, and as we go taihvard the ix>uchea are less and lesi ad^ anced in development, until, as shown in Fig. !p4, they merge into tht general eutoderni asaband of cells, Mes, the last of which is the iiu'.wblti.st, Mh, a large granular cell unite distinct from the remaining cells of the Itantl or pouches. The pouches are the primitive segments {['ifu-tfiiietiti; mesoblastic somites of Balfour). In Amphioxus, then, the mewKlerm ai-isewfnnn the entoclerm along two liiifs, and is divided into paii-eil hollow segments I>efore it is separated from the entoderm. Some writrrs, especially the brothers Hertwig, think tliis pnxvsH of development to l)e primitive, and that the vertcliriite tyi>e is derived from it. In true vertebrates the mesodenn arises mi each side, but also in the axis, and InK-omes two masses when the medullary gi-oove imd iiotochonl appear ; in Amphioxus the medullary plate and notochord appear very early, and the division of the mesoderm may be due to that fact. Amphioxus is imdoubtedly a lower type, but whether it really preserves the older type of development in its purity is doubtful; indeed it is probably a tunicate rather than a vertebrate.

Hatschek, in a series of brilliant investigations, has shown that in manv bilaterally symmetrical invertebrates the mesoderm arises as two bands of cells, which subsequently divide into a series of closed sacks (segments), and which during their own formation terminate each in a single large posterior cell (mesoblast) , which throws oflf cells to add to the mesodermal band (germ-band, Keimstreif) , This mesoblast J by its appearance and position, appears to be a derivative of the entoderm. As a matter of speculisition we may assume that in Amphioxus we have the germ-bands, but characterized by an exceedingly precocious segmentation. We can further assume that in vertebrates we have the germ-bands also, but that they are modified, 1, by the loss of the distinct terminal mesoblast; 2, by precocious fusion in the axial line, and 3, by extremely retarded segmentation. A great deal may undoubtedly be said in favor of these three assumptions, which together constitute that theory of the vertebrate mesoderm which, of the many theories, that have been advanced, is most likely, in my opinion, to prove of permanent value.

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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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