1897 Human Embryology 10

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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 X. Origin of the Blood, Blood-Vessels, and Heart

Thb circulatory system is developed from two anlages which are at first independent. The heart arises in the cervical region of the embryo ; the blood-vessels and first blood-cells in the extra-embryonic area vasculosa ; the blood-vessels subsequently grow into the embryo and unite with the heart. The heart begins to beat before the vessels are connected with it, so that as soon as the connection is established the circulation begins. The heart contains at first only a clear fluid ; after the circulation has begun blood-cells come in through the vessels from the area vasculosa. The first blood-cells have a reddish color and a round nucleus. Somewhat later the colorless granular leucocytes appear, but where they arise is uncertain. In all vertebrates except mammals the red cells persist throughout life, but in mammals they are confined to the foetal period, during which they are gradually replaced by the non-nucleated red-blood globules (plastids) . Much confusion exists as to the nature and development of the blood, because the great majority of writers have ignored the important fact that the mammalian adult blood-globules are a new. actjuisition of that class and are not homologous with the red-blood corpuscles of other vertebrates. Mammals have three kinds of blood corpuscles: red cells, leucocytes, and the adult red globules; all other vertebrates have two kinds only.

An immense deal has been written on the development of the blood in the embryo, and there is perhaps no other question in embryology which has been so much studied and yet left with such a variety of opinions as to its right answer. In the following pages I have endeavored to collate what seem to me the best-established results ; but until some one subjects the literature of the subject to a critical revision, based on a thorough comparative investigation of the development of the blood and blood-vessels throughout the vert^^brate series, we can hardly expect a satisfactory history of the embryonic blood.

We have to distinguish between the primary and secondary vascular anlages.

I. Blood-Vessels and Blood

Primary Vasctdar Anlages

These are cords of cells which api)ear first in the area vasculosa and rapidly extend into the embryo; the cords form a network ; scattered clusters of cells in the cords very early assume the hemoglobin color and appear as reddish-yellow spots which have long been known, and are described by Pander, VonBaer, 28.2, Remak, 50.1, Prevostet Lebert, 44.1, and others. We owe to His, 68.1, 05-103, the first exact account of the origin of blood-veBsels in the chick ; since then the studies of BiBse, 79. 1, Gotte, 74. 1, Kolliker {" Entwickelung^jee."), Balfour,73. 1, J. Kollmann, 84.3, Uekow, 87.1, and others have added a little to the descriptions by His. It is now demonstrated that the blood arises in amniota from the mesoderm and not from the yolk, as was, I believe, first suggested for teleosts by Lereboullet and recently by Ryder. The exact history of the first blood-vessels has yet to be studied in other amniota than the chick.

In the chick the distal portion of the mesoderm has no coelomatic cavity when the development of the blood begins; the mesoderm lies close against the entoderm or germinal waU (Keimuall). The juxtaposition of the two layers has leil His and others to consider that theentodenn or yolk gave off the cells which form the mesoderm of the area vasculoHJi. This portion I of the mesoderm was early distinguished by German writers under the name of Gefiissachicht or vascular l^er (f e u illet angiopias' itmie), and has been called the blood. - germ (Bluikeim) ; by His it is identified as a st^^e of the parablnst, see Chap, ter VI. The first indication of the blood-vessels is a reticulate appearance of the layer, which can be recognized in surface views at the end of the first day and rapidly increases in extent and distinctness during the second day of incubation. As soon as there are several primitive segment.^ the network shows traces of coloration in irregularly shaped retldish-yellow spots which are largest smd most numerous around the caudal end of the emhrj'o ; these spots are the so-called blood-islands, Fig. 124. The network appearance is due to thickenings of the mesoderm, as is evident from sections. The two primary layers are separated so that the mesodermic thickenings lie between them. Between the thickenings are irregular lacuna?. Fig. 124, b b, which are only partly filled with mesodermic cells; these lacunie bj' their subsequent expansion and fusion develop during the latter half of the second day the coelom of the area vasculosa, and always so that the thickenings (or bloodvessels) art; on the entotlermal side. Fig. 12(i. In other words, as soon as the two leaves of the mesoderm are differentiated in the area vasculosa the blood-vessels are found exclusively in the sjilanclmic leaf. In the sheep they appear also in the somatic leaf, R. Bonnet, 89. 1, 50, or future amnion, but they soon disappear and never contain any blood -corpuscles. The network of blood-vessels of the vascular area form at first a thick network without distinction of stem or branch, and are all in one layer, none overlying the others (EoUiber, ■' Gmndriss," p. 00), Fig. 1 25 ; the edgeof the area ia marked by a tjingle large vessel which is known as the vena, or better, shins terminalis, Fig. I'iS, vt. I have spoken uf vessels, /t— but up to this time the vascular luilages are solid. The vena terminalis persists fur some time as the distal boundary of the area, while it is spreading farther and fartuerover the yolk, but by the end of the fourth day it is no longer distinguishable as a distinct structure (Prevost et Lebert, 44.3, 240). The vena terminalis ultimately becomes connected with the venous system of the chick, hut in rabbits with the arterial system; for this reason tlio term sinus is to be preferred tii vena as applied to tliis vessel.

The blood-islands are six>ts wliere their is a cluster of cells which TCmiiin attached to the walls of t)ie ves.scis in the urcii vasculosa (see Fig. 120, bl. /«). The <'el]s develop lupincglobin in their interior, heuce the clusters have a nxldish cnlor, which renders the islands very conspicuous in surface views of fitssh specimens. The blood islands of the chick a]>peHr first in the uiwi ojiaca, and almost immediately after in the pellucida also. They havts at first a roundetl or bnmching form. Fig. I'ii; in the inner part of the layer they are small and stand alone; toward the periplicry tlicy are larger, closer set, and mon; united with one another; their developnmnt is greater iiround the caudal end of the embryo. they are situated, chiefiy, at the nodes of the vascidar network. When the solid vascular cords acquire a lumen, thi; islands. Fig. Vii\, hi. i-i. remain attached t^> one side of the vessel, like a thickening of its wall. The cells of the inlands idtimately liecoino free hlotHl-corpuscles.

the (/roirfh of ihe primtin/ iiitliigrs takes phice by the development of buds from the vessels already formetl, as first slmwii accurately by Prevost et Lel»ert, 44.3, ?:!!l; these bads are rounded or {iiinted and elongated, forming as it were spurs: they often end by meeting one another and uniting: they are usually hollow from the first, and after they met?t one another or an adjacent vessel, the cavities Ijecomt! continuous and thus the vascular network is extendetl. A. Ooette, however, maintains, 76. 1, 4'.C, that the network arrangement ezieta from the start in all vertebrates, and that the apparent budding is due to the prt^rees of vascular differentiation into indifferent mesenchymal cells.

In mamjnals the solid primary' anlages appear in the extra embryonic area vasculosa, and extend later into the embryo. So far as known to me there has exact investigation of their history. They present well-marked blood -islands, which are thickenings of the mesoderm, and make their first appearance in rabbit embryo of the eighth day just before the first primitive segments (K61H ker," Entwickelungsges.," 'iW) ,

The growth of the network in the rabbit by the formation of solid buds which become hollow has been described bv Wisso

Growth of the Veasels into the Embryo

The fact that the vessels penetrate the embryo after they have appeared in the area vasculosa was fir^t discovered by His, 68.1, (t!l, and is now a familiar phenomenon. It is evident that this penetration may take place in two ways : it may be a progressive differentiation of cells already present (r/. Goette, 76.1, 5311). or it may be an actual ingrowth of vaso- formative tissue; the balance of evidence is in favor of the latter alternative, which accordinglj', following His in this respect, the ma°|i jority of eml)rj-ol(^sts have §5^ adopted. In the chick the vas|"^.S ciilar differentiation extends I Y^^ from the area oiKica to the area ■■' - pellucida, ami thence into the _^ , _ , bodv proper of the cmbr^'o. C| 3 But in the lizard {Strahl, Mar?| burg, Sitzber., 1S»3, 00-71) the vessels appear first in the an'a pellucida and thence extend into the area opaca and the embr^'o. The entrance of the vessels into the embrj-o chick b^ins toward the end of the second day. It is effected, according to His, 68.1, 99, by buds, which are at first solid cords, and grow toward the embryo, uniting as they extend into a network; the hollowing out of the cords likewise progresses centripetally. The penetrating vessels follow certain prescribed paths. A part of the vessels run along the posterior edge of the amnio-cardial vesicles, and enter into connection with the posterior end of the heart which has meanwhile been developed — owing to the early separation of the head end of the embr>o from the yolk this is the only part of the heart the vessels can reach directly. While the vessels are approaching the heart their differentiation into various sizes is going on, the smallest ones to remain as capillaries, the larger ones to become arteries or veins ; this differentiation, which has yet to be followed step by step, leads to there being only two main vessels, the so-called omphalo-mesaraic veins, which actually open into the hind end of the heart. Another set of vessels penetrates along the splanchnopleure of the rump on each side, until they attain the small space l)etween the notochord, myotome, and entodenn, where they fuse (Turetig, 86.1), so as to form a longitudinal vessel, the anlage of the aorta desceitdeus^ which is primitively double. The aorta appears first at the head end of the rump and hence its development progresses backward ; it also grows forwanl over the heart, l)onds over ventrally just behind the mouth, and, passing aromid the blind end of the vorderdarm, approaches the median line and unites with the cephalic end of the tubular heart. An utterly different history of the origin of the aorta, namel}", from the median dorsiil wall of the archenteron, is asserted by C. K. Hoffmann, 82.1, for the dog-fish. The heart begins to beat before the vessels unite with it ; the first' blo<xl-cells have alreadv hoen formed; hence as soon as the union is accomplished the bloodcirculation starts up, the blood passing through the aorta to the rump, thence by numerous lateral branches to the area vasculosa, and returning by the two omphalo-mesiiraic veins to the heart. The course and mo<lifications of the primitive circulation are descril^ and figured in Chapter XIV., on the germinal area.

Origin of the First Red Blood-Cells

I consider it prol>able that the red blood-cells of all vertebrates arise, as has been maintained by H. Ziegler, 89.1, by proliferation of the endothelial lining of the vessels. This conclusion is based — I, ujion the fact that in various vertebrates, notably in Ikjuv fishes, elasmobranchs, and all amniota, certain parts of the vascular system are at first solid cords of cells, and of these conTs the central portion lx»comes blrKwl-cells, the ]K»ripheral portion the vascular wall; it S(H?nis to me that the right interpret4ition is to regard the central cells its l)elonging with the outer cells, and therefore etiuivalent to the product of an endothelial proliferation ; t>, upon the origin of red cells from the walls of the venous ca])illaries of the !)ony marrow of binls (J. Denys, 87.1). In all these cases the blood-cell in a liberated specialized endothelial cell, A. Go(>tte is the principiU op]x>nent of this view, and has maintained that in Petromyzon, 80.1, r»i>, Amphibia, 76.1, 5.58, and birds, 74.1, 18()-1J^(), the blood-cells of the embryo have an origin different from the endothelium, the former arising from the yolk, the latter from the mesoderm. Although Goette is one of the very best of embryological observers, I cannot agree with him on this point, for I feel satisfied that he is in error as regard the chick, while in regard to the lamprey and the land frogs it is possible that Qoette's observations are incomplete — certainly his descriptions are less clear than those of the origin of the blood-cells within the vascular anlages. It must be added that Davidoff has maintained, 84. 1, that in the salamander the blood-cells arise from the surface of the yolk; but his statements need, I think, verification.

  • It is similar that so oIdhc an olMerveroA Balfour nhould have maintained, as h<» did. TS.1. that th\n of saurorwida are metamorphoHed nuclei, and this view is still adheml tr> in his "ElemMits." ad ed. IWW.. Balfour's error was <lue to the fa*rt that the cells, when flmt wt free, hove a minimum nuautity of jinitoplasm around the nucleus, ami this he did not olMerve: the nuclei have tt>o at first a very distinct larjfe nucle«>his. which Balfour \*TonKly assumed to represent the nucleus of the future corpuscle.

The blood-cells of teleosfs arise, at first at least, in certain large vessels within the embryo (Wenckebach, H. Ziegler, 87.1,89.1, compare also H. Ambert, 66.1), which are formed as solid cords, the central cells of which are metamorphosed into blood-corpuscles. At the time the circulation begins there are no blood-vessels over the yolk, but definite blood-channels, which are merely grooves on the yolk or passages between the yolk and the ectoderm ; these channels flubseciuently acquire mesenchymal walls when the mesoderm grows out over the yolk. Owing to this peculiarity of the earlj' vitelline circulation blood-cells appear over the yolk before there are bloodvessels, and the observation of this fact seems to have led several observers to the error of attributing the origin of the blood-cells to the yolk or the superficial layer thereof (Kupffer's periblast) . For a synopsis of the various opinions see Mcintosh and Prince, 90.1, 783-783. In elasmobranchs (J. KoUmann, 85.1, 297) there are mesodermal blood-islands, which expand and unite, forming a network in the area opaca ; the vessels are at first solid, the central cells become blood-cells, the peripheral cells endothelial walls ; so far as observations go it is possible, however, that all the cells of the bloodislands become blood-cells, and that the endothelium is simply an overgrowth of mesenchyma, but in view of the development in other vertebrates this possibility has little probability. The development of the blood in reptiles and mammals needs thorough study, but we know that it is closely similar to that in birds. In the chick^ as stated above, the cells of the blood-islands form the first blood-cells, and this statement probably applies also to all amniota.

For the origin of blood-cells in the embryo see the following section.

Secondary Vascular Anlages

These are buds which arise from the vessels already present in the embryo, similar to the buds already described in the area vasculosa. There being no real division between the primary and secondary anlages the distinction is used merely for convenience of description. The secondarj^ anlages, like the primary, give rise to the endothelium of the wall only ; when a vessel becomes an artery or a vein the media and adventitia are added by differentiation of the surrounding mesenchyma. The secondary anlages can l)e found in mammals in various parts of the body during embryonic life, and even after birth, and in Amphibia may be studied during the larval period ; the tail of tadpoles being a favorite object for this purpose (Golubew, 69.1). The secondary anlages were, so far as I know, first accurately described in batrachians by Prevost et Lebert, 44. 1 ; they were followed two years later by Kolliker, 46.2, see also Qolubew, 69. 1, Arnold, 71. 1, and Ranvier's " Traite technique," r»18, G23. In mammals they have betin well described by Ranvier, 74.2^ E. A. Schafer, 74.1, Kolliker (Entwickelungsges. /' 171, "Gnmdriss," 63), and others.

The secondary anlages appear as thom-shapeil points projecting more or less nearly at right angles from the walls of capillaries alreaily formed. A. Goette, 75. 1, 544, has maintained that these are not real outgrowths, but differentiations of intercellular processes present ah initio in the mesenchyma. These point« rapidly elongate into fine threads, which may join the wall of another capillary or the tip of another point ; Golubew states that when two points imite in the frog, they overlap and then unite by their sides ; while the point is gTi^wing the cavity of the parent capillary extends into the bas(» of the point, and penetrates farther and farther, so that the thre^ul-like point becomes gradually enlarged into a capillary bloodvessel. The capillaries formed in this way show a marked tendency to form loops.

Very similar is the account quoted below by E. A. Schaeffer in Quain's Anatomy," ninth edition, II., 198, 199) : ** Within the body of the embryo vessels are formed in like manner from cells belonging to the connective tissue. One of the most favorable objects for the study of the development of the blood-vessels and their contained blood-corpuscles is afforded by the subcutaneous tissue of the newl)om rat, especially those parts in which fat is being deposited. Here we may ol^serve that many of the connective-tissue corpuscles are much vacuolateil, and that the protoplasm of some of them presents a decided reddish tinge. In others the red matter has become condensed in the form of globules within the colls, varying in size from minute sjx,»cks to spheroids of the diameter of a bl(K)d -corpuscle or more. At some parts the tissue is completely studded with these cells, each containing a number of such spheroids, and forming, as it w(»re, * nests' of blocnl-cofpuscles or minute *blcK id-islands.' After a time the cells Ijecome elongated and pointed at tlu^ir ends, sending out prfxresses also to unite with neighboring cells. At the same time the vacuoles in their interior l)ecome enlargCMl, and coalesc*e t<^ funn a cavity with tho cell in which the ivddish globules, which are now Incoming disc-shai)e(l, are found. Finally, the cavity extends through the cell jirocesses into those of neighlM)ring cells and into those sent out from pre-existing capillaries, but a more or less extensive capiUary network is often formed long Ix^fore the connection with the rest of the vascular system is estiiblish*Ml. Young capillaries do not exhibit the well-known lines when treat e<l with nitrate of silver for the differentiation of the holloweil cells and cell -processes into flattened cellular elements is usually a sul)s(HiU(»nt proc*ess. The mode of extension of the vascular system in growing parts of older animals, as well as in morbid new fonnations, is (|uite similar to that here descrilKnl, except that blo<Kl-corpuscles are not developed within the cells which are fonning the bl(K)d-vessels."

The development of new capillaries in the manner just described also takes place from the vessels fonnwl by vasoformative cells.

The secondary vascular anlages of the foetal liver have been specially studiinl by P. Kuborn, 80.1; they correspond to the so called foetal hepatic giant cells of early authors, and give rise to vascular walls, red cells, and later (embryos of three or four centimetres) to the red plastids, compare p. 221.

Vasoformative Cells

In all secondary anlages of the vessels we have outgrowths of vessels already present; there are also vessels developed from special vasoformative cells, which have no connection with previous vessels ; the origin of the vasoformative cells has still to be ascertained, but it may be safely asserted that they are derived from the mesenchyma. These cells were, I believe, discovered by L. Ranvier (74.2, and "Traite technique," G25), who studied them in the omentum of the rabbit before and after birth. He found small spots of milky appearance, which he designates as " taches lai tenses," and which contain ordinary connective-tissue corpuscles, and fibrillaB, numerous leucoc}i«s, and vasoformative cells. The last, in rabbits from two to eight weeks old, are finely granular, branching often anastomosing, elongated cells with elongated nuclei; earlier they are scattered, spindle-shaped cells. Soon a capillary from the neighborhood grows in and unites with the vasoformative network, and thereupon the excavation of the network begins, the lumen of the capillary gradually extending throughout the cluster of vasoformative cells.

Primitive Blood- Vessels

The first vessels consist merely of a wall of protoplasm with scattered nuclei, and accordingly are all essentially alike in structure; the first differentiation is one of size only, the vessels that are to become arteries and veins rapidly increasing their calibre, while the mesenchj- ma around them is still undifferentiated. The protoplasmatic wall in cross-sections of a vessel is thick enough to contain a nucleus. The next step in development is the thinning out of the layer, so that the nuclei become protuberant as in the adult endothelium ; at the same time the protoplasm becomes divided into distinct cell territories, and intercellular lines are developed and may be impregnated with nitrate of silver, as in the adult.

The vessels grow by the multiplication of the cells of their walls. ^W. Flemming, 80. 1, has sho^vn that in the capillaries the nuclei undergo karj-okinetic division, and that the division of the protoplasm takes place later.

The distribution an<l metamorphoses of the principal vessels are discussed in Chapter XIV.

The red blood-cells are the only elements contained in the blood during the earliest stages of the vertebrate embryo. When the circulation begins the numl)er of corpuscles is small, but rapidly increases there^ifter. The cells are at first round (in probably all vertebrates) ; in the chick they measure from 8.3 to 12.5 /i. The nucleus is large, more or less nearly spherical, and surrounded by a layer of protoplasm (Minot, 122), which is so thin as to have been often overl(X)ked. The cells at first are granular and slightly colored (Prevost et Lebert, 44.3, 241; KoUiker, Grundriss," 03), and then become more colored and homogeneous, scarcely showing the nucleus during life, though it comes out ver^' clearly as soon as the corpuscles are removed from the vessels or acted upon by hardening reagents. The nucleus in hardened corpuscles stains deeply. In amphibians the 3'oung bUxKl-cells, like all the other cell8 of the embryo, contain nimieroua yolk granules; as the granules disappear the nuclei and lx)dies of the cells both acquire a more homogeneous and opalescent appearance, and at the same time become flattened, elongated, and colored (A. Goette, 76.1, 770).

The primitive form of the vertebrate red-blood cell is probably spherical, or at least spheroidal, and the characteristic mature shape is not assmned until later, as I have learned from my own observations on a considerable variety of embryos. This statement is further supported by A. Goette's observations on Petromyzon, 90. 1, 6G, and Bombinator, 75.1,538. In the chick the mature elliptical fonn begins to predominate during the fourth day; the earlier round form is still encountered for several days, but it gradually becomes rarer (Prevost et Lebert, 44.3, 242).

Minot, 122, has outlined the progressive differentiation of the red cells in sharks, sklamanders, chicks, and rabbits. The following description refers primarily to the chick : By following the development we find that the protoplasm enlarges for several days, and that during the same time there is a progressive diminution in size of the nucleus, which, however, is completed before the layer of protoplasm reaches its ultimate size. The nucleus is at first granular, and its nucleolus, or nucleoli, stand out clearly ; as the nucleolus shrinks it becomes round and is colored darkly and almost imiformly by the usual nuclear stains. This species of blood-corpuscle occurs in all vertebrates and represents the genuine blood-cells. The blood-cells of mammals pass through the same metamorphoses as those of birds. For example, in rabbit embryos, the cells have reached the ichthyopsidan stage on the eighth day; two days later the nucleus is already smaller, and by the thirteenth day has shrunk to its final dimensions. According to the above description we can distinguish three principal stages: I, young cells with very little protoplasm; 2, old cells with much protoplasm and granular nucleus; 3, modified cells with shrunken nucleus, which colors darkly and uniformly, Fig. 127. I do not know whether the first form occurs in any living adult vertebrate, although the assumption seems justified that they are the primitive form. On the other hand, the second stage is obviously characteristic of the Ichthyopsida in general, while the third fonn is typical for the Sauropsida. Therefore, the develrament of the bloodcells in amniota offers a new confirmation of Louis Agassiz' law (Haeckel's biogenetisches Grundgesotz) .

Multiplication of the red cells by division was recorded by Remak, 50. 1, 104, and has since been frequently observed. Special attention was directed to its occurrence oy Peremeschko in 1879, 79.1, 81.1, and by Bizzozero (Cbl. med. Wiss.^ 1881, Moleschott's " Unters. zur Naturlehre," XIII.) in 1881, and has since been studied by Bizzozero et Torre, 84.1, Bizzozero, 84.1, Funcke, 80.1, Eberth and Aly, 85.1, A. Mosso, 88.2, and others. The division is indirect or karj'okinetic, and takes place across the longitudinal axis of the corpuscle, with which the nuclear spindle is parallel. The process has been observed in bony fishes, amphibians, adult Sauropsida, and in amniote embrj^os. The division occurs only in young or partly differentiated corpuscles; the divisions, for example, are abundant in the blood of the chick of from three to five days; the sixth day they are rarer, the tenth seldom, and after hatching are not found in the circulating blood at all (Funcke, /. r.). It is, accordingly, safe to assume that the proliferation of the red cells is typical for all vertebrates. Their number is further increased by additions from various sources in the embryonic and (adult non-mammalian) vertebrates; but, so far as at present known, the mammals have only the red cells, which arise directly from the primary vascular anlages, therefore the discussion of the maintenance of the supply of red cells falls outside our scope. The problem has bet»n much debated ; the investigation which seems to me to h?ive led to the best results is that of J. Denys, 87. 1. For the reader's convenience I cite also the following authorities, but the list is ver\' incomplete : Bayerl, 84.1, W. H. Howell, 88.1; Lowit, 87.1, 91.1, E. Neumann, 74.1, Malassez, 82.1; Obrastzow, 81.1; G. Pouchet, 80.1; and Rindfieisch, 80.1. For additional references see Quain's "Anatomy, ninth edition, II., 40.

Disappearance of the Red Cells

The red cells form the permanent red-blood globules in all vertebrates except the mammals. In mammals they disappear during embryonic life or soon after birth. Although they {lersist ft)r a long period, it will be convenient to state here what little is known of their history. How they disaj)pear is not known, although sevend authors have maintiiined that they are transformed into red plastids, but this opinion seems to me ill founded. \V. H. Howell, 90.1, rei)orts the interesting discovery that the luicleus of the mature red cells is extruded in mammals leaving the body of the cell ; in consetjuence he maintains the plausible conclusion that the extrusion is the means of developing the non-nucleated red corpuscles, but I am moi-o inclined to regard it as a step in the degeneration and destruction of the red (vlls. In the human embryo at on(> month the red cells are the only bl<x)d-corpuscles ; at two months the\' are the most numerous, although the plastids have begun to appear; at three months they form only a small minority of the corpuscles.

Origin of Leucocytes

The origin of the first colorless corpuscles in the embryo is still unctTtain. The blood is found to contain for some time only the? red cells, the leuc»ocytes appearing in the chick (Prevost et Lelx^rt, 44.3, 243), about the eighth day of incubation; in the rabbit, it is said, al)out the ninth day, and in elasmobranchs not until the embryo is well advanced in development, A. Mosso, 88.2. It is to be note<i that after the blood-vessels and red blcKxl-cells the leuccxytes are the first cells to be differentiated from the mesenchyma, the remaining mesenchymal tissues (Chapter XIX.) being differentiate<l gradually and to a large extent simultaneously. So far as I know, the subject has never been carefully investigated, nor is there even any exact description of the appearance and number of the first leucocvtes.

After the lymph-glands api)ear they probably assume the function of ])roducing leucocytes ; but the process in embryonic glands has still to be studied, and accordingly for further information the rt>ader is r(»ferred to the standard histologies. That the leucocytes multiply bv direct or akinetic division has been recorded bv several observers, L. Ranvier, J. Arnold, 84.1, and others.

Origin of Mammalian Blood -Globules or Red Plastids

There are many opinions as to the origin of the non -nucleated retl blood-globules of mammals. The best-founded conclusion is, it set^ms to me, that of E. A. Schafer, who traces them to local differentiations of the protoplasm of the vasifactive cells. This viewmakes the globules comparable to the plastids of botanists, such, for instance, as the chlorophyll granules. As the terms " globules" and " corpuscles" have been applied indiscriminately to all the formed elements of blood, and«is it is desirable to have a simple term which shall also indicate the morphological separation from the other "blood-corpuscles," I shall apply the term ^^ red plastids*^ to the non-nucleated mammalian adult red globules. The chief opinion rivalling Schafer's is that the red plastids are derived from nucleated corpuscles, which have lost their nuclei and shrunk, the plastids being always much smaller than the red cells. This view has been specially advocated by KoUiker, "Gewebelehre," 5te Aufl., 1867, p. 638, is found in several subsequent writers, and has been very recently brought forward by Casimoro Mondino, 88.1, but sufficient observation to justify it has not been furnished in my judgment. The strongest evidence in favor of the conversion of nucleated corpuscles into plastids is that which is presented by Howell, 90. 1 , and mentioned p. 221. Similar to this view is that which traces the plastids to modifications of leucocytes occurring after birth, F. Sanfelice, 89. 1 ; the white cells are supposed to shrink, lose their nuclei, and become charged with haemoglobin. Yet another opinion aflHrms that the marrow of bones produces from certain of its cells the red plastids, but the defenders of this opinion are by no means agreed among themselves as to how. For a good synopsis of the conflicting theories see Schafer in Quain's "Anatomy," tenth edition. Vol. I., Pt. II.

The first red plastids certainly arise in the vasifactive cells in various parts of the embryo. Schafer in Quain's " Anatomy," ninth edition, II., 36-37, giv^s the following description of the process : " A part of the protoplasm of the cell acquires a reddish tinge, and after a time the color^ substance becomes condensed in the form of globules within the cells, varying in size from a minute speck to a spheroid of the diameter of a blood-corpuscle, or even larger; but gradually the size becomes more uniform. Some parts of the embryonic connective tissue, especially where a vascular tissue such iis the fat is about to be developed, are completely studded with cells like these, occupied by a number of colored spheroids and forming nests of blood-corpuscles, or minute 'blood-islands.' After a time the cells become elongated and pointed at their ends, and processes grow out to join prolongations of neighboring blood-vessels or of similar cells. At the same time vacuoles form wi&in them, and becoming enlarged coalesce to form a cavity filled with fluid in w^hich the reddish globules, which are now becoming disc-shaped, float. Finally, the cavity extends through the cell processes into those of neighboring cells, and a vascular network is produced, and this becomes eventually united with pre-existing bl(X)d-vesselft, so that the blood-corpuscles which have b«en formed within the c»ells in the manner described get into the general circulation. This 'intracellular ' mode of development of red blood-corpuscles ceases in most animals before birth, although in those which, like the rat, are bom very immature it may be continued for a few days after birth. Subsequenth', although new vessels are found in the same v/ay, blood-corpuscles are not produoed within them, and it becomes necessary to seek for some other source of origin of the red-blood discs, both during the remainder of the period of growth, and also during adult life, for it is certain that the blood-oorpuscles are not exempted from the continual expenditure and fresh supply which affect all the other tissues of the body."

Very early in embryonic life the liver, as first pointed out by Kolliker, and more fully demonstrated by Neumann, 74.1, becomes the principal seat of blood formation. The secondary vascular anlages are very prominent in the fcetal liver and in sheep embryos of four centimetres and more in length. P. Kubom, 90.1, has traced the development of red plastids from the protoj)la8m onl}', as described by Schafer. A similar rosult is reached by R. Nicolaidos, 91.1, from studying the production of red plastids in the mesentery of young guinea-pigs, see also Wissosky, 77. 1. The process of plastid development is easily followed in the mesentery of the human foetus.

It seems to me probable that research will ultimately establish the origin of red plastids in the adult also, as intracellular protoplasmatic bodies entirely distinct from the nuclei, and in no way to be homologized with cells. Kultschitzki, however (see Hofmann-Schwalbe's Jahresher,^ 1883, 58-50) > asserts that in the lymph -glands of the rabbit the red plastids arise within cells by metamorphosis of the nuclei ; to nuclei Balfour traced, he suppostnl, the rtnl cells of birds, compare p. 215, foot-note.

Origin of the Blood-Plates

C. ilondinoand L. Sala, 88.1, affirm that the blood-plates multiply by division, and being nucleated in the non-mammalian vorU»l)rates, according to these authors, they divide karj'okinetically ; while in mammals the plates have no nucleus, but the larger plates have chromatine granules, which, however, divide as do the plates. They st^ito that the plates ai'e present in mammalian blcKxl as S(X>n its it begins to circulate. In the French resume of their work {Arch, Ifdl. ii/o/., XII., 304), they stute that Fusiiri has confirmed their observations in an article in the Ri forma mediva^ 1 3 Agosta, 1S8'J. I (iuestiou most decidedly the trustworthiness of these statements, for the autlior's figures suggest at once that they have mistaken distorted bloinl-globules for blood-plates. No other ol)servations on f(jetfil l)l(HHl-j)lates are known to me. It should be added that L. Lilienfeld, 92. 1, hasadvance<l the hypothesis that the jJates are derived from leuc(X*yte nuclei, while Howell, 90. 1, suggests that they are the extruded nuclei of red cells.

Morphology of the Blood-Corpuscles. — The following conceptions liave been ad v< >cated by Minot, 122. The prece<ling sections show that the vertelwate ])lo<Hl-corpuscles are of three kinds: 1, Red cells; 2, White cells; 3, Plastids. The red and white cells occur in all (?) vertebrates; the plastids are confined to the mammals. the red cells ])resc»nt thre(» chief modifications ; whether the primitive fonn occurs in any living adult vertebrate I do not know ; the second form is ])orsistent in the Ichthyoiwida, the third form in the Sauropsida. A(*cording to this we must distinguish :

A. One-celled blood., first stage by all vertebrates: the blood contains only red cells with little protoplasm.

B, Two-celled blood, having reil and white cells. The red cells have either a large, coarsely granular nucleus (Ichthyopsida) or a smaller, darkly staining nucleuH (Sani-opHlda, mammalian embryos) .

C. Plastid blood, without red cells, but with white cells and red plastids; occurs only in adult mammals. Mammalian blood in its development passes through these stages, as well as through the two phases of stage B, all in their natural sequence; the ontogenetic order follows the phylogenetic. It seems not improbable that an animal may yet be found with blood intermediate between B and C in the adult stage.

II. Origin of the Heart

The heart, as has been stated, ia developed independently of the blood and blood-vessels ; it contains at first a clear fluid, and liegins beating before the blood-vessels from the area vasculosa have joined it. The primitive fonu of the heart is a straight median tube on the ventral side of the cervical region ; the cephalic end of the tube is connected with the arterial system of the embiyo, while the caudal end ia primitively connected with the venous system of tho yolk. These relations may be traced in all Aertebrates, hence the heart arises as the active oi^n of communication between the yolk or primitive food supply and the embryo.

Primitive Node of Development of the Heart

In regard to the development of the heart wo have to distinguish the mode still preserved in the primitive vertebrates (marsipohranchs, ganoids, and amphibians), elasniobranchs, and in some but not all t«leost8 (Mcintosh and Prince, 90.1, 7T5), from the mode in the amniota. In the tirst mode the heart arises in tho median line; in the second mode the heart arises from two lateral anlages, which subsequently unite in the median lino. The dilTerenco is not a fundamental one, hut is correlated, as first pointed out by Balfour, with the earlier or later separation of the cephalic end of the embryo from the yolk; when that separation is retarded the heart is differentiated before the neck of the embryo is folded off from the yolk, (rompare Copter XIII. ; this delay occurs in varj'ing degrees in all amniota. The following account of the origin of the heart in Amphibia is based on C. Rabl, Coe 6.0. ^^ 86.1, who cites the earlier authorities. The head of the emhrj-o early becomes free and projects si> far that the neck is free from the yolk also. The mesoderm ' extends for^vard on each side between ectoderm and entoderm, and has a coeiomatic cavity on each side, Fig. 128, C'oe. The two wings of mesoderm do not, however, meet on the median ventnd line, being separated by a ridge. Eii, i)f entoderm by which the inner germ-layer comes into inmiediate contact with the ectoderm, Ec.

Whether this ridge is preserved to form the endothelium of the heart or is resorbed into the general entoderm is not p<Mitively known. In a later Htage, Fig, Vi'J, the two mesodermio wings have met in the median line U'low the intestinal canal ; the coelom lias expanded j between the mesothelium of each side in tlic median line is a amall mass of cells, Ht, which 8<xin shows a central lumen, which becomes the cavity of the heart, while the cells around give rise to the future endothelium; the cnduthcliiun is still in contact with the entoderm. Below the heart the uiesothclia are in actual contact, forming a double wall, which sixin brcftks through, so that the coelom on each side ojwns into the other, or, in other wonls, there is now a single pericanlial cavity. The heart has become a two-layered tube; the inner laj-cr consists of eudothelium, the origin of which is discussed in a separate itamgraph Itelow; the outer layer consists of mesotheliimi, which gives rise fei the muscular wall of the heart. Later the mesothelium cl<ises over the dorsal side of the endothelium, thus finally seiMiDiting it from the entoderm. Still later the tubular heart loses its suspension fnim the dorsal side of the pericardial cavity and is attached only at its anterior or cephalic and posterior or caudal extremities, and hence is free to bend and twist within the pericardial cavity in the manner nocessary for the evolution of the eart's adult form.

Amnlote Mode of Development of the Heart

Observations on the heart are to be found in many of the older writers on embrj-ology, notably in Von Btier, Prevost et Lcbert, Reinak, Bischoff, and Coste, but until the introduction of section cutting the details of the process conld not In' observed. The foundations of our present knowledge were laid by W. His, 88.1, 8;)-8.^, and the subject was further elucidated by Kolliker's invaluable ol)8er^■ations on tiie chick and rabbit, n?corde<l in his "Entwickelungsgeachichte;" Gasser, 77.3, has publishwl an admirable dcacrii>tion with figures of the development in the chick; there are besides numerous references to the heiirt scattered in recent litemture: see, for instance, Hensen, 76.1; Heape, 86.2; Selenka, 86.1, ft al.

In the amniota the cephalic coelom very early dilates to a much greater degree th';n the c<eloin elsewhere, llnis developing on each side the so-called PariehtUiohle of Gennan writers, for which I have proposed the name of amnio-cardiul vesicle. In the chick the early and extreme dibitatioii of this cavity is well known, and is iutimately correlated both witli the closure of the archenteron to form the cervical entodermic canaJ (loz-Jerdn/w)) and also with thedevelopment of the heart and the origin of the amnion. In the chick the dilatation forces the splanchimpleuro (splanchnic niesohlast and entodenn) downward on each side; then bends the splanchnopleure in under the embryo until the two membranes meet in the median Hue and fuse ; their fusion shuts off the Vorderdarm from the yolk and leaves it as a flatteiie<1 cunul, Fig. 1 i!)A, Fh; for further details see Chapter XII. The layer of mesothelium bounding the cceloni is everj-where distinct; the mesenchynia is well develooeil all alx>ut the mcdullarjcanal and notochoi-d, Fig. l^OA, but is almost entirely absent from the walls of ainuio-eardial vesicles, until we reach the distal vasculaf area, consequently when the vesicles expand the mesothelium is brouglit close against that ix>rtioii of the entodenn which is destined to form the Vorderdarm ; where the contact takes place there appear between the cntotlemi and mesothelium a few very irregularlygroupetl mesencliymal cells. Fig. T-illA, Kudo; these are theanlage of thtt endothelial lining of tlio heart, or Kndofhelkerz of German embrjologists. The mcsothclimn of each side meets its fellow in the meiliau ventral line, forming a thin partition or ventral mesocardium. Fig. I'^ftA, which subsequently breaks through; from the ventnU wall of the Vorderilann, Pb, the mesothelimu bulges out as a much-thickened layer, jh»/'*, which develops into the muscular wall of the heai't, while between this wall and the entodenn of the Vorderdarm He the mesenchymal cells. Development proceeds by the mesothelial fold becoming more protuberant on each side, and the mesenchj-mal cells assuming the endothelial character, coming to bound several irreguliir cavities on each side. Fig. i:jn, Kn.ht; these cavities soon fuse into two main eavities rinming longitudinally; as the two cavities enlarge they meet in the median line and remain sciiarated at first by a wall of two layers of endothelium; this wall soon breaks through and there results a single median tube of endothelium connected, by long processes of cells, acroes quite a wide space with tba mesc^elium. ELicellent figures of all these changes are given by Qasser, 77.3. The heart is now a double tube, connected by the meeotbelium with the tissues above and below ; but soon the connection on the ventral side is severed, and a little later that on the dorsal, but the attachments are retained as in ampliibia at both ends of the tube. A section through the end of the heart is shown in Fig. 130; the ventral mesocardium is entirely lost; the doi-»ul id jtreaerved, as also at the opposite end of the heart, thougli not in its middle; the thick meeothelial wall or muscular heart is widely removed from the thin inner endothelial heart (Endothelherz) .

From the preceding account it appears that, owing to the development of the heart beginning before the Vonlerdarm closes, the heart is distinctly double in origin, though all trace of the duplex condition is quickly lost. In mammals the double stage lasts longer, the Vorderdarm being closed still later.

Our knowledge of the origin of the heart in viammals rests chiefly on the observations of Kolliker upon rabbits; this paragraph is therefore based on the description given in Kolliker's " Grundrias," p. 'Mi, 120. Traces of the heart can be recognized in embryos with five protovertebratiB, and the two anlages are well advanced in embryos with eight to ten segments, and in surface views, Fig. 114, may be seen at either side of the head, bending anteriorly toward the median line, and each connected posteriorly with the developing omphalo-mesaraic vein of the same side; one can also distinguish the parietal coelomatic cavity about the heart. A transverse section through the region uf the heart presents a verj- uniform picture in" . all mammals thus far studied ; <'injipare Fig. 05 of the opossum with Fig. 114 of the rabbit. The parietal coelom or amnio-cardial vesicle is small as compared with that of the chick, Fig. 117, and lies quite distant from the median line ; the splanchnic mesothelium forms a large fold, which projects into and nearly fills up the coelomatic cavity ; this fold forms, as, in the chick, one-half of the muscular heart; in the interior of this fold lies the endothelial heart, which sends out processes by which it is connected with the surrounding mesothelium. By the bending down of the layers and the expansion of the coelom the Vorderdarm is shut oflE and the two lateral heart anlages are brought together in the median line below the Vorderdarm, and there fuse into a single structure ; the fusion takes place in such a manner that the two mesothelial folds unite by their edges to form a single thick tubular wall around the double endothelial heart; it is not long, however, before the two endothelial tubes also fuse into one. As in the chick the two mesothelia, when the median heart arises, form a membnme (mesocardimn), by which the heart is attached to the tissues above and below ; both mesocardial membranes break through, putting the two coelomatic cavities into communication and leaving the tubular heart sus|)ended by its ends.

In amniota the heart arises from a double anlage, which by the bending down of the splanclmopleure of the Vorderdarm becomes a single median anlage, as in amphibians; C. K. Hoffmann, 84.3, has asserted that in snakes the heart arises from one of the lateral anlages, but Junglow, 89.1, has rendered it probable that this is merely a blunder of observation. The median heart is at first a nearly straight tube attached by each end to the wall of the pericardial coelom, and connected in front with the aorti© and behind with the omphalo-mesaraic veins ; the tube is double, consisting of a thin inner endothelial wall of mesenchymal origin separated by a considerable space from the outer thicker mesothelial layer, from which the muscular tissue of the heart arises.

Origin of the Endothelium of the Heart

This is still unsettled. As we have seen, the endothelium has upon its first appearance nothing of an endothelial character, but resembles instead the cells of the mesenchyma at the time; in amphibia they are large and rounded and charged with yolk granules; in amniota they are more like embryonic connecti v e-tissue cells. These cells always appear between the entoderm of the cervical archenteron (Vorderdarm of Von Baer) and the mesodenn bounding the coelom, and when they first appear there are no other cells near them between the mesothelium and entoderm, compare Figs. 128 and 129. Whence do these cells come? I consider it probable that they are the forward extension of the vascular anlages of the omphalo-mesaraic veins and that just as the endothelial m^rte are formed by the ingrowth of loose strings of cells so are the two veins, and these uniting in the median line form the endothelial heart. This view is hypothetical. A variety of other conflicting views have been advanced, of which the following may be noted. Balfour, '* Elements, 85, 89, thinks the cells come from the neighboring mesoblast, as ( )ellacher had previously considered was probable in teleosts, 73. 1, 84. Goette has maintained that in Petromyzon, 90.1, teleosts and amphibians, 75.1, the cells come directly from the entoderm, and C. K. Hoffmann, 92.1, maintains the origin of the heart to be entodermal in elasmobranchs. Rabl, expresses himself very cautiously, but inclines to the view that the cells come from the entoderm, and in regard to the sharki) he is uncertain, 89.2, 225. J. Ruckert, 88.2, believes that the cells which become the endothelium are thrown oflE in elasmobranchs from both the entoderm and mesoderm at the points where the cells first appear. Finally, F. Schwink, 90.1, asserts that in amphibia the cells are derived neither from the neighboring entoderm noi* mesoderm, but that they grow in from the mass of yolk-cells. Schwink's observations seem very careful, and may turn out to confirm the hypothesis of the origin of the endothelial heart from the omphalo-mesaraic veins uniting.

Origin of the Vascular System

O. Biitschli, 83.3, has advanced an hypothesis of the phylogenetic origin of the heart and blood-vessels which has much plausibility. He suggests that the heart is a remnant of the primitive or segmentation cavity of the embryo, and is not derived from the secondary or permanent body cavity (schizocoele or enterccjele). He endeavors to reconcile this view with the accounts of the development of the heart in vertebrates, maintaining that it probably arises as a fissure in the mesodenn, remaining as a permanent part from the temporary primitive cavit}'. More support for the hypothesis is found in arthropods; for it haa been observed in several forms that the two edges of the mesoderm approach one another in the median dorsal line, leaving a space between them which belongs to the primitive cavit3\ This space becomes the heart. Sometimes it is cut oflE before, sometimes after, the mesoderm is split into segments. These observations were upon the bee (Biitschli), Oeophilus (Metschinkoff), and Branchipus (Claus) . An investigation to answer the problem propounded b}' Biitschli would, it may Ix) safely said, prove fruitful and interesting. For further speculations in this direction see Schimkevitsch, 85.1.

As to the evolution of the viiscular system the course of development in the embryo indicates, it seems to me, that the immediate ancestors of vertebrates had no capillary vessels, but only a few large afferent and efferent trunks with a few anastomoses, as is now found in many annelids. With the acquisition of the large yolk the development of accessory blood-channels over the surface of the yolk presumably followed to secure, more efficently, nutrition for the embryo. These first channels were, if we may rely on the ontogenetic indiaitions, grooves on the surface of the yolk bounded on one side by mesenchymal cells, bv the further differentiation of which the grooves be^come endothelial tubes; in this manner we can account for the blood- vessels appearing first in the extra-embryonic area. Sin(;e the blood-cells are developed from the walls of the vessels, it is possible that the walls may have acquired haemoglobin, and the cells then have been set free l)y a further evolution, but it is perhaps actually possible that the isolation of the blood-cells from their matrix (the vascular wall) may have preceded the acquisition of haemoglobin.

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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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