1897 Human Embryology 8

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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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Part III. The Embryo.

Chapter VIII. The Medullary Groove, Notochord and Neurenteric Canals

In all vertebrates there occur two primary axial structures in verjearly embryonic stagen : oue is the medullary canal, derived from the ectoderm; the other is the notochord, derived from the entoderm : as soon as these two anlages have appeared the mesoderm disappears from the median line, and the previously continuous sheet of mesoderm becomes divided into two wings. Connected with the early history of the medullary canal and notochord are the temporary passages known as the neurenteric canals. For these reasons these three subjects are best treated together,


I. The Medullary Groove

I. The Medullary Plate

By this name we ^s designate the central axial portion of the ectoderm, || which early becomes distinguished by its greater f| thickness from the remaining portions of the layer ^3 and which gives rise later to the nervous system. a? The ectoderm of the mammalian embrj-onic shield 'f and of the saiiropsidan embryonic area lias at first, 1'^ it will be lemembered, a considerable thickness, for .^| it consists of ciiboidal or low cylinder epithelial ^| cells. The stage which follows next after the for- -* jj matlon of the primitive axis is characterized l)y the | j l^adual thinning out <)f the ectoderm over the peri- |,s pheral portions of the shield or area, wliile in tlic 3 § neighborhood of the axiid line the full diameter of -■ ^ the outer genn-layer is not merely retained, but is ?? actually increased. For a time there is a gradual ^ ? passage between the thicker and thinner imrts, hut ^^ a.s development prt^^-sses the deman-ation rapidly f ^ becomes shar|)er. Fig. !).'>, Md. Sfxin after its for- r. 5 mation, the interval varying according to the spe- ^5 eies, the medullar)- plate increase's its thickness 5= s everywhere except along the mtnlian line, thus l>i'- p'. coming double ; the thin median jiart often shows a slight groove which is known as the dorssil fur- ■ row {RficKenfurche) . * This fiirn>w does nut extend J clear to the cephalic end of the plate, because thei-e | the lateral thicker bands are continuous with oue ' another, the front end of the plate being rounded and clearly limited


The medullary plate appears only in the region of the head-process in amniota, and an the process grows backward at the expense of the

Primitive streak the medullary plat« follows, hence it is unequally eveloped throughout its longitudinal extent, being always more advanced headward and lees advanced tailward; hence it is tiiat while it is developing its posteridr extremity is always vague and fades out into the imdifferentiated ectoderm. So great is this inequality in mammals that we find the front end of the plate transformed into the medullary groove before the hind end is differentiated. The stage of development in which there is a well-marked primitive streak and in front of it a medullary plate overljTng the headprocess occurs in the rabbit at the beginning of the eighth day. At its hind end the plate extends so as to partly cover the primitive streak, while in front its edges already rise slightly, BO that it constitutes a minute shallow trough. For figures of a similar stage, age unknown, in the mole, see W. Heape, 83. 1, Figs. 13 and 14. In older writers we find figures representing the medullary plate (or groove) and the primitive streak as one structure, and the dorsal furrow in the middle of the plate as the , continuation of the primitive groove. To illustrate this eri™.;...™ Bs o -tiu^.c isiuuiiuuus imc. ror I presBut a copy, Fig. 96, of one of Biechoff's figures of the rabbit's ovum, in which no distinction is made between the two grooves, although in reality the dorsal groove stops in front of the primitive groove, the anterior end of which is often bent to one side.


In the Sauropsida the medullary plate is verj- similar to that of mammals. In both birds and lizards it can be seen that not the whole of the axial band of thicker ectoderm, but only the parts nearest the median line, share in the actual formation of the medullarj- groove. The differentiation is begun as in mammals by the thinning out of the ectoderm in the peripheral regions, until it becomes a thin pavement epithelium, while about the axis the cells become elongateil vertically; pyramidal cells, with the apex external, idternatiug with those with the apex internal, thus producing a peculiar appearance on sectioni* and causing the nuclei to form two layers; the single cells are, of course, irregidar in shape. In birds and reptiles, as in mammals, the medullary plate overlies the headpi-ocess and l)ecomes well marked off in front, while it is still being difffrentiatfd iH»steriorly, compare Fig. !t7.


The Medullary Groove

Almost or quite as soon as the motlullary plat*- iw Ei-nned, it.-* edge boeomea elevated in front and ■ou uach side; hent^e it fonns an open trough, known as the medullry groove, Fig. 'J', Md.gr. During this process the medullary ectoderm iQcreases considerably in thickness, and at the same time the nuclei moltiply and lie irregularly scattered at varying heights. The ectoderm (uongside the medullary plate or groove thins out still farther Inasmuch as the development is most rapid in the


roll.. Emhryn A J. M-<t['Hi-> iLruuuti oiM 'if tlif >M«nienlK: B. K-cti.Hi iliv jTiinilivi- »ln-uk. ihl. t/r. uii-liillar)' □1^ gi: '•□toil'-nii. y bIkiui

head end of the embryo thtre nrnios a stage in which there is ii wellmarked lucdull ii\ yr >o\e in fr nt, a medullar}- plate behind that, and H priniiti\e -treak at tht Imid end of the enibryu; but wlwii the streak Ims disjj |k trel the inwlulhiry groi>ve is found to extend the entiFL kngth it the tmbrj >. theiv is theu a stage in which by means of a series of transverse sections. Fig. 97, of the embryo, we may study the successiTe steps in the development of the medullary groove. This stage is found in the rabbit at nine days; in the chick at thirty to forty hours of normal incubation.

The medullary groove gradually deepens, its sides rising higher and higher and arching more and more toward one another imtil the edges meet and coalesce, thus changing the groove into a tube. The process is illustrated by the series of sections through a chicken embryo with seven segments shown in Fig. 97.

In some mammals the medullary groove becomes well developed. Fig. "" j before the medullary plat« is clearly marked off by the thinning out of the ectoderm aloi^side of it; the groove is also much larger, Fig. 98, in proportion to the size of the embryo, Fig. 99, than is the case in the large ova of birds and reptiles. The anterior end of the groove is wide open and expanded on each side ; this lateral spreading is the anlage of the optic diverticulum, Fig. 99, op, and is transformed later into the optic vesicle, which is an essential component of the future eye, A section through the optic grooves of a mole embryo a trifle older than Heape's stage, F, Fig. 99, is shown in Fig. -^ 100, The medullary plate is thickened and shows a median lesser, and two lateral greater depressions ; the former, Md, is the medullary groove proper; the latter, op, do not participate in the brain formation, but in that of the eye; at the edge of the optic anlage the plate passes abruptly into the much thinner entoderm. For some distance behind the optic anlage the edges of the medullary groove are almost in contact, Fig. 99, but farther back the grove is again wide open; this widely open part ia known as the sinus rhomboKlalis, which is not to be confused with the sinus rhomhoidalis of the neck, for the term is also applied to the cavity of the embrj-onic fourth ventricle nf the brain ; the sinus here described belongs to the future lumbar r^on. The swelling in the floor at the hind end of the sinus is (Ktused by the inesoblast of the front end of the primitive streak.


derm; Uet, nweoderm; Etit^ eotodeniL

H«pe. op.

tk-ula ; Jut, rntKlulIaz? riilKP or «Jice ot mrduilary RToove: Wd medullary BTOOve wlclely open.


On either aide of a rabbit or oposRum embryo, in a stage a little more advanced than in Fig. !'!>, just behind the open anterior end of the canal, there extends a longitudinal ridgo correaponding to the anlajge of one of the two tubes which will eventually form the heart, see Chapter XI. The lateral heart anlago of the opossum is shown in section. Fig. 05, Ht.


In a mole embryo, a little older than Fig, ftti, the hinder portion of the medullary canal is much tho same ai before; anteriorly, however, development has progressed and the edgos of the medullary folds have come together and partially fused at the anterior end of the embryo, owing to the more rapid growth of the sides than of the floor of the canal aa pointetl out above. At the extreme end, however, a pore is left. At this stage, therefore, the neural canal is still open to the exterior, both anteriorly and posteriorly. The optic grooves are now closed, and have given rise to the optic vesicles; these are shown as two bud -like vesicles projecting outAvard and backward and slightly dovoiward from the front end of the neural tube; behind them the swelling of the fore-brain is discernible, while still farther backward and at the edge of the Ixxly of the embryo the two tubes of the heart are indicated. Tho fi>lding oflE of the embryo from the yolk-siic has at this stage made some progress, and, indeed, the whole of the head of the embryo now projects freely alxivo the blastodermic vesicle. In the next stage {H, embryo 2.2 mm.) of the mole the edges of the medullary plate have met and uniteil, making the medullary gnxive in front into a canal, but the sinus rhomboidalis is still open, though beginning to dose The ckwure of tho groove begins in the cervical regi in and spre ida for^v ird and more slowly backward; where the closure t tki-s place la. t in front is kno^v^l as the nerti'Dporns; the position tt the neuroporus is presumablj the same m all amniota if not in all \ertel rites Van Wijhe, 84.1, finds thit in the duck the connection with the ectoderm is retained in front longfst m the re gion of the first cerebral ^ t icle ind not in that of the mid-br iin so tli it it has nothing to do, as r nie line suggested, with the devcl pnu nt of the pineal gland (epiphysis) . fhia ci nntx tion represents the final ant« noi clos ure; ^ an Wijhe Hpeculites that it was an opening in the ancestor f vertebrate and terms it the anttnor neuroporus.

The ineihiUnvij (trnoie f 4tiiphi bin has been mi)re fuli> studied th in j tluitof anyotbercliiss. Tht m stcom j)Iete history is that gi\en 1\ AIe\ Ooette for Bombinator, 75

79.1, S. F. Claikc, 80.1 Ruscnm "\I(Kimn Tin Ion 78 1 Ecker s •'Icones," Taf. XX III,, and othtrs In dl Amphibi i the me<liillary plat*! is very wide, indetd Fi^ lii| bcm^ broadest in front


Its margin is thrown up into a sliglit but broad ridge; when the plate closes to form a canal the surfaces of the marginal ridges ^jrow together and the surfac'e of the plates within the ridges becomes the surface of the central canal. In all Amphibia the central doraal groove, mg, is very distinct. As the ectoderm of the amphibian ovum very early becomes distinctly two-layered, it results that in the medullary plate the two layers can be recognized from the start; the outer layer (Goette's Deckachichf, Balfour's epidermic stratum) of course lines the medullary cavity and alone forms the epithelium of tho cen tral canal. When the groove closes the lumen of the canal is nearly circular bi section, but it soon changes into anarrotv vertical slit similar to f g„ the lumen of the amniote canal. , — -|:e™>- The round cavity is due to the *■ ch. DouA way in which the medullary isiea in & rew or me oeiis. 'pl^tcS CUrl Up, as shoWU iu Fig. 102. As pointed out by Alex. Otoette, 76.1, ICO, the lateral portion of the medullar}' plate arisen by delaminatioQ, a peculiarity which has an important bearing, I think, on the discussion of the evolution of the medullary canal, see p. 179. Finally, in the Amphibia, the medullary plate extends to the middle of the blastopore, and, it is maintained by some writers, eitf-nds beyond it, so as to completely surround it. This point is recurred to in connection with the liiatory of the anus.

The Medullary Canal

The medidlary canal, as stated, arises by the closure of the groove. The canal closes in the cer\"ical region first, hence it has at one time two free openings; as the closure pro^pesses the anterior region ie completed, while tho sinus rhomboidalis IB still open ; moreover, we see that the anterior end achieves considerable differentiation before the posterior end of the canal is closed. Of the entire length of the primitive canal about one-half is the anlago of the brain, while the other half forms the spinal cord. In the development of the brain the transverse expansion of the canal is most conspicuous, while in the development of the spinal cord the elongation of the canal predomiuates. The dilatation of the brain part begins very early, and comprises at first a general dilatation of the whole aiilage, and, second, special and greater dilatation of three regions; the three dilatations are known as the three primary cerebral vesicles (ifi'rHWass/i), and are designated as /«»<'-fcrfn'/i {Vorderhirn, prosencephalon), mid-brain {Mittelhini, mesencephalon), and hind-brain (77iM/e?7K'rH,metecephalon), respectively. The first vesicle is much the widest, and appears in mammals and probably in all vertebrates verj- early ; in mammals it shows itself plainly in the medullary groove as already noted. When the groove closes the canal is of contBe attached to the ectoderm, Fijj. !iJ, but this coimection ia soon sevcre<I, and the mwhillary, or, aa it also called, neural canal, becomes an indei>endent structure lying inside the external ectoderm of the embryo, and surrounded by mesodermic oells, which subsequently grow in l)etween the canHl and the ectoderm 80 that the canal comes to lie farther and farther away from the surface, Fig. 10:J, The structure of the medullary canal in early stages has been as yet but iinperfeiitly studied. The wall increases steadily in thickness, except in certain ]Htrts of the brain. Where it thickens ita nuclei multiply and form several irregular layers ; the cell bodies around the nuclei are small and connected by niunerous processes, so as to

f>roduce a protopla-sinic network; the protoplasm and nuclei next the umen early assume the character of epithelial cells, so that the cavity of the medullary canal is line<l by a distinct epithelial layer; this layer corresponds to the outside layer of the ectoderm (epidermis) ; ia some parts — aa, for instance, the dorsal wall of the fourth ventricle — the single epithelial layers constitute the entire medullary wall. The ikfd,


rcuial: ^g, priniiilvp »epnem i; Spl. spUnchnopleun' , Snt. n


non-epithelial cells of the canal l>ecome, as described in Chapter XXVtl,, ganglion cells. The nuclei of the medullary canal wall are oval, their long axis being more or less nearly })eri>endicular to the surface of the canal; each nucleus contains one or several nucleoli. The canal is primarily oval in section, but its lumen is a narrow tisanre. Fig. 103, hence the walls are thickest at the sides, and thinner dorsally and ventrally; this (leculiarity dominates to a marked degree the subsequent development of the brain and Hpinal cord.

XlTolution of the Medullary Canal. *— Under this head we have to consider, tirst, what is the primitive vertebrate type of the central nervous system ; second, what genetic relation existed between the vertebrate and the invertebrate type.

Theopinion generally accepted by embrj-ologista is that the typical vertebrate canal is funned bj- the closure of the medullary groove.


This view is advocated by Balfour, and has heen bo thoroughly accepted hy Adam Sedgwick, that he hasmadeit the basisof a speculation, 83. 1 , on the original function of the canal ; he Bupposes that it was open behind tind excretory ; the cilia which are found in the central canal of the spinal cord originally sened to produce the excretory current. Van W iihe, 84. 1 , has advanced independently almost the same hypothesis. Both of these speculations overlook the serious difficulty of assuming that the canal is primitive, while in the lowest vertebrates it is clearly a secondary modification. In Petromyzon, Lepidosteus, and Teleosts, the medullar)' plate, instead of becoming the floor of an external groove, forma a solid keel-like projection toward the ventral surface. This keel subse»niently becomes separated from the superficial layers of the ectoderm, and afterward a central canal is developed in it. In the ganoids, which approach the elasmobranchs in structure, there ia, as shoivn by Stilensky, 81.1, a medullary groove of peculiar form, which su^ests a transition from the solid keel to the open grixive; again in Amphibia there is evidence that the dclamination is still preserved to a slight extent in that group. These coDsiderations leiid me to the hypothesis that the nervous system of vertebrates wa.s primitively a solid axial thickening of the ectoderm, and within the class of gjmoids became motlified into a groove perhaps simply by more precocious development of the central canal ; the groove type has been kept in elasmoliranchs, amphibians, and anmiuta. Baftour, "Comp. Erabryol.," II,, ;itt-i, thus defends the ui)posite view : " It seems nhuost certain that the formation of the central ner\ous system from a solid keel-like thickening of the epidennis is a derived and secondary mode, and that the folding of the meduUarj- plate into n canal is primitive. Apart from its greater fre<iuency the latter miMie of formation of the central nervous system is shown to be tlio primitive type by the fact that it offers a simple explanation of the presence of the central canal of the nervous system; while the exi.stence of such a canal cannttt easily be explained on the iissumption th.it the ct ntral ner\ ous system was originally devehipKl IS a teel likethukcning of theepiblast" (epiblast-ectoderm) . It is not possible at present to ilecide jiositively -between (lie two views, but the view whit'h I am inclined to adopt is further .iustified by the development of the central nei'vous systeni in Annelids, wliich is fonned by the coaleswncc of a pair of linear cords; these cords arise each side of a ciliated longitudinal furrow, first as a single row ;i?Si^nri\;^iirrfTh;"ViiTM;,T"irnT'^ii;™;r.ii<"Vim "f f'totlemial cells, subsequently as Several rows which still united to the external ectoderm they extend towanl one another inside the ciliated cells of the furrow, and unite in a single nervous baud. The origin of the HnneliFlau uerve conl is illustrated by Fig. 11)4, which represents a transverse section of the embryo of an earthworm, at a stage during which the cells, n ?i, that are to form the nerve cords are still part of the superficial ectoderm, -E7c, though their future separation is already indicated. In leeches and arthropods the development is very similar. In all these cases the bands split off from the ectoderm. It appears then that in the nearest* invertebrate allies of the vertebrates, the nervous system develops as a thickening along the inner surface of the ectoderm, and delaminates from that layer. It seems to me very natural to suppose, therefore, that the strikingly similar process in the lowest vertebrates is the primitive one, and that the canalization of the medullary plate was evolved within the vertebrate series.


I have assumed that the ventral nerve cords of annelids are homologous with the medullary canal, a view that is now generally accepted by embryologists. Balfour (Works, I., 393, and "Comp. Embryol.** II., 311) has suggested a more complicated relation in his hypothesis that the lateral nerve trunks, which are known in many of the lower worms {e. (/., nemerteans, have fused on the ventral siae in annelids, but on the dorsal side of the body in the vermian ancestors of vertebrates. In favor of this ingenious surmise no evidence has since been found. Hubrecht denies the homology of the annelidan nerve chain and the vertebrate medulla; he considers, 87.1, 620-624, that the more primitive condition is represented by certain nemertean worms, which, beside two main lateral nerves, have a small longitudinal median nerve; the lateral nerves gave rise to the nerve chain of annelids by their fusion, the median nerve to the medulla of the ancestors of vertebrates. As no intermediate forms, either adult types or embryonic stages, are known to represent any phases of this double metamorphosis, I cannot admit that Hubrecht's bold speculation invalidates what seems to me the well-established homology between annelids and vertebrates.

The remarkable hj^pothesis of W. H. Gaskell, 90, 1, that the medidlary canal is homologous with, and deriveil from, the entodermal canal of Crustacea, seems to me unwarrantable.

II. The Notochord

As the notochord is a purely embryonic structure, I present its complete history here.

The notochord (chorda dorsalis, Wirbelsaite) is a rod of peculiar tissue, constituting the primitive axial skeleton of vertebrates. It begins immediately behind the pituitary body (hypophysis) and extends to the caudal extremity. It occurs as a permanent structure in the lower type, and as a temix^rary one in the embrj^os of amphibia and amniota, including man. Comparative embryology has shown that it is a greatly modified epitheliiU band which arises in the median dorsal line of the entoderm, being in position and mode of development analogous to the ectodermal medullary canal, or primitive tubular nervous system.

Numerous embryological articles contain observations on the notochord. The following references may assist students. The best general discussion is by Balfour, in his " Comparative Embryolog}';" the best observations on its origin in mammals is by Heape, 83.1, for descriptions of the chorda canal see Lieberkuhn, 82.1, 84.1; Carius, 88.1, and VanBeneden, 88.3; for its histolog}- , W. Miiller, 71.2; for its histogenesis, A. Goette, 75. 1, 349-361 ; for its anterior anatomical relations see Mihalkowics, 74.1, T6.1, Froriep, 82.1, Rabl-Riickhard, 80. 1 , and Romiti, 86. 1 ; for its atrophy in mammals see Leboucq, 80. 1: for its evolution see Ehlers, 86. 1.

  • With, of course, the pomlble exception of Amphioxua.

Origin from Notochordal Canal

The notochord appears very early in the course of development; its differentiation from the median dorsal wall of the notochordal canal begins at the time when the medullary groove is not fully marked out posteriorly, and is nowhere closed. The notochordal anlage can bo first detected just in front of the primitive streak as an axial band of cells, which at first is not well marked off from the mesoderm ; this band forms the median dorsal wall of the blastoporic canal in all vertebrates in which that canal has been identified. The differentiation of the notochordal cells begins usually at the anterior end of the canal and progresses backward, as the blastopore moves backward during concrescence. The differentiation varies as to the time of its beginning; it may begin in the unconcresced embryonic rim, as in Scyllium, or much later, as in Lacerta.


As the medullary groove (or keel) deepens, it pushes down toward the mid-gut until it comes into actual contact with the notochordal epithelial band, thus dividing the mesoderm into two lateral masses, Fig. HT, one on each side; this also leads to the temporary transverse stretching of the notochord.


Lieberkuhn, 82. 1 , 84. 1 , has directe<l attention to a specialpeculiarity in the early development of the notochord in mammals. The notochordal canal is formed throughout its length and then breaks through at various points to fuse with the yolk cavity, so that it may be described as a tube running along the median line, and having an irregular series of openings on its ventral side. The canal is lined by epithelium, which is thickened on the dorsal side to form the anlage of the notochord. In transverse section the chorda appears according to the level of the section to constitute part of a furrow or a canal (compare also Heape, /. r.,jp. 441, Fig. 40, 41). Lieberkuhn calls this canal mesoblastic, and KoUiker follows him in so doing, but this opinion seems to me based upon misconception. Indeed, C. Giacomini's researches, 88.1, show that the canal terminates in the rabbit in a blastopore, and Van Beneden, 88.3, has emphasized the fact that the canal helps to form the definitive archenteron. After the notochordal canal has fused with the yolk cavity-, the notochordal anlage is, of course, incorporated in the entoderm of the main archenteric cavity, and api)ears as the median dorsal portion of the entoderm. It early acquires a sharp demarcation and becomes considei*ably thicker. Fig. 105, than the adjoining entoderm, and forms a distinct though shallow groove.

Separation from the Entoderm. — The notochordal band separates off and the entoderm proper closes across under it, so that the notochordal band lies l>etween the entodenn and the fl(X)r of the medullary groove (or later canal) as shown in Figs. lOG, 103, and 97


A. This separation does not take place at tho anterior extremity of the chorda until somewhat later, so that for a considerable period its front end remains fused with the walls of the archenteron, Fig. 106. Setenka, 87. 1, observed that this front end of the notochord becomes dilated in the opossum and hollow; the hollow end subsequently forms an irregular sac opening into the anterior end of the intestinal cavity; Selenka names the sac the Oannienatasche ; it opens behind the partition which closes the mouth and is entirely distinct ,„., ..,..r«'.'r"i£[^':SuiS;f|^^j,^, fro"^ the hypophysalevasploru or body cavity: £i^«iuxienn; nrS, gtnation. Further in ves tigations led to tho discovery of traces of a similar canalization of the front end of the notuchord in other vertebrates (Selenka, 88. 1) . the peculiarity shows conclusively that the connection of the iiotochoi-d with the hypophysis is secondary, and that, therefore, Huhrecht's li\-iM»t!i('sis as to ttie evolution of the notochord is untenable.


The separation from the entoderm is effected, at least in mammals, by the entoderm proi)er showing itself under the notochord t«^vard we median line, and when the cells from one side meet those of the other they unite with them and form a continuous sheet of entoderm below the notochord cells. It is probable that tlic separation begins in all vertebrates, as it has been shown to do in several cases, l>efore the whole length of the notochord is fonneil. and progreaspB headward; see, for exam pie, 51c I ntosl 1 and Prince's account of the process in teleosts, 90.1, 743. So, also, in Triton til


Embryo,


g>atris, Bamlieke, 0.2, 90, fimnd that the separation of the notochord from the entoderm takes place earlier than in the Vrodela, and progresses from in l)ack forward. After the separation pigment {iranulef* appear in the central jx>rtion of tho chorda, an important tibservation. since certain writers have held, I believe erroneously, that the presence of pigment provps that the not^jchord must be derived from the ectoderm, which in usually pigmente<1 in amphibian ova.


After its separation the chorda is a narrow band of cells starting anteriorly from the wall of the alimentary tract and running backward to the blastopore. So long as the blastoporic canal is open, the chorda terminates in the entodermic epithelium lining the canal. For a certain period the chorda continues growing tailward by accretions of cells from the walls of the blastoporic passage, and after the canal is permanently obliterated the chorda may still continue its lengthening by acquisitions at its caudal end of additional cells from the primitive streak ; such cells may, however, properly be regarded as coming from the entodermic lining of the blastopore. We can, then, distinguish two portions of the notochord ; the first arising from the epithelium of the notochordal canal, the second presumably from the cellular wall of the obliterated blastopore. Braun and others have sought to attribute essential importance to these diflferencres, but it seems to me improperly. It is more reasonable to say that the chorda arises in the amniota, as in the lower forms, directly from the entoderm, but presents certain secondary modifications in its development.


After it is once formed as a band of cells the notochord passes through various changes of form, but ultimately becomes a cylindrical rod with tapering extremities. It attains considerable size in the embryos of most vertebrates, but in those of placental mammals it is always small, particularly so in the mole (Heape, 83. 1). It is probable that in mammals the notochoi'd, when first separated from the entoderm, is a broad flat band, as if compressed between the medullary canal and entoderm {cf. Kolliker, " Entwickelunr^gesch.," Figs. 194 to 197, and also Heape, 86.2, PI. XIII., Figs. 3G to 42). The band then draws together, diminishing its transverse and increasing its vertical diameter, until it has accjuired a rounded form ;* finally its outline becomes circular in cross-section. This series of changes begins near the anterior end of the chorda, and progresses both forward and backward. The nuclei of the notochord tend to gather at first in the central portion of the chorda, but in later stages (shark embryos with fifty and sixty myotomes) the nuclei are found situated peripherally, Rabl, 89.2, 249. The mesoderm early grows in between the entoderm and the notochord, which, however, for a considerable time remains close to the medullary tube. Later the mesoderm penetrates also between the notochord and medulla. The laver of mesodermic cells immediately around the notochord, which are of the well-known mesenchymal t\^, forms a Bi)ecial sheath, which at first comprises only a single layer of cells, at least in batrachia (Goette, 76.1, 357, Fig. 187). This is the commencement of the so-ciUled outer chorda sheath ; it subsequently becomes much thicker. In the lower types it is sometimes an imjx^rtant axial structure; but in most cases it is replaced by cartilage, and in all the amniota the cartilage is replaced by the osseous vertebriE, the intervertebral ligaments, etc. The formation of the vertebral column involves the disappearance of the notochord as described below.

Notochord of Teleosts

The medullary kejel or great neural axial thickening of teleosts extends to the entoderm; the cells at the bottom of this keel next the entoderm give rise to the chorda. There being, it is said, no open blastoporic canal in the bony fishes, we can only trace the cells back into the undifferentiated mass of cells with which ectoderm and entoderm also fuse, and which lies at the hind end of the embryo. According to the most generally received opinion, the cells of the notochord arise from the entoderm, and their ' fusion with the ectoderm of the medullary keel is temporarj' only. The teleostean chorda separates first from the mesoderm, second from the entoderm, and third from the ectoderm. The development in Lepidosteus is similar. The modifications we here encounter will probably be traceil back to the general vertebrate type. For discussion of the subject and citations of earlier authorities, see Mcintosh and Prince, 90.1, 740-745.


  • A splendid description of the aelachian notochord at this sta^^ is K^veu by 0. Rabl. 80.2,


Shape and Relations to Other Parts

As soon as the headbend (first cerebral flexure) appears. Fig. 107, the notochord becomes correspondingly bent, and its bend lies close to Rathke's pocket. Fig. 107, ny. From Selenka's Guamen' tasche there now runs upward and forward a short limb of the notochord, which subsequently atrophies. This limb ma}^ remain regular or it may grow and become somewhat irregular before it atrophies; after it is gone the chorda has a new or secondary anterior extremity^ which Romiti, 86.1, finds in the chick embryo at the end of the fourth and during the fifth day of incubation to be united with an irregular solid cord of cells which grows out from the epithelium of the hypophysis. The cord soon disappears. Its significance is quite unknown. Romitl SUg- brain; wi6, mhlbraln; fb. fore-brain;



Fig. 107.— Rabbit Embryo of Amni. ; Median I^mgitudinal Sc^ion of the Head. The connection between the montli 3/, and pharynx rnt, is junt ett• j. 1 -- -D 'x* ta))ll8he<l; Mt'fc. notochonl; hb^ hind

gestethat it may produce a strain resulting in the pulling out of the hypophysal evagination. This notion seems to me untenable, since the hypophysal invagination begins lK*fore there is any union with the notochord. The cranial portion of the notochord has not only the tend shown in Fig. loT, but also follows the other curves of the head ; it takes a sinuous course besides within the base of the cranium ; finally, in the region corresponding to the middle third of the spheno-occipital cartilage, it makes a great dip ventralward. The sheath of the notochord in the cranial region is converted into the spheno-occipital cartilage ; at the dip just mentioned, however, the notochord lies entirely below the cartilage close against the wall of the pharj-nx (Froriep, 82.1, Romiti, 86.1). Writers before Froriep had represented the chorda as having disappeared at the bottom of the dip.


The anterior termination of the notochord has l)een carefully studied by Prenant, 91.2, x>0;j, who finds that it has (pig and rabbit) no connection with the hypophysis, but may have a secondary temporary connection with the entoderm just behind Seesel's pocket, and that the part of the notochord nearest the hj^pophysis very early degenerates, leaving the notochord to terminate above Seesel's pocket ; according to this view the so-called prce-chordHl region primitively contains the notochord. The secondary anterior termination of the notoohord is close to the infundibtilum (and future pituitary body), and it is customary for subsequent Btages to divide the head and skull into a prcE-pituitary and a post-pituitary region; the latter region alone contains the notochord, after very early stages.

Aist(^eneBia.' — After the notochord has been formed as a rod of cells, its cells undergo a process of histolt^cal differentiation, unique in vertebrates. The cells at first become greatly compressetl in the line of length of the chorda, and lience appear quite thin in longitudinal sections. Fig. 108, hardly greater in diameter than their own nuclei. The flattened cells are next converted into a highly characteristic reticulum by vacuolization. Thus, in the chick, by the third day some of the central cells become vacuolated, while the peripheral cells are still normal ; at tirst, as in the frog, there seems to be only one large vacuole in each cell, Fig. 108, B. Around the vacuole is a peripheral layer of graimlar protoplasm, in which the nucleus lies imbedded, while the vacuoloH themselves are filled with a perfectly clear and transparent material, which is supposed to be fluid in its natural condition. During the fourth day (chick) all tlio cells Iwcome vacuolated, with the exception of a single layer tif flattened cells at the periphery. (In the anura, it is said, there is no distinct peripheral layer of protoplasntatic cells.) The vacuoles go on enrging until by the sixth day they have so much increased at the expense of the protoplasm that only a very thin layer of the latter is left at the circumference of the cell, at one pirt of which, where thero is generally more protoplasm than elsewhere, the remains of a nucleus may generally be detected. Thus the notwrhonl l>ecome9 transformed into a spongy reticuluih. the meshos of whi(;h corresixind to the vacuoles of the cells and the septa to the i-emains of their cell walls (Foster and Balfour) . As f Joette has pointed out, the process is accompanied by an expansion of the cells which is the main factor in the widening and lengthening of the no^ichonl, which goes on pari passu with the growth of the surrounding tissue.



Fig. , — Lon


The histf^cenetic process is 8tate<l to l)e essentially similar in mammals (W. MuUer, 71.2,337, 338). There is the central layer of vacuolatetl cells and the peripheral layer of protojtlasmatic cells.


The latter are, however, ultimately converteil into vacuolatetl cella. The cell walls are iKjrforate, having fine pores that correspond prohably to in tercel lultir brides of prot^tplasm. The innvr chorda sheath appears early, and is to he re^^arded as an anhistic basement membrane secreted by the iiotucbordal cells.

Disappearance

The disappearance of the notochord in man commences with the necynd month of foetal life. The first step in an alteration of t)ie characteristic histoli^cal structure, accompanied by shrinking of the tissues, so that a clear f |iace appears around it. The inner cliorda sheath is lost. The cell \vaUs disappear, the tissue becomes granular, and breaks up into multinucleate, irregularly recticulate masses. Fig, lOtI, which are griwlually resorbed (Leboucfi, 80.1). In mammals the resorption progresses more rapidly in the cores of the vertebrae than in the intervertebral spaces, and again more rapidly at the ends than in the centre of f q i>- m- em dk \otoch m Tifwie.from each vertebni; hence the chorda persists a little longer in the centre of the vertebra, and considerably longer in the intervertebral sjiaces; in these Ia.st the final remnants of the chorda nuiy be delected in mjm even afterbirth. The cavity l>et^veen the vertebral cartilages is a new structure and is not the space left by the notochord, as has b«H)n sometimes asserted. It appears, Jiowever, that the resorption of the cliorda may leave a small sjiace, which becomes includetl in the intervertebral cavity. A iKK-uliar feature is the frequent i)ersistence of caliified cartilage immediately around the notochord in ossifying vertebras.

Morphology

The notochord was for a long time supposed to be exclusively characteristic of vertebrates. It is now known to exist in Amphioxus, which is not a true vertebrate, and in the Tunicata. Morpholi^ists have long Itelieved that it must have some homologue among the oi^ms of invertebrates. The development of the notochord in the lower vertebrates indicates very plainly what must have been the general chitracter of such an homologous invertebrate organ. In certain fishes and amphibia the notochord has been as.%rted to arise as a furrow along the median dorsal line of the entoderm ; the furrow deeix'ns and then cloi^ over to foi-m a rod sejiarate fn>m the entodermic canal projjer. The notochordal rod retains for a time its anterior and jxistcrior connections with the entfKlenn. It is usually regarded fis inori>hologically a solid canal, a view very oi^-n to doubt. Ultimately the ends become detached, and so arises the solid isolato<l chonla. In the higher vertebrates the course of deveIo]iment is similar, although several of the primitive features in the formation of the chonhi are obscured. Ehlers, 85.1, haapoint«:^d out that in various invertebrates there is a similar canal, the " Neltendarm " af (iernian writers, which is derived from the entoderm and <roniui.'ted anteriorly and i>i)sterii>rly with the entodermal cavity. It is a very plausible su^estion, which homologizes the vertebrate notochord with the invertebrate "Nebendarui." ulo'ecbt has sought to homologize the notochord with the proboBciB of nemertean worms. There is not a single fact which seems tt> me to justify, even remotely, this attempt at guess-work phyl<^eny, nor can I hnd any resemblance of the notochord with the structure in Balanc^lossus with which Bateson has sought to homulogize it.

III. Neurenteric Canals

The term neurenieric canal is used to designate an open communication between the archenteric cavity and the medullary canal. Quch communications are found only in early stages ; they always pass through the anterior end of the primitive streak and lead, therefore, into the posterior end of the medullar^' canal or groove; they are present only during a short period. Much confusion has existed in regard to these canals, of which as many as three have been distinguished by M. Braun, 82.3, while several writers recognize two.

The true Neurenteric Canal is probably the blastoporic canal proper, and is to be identified by the notochord terminating in its ^ wall. As stated in Part II. of this chapter, the " chorda-canal" of mammals is the " blastoporic" canal, and therefore also includes the neurenteric canal. As previously described, the ■ blastopore is tiie opening of the notochordal canal at the ante* rior end of the primitive streak. The neural ridges or medullary folds extend around and behind or across the blastopore, which, therefore, opens into the posterior extremity of the medullary groove. If now the canal is open jit the period of development when tlio medullary groove is deep or has already closed over, making the medullary canal, then thei-e is a direct communication between the entodermal canal on the one hand and the spinal canal on the ;; j,^',^;^^^ other. We owe to Balfour the jniy partially cut, but wai identification of tills canal as °lS*^rtXnTof°bi"«t?^r;ir'"I; the blastopore. It may ivith JH.I, «F i«^rew/rtTci^i"*.X?D™hlm"'""' propriety l»e termed the true neurenteric canal, or the canal of Kowalewsky from its discoverer. Kowalewskj' first found it in AmphioxuB, and subsequently demonstrated its occurrence in various fish t>'pes.

This canal is well known in E^Iasmobranclis and Sauropsida under the name of the blastoporic canal. It has recentlr been shown to be present in Petromyzon by A. Gktette, 90.1. In teleosts it is rudimentary, the passage being only imperfectly indicated (»ee Mcintosh and Prince, 90.1, iSi-TSti). In the Amphibia its relations are more clearly understood than in any other type. According to H. E. Durham, 86.1, it can be well seen iu longitudinal sections of earlf stages of the frog. Fig. 110, as a short canal, iie, opening widely into the entodermic cavit^'. This canal has also been described in Bombinator by A. Goette, 76.1, and in Triton and Rana by F. Schanz, 87.1. Schanz was the first to clearly discriminate between the anal or false blastoporic and the neurenteric or true blastoporic canal. In birds the neurenteric canal was first described by Gasser. 79.1, in goose embn-OB, and since then has been found by Braun in several other birds, though as an open passage it appears to be usually obliterated, an in the chick. Fig. 1 1 1 represents a transverse section which passes through the (jasserian or neurenteric canal of the jaroquet.


Fig 111. — TmnflviTiip S«tiun i>r nn tiii)irvt ( MelO|HlI(acuii> to i nr trw Ni-iirmttTlo Cuuul. lie, KL-t<>cli-nii : yin, iiiyiiliimr: Mil. ni«liilliu-y caiwi: en. iHiiociKmi nbercval ■»■ tU» Hbtirt ni-ureDtcrli: van*), hi-: Knt, <«tu<lrnii; mm, iiiriodtrrm. After Hui


Braun lias maintained that in various birds there are two neurenteric auMils rtH;ognizable, which lie near togetlicr. Braun states that in the duck and Mobicilla the two canals are separated both in the times and position of their w-cum^nco, and that in the Australian panxjuetthey are prt>sent simulttuieuusly. I). Schwarz, 89. 1, criticises Braun 's ob!*er^'aliinis and concludes that there is really no second canal. I am iiiclinwl to accept this conclusion,

In miinmiiils tho oireii blastoporic canal has been seen by verj- few observers; it has Ijoi'h carefully stutliKl in the rabbit by C. Giacomini, 88.1, who shows that the fnmt ]X)rtion persists for u time as the chorda-canal, while the hind portion, nmning tlintugh the primitive streak, (iiiTesi>onds to the neurenteric canal and is obliterated quite early.

The Aual Canal is also sometimes calle<] the neurenteric canal, and seems to have been especially the nubject of misconception. Its full history is given later. Its morphological relations are probably correctly indicated by the observations of Fr. Schanz, 87.1, which have Xieen confirmed by T. H. Morgan, 80.2, by RobinsoD and Assheton, 91.1, and others. To explain these relations wo may start from the stage in amphibian ova, in which the anus of Rusconi is almost closed over ; the true blastopore lies at its front edge. As the anus of Rusconi contracts its aperture appears more and more as a mere enlargement of true blastopore, and it is at this stage commonly spoken of as the blastopore ; to preserve the distinction we may name this opening the secondary blastopore. Alice Johnson, 84. 1, had shown that the permanent anus is derived in Triton from this secondary blastopore. H. E. Durham, 86.1, observed that there are two passages in the frog at a little later stage. Pig. 110. Schanz, I, c, found that the medullary ridges meet at their hind ends across the secondary blastopore and divide it into two openings, the anterior, the true blastoporic or neurenteric, and the posterior, the anal opening. In many Amphibia the anal canal is often temporarily closed by the tissue growing across it ; in later stages the ectoderm forms a slight invagination to develop the anus proper, the partition between it and the archenteron breaking through. The partition is called the anal plate.


The relations of the anal canal in Sauropsida are not yet well ascertained. It is represented by the anal plate, consisting only of ectoderm and entodenn. Although this anal plate (Afterliaut) has not been actually i)roved to be homologous with the tissue which temporarily closes the anal canal in amphibians, yet it is hardly possible to question the correctness of the homology, for it separates the anal invagination from the archenteron and suteecjuently ruptures. The anal membrane recurs in mammals, and if it represents the anal canal in one case it d(^s also in the other. C. Giacomini, 88.1, 287, 288, states that in rabbit embryos with several myotomes the anal plate is grown into a short cord of cells, in which there appears a temporary lumen — this lumen he calls the anal canal. After the canal has disappeared the anal membrane is again found to consist of two epithelial plates, the rupture of which forms the true anal perforation.


Braun's Third Canal

The third canal, which was first described by Braun, 82.3, is said to occur in older embryos. D. Schwarz, 89. 1, 211, denies its existence altogether. The "Enddarm" of Gasser and KoUiker becomes the ** Schwanzdarm" (postanal gut, Balfour) of older embryos, which soon becomes divid^, at least in birds, into a dilated terminal portion and a narrower neck communicating with the intestine proper. The posterior section then subdivides, and its narrow end-segment lengthens out and unites ^vith the spinal cord. This jjassage we may designate as Braun's canal. It is not improbsible that it is homologous with the amnioallantoic canal of Gasser, 82.2, which Rauber, 83.2, has nicknamed Cochin-China canal, after a l)ree<l of hens in which it seems most constant. In the one cjise wo may suppose the canal to open after, in the other before, the closure of the posterior end of the medullary groove. If the homology is correct it may be further said that the canal is identical with Kui)flFer's myelo-allantoidean canal: it cannot be brought into relation with the development of the allantois, as believed by Kupffer, 82.2, 83.1, as the allantois and end-dann are both formed before the canal appears.


Significance of the Neurenteric Canal

As to the morphology and physiology of the canal we know almost nothing. The suggestion of Sedgwick and Van Wijhe, that it is the excretory opening of the tubular nervous system, has already been noticed, p. 179 ; it will suffice to recall here that no valid evidence in favor of their hypotheses has been foimd yet. There is no adult form known in which the neurenteric canal persists ; were there such an animal we might hope to discover the fimction of the canal by observ^ation. Morphologically the neurenteric canal, so far as I can judge from present evidence, is part of the persistent blastoporic canal, which IS included in the medullary groove, and by the closure of the groove becomes shut off from the exterior. Why the secondary blastopore (prostoma) should be divided into the two openings, the neurenteric and anal, we do not know.

It seems not impossible that a persistent neurenteric canal may occur as an excessively rare anomaly in the adult.




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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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