Book - A Text-book of Embryology 4
Chapter IV. The Beginning Differentiation of the Embryo; The Neural Canal; The Chorda Dorsalis; The Mesoblastic Somites
Heisler JC. A text-book of embryology for students of medicine. 3rd Edn. (1907) W.B. Saunders Co. London.
Early Draft Version of a 1907 Historic Textbook. Currently no figures included and please note this includes many typographical errors generated by the automated text conversion procedure. This notice removed when editing process completed.
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The germ, in the stages thus far considered, has the form of a hollow vesicle more or less irn^giilarly spherical. It will he seen, in following the further history of development, that the layers of cells constituting the walls of the vesicle give rise to the alterations of external form and to the rudiments of the various organs of later stages hy processes which, though seemingly com])lex, are referable to certain simple fundamental principles. It is, namely, in the unequal growth of different parts of the gt^rm, in outfoldings and infoldings, and in the furrowing and constricting-off of parts, as well as in the adaptation of structure to function, that we find an explanation of the various develo})mental processes.
The first indication of the formation of the embryo and of its differentiation from the parts of the germ that are destined to produce, wholly or in part, the several extra-embryonic structures, is the marking out of the embryonic area by the thickening of the cells of the vesicle-wall in a definitely circumscribed region. The structures designated as extra-embryonic are the umbilical vesicle, the amnion, the allantois, and the fetal part of the placenta. The development of these and the production of the external form of the body of the embryo will be considered in the next chapter.
The primitive streak and its head-process have been already described. After their appearance the further evolution of the embryonic bodv is closelv associated with three fundamentally important processes — namely, the formation of the neural canal, of the chorda dorsalis, and of the mesoblastic somites.
The Neural or Medullary Canal
The neural canal is an elongatwl tulic Ij iiig Iji'iK'atli the ectoderm in the median longitudinal axis ol' the eniltryonio body, its [wsition ct)rre)ipi)ndinf; tu that of the future e>pinat canal. Its walls are cump4i!^ed of cylindrical epithelial cellw.
To follow the development of the medullary canal, it is necessary to study the surface np]»earanee of the ovimi at. the stage when the mesoderm is beginning to gniw out from the r^ion of the head-process of the primitive streak. Upon the surface of such a germ (Fig. 35), one may see the primitive iiitreak and, in front of it, also in the median line of the embryonic area, the head-procesB of the primitive streak. The ectotlermtc cells overlying the head-process thicken so as to
become columnar, while tliose on each side of it Ix'come flattened. This differcntiiition results in the prtMluctitm uf a relatively thick axial plat« of ectoderm, the medullary plate, which is present at the hcRinning of the ei^rhth day in the rabbit's germ, and in the human germ at about the fimi'teeiith day. Almost as soon as the plate is formed, its liitm-al and anterior edges b^n tu curl up, producing the mednlUiy for* thinner ectoderm; these projections constitute the mednllarr fold!. A Hitrf'aee view shows the medullary folds to he (MintitiiioUM witli euch other ia front, while their posterior MwU iiri! w|mrutwl and embrace l>etween them the front end iif ihi! priniilivti Htrcuk (Fig. 41). Since the formation of \\ii\m: ntrii'^liiri-N '\r, always mure advanced in the ant«rior part (»(' th" cfiiliryoiiii! iin'u, their posterior extremities are not Mhiirply 'li'tln(«i hut fiwle away (Fig. 41). The edges of the tnithiihiry plnte <>»iitinnc to curl until they meet, when they llliil4-, forriiiliK tJie mftdullary or neural canal (Figs. 44 and
I'll 'V\w mm I II I III I'v tiililM mill plate (continuing to advance hivvmtl llu> lull imkI nl' \\w i<iiilirvi)nii> area, and the closure (>l llii> dihi' UilvliiH |iliii<4' iVmi lii'fon' Imokward, the entire tulii(lllvi>«li«4tk la iiimli' ltMllpiiip|H<nrliy lieing included within liiivliiu: grown tiiwnnl each other a Aw^xi um^> t'lltav \\w uiil»ii of th<> ciigcH of the me<Inllary yi»\W "I'w iiihum'wi' tl«> |wrrliilly riirnii'd neural tube. By liijilrt itiid their subsequent v^\tK. 1 1 iii^', lhi> t4>iii)'U'Usl ut'tinil 1tilH> <>oni(<M to lie under the mUm\' viUhKhh. iu ^xuktHVthm wllh wliioli is afterward lost It is apparent, therefore, that the neural tube is a structure whose walls are cumposed of ectodermic cells, and that it has originated from the ectoderm by what may be called a process of infolding.
The medullary canal is the fundament of the entire adult nervous system. The first step in the conversion of a structure so simple into one so complex consists in the rlilatation of the cephalic end of the ucural tube and the subsequent division of this dilated extremity into three imperfectly separated compartments, named respectively the fbre-brain, the mid-brain, and the Iiind-brain Tesiclea. It is by the multiplicatiou and
Fig. IS.— TiSDSvene lection or a gcTenteen-and-A'balf-daT Bheep-embrro (Bonnel).
specialization of the cells composing the walls of Hie medullary tube that the cerebrospinal axis is produced, the brainvesicles giving rise to the brain-mass, while the remainder of the tube produces the spinal cord. Approximately one-half of the length of the (u!>e is devoted to the formation of the brain, the other half forming the spinal cord.
The nenral tube closes first in the future cervical region, the cephalic part of the canal remaining o]>en for a time. FVom the neck region the closure of the tube progresses toward either end of the embryo.
The Notochord or Chorda Dorsalis
The notochord is a solid cylindrical column of cells lying parallel with the medullary tube, on the dorsal side of the archenteric cavity.
IIh position is that of a line passing througli tlio centers of the bodies of the future vertebrie, the development of the chorda occurs at the .same time aa that of the neural tube, and ill a verj- similar manner. A thickening of ihe cells of the entoderm in a longitudinal line extending along the dorsal aspect of the weleateron produces the c}iordal plate. Along cither edge of the chordal plate a small fold of entoderm projects veutralward. By the curling around of the edgcii of the chordal plate, the latter becomes a solid cylinder of cells, which is isepunited from the entoderm proper by the union of the chordal folds, as shown in Figs. 44 and 4-5.
The appearance of the notoehord is the first indication of the axis of the embryo, since around it the permanent spinal column is built up. The relative size of the chorda is lees in the higher vertebrates than in the lower members of this group. It is (me of the distinctive features of a vertebrated animal.
The chorda is essentially an ombrjonie structure, since it gives rise to no adult organ. Its only representative in postnatal life is the pulpy substance in the centers of the intervertebral disks. It is a [KTroanent structure in one vertebrate only, the amphioxus. In this animal it is the representative of the spinal (*lumn of higher vertebrates. The notoehord affords another illustration of the principle that higher organisms rp|)eat, in their development, the structure of the lower members of the group to which they k-lon^r.
The Neurenteiic Canal
The nenrenteric canal is closely associattnl with the ilevelopmcnt of the medullary canal and with the disappearam-e of the primitive groove. We have learned thatthe blastopore is the orifice through which the ccelenteroii ojiens to the exterior, and also that in birds and mammals the position of the blastopore, as indicated by the presence of the terminal ridge, corresponds to the anterior end of the primitive streak, and therefore of the primitive groove. Reference to Fig. 41 will show that the medullnry folds have extended so far posteriorly that they embrace between them the primitive groove; therelore when they unite to form the neural canal, the primitive streak falls within its limits.
In a gastrula with an ojKin blastopore, such as that of the amphioxus and those of amphibians, the blastopore is included between the me<lullary folds, and, after the completion of the neural canal, it constitutes an avenue of communication between the latter and the coelenterou or primitive enteric cavitv; this communication is the nenrenteric canal. In mammals, as also in birds, reptiles, and selachians, classes in which the primitive streak is the representiitive of the closed blastopore, a small canal is found at the anterior end of the primitive groove, passing through Hensen's node, and opening into the coelenteron. With the covering in of the primitive groove by the medullary folds, this canal becomes the neurenteric canal. According to Graf Spec, a ueurenteric canal is found in the human embryo, as well as in the groups above mentioned. The canal is a temporary structure and gives rise to no organ of the adult.
The Somites or Primitive Segments
The mesoblastic somites are cuboidal masses of cells, arranged in two parallel rows, one on each side of the notochord, extending the entire length of the l)ody of the embryo. They are sometimes called protovetiebvcey but this term if use<l at all should be restricted to a subdivision of them that appears later.
The development of the somites was incidentally referred to in the descripticm of the mesoderm. As mentioned in that connection, the paraxial plates of mesoderm, lying as parallel longitudinal columns, one on each side of the notochord, break up, each one into its corresponding series of primitive segments. The division throughout the entire length of the boily takes place not simultaneously, but consecutively, beginning at the head-end.
The segmentation of the axial mesoderm is indicated by certain surface markings. The surface of the embryonal area, at the stage when the primitive streak and the medullary groove are present, shows a dark zone on either side of the median line, the so-called stem-zone, which marks the limits of the iixial pliit« uf mesoderm (Fig. 46) ; the position of the lateral platos in indicated by the peripheral lighter puietal zone. The stem-zone soon exhibits, on eucli side of the primitive streak and medullary groove, a series of parullel transverse lines, produced by the transverse furrowing of the axial plates, preparatory to their divisiou into the primitive e^ments. The first pair of somites is formed in the future cervicid region, before the medullary folds have united to form the neural tube, and when tiie primitive streak is yet preseut. After the appeuranee of the first ]>air, the forma
Fig,— fi«bbll embryo of the ninth A»j, seen from Ihi- doraal side (aflet KClItker). Mignlflert SI illiiraate™. The Blem-Bcinu («i:i und Uieiittrieml lone (pil are lo be diHtlnKiilBhccI. In the fonner 8 pairs of prlmlllve scEtnentii have been estobllBhed al the aide of the ehorda and nentral tube: op. area pellnclda: r/, medullary trnrare: fA, fore-brain: nft, eye-vealele: nA, mM-braln: M>. hind-brain: uH>, primitive eemaetitT id, stem-Hine : jv, partetsl mnei A. heart', »*, pericardial part of the body-cavity: vd, marjcin of the entrance to the benil-inic tvorStre Da/mpfotie'. acen through the overljrinit stmclnn's; nf. amniotic fold: ™. vena omphalomeienterlea.
tion of other segments proeceds hradward and tailward. In selachians the number of head-segments has been shown to be nine; in higher vertebrates the number is possibly less. The trunk-segments are added in regular order from the neck-region to the tail-end of the embryo. In the hiimau embryo there are thirty-eight (wirs of neck and trunk somites and ix;rhaps four pair* in the i»coipital region of the head.
The first somites appear on the eighth day in the nibbii, and between the twentieth and twenly-semnd hours in the chick. While they are forming the nciinil canal is closing, the notochord is difFerentiating from the entoderm, and the lateral plates of mesoderm are splitting to form the bodycavitv or ccelom.
In structure the primitive segments of lower vertebrates consist of columnar cells arranged around a central cavity (Figs. 43 and 45). The cavity, in the amphioxus, communicates for a time with the ooelenteron, since the segments are in this case developed as entodermic evaginations ; in selachians, the method of formation of whose primitive segments may be regarded as the primitive method for vertebrates, the cavity is for a time in communication with the body-cavity, since the segments in these animals develop as if by evagination from the dorsjil side of the mesoderm after it has separated into its parietal and visceral layers and before it has divided into the axial and lateral plates. The size of the cavity is quite variable; in some cases, as in the Amniota, it is almost if not entirelv obliterated bv the encroachment of the cells of the walls of the somite.
Belonging to the somite, though not apparent on the surface, is a mass of cells which connects, for some time, the somite proper with the lateral plate (Fig. 45). This is known as the intermediate cell-mass or middle plate. Later, the separation of these is effected, the mesial part of the somite being the myotome, the intermediate cell-mass becoming the nephrotome. Each one of these parts contains a cavity, that of the myotome being called the myocoBl. From the inner, mesial side of the myotome, embryonic connective-tissue cells (mesenchyme) develop, constituting the sclerotome, or skeletogenous tissue. The sclerotomes, made up of loosely-arranged embryonal connective tissue, grow around the medullary canal and chorda dorsalis, spreading out and fusing with each other. Subsequently this tissue produces the vertebral column and its associated ligamentous and cartilaginous structures. The outer part of the myotome, sometimes called the cutis plate, gives rise to the corium of the skin of the trunk or perhaps to muscular tissue. The remaining part of the myotome, that situated dorsolate rally, constitutes the muscle plate or myotome proper ; it gives rise to the voluntary musculature of the trunk.
The segmentatioii of the body of the embryo is an embryological process of great significance.
The segmented condition is common to the developmental stage of all true vertebrates, and in some invertebrates it persists throughout adult life. The development of the axial skeleton and of the muscular system, it will be seen later, bears an important relation to the process of segmentation, as does also the evolution of the genito-urinary system.
Upon reflection, it will be seen that in the region of the embryo corresponding to the future neck and trunk, the segmentation affects only the dorsal part of the body, while the ventral mesoderm, the so-called lateral plate, which contains the ccielom, remains unsegmented. On the other hand, in the head-region, the segmentation is both dorsal and ventml, the fornx^r being in series with the trunk-segments, while the latter, affecting the ventral mesoderm, and therefore also, in the corresponding region, the coelom, produces j*ognu»nts known as branchiomeres, in connection with which tho vistH^nil arches are developed (see Chapter VII.).
The division of the primitive segments to the differentiation t»f the skeleton and of the musculature of the trunk, and ttl«o of the visceral arches to the muscles of the jaws, will be inm^idrrtnl in subsequent chapters.