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CHAPTER VII. THE FURTHER DEVELOPMENT OF THE EXTERNAL FORM OF THE BODY

Heisler JC. A text-book of embryology for students of medicine. 3rd Edn. (1907) W.B. Saunders Co. London.

   Text-book of Embryology 1907: 1 Male and Female Sexual Elements - Fertilization | 2 Ovum Segmentation - Blastodermic Vesicle | 3 Germ-layers - Primitive Streak | 4 Embryo Differentiation - Neural Canal - Somites | 5 Body-wall - Intestinal Canal - Fetal Membranes | 6 Decidual Ovum Embedding - Placenta - Umbilical Cord | 7 External Body Form | 8 Connective Tissues - Lymphatic System | 9 Face and Mouth | 10 Vascular System | 11 Digestive System | 12 Respiratory System | 13 Genito-urinary System | 14 Skin and Appendages | 15 Nervous System | 16 Sense Organs | 17 Muscular System | 18 Skeleton and Limbs


Having traced the growth of the germ to the time when the body of the embryo becomes definitely diiferentiated from the embryonic a})|)endages or fetal membranes, the development of the individual organs and tissues may be taken up. The discussion of this latter subject, especially of that part of it pertaining to the structures on the exterior of the body, involves a consideration of the external form of the embryo and fetus during the successive stages of growth.

In the preceding chapters it was pointed out that the cells of the segmented ovum arranged themselves in such a manner as to form a hollow vesicle, the blastodermic vesicle (Plate I.) ; that this vesicle, having at first a single-layered wall, came to consist of two layers of cells, the ectoderm and the entoderm ; and that, finally, a third, intervening layer, the mesoderm, made its appearance. It was shown, further, that the thickened portion of the vesicle wall, the embryonic area, became more and more differentiated from the remainder, and that, by certain processes of folding, this area was made to assume the definite form of the embryonic body, while from the other parts of the vesicle-walls the fetal membranes were produced (Plate II.). It may be well to remind the reader again that when the body of the embryo has become closed off from the fetal membranes, this body is an irregularly tubular structure whose walls are the somatopleure and whose enclosed space is the body-cavity, and that within it are two other tubes, a larger, the gut-tract, formed by the splanchnopleure, and a smaller ectodermic tube, the neural canal.


While, as a inattor of coiivcniciKjo, tho (]('scri]>tion of the individual organs is taken up after tracing the course of development to this stage, it should he borne in mind that the rudiments of some of them are already distinguishable before the germ-layers become infolded to form the IxKly-wall and the gut-tract. It will facilitate a comprehension of the gi'ueral principles concerne<l in the origin of the different |>Jirts of the body to R»fer to the tabulated statement of the derivativi»s of the three primary germ-layers as presenteil in Chapter III.


In ciMisidering the external form of the prcnluct of conception, one may adopt the classification of I lis, referred to in the first chapter. This author divides the jHTiod of development into thn»i» stages, of which the ///W, the stage of the onuiuor the blastodermic stage, comprises the first an<l second we^ks of inira-uterine gri»vth ; the Htroiufy the stage of the «BihiTO« extends frimi the seci>nd to the fifth wi»ek ; and the nhtnL or tte fetal stage, includes the time bi^twwn the fifth w<^k aud the end of gestation.

THE STAGE OF THE OVUM.

Durii^ :ht:* t'»rtnii:lit allotteil to this first staire of develop invut nxMT chie various changi»< l\v whirh the impn»gnateil

•\'im H'»tui:>f^ th-.' fi^rm of a hollow <phen\ de<ignat«»<l the

fnoi-vtiiiii- 'i* ■^i:i>ciHK'rTnir vehicle. Tlu* M-rir^of tnin^torma >Mi> wa^ u»'ii Fi"i*'rtb^il in Chapter II. In ihi> pla«v it will

'^ ^ttiKi:««iii I' r'vr c.» tht* external ch:inictrr< of the blasto i«gHK «*»a» t> A'CVCcxl in Fisr^. 4!>», whi.*h ri*pre<tMit



was estimated tu be alitml twelve days old. Its fonn van that of a spliore' snmewliut fijittf nod, its short and long diamot'.Ts inciisurins resiwcti vi'ly 3.3 mm. und 5.5 mm. T!ie flattened Bur&ces were t^llloolll, ^vhile the equatorial sons wns



!.' ;-. hloiiii-iiLii Liiiij( nil ilif iiiiii-r iMiliir iwrlloii of the ftiMriimlB

l'.E. uledliG epiLhulluiii^ Oii., duUdiut reQeia: TV, Inipbobliul; (b, nuKriut oaplllary: r)r, gland of uterine muMwa: /U.Jl.UciiaicIn (he traphoblail, conUlnIng malemil blood: K.A, nits' of embryil; Om/i. itvcldus rtuinparta : if, fbUI mEdohlut : L'.Z. liiti!rgliiu<liil*r llMue ur mticcwi. In which enrly dccidunl n'lln are


beset with villi istic of the hill


Tile I'arlv apiieiir.i

It 13 during the early jKirt of the second stage, at about the fourteenth day, that the somatopleuric layer of the lilastodermic vesicle becomes folded in to prodtiee the walls of the


erabrvouic body. Fig, 67 allows a human etnbrvo of about the tifteenth day, wbose form ia as yet imnerfectiy differentiate, the ventral wall of tlio bixly Iwing incomplete, tiince the giit-trart Is tttitl in wmimimication with the iimbiliCftl viwioie throughout almost the entire lungth of the embryo. The Iwck and sides of the embn-o arp enveloped by the unnlon, and the dorsal outline is concave. The caudal pole



fM, M~HuHiiiii*itibrri>ifrKlii>ul (liu thltlvenlh asy (Rti) Tbemudiit lll>WUll>r|ri>liouiiii*Dtvil trlUi Ihv tilutcidiinnlc! vmicle bj miianBof tlii! alid ku wIIuhMi' lUUi Ui» ainnlan nl randy romplrtcly endtoiri the vtghryn.a tvTtiH iltHlllnHiiiiii^iiminiinlrBli-i throuKhoUl th«it«lvt partofthi: mllnl i


U mvn t« \\f winni't'twl by means of the ullantoip sliilk with ninu, which Intter Btnictiire, however, is not


th.i


iilllv.


i'fl>iv.(>nliil ill lliK rijrnre. The concavity of the dorsal ontliiu> in |Ht>nliiii- (.1 the hiiman embryo of this stage. The (luvi<lopini<n( III' thiT Iicud i'h eUmcly iiHB0ciat*'d wilb the dilatalitui uf \\w .'cphaiic end of the neiinil hibe and the subse(picnt divi-i.iH of thin dilated extremity into the thrt^ primary bmln-viwioh'M, tho forc-bmin, the mid-bruin, and the hindbmin. The oral pit, the Brut indication of ibe future mouth, ia )>rvMii]t in the curly part of this stage ; it is a depression the rudiments respectively of the crystalline lens and of the membranous internal ear; at this time also the visceral arches and clefts first become distinguishable. On the twenty-first day, the rudiments of the limbs appear as little bud-like processes springing from the trunk. The conspicuous projection on the ventral surface between the now almost completed yolk-sac and the cephalic end of the body is produced by the primitive heart (Fig. 59, 10, 11, and 12).


Until the twenty-first day the ombrvonic body is erect. Between the twenty-first and twenty- third days a marked alteration in the appearance of the germ is brought about by a pronounced bending of the long axis of the embryonic body (Fig. 59). The degree of curvature is such that the caudal and cephalic extremities overlap. The flexion reaches its maximum degree by the twenty-third day. The curved dorsal outline is referable to four well-marked flexions, the position of the most anterior, or cephalic flexure, corresponding to that of the future sella turcica and being indicated by the projection of the mid-brain vesicle (Fig. (>2, III.) ; at this point the anterior part of the head is bent almost sufficiently to form a right angle with tho posterior half. A second or cervical flexure is found in the future neck-region, while further caudad are seen the less pronounced dorsal and coccygeal curves.


The fourth week marks the period of the most active growth of the embryo. Afler the twenty-third day, the body as a whole uncoils somewhat, although in the latter half of the fourth week the individual flexures noted above become more conspicuous.

The Visceral Arches and Clefts

The visceral arches, with the intervening visceral clefts, constitute a conspicuous feature of the extemal appearance of the embryo during this stage. These arches are a series of fivx» approximately parallel ridges appearing upon each side of the future neck-region and extending obliquely downward and forward toward the ventral surface of the embryo (Figs. f)0 and 62). Tho four furrows lying between the five visceral arches are the visceral clefts. A coronal section of the neck


UTa)u||Ulur ui prltntllvu l>sai


111* tu tkutUt: urtlii's (U^i ' "^- ">'■< uaxlltorr and mandlbuUr 'ml anh : o l-a IV, Bnl to Iburth wirlle anhiM : fv. cc. priuil* liial vain*; dC. fliicl "f Cuvlur; al. r. ■trioai and ventricle of iii'llin" •■a: ni. da. vtntral «nd donal anrtK: mi, oi, optic end region (Fig. 61) — a section in a plane parallel with the ventral surface — shows that the furrows seen on the Dctodemiie surface correspond in position to a like number of deei>er grooves on the inner or entodermic surface. The inner furrows are out^ptK^ketiiigs of the entoderm lining the pliaryn^real region cif tiie furc-gut ; they are referred to as the pharyngireal poucheB or throat-pockets to distinguish them from the outer clefts. At the iiottoni of the clefts tiie ectoderm is in contact with the entoilerra, the meaoderra being absent; these two layers constitute the closliig membrane. The visceral arches or ridges consist of tliickened masses of niesodermic tissue covered outwardly and inwardly respectively



FlQ.

phsrynp'Bl end of gut-lrsot from behind (froi

eniljryo ot a.iinm.: B, of 4.ffi mm. (about 2510

ceni ftirrawa; I', bIdus privcerTlHlls, compriiing Ilifrd and tuurth oi

, *. !l, (, rlMoral Bwhos. each witb lis »jB™™i-ureh vesasl
B. lubtrc

7, orinpo of lirj-Qi ; *, pulmonnry evagioatlou.


by the ectoderm anil the entoderm. Each arch contains an artery, the visceral-arcli vesBel. these five pairs of visceralarch vessels arise by :i common stem, the tnmcns arteriosus, from the primitive Iieurt.'

The morphological significance of the visceral arcbes and clefts may be a]ipreeiated by a comparison of the conditions obtaining in lower types. While in birds and mammals the

' Fur U11 account of llii: luetaiiiorphusis of the viac!«rnl-Hrc]i vessels inlo the udiilL arlcriee uf llie ihniit mid neck llie render in referred Lu Chapter


number of the lAetia 13 four, in reptiles, amphibiaiis, and bony fishes, Hve clefts appear, and In some fi.shes (selachians) the number is six. In alt aqnatiu verteiirates, the thio epithelial closing membranes nipture, thus establishing communications between t!ie alimentary tract ami the exterior, tlirough which ojienings water passes in and out. The margins of the cleibi — except the first or hyoraandibular cleft — become the scat of a rich supplv i>f capillary blood-vessels, the blood of which obtains oxygen fmm the water and yields to the latter its carbon dioxid; while the visceral arches, excluding the first and second, become known in these classes as brancbial arches from their producing bony arches which support the branchiEe or gilis. With the exceptions noted, the viseenil arches and elerts with their capillary plexusea therefore functionate in these classes as a respiratory ap»,J pa rat us.

When, in the course of evolution, certain of the vert«-i brates assume an aerial existenw, in consequence of whicbl they acquire a breathing mechanism adapted to such a model of life, the respiratory function of the clet^s or branchis^a ceases, and they either disapjiear entirely or constitute merely.! rudimentary structures of the adult. The so-called clefts in f man aie never actual openings, the closing membrane always- 1 being present (His, KoUiker, Piersol, Born), To express the i morphology of the visceral clefts* briefly, they are permanent J structures in flshes and in tailed Amphibia; they are present ' during the larval stage of other Amphibia, while in bird» \ and mammals they are found only in embryonic life.

The growth of the visceral arches and clefts bears an intimate relation to the difTerentiatiou of the head- and the neckr^ions of theembryo. They first make their appearance at about the twenty-third ilay and attain their greatest development by the end of the fourth we^k. Both the arches and the clefts appear earliest and are best developed at the cephalic end of the scries, the fifth arch being exceedingly illdefinecl. During the fifth week the obliteration of the arches and clefts as such begins, since certain of them become metamorphosed into permanent structure;^ wliile the 1 undergo regression.


The Metamorphosis of the Visceral Arches and Clefts. — The first visceral arch becomes (livide<l into an upper i)art, the maxillary arch, and a lower ])ortion, the mandibular or jaw-arch (Fig. 62). The maxillary arches or processes of the two sides unite^ at their anterior ends, with the intervening nasofrontal process (Fig. 67, and in tliis way is formed the upper l)oundary of the mouth-cavity ; the mandibular processes become joined with each other anteriorly and constitute the inferior boundary of this cavity. The maxillary processes become the superior maxillie, while the mandibular [)rocesses l)ecome the lower jaws. The mesodermic core of the mass of tissue constituting the mandibular arch divides into three sections, of which the two situated at the proximal end of the arch are quite small and give rise respectively to the incus and the greater part of the malleus ; the large distal segment is a slender cartilaginous rod, Meckel's cartilage, whose proximal extremity becomes the processus gracilis of the malleus (see Chapter XVIII.).

The second visceral, or anterior hyoid arch becomes obliterated as such, although a bar of cartilage which it contains — Beichert's cartilage — gives rise by its proximal extremity to the stapes,^ while the remaining portion becomes metamorphosed into the styloid process, the stylohyoid ligament, and the lesser cornu of the hyoid bone.

The third or posterior hyoid arch, which corresponds with the first branchial arch of fishes, likewise loses its identity as a surface marking, while the bar of cartilage it contains becomes the body and greater cornu of the hyoid bone.

The fourth and fifth arches coalesce with the adjacent tissues, producing no special structures.

The first outer cleft, known as the hyomandibular cleft, suffers obliteration except at its dorsal extremity, where the tissues forming its margins produce the external ear. The remaining three outer clefts disappear in the following manner : the fourth outer cleft becomes covered and hidden by the fourth arch, and the third and second clefts are successively

  • Reichert, (iegenbaur, Ilertwig ; or to the ring of the stapes according

to Salensky, (jradenigo, and Rabl.


tiuried by the growth of the third and second arches. The sinking-in of the lower arches and clefts (Fig. 61) results in



fc. »•.»». 1-^:1. J", limb*;. iJ.."11«nlolc »li.lk:rA. vil


« on the lateral surface of the ^ft^^i^^B*^«F^ 6*2, »p), ^v)lieh snUeqiienlly



is made to disappear by the coalescence of its edges. Occasionally this sinus, instead of becoming completely obliterate<l, persists, and the thin layer of tissue forming its bottom ruptures — possibly spontaneously or perhaps more probably as the result of exploratory probing — constituting the anomaly known as cervical fistula. Such a fistula establishes an opening into the esophagus.

The first inner cleft or first pharyngeal pouch becomes metamorphosed into the middle ear and the Eustachian tube, the closing membrane, which separates it from the outer cleft, forming the membrana tympani. The second pharyngeal pouches produce no special structures, but the adjacent tissues give rise to the epithelial parts of the middle lobe of the thyroid body and to the posterior third of the tongue, in the manner more fully indicated on pp. 143 and 226. The third inner cleft produces the thymus body, while from the fourth results the lateral lobes of the thyroid bod v.

The configuration of the face, depending as it does so largely upon the development of the boundaries of the nose and of the mouth, is closely associated with the growth of the first pair of visceral arches. The earliest indication of the mouth, the oral pit, appears at about the twelfth day as a shallow depression on the ventral surface of the embryonic body l)etween the fore-brain vesicle and the prominence caused by the primitive heart (Fig. 59, 3 to 5). This depression is deepened by the growth of the tissues surrounding it, as also by the flexure of the head, which occurs at the twentv-first dav. In the third week, therefore, the oral pit is a five-sided fossa, being bounded above by the nasofrontal process, which has grown down from the elevation of the fore-brain, laterally by the maxillary processes, and below by the mandibular arches (Fig. 67, ^1). The pharsmgeal membrane, which consists of opposed ectoderm and entoderm and which separates the primitive oral cavity from the gut-tract (Fig. 66, rA), ruptures at the time of the appearance of the third branchial arch.

By the end of the third week, the communication between the yolk-sac and the gut-tract has become reduced to the relatively small vitelline duct. At the twenty-fifth day the



enihrvo j»n'sc»iits a well-(lov(*l()i>o(l tail. By tli<» termination of the fourth week the volk-sac has attained its maxiinum size, and the presence nf the s<nnite.s is indieateil hy transverse ])ai*:inel lines on the dorsal snrfaee of tlu^ IxkIv.

THE STAGE OF THE FETUS.

This sta^e odinprises tin* time between the beginning of the second inniith and the end of jircirnancy.

Dnrin^ the second month tiic rate of irrowth is far less nipid than in the jm'ccdinj^ stap*. The marked enrvatnre ol' the h»n^ axis (»f tlu* ImmIv jVradnMlly dimi>hes, the embryo assnming a more (;re<a [)osture. Owin^r t-> the partial disaj)|M*aninc(^ of the cervical tU'xure, the iiead l)econie< raised.

I>nrin«^ \\w. fifth week tiie vitelline duct is >een to bo lonjx iind slendei , the umbilical cord lias become longer and nion* snind and mav contain u coil of intestine ; the abdomen is very |)i*ominent, and in the neck-region is a characteristic dorsjii concavity. At tiiis time al>o the nasal pits Ikhmmuc conspicuous as depressions situated on either side of llu* nasofrontal process (Fig. (57j. Th<* nasofrontal jnvx'css ini':in\vldle undergtN'sdiiferentiation int«> the globular processes, which constitute the inner boundaries of the na<al ]»its, and the lateral frontal processes, which limit these tlepre^^ions exter


Vi« W llttmM»m\itytMiftl»ulrix weeks. cnlnrL'id ihr.-.- lii;., . \u.

\»\\\ a\u\ M'\»rAW\W'm fn>m the dcpn'ssicMi- tni- ih. . y..^. Aw twM\ \wu ftw hl\\\ iu coramuuiiration WUw \\\\\\ the



primitive oral cavity. The lacrimal groove is well-markeil at this stage, and tho external auditory meatus is indicated. The mandibles become united mesially at about the thirtyfourth day. The third and fourth gill-clefts have by this time disappeared in the cervical sinus. The paddle-like limbbuds have lengthened and present, at first, a division into two segmcHits, of which the distal is destined to become the hand or foot, while the proximal jwrtion undergoes segmentation a little later into the arm and forearm or thigh and leg ; by the thirty-second day, the hand, now showing differentiation into a thicker proximal and a thinner terminal part, exhibits the first traces of digitiition, in the form of parallel longitudinal markings which soon become grooves and, later, clefts. The develoj)ment of the upper extremities precedes that of the lower by twelve or fourteen days.

During the sixth week the head assumes more nearly its normal [)()sition, and for this reason the apparent length of the fetus is considerably increased, the dorsiil concavity in the neck-region being ahnost obliterated ; the rudiments of the eyelids and of the concha become recognizable, and the various parts of the face assume more definite shape. By the fortieth day the oral cavity has become separated from the nasal pits by the union of the nasofrontal process with the maxillary process(\s, and the external boundaries of the nostrils have become marked out by the meeting of each lat(^ral frontal j)rocess with the corresponding maxillary ])rocess. As a result of these changes, the nose, although still very broad, begins to assume characteristic form. During this week also the fingers are seen as separate outgrowths, while in the seventh week the rudiments of their nails become evident.

Toward the end of the second month — about the fiftieth to the fifty-third day — the toes are just beginning to separate, the protrusion of the intestine at the umbilicus is at its maximum, the palpebral conjunctiva separates from tiie cornea, and the rudimentary tail begins to disiippear.

The eighth week witnesses the total disaj)pea ranee of the free tail, the formation of the septum that divides the cloaca info the rectum and the f^nito-urinarv passage, and the presence of the project iiig genital tubercle with the accompanying genital folds and genital ridges. The external genitals as yet show, no distinction of sex. Fnini the end of the second month to the time of birtli, fetal growth is, in great measure, merely the further develojmient of organs already mapped out; it is held by many authoritifs, therefore, that if mal format ion.s are ever due to maternal impreasions, such impre.-isions could be oiierative only hi the event



of having been i-eceivcil prior to the eighth week of gestation.

Dnring the tJilrd month, the face, although definitely formed, still presents thick lips, a pointed chin, and a rather bniad and triangular nnse. At this time the Wtaha are wellformed and assume a eharaeteristie attitude, and the fiugiTs and toes are provided with imperfect nails. The external genitals, which, until the close of the second month, preserved the indifferent type, now begin to show sexual distinction.

In the fourtli month, a growth of fine hair, the lanngo, appear> npiwi llie sculp anfl some other parts of the body;


the anus ojiens ; the intestine rece^Iea within the abdomen; and the external generative organs present well-marked sexual characteristics.

The fifth month marks the inauguration of active fetal movements and the appearance of a more plentiful growth of colorless hair.

In the sixth month the fetal bo(]y becomes coaled with the Temix caseoaa, a modified sebaceous secretion whose func


tion is the protection of the epidermis fnmi maceration in the amniotic fluid. The eyebrows and eyelashes also appear about this time.

The aeventh month witnesses the appearance of the lanugo, orembryonal down, upon practically the entire surface of the body ; the testes of the male fetus are in the inguinal canal or at the internal abdominal ring; and the nails break through their epiilermal covering. Children born at the end of tlic seventli month niav survive, but usuallv thev do not.

In th(» eighth month the lanugo begins to disappear.

In the ninth month the testicles are found in the scrotum, while, in the ease of Xhv. femah*, the labia majora are in contact with each other. The contents of the intestinal canal, the meconium, consisting of intestinal and hepatic secretions mingled with epidermal cells and hairs swallowed bv the fditus, is now of a dark greenish color. The umbilicus is almost exactlv in the middle of the bo<lv.

The weight of the fetus at full term is fnmi 3 to 3.5 kilograms (from to 7 ]K)unds), the average* weight of the male child being about ten ounces greater than that of the female. While variati(ms from these figures are not uncommon, statements of excessive weight are to be received with reservation, since it has been found, ujxm careful observation by comjH?tent authorities, that the weight of a new-born infant rarely excenls ten poimds. The weight of the chihl, besides dej>ending u|M)n the ])hysical condition of both parents, is influenced bv the age of the mother, young wcmien having the smallest, and women between the ages of thirty and thirtyfive having the heaviest children ; by the nimiber of ])revious pn-gnancies, the weight being gn»aler with each succeeding iirt^nw»ev. pnividt^l the successive children are of the same Ti^.x :.r:d ar\* not lK>rn at t(K» short intervals ; ami also by the veir:.: Ga>>uiT' and height (FrankmhaiiMn) of the mother, Mv •^.*-' >:n:r a dinrt (juc. Min(>t bcH<*V('s that these vui'«i> T.f •iixx^ o|K*rate chiefly by prol(Hi<:iiiL^ or abbreviat-.-• T?r n:'"'» ti .:' 5:*Maiion, and that therefore the variati«»ns T T^^-u iL i^^V. Jirv* rt»fenible to tw«. prineipal causes — /,^..^.., ..^ ,1 '\^* s-^ ^\ birth, and variatit»n> in the rate of

Mggfc M lite li^ns Ht the time of birth ir* about oO

1^ jf ;u •jnbryo or fetus may l»e esti liftir**-*:-^ 'pcvuliar to each >tai:e a- above

.iici. * Hi;. 'iK- nde tbnnulatetl by JIaa-e. Me iiid of the liflh niMiitli, tin


square of the age in months equals the length in centimeters, while after the fifth montli, the length expressed in centimeters equals the age in months multiplied by five. Thus a fetus of four months would have a length of 16 centimeters; while one of six months would be 30 centimeters long. Hence, the age in months is the square root of the number expressing the length in centimeters; or, if the length exceeds 30 centimeters, the age in months is one-fifth of the length expressed in centimeters.

Reference has been made in Chapter I., page 40, to the relation between conception and menstruation, and to the manner of estimating the age of the product of gestation, based upon this relation.