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| =CHAPTER VII. THE FURTHER DEVELOPMENT OF THE EXTERNAL FORM OF THE BODY= | | =CHAPTER VII. THE FURTHER DEVELOPMENT OF THE EXTERNAL FORM OF THE BODY= |
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| While, as a inattor of coiivcniciKjo, tho (]('scri]>tion of | | While, as a inattor of coiivcniciKjo, tho (]('scri]>tion of the individual organs is taken up after tracing the course |
| the individual organs is taken up after tracing the course | |
| of development to this stage, it should he borne in mind | | 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 | | that the rudiments of some of them are already distinguishable before the germ-layers become infolded to form |
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| >Mi> wa^ u»'ii Fi"i*'rtb^il in Chapter II. In ihi> pla«v it will | | >Mi> wa^ u»'ii Fi"i*'rtb^il in Chapter II. In ihi> pla«v it will |
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| '^ ^ttiKi:««iii I' r'vr c.» tht* external ch:inictrr< of the blasto | | '^ ^ttiKi:««iii I' r'vr c.» tht* external ch:inictrr< of the blaston Fisr^. 4!>», whi.*h ri*pre<tMit |
| i«gHK ''«*»a» t> A'CVCcxl in Fisr^. 4!>», whi.*h ri*pre<tMit
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| | | was estimated tu be alitm |
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| 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
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| flattened Bur&ces were t^llloolll, ^vhile the equatorial sons wns
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| !.' ;-. hloiiii-iiLii Liiiij( nil ilif iiiiii-r iMiliir iwrlloii of the ftiMriimlB
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| l'.E. uledliG epiLhulluiii^ Oii., duUdiut reQeia: TV, Inipbobliul; (b, nuKriut
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| 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
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| mEdohlut : L'.Z. liiti!rgliiu<liil*r llMue ur mticcwi. In which enrly dccidunl n'lln are
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| beset with villi
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| istic of the hill
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| Tile I'arlv apiieiir.i
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| It 13 during the early jKirt of the second stage, at about
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| the fourteenth day, that the somatopleuric layer of the lilastodermic vesicle becomes folded in to prodtiee the walls of the
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| erabrvouic body. Fig, 67 allows a human etnbrvo of about
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| the tifteenth day, wbose form ia as yet imnerfectiy differentiate, the ventral wall of tlio bixly Iwing incomplete,
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| tiince the giit-trart Is tttitl in wmimimication with the iimbiliCftl viwioie throughout almost the entire lungth of the embryo.
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| The Iwck and sides of the embn-o arp enveloped by the
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| unnlon, and the dorsal outline is concave. The caudal pole
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| fM, M~HuHiiiii*itibrri>ifrKlii>ul (liu thltlvenlh asy (Rti) Tbemudiit
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| lll>WUll>r|ri>liouiiii*Dtvil trlUi Ihv tilutcidiinnlc! vmicle bj miianBof tlii! alid
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| ku wIIuhMi' lUUi Ui» ainnlan nl randy romplrtcly endtoiri the vtghryn.a
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| tvTtiH iltHlllnHiiiiii^iiminiinlrBli-i throuKhoUl th«it«lvt partofthi: mllnl i
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| U mvn t« \\f winni't'twl by means of the ullantoip sliilk with
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| ninu, which Intter Btnictiire, however, is not
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| th.i
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| iilllv.
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| i'fl>iv.(>nliil ill lliK rijrnre. The concavity of the dorsal ontliiu> in |Ht>nliiii- (.1 the hiiman embryo of this stage. The
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| (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
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| bmln-viwioh'M, tho forc-bmin, the mid-bruin, and the hindbmin. The oral pit, the Brut indication of ibe future mouth,
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| ia )>rvMii]t in the curly part of this stage ; it is a depression the rudiments respectively of the crystalline lens and of the
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| membranous internal ear; at this time also the visceral arches
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| and clefts first become distinguishable. On the twenty-first
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| day, the rudiments of the limbs appear as little bud-like
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| 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
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| by the primitive heart (Fig. 59, 10, 11, and 12).
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| Until the twenty-first day the ombrvonic body is erect.
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| Between the twenty-first and twenty- third days a marked
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| alteration in the appearance of the germ is brought about by
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| a pronounced bending of the long axis of the embryonic
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| body (Fig. 59). The degree of curvature is such that the
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| caudal and cephalic extremities overlap. The flexion reaches
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| its maximum degree by the twenty-third day. The curved
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| dorsal outline is referable to four well-marked flexions, the
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| position of the most anterior, or cephalic flexure, corresponding to that of the future sella turcica and being indicated by
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| the projection of the mid-brain vesicle (Fig. (>2, III.) ; at this
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| point the anterior part of the head is bent almost sufficiently
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| to form a right angle with tho posterior half. A second or
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| cervical flexure is found in the future neck-region, while
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| further caudad are seen the less pronounced dorsal and coccygeal curves.
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| The fourth week marks the period of the most active
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| growth of the embryo. Afler the twenty-third day, the
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| body as a whole uncoils somewhat, although in the latter
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| half of the fourth week the individual flexures noted above
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| become more conspicuous.
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| ==The Visceral Arches and Clefts==
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| The visceral arches, with the intervening visceral clefts, constitute a conspicuous
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| feature of the extemal appearance of the embryo during this
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| stage. These arches are a series of fivx» approximately
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| parallel ridges appearing upon each side of the future
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| neck-region and extending obliquely downward and forward toward the ventral surface of the embryo (Figs. f)0
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| and 62). Tho four furrows lying between the five visceral
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| arches are the visceral clefts. A coronal section of the neck
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| UTa)u||Ulur ui
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| prltntllvu l>sai
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| 111* tu tkutUt: urtlii's (U^i ' "^- ">'■< uaxlltorr and mandlbuUr
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| 'ml anh : o l-a IV, Bnl to Iburth wirlle anhiM : fv. cc. priuil*
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| liial vain*; dC. fliicl "f Cuvlur; al. r. ■trioai and ventricle of
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| 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
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| surface — shows that the furrows seen on the Dctodemiie
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| surface correspond in position to a like number of deei>er
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| grooves on the inner or entodermic surface. The inner
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| furrows are out^ptK^ketiiigs of the entoderm lining the pliaryn^real region cif tiie furc-gut ; they are referred to as the
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| pharyngireal poucheB or throat-pockets to distinguish them from
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| the outer clefts. At the iiottoni of the clefts tiie ectoderm is
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| in contact with the entoilerra, the meaoderra being absent;
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| these two layers constitute the closliig membrane. The visceral arches or ridges consist of tliickened masses of niesodermic tissue covered outwardly and inwardly respectively
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| FlQ.
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| phsrynp'Bl end of gut-lrsot from behind (froi
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| eniljryo ot a.iinm.: B, of 4.ffi mm. (about 2510
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| ceni ftirrawa; I', bIdus privcerTlHlls, compriiing Ilifrd and tuurth oi
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| ;, *. !l, (, rlMoral Bwhos. each witb lis »jB™™i-ureh vesasl : B. lubtrc
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| 7, orinpo of lirj-Qi ; *, pulmonnry evagioatlou.
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| by the ectoderm anil the entoderm. Each arch contains an
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| artery, the visceral-arcli vesBel. these five pairs of visceralarch vessels arise by :i common stem, the tnmcns arteriosus,
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| from the primitive Iieurt.'
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| The morphological significance of the visceral arcbes and
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| clefts may be a]ipreeiated by a comparison of the conditions
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| obtaining in lower types. While in birds and mammals the
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| ' Fur U11 account of llii: luetaiiiorphusis of the viac!«rnl-Hrc]i vessels inlo
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| the udiilL arlcriee uf llie ihniit mid neck llie render in referred Lu Chapter
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| number of the lAetia 13 four, in reptiles, amphibiaiis, and
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| bony fishes, Hve clefts appear, and In some fi.shes (selachians)
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| the number is six. In alt aqnatiu verteiirates, the thio
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| epithelial closing membranes nipture, thus establishing communications between t!ie alimentary tract ami the exterior,
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| tlirough which ojienings water passes in and out. The margins of the cleibi — except the first or hyoraandibular cleft —
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| become the scat of a rich supplv i>f capillary blood-vessels,
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| the blood of which obtains oxygen fmm the water and yields
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| to the latter its carbon dioxid; while the visceral arches,
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| excluding the first and second, become known in these classes
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| as brancbial arches from their producing bony arches which
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| support the branchiEe or gilis. With the exceptions noted,
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| the viseenil arches and elerts with their capillary plexusea
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| therefore functionate in these classes as a respiratory ap»,J
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| pa rat us.
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| When, in the course of evolution, certain of the vert«-i
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| brates assume an aerial existenw, in consequence of whicbl
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| they acquire a breathing mechanism adapted to such a model
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| of life, the respiratory function of the clet^s or branchis^a
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| ceases, and they either disapjiear entirely or constitute merely.!
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| rudimentary structures of the adult. The so-called clefts in f
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| man aie never actual openings, the closing membrane always- 1
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| being present (His, KoUiker, Piersol, Born), To express the i
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| morphology of the visceral clefts* briefly, they are permanent J
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| structures in flshes and in tailed Amphibia; they are present '
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| during the larval stage of other Amphibia, while in bird» \
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| and mammals they are found only in embryonic life.
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| 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
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| about the twenty-third ilay and attain their greatest development by the end of the fourth we^k. Both the arches and
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| 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
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| and clefts as such begins, since certain of them become metamorphosed into permanent structure;^ wliile the 1
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| undergo regression.
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| The Metamorphosis of the Visceral Arches and Clefts. —
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| The first visceral arch becomes (livide<l into an upper i)art,
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| the maxillary arch, and a lower ])ortion, the mandibular or
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| jaw-arch (Fig. 62). The maxillary arches or processes of
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| the two sides unite^ at their anterior ends, with the intervening nasofrontal process (Fig. 67, and in tliis way is formed
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| the upper l)oundary of the mouth-cavity ; the mandibular
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| processes become joined with each other anteriorly and constitute the inferior boundary of this cavity. The maxillary
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| processes become the superior maxillie, while the mandibular
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| [)rocesses l)ecome the lower jaws. The mesodermic core of
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| the mass of tissue constituting the mandibular arch divides
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| into three sections, of which the two situated at the proximal
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| end of the arch are quite small and give rise respectively to
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| the incus and the greater part of the malleus ; the large distal
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| segment is a slender cartilaginous rod, Meckel's cartilage,
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| whose proximal extremity becomes the processus gracilis of
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| the malleus (see Chapter XVIII.).
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| The second visceral, or anterior hyoid arch becomes obliterated as such, although a bar of cartilage which it contains —
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| Beichert's cartilage — gives rise by its proximal extremity to
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| the stapes,^ while the remaining portion becomes metamorphosed into the styloid process, the stylohyoid ligament, and
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| the lesser cornu of the hyoid bone.
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| The third or posterior hyoid arch, which corresponds with
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| the first branchial arch of fishes, likewise loses its identity
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| as a surface marking, while the bar of cartilage it contains
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| becomes the body and greater cornu of the hyoid bone.
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| The fourth and fifth arches coalesce with the adjacent tissues, producing no special structures.
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| The first outer cleft, known as the hyomandibular cleft, suffers obliteration except at its dorsal extremity, where the
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| tissues forming its margins produce the external ear. The
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| remaining three outer clefts disappear in the following manner : the fourth outer cleft becomes covered and hidden by the
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| fourth arch, and the third and second clefts are successively
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| * Reichert, (iegenbaur, Ilertwig ; or to the ring of the stapes according
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| to Salensky, (jradenigo, and Rabl.
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| tiuried by the growth of the third and second arches. The
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| sinking-in of the lower arches and clefts (Fig. 61) results in
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| fc. »•.»». 1-^:1. J", limb*;. iJ.."11«nlolc »li.lk:rA. vil
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| « on the lateral surface of the
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| ^ft^^i^^B*^«F^ 6*2, »p), ^v)lieh snUeqiienlly
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| 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
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| ruptures — possibly spontaneously or perhaps more probably
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| as the result of exploratory probing — constituting the anomaly known as cervical fistula. Such a fistula establishes an
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| opening into the esophagus.
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| The first inner cleft or first pharyngeal pouch becomes metamorphosed into the middle ear and the Eustachian tube, the
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| closing membrane, which separates it from the outer cleft,
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| forming the membrana tympani. The second pharyngeal
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| pouches produce no special structures, but the adjacent tissues
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| give rise to the epithelial parts of the middle lobe of the thyroid body and to the posterior third of the tongue, in the
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| manner more fully indicated on pp. 143 and 226. The third
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| inner cleft produces the thymus body, while from the fourth
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| results the lateral lobes of the thyroid bod v.
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| The configuration of the face, depending as it does so largely
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| upon the development of the boundaries of the nose and of
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| the mouth, is closely associated with the growth of the first
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| pair of visceral arches. The earliest indication of the mouth,
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| 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
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| primitive heart (Fig. 59, 3 to 5). This depression is deepened
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| by the growth of the tissues surrounding it, as also by the
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| flexure of the head, which occurs at the twentv-first dav. In
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| the third week, therefore, the oral pit is a five-sided fossa,
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| being bounded above by the nasofrontal process, which has
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| grown down from the elevation of the fore-brain, laterally by
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| the maxillary processes, and below by the mandibular arches
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| (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
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| time of the appearance of the third branchial arch.
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| By the end of the third week, the communication between
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| the yolk-sac and the gut-tract has become reduced to the
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| relatively small vitelline duct. At the twenty-fifth day the
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| enihrvo j»n'sc»iits a well-(lov(*l()i>o(l tail. By tli<» termination
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| of the fourth week the volk-sac has attained its maxiinum
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| size, and the presence nf the s<nnite.s is indieateil hy transverse ])ai*:inel lines on the dorsal snrfaee of tlu^ IxkIv.
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| THE STAGE OF THE FETUS.
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| This sta^e odinprises tin* time between the beginning of
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| the second inniith and the end of jircirnancy.
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| Dnrin^ the second month tiic rate of irrowth is far less
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| nipid than in the jm'ccdinj^ stap*. The marked enrvatnre
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| ol' the h»n^ axis (»f tlu* ImmIv jVradnMlly dimi>hes, the embryo
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| assnming a more (;re<a [)osture. Owin^r t-> the partial disaj)|M*aninc(^ of the cervical tU'xure, the iiead l)econie< raised.
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| I>nrin«^ \\w. fifth week tiie vitelline duct is >een to bo
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| lonjx iind slendei , the umbilical cord lias become longer and
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| 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
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| Ikhmmuc conspicuous as depressions situated on either side of
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| llu* nasofrontal process (Fig. (57j. Th<* nasofrontal jnvx'css
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| ini':in\vldle undergtN'sdiiferentiation int«> the globular processes,
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| which constitute the inner boundaries of the na<al ]»its, and the
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| lateral frontal processes, which limit these tlepre^^ions exter
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| Vi« W llttmM»m\itytMiftl»ulrix weeks. cnlnrL'id ihr.-.- lii;., . \u.
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| \»\\\ a\u\ M'\»rAW\W'm fn>m the dcpn'ssicMi- tni- ih. . y..^.
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| Aw twM\ \wu ftw hl\\\ iu coramuuiiration WUw \\\\\\ the
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| primitive oral cavity. The lacrimal groove is well-markeil
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| at this stage, and tho external auditory meatus is indicated.
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| The mandibles become united mesially at about the thirtyfourth day. The third and fourth gill-clefts have by this
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| time disappeared in the cervical sinus. The paddle-like limbbuds have lengthened and present, at first, a division into two
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| segmcHits, of which the distal is destined to become the hand
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| or foot, while the proximal jwrtion undergoes segmentation
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| a little later into the arm and forearm or thigh and leg ; by
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| the thirty-second day, the hand, now showing differentiation
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| into a thicker proximal and a thinner terminal part, exhibits
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| the first traces of digitiition, in the form of parallel longitudinal markings which soon become grooves and, later,
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| clefts. The develoj)ment of the upper extremities precedes
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| that of the lower by twelve or fourteen days.
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| During the sixth week the head assumes more nearly its
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| normal [)()sition, and for this reason the apparent length of
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| the fetus is considerably increased, the dorsiil concavity in
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| the neck-region being ahnost obliterated ; the rudiments of
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| the eyelids and of the concha become recognizable, and the
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| various parts of the face assume more definite shape. By
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| the fortieth day the oral cavity has become separated from
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| the nasal pits by the union of the nasofrontal process with
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| the maxillary process(\s, and the external boundaries of the
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| nostrils have become marked out by the meeting of each
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| lat(^ral frontal j)rocess with the corresponding maxillary
| |
| ])rocess. As a result of these changes, the nose, although
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| still very broad, begins to assume characteristic form.
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| During this week also the fingers are seen as separate outgrowths, while in the seventh week the rudiments of their
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| nails become evident.
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| Toward the end of the second month — about the fiftieth
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| 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.
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| 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
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| the presence of the project iiig genital tubercle with the accompanying genital folds and genital ridges. The external
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| genitals as yet show, no distinction of sex. Fnini the end
| |
| of the second month to the time of birtli, fetal growth is, in
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| 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
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| of having been i-eceivcil prior to the eighth week of gestation.
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| Dnring the tJilrd month, the face, although definitely
| |
| formed, still presents thick lips, a pointed chin, and a rather
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| 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
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| genitals, which, until the close of the second month, preserved the indifferent type, now begin to show sexual
| |
| distinction.
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| In the fourtli month, a growth of fine hair, the lanngo,
| |
| appear> npiwi llie sculp anfl some other parts of the body;
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| the anus ojiens ; the intestine rece^Iea within the abdomen;
| |
| and the external generative organs present well-marked
| |
| sexual characteristics.
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| The fifth month marks the inauguration of active fetal
| |
| movements and the appearance of a more plentiful growth
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| of colorless hair.
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| In the sixth month the fetal bo(]y becomes coaled with
| |
| the Temix caseoaa, a modified sebaceous secretion whose func
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| | |
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| tion is the protection of the epidermis fnmi maceration in the
| |
| amniotic fluid. The eyebrows and eyelashes also appear
| |
| about this time.
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| 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.
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| 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
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| contact with each other. The contents of the intestinal canal,
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| the meconium, consisting of intestinal and hepatic secretions
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| mingled with epidermal cells and hairs swallowed bv the
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| fditus, is now of a dark greenish color. The umbilicus is
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| almost exactlv in the middle of the bo<lv.
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| | |
| 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
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| child being about ten ounces greater than that of the female.
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| 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
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| 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
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| smallest, and women between the ages of thirty and thirtyfive having the heaviest children ; by the nimiber of ])revious
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| pn-gnancies, the weight being gn»aler with each succeeding
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| iirt^nw»ev. pnividt^l the successive children are of the same
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| Ti^.x :.r:d ar\* not lK>rn at t(K» short intervals ; ami also by the
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| veir:.: Ga>>uiT' and height (FrankmhaiiMn) of the mother,
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| Mv •^.*-' >:n:r a dinrt (juc. Min(>t bcH<*V('s that these
| |
| vu''i'«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 —
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| /,^..^.., ..^ ,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
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| 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
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| manner of estimating the age of the product of gestation,
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| based upon this relation.
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