Book - A Text-book of Embryology 18

From Embryology


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

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Although the skeleton is the framework of the body in the aiiatoiuieal or mechanical sense, it is not so embryologically, since its development is not begun, at least not to any important extent, until nearly all the principal organs are well differentiated, and its growth is largely subsidiary to that of the structures which, in the mature state, it supports and protects. Morphologists speak of the exoskeleton and the endoskeleton, the former having reference to the hard structures found superficial to the soft parts, for whose protection they serve, such as the canipace of the lobster, and the hartl scales of certain fishes ; while the latter term si<rnifies the cartilairinous or bony structures found within the bodies of most vertebrate animals. Kven in the highest vertebrates, certain bones, such as those of the vault of the cranium, are usually cousi(l(»r(»d by morphologists as being thi' representatives of part of the cx<>skeleton of lower types.

The skeleton, using the W(>rd in its ordinary sense, consists of the axial skeleton and the appendicular skeleton, or skeleton of the limbs, '^fhe former, inclu<liug the head and the trunk, is connnon to all vert(»brates ; the latter is not found in the lowest members of tliis class and hence is to \ye regarded as a later ac(|uisition in the evolution of the skeleton.

In studying the development of t]w skeleton, as in considering thatof otht'r systems an<l organs, cleaner conceptions of the y:rowth of the individual mav be obtained bv comparing it witli the evolution of the ty[)e. For example, the simplest form of skeletal apparatus is that of the amphioxns.

In this animal the only representative of the skeleton is the notochord, a cyliiulrical rod composed of cellular or gelatinous tissue in which neither cliondrification nor ossification ever takes place. Such an animal furnishes an example of the notochordal stage of the skeleton. the surrounding of the chorda with a sheath of embryonal connective tissue, by which it is strengthened and thereby better fitted to serve as the body-axis, furnishes the membranous type of skeleton, a stage a little farther advanced than the preceding. The next higher type of skeleton is the cartilaginous form. In tliiscase the eml)ryonal (ronnective tissue has undergone transformation into cartilage, at which p%int development is arrested, the stage of ossification never being attained. The cartilaginous type of skeleton is illustrated by that of the selachian (sharks and dog-fish).

The third and highest type of skeleton is the osseous. This results from the replacement of the cartilaginous tissue l)y bone. The process of ossification does not, however, affect every part of the cartilaginous skeleton, there being some portions of the latter which remain permanently unossified. As there are, throughout the vertebrate series of animals, various gra(hitions in the degree of differentiation of the skeleton, so in the course of development does the osseous system of every higher vertebrate pass througli these stages from the simplest condition, that of the notochordal skeleton, to the highest form of the almost completely ossified skeletal apparatus.

The Axial Skeleton

The axial skeleton, as stated above, includes the bones of the trunk and those of the head. Logically the development of the former will first claim attention.

The Development of the Trunk. The Stage of the Chorda.— The formation of the (jhorda dorsalis or notochord is the earliest indication of the axis of the embryonic body and it will be recalled that it is also one of the earliest embryological processes. The mode of development of the chorda from the entodermal epithelium has been described at p. 73. The chorda serves the purpose, as it were, of an axis about which the permanent vertebral column and a part of the skull are, at a much later date, built up. The anterior or headward termination of the chorda corresponds to the position of the later hypophyais, or pituitary body, and thus the chorda is coextensive, not only with the vertebral column, but also with a portion of the cranium. The cells of the chorda enlarge and become distended with fluid, the protoplasm of each cell being reduced to a thin layer. The peripheral cells, however, constituting a distinct layer, the chordal epithelinm, remain small, and it is by their proliferation that the •horda increases in size. In the amphioxus the chorda is the only " skeleton *' that is ever acquired, and in this animal it is a permanent structure. In all other vertebrates it becomes surrounded by embryonal connective tissue, mesenchyme, which latter undergoes chondrification, and in the higher types ossification also. While in some of the lower vertebrates, as in certain classes of fishes, the chonla persists as a structure of more or less importance, in the higher members of the series, birds and mammals, it retrogrades as the processes of chondrification and ossification go on, until finally it is represented only by the pulpy centers of the intervertebral disks.

The Membranous Stage. — The notochordal stage of the development of the vertebral column is succeeded by the menihninous staire. The transformation is effected bv the appearance of an ensheathing mass composed of embryonal connective-tissue cells which surround not onlv the chorda but also the neural canal or fundament of the nervous svstem (Fig. 17-1, nh). The source of this embryonal connective tissue or mesenchyme bears an imi)ortant relation to the primitive segments. As the develo[)nient of the primitive segments was described in the last chapter, and also in Chapter IV., it will suffice to remind the reader that each primitive segment undergo(»s differentiation into the myotome or muscleplate, the cutis-plate, the nephrotome, and the sclerotome (Fig. 174), the sclerotouK? occupying the mesial surface of the segment and lying in close proximity to the chorda.

While the myotome originates from the flattened or mesothelial cells of the primitive segment, the sclerotome is made up of cells of the type characteristic of young-growing connective tissue — that is, of the mesenchymal part of the primitive segments as distinguished from their mesothelium. Owing to the rapid multiplication of its cells, each sclerotome spreads out headward and caudalward, and dorsad and ventrad, surrounding both the chorda and the neural canal, until both these structures become enclosed in a common, continuous sheath of embryonal connective tissue. That part of this tissue which surrounds the chorda is often designated the skeletogenous sheath of the chorda and also the membranous primordial vertebral column. The cells of the sclerotomes not only surround the chorda and the neural canal, but they also spread out laterally into the intervals between the muscle-segments to constitute the ligamenta intermuscnlaria or the bands or strips of connective tissue which separate adjacent muscle segments from each other (Fig. 1 75). It is worthy of note that while this skeletogenous sheath of the chorda originates from segmented structures, the somites or primitive segments, and is to that extent related to the segmentation of the body, it now presents no trace of segmentation.

Fig. 175.— Frontal projection fVom a series of sections through a cow embryo of 8.8 mm.(0.35 in.). (From Bonnet, after Froriep.)

Very soon, however, this ensheathing membranous tissue exhibits areas of condensation alternating regularly with less dense areas. Eaeh such condensed area has the form of u somewhat obli(|uely i)lace<l bow or half-arch. This halfareli of condensed mesenchymal tissue is called the primitive vertebral bow by Froriep (^Figs. 175 and 176), whose investigations upon chick and cow embryos established most of the facts known concerning the development of the vertebra?.^ The median portion of the lx>w is on the ventral side of the cliorda and is known as the hypochordal brace. The lateral extremities of the bow abut against the eorresj>onding myotomes, eadi extremity becoming bifurcated. The dorsal limb of the bifurcation, the neural process, extends gradually over the dorsal surface of the primitive spinal cord, forming the neural arch; while the ventral limb advances ventrad, foreshadowing the hemal arch or costal process of the vertebra, or, as regards the thoracic region of the body, the future rib.' Both dorsal and ventral processes grow into the intervals between iidjaoeiit myotomes and hence are intersegmental, that i>, they, n> well as the vertebral \m)\\ from which they >j>rintr, (•orre>pon(l to the intervals between the primitive segments of the body. Sub>e(|iieiitly these j)roeesses of the bow uive rise to the variou«- processes of the ecmipleted vertebra. The median part ol' the bow, the hypochordal brace, subseqncMitly becomes cartilaginous and assists in forniiii<r th(» bodv of the vertebra in l)inl>, but in mammals it reuiaius unchondrified and becomes an inconspicuous and transitory part of the intervertebral ligament — the future intervertebral disk — exce]>t in the case of the first cervical vertebra, the atlas, the ventral arch of which it furnishes. The nieinbran(nis anlage of x\w cartilaginous body of the vertebra is fcnind in a special condensation of the mesen ' M<»re rcH'ently the ]>r(>cess lias K'cii stiKlicd m tlu* human embryo by IJiinh-eii. Afurrn'tin Jourunl nf Annfonv/. vol iv , No. 'J, \W\.

' Mnr]»h(>l()irir:illy, each vertehni is po^n^ssod of a ii'iirnl nrrh^ f(f>r tlio protection of the spnial cord ; and a bnnni nrrh^ for the protection of the ortnms of emulation, re<y)i I'll turn, and diirc^tion, the ril)> of man and the higher Vi'rtebnites U'lny the ])er»^istent hemal arches in tfie rei^ion of the tfiorax.

eliymatous slieath of tlio i-lionla on tin; caudal aide of the liyi>ocliortlal brace. The intarrertebral ligament is develowd fnmi the (erichonlal tissue on the dorsal side of the liyi>ocliordal bniee. Banleen's piimilivc rlluk iiiultides this aniiigc nf the iiitorvtTtebnil ligament phis the hyiwK'lionial brace of Froriep, which latter Bardceii regards as a transitory thicken of e n r 1 gi. Ilk

The Cartilaginous Stage.— This stage of the devcloi>niciit (if titc spine is bi-onght abont by the metamorphosis of IKirts of the niembrannus vertebral column into the cartilaKinons vertebra. Other and alternating \arts of the same strncttiro furnish the Intervertebral disks and the ligaments that hind together the individual elements of the spine. 'J'he hisliilogic:il changes neeessiry to effect the transformation of the embryonal connective tissue into cartilage are, briefly, the moving ajwrt of the cells and the modification of both the cells and the intercellular substance, the latter acquiring the characteristic qualities of the matrix of cartilage.

For each vertebral bodr there are two centers of chontlrifieation, one on each ^ide of the chorda within the mass of tissue referred to above (Fig, 176), The formation of cartilage l>egins in the second month. The two centers are soon connected with each other by a third, which lies on the ventral side of the chorda, the three forming now a cartilaginons half

crliuder wliicli js later coniplete<l by the development of c tilage on tLe dorsal side of the chorda (Fig. 177). Accord- ' ing to Bardeen, the tartilage of llie body grows at the exjwhse of the primitive disk anterior to (above) it. At the time when the chorda is completely encased in cartilage the Bi>inal cord is still ensheathed by merely membranous tig Before the end of the second month the neural uches of the J vertebrte are indicated by small isolated niassi's of cartila) which develop in the connective tissue BUrrounding the spinal I cord, the lateral parts of the membranous vertebral bows. 1

Parachordal cartilage

In the eighth week these fuse with the bodies and appear I llieri as projections from them. By the end of the third I month the processes, or neural arches, have grown snfficiently ] to" meet with their fellows on the dorsal side of the spinal J conl, and in the fourth month the corresponding arches of , the two sides become niii ted, thuscompletingthecartilariiuma I sbeath of tbe cord.

The masses nf connective tissue occupying the intervalsj

between the vertebral bodies, originating, as stated above, in condensations of the mesenchymal sheath of the chorda on the dorsal aspect of the hypochordal braces, become the intervertebral ligaments (Bardeen's primitive disks) upon their fusion, in mammals, with the hypochordal braces. Subsequently they become the intervertebral disks. The tissue between the cartilaginous arches becomes differentiated into the ligamenta subflava.

While the unsegmented skeletogenous sheath of the chorda is gradually differentiating into the separate elements of the cartilaginous vertebral column, the chorda itself begins to retrograde. Within the bodies of the vertebrae its development is completely arrested, while those portions of it contained within the intervertebral disks continue to grow. The chorda at this stage consequently shows alternating enlargements and constrictions. In certain fishes it persists as a structure of more or less importance. In vertebrates above cartilaginous fishes, all traces of the parts of the chorda within the vertebral bodies are lost as soon as ossification occurs, while in the intervertebral disks parts of it jxjrsist as the soft pulpy cores of the latter.

Thus the cartilaginous vertebral bodies or centra originate in masses of mesenchyme situated between the primitive vertebral bows and are, according to Froriep, segmental, that is, they correspond in position with the muscle-segments, each centrum being developed within the limits of a single segment ; while the processes develop from the lateral parts of the vertebral bow and later unite with the body. Bardeen, on the other hand, refers the origin of each vertebral body to two segments, since, according to his observations, the body grows at the expense of the next anterior primitive disk.

The cartilaginous trunk is completed by the chondrifieation of the ligamenta intermuscularia to form the cartilaginous thorax.

The Osseous Stag^e. — The process of ossification begins in certain parts of the trunk at the end of the second month.

W\\\vv \\w work (»f olioiulrifiration is entirely completed. As i)ii« liiisioloirical details of Ixuio-foriiintion are to be found in I 111' h'\l-lMM)ks of lii>toloirv, it will not 1)C necessary to enter into till' suhjcct lu»ix'. The place's in any individual cartilage wlu-rr tissificMtion houiiis aix» called the cHintcrs of ossification. I'lh' proiM'ss is oni' of siii)stitutk>n, the cartilage becoming broki'ii down an<l absorlunl as the fonnati«)n of bone goes on. riit' oBHification of each vertebra is l)ogun at three conliT-n, onr in the bodv and one in cjieh arch. The centers l»»r the arches appear in the x'venth week. The centers ti>r ilic bodies a)){M'ar a little later and are found first in the d»n-;d MTtehiie, ap|)«'arinLr sueeessively later in the vei"t<»bi.i- I'iiiihrr lip and farther down. The ossified arches unite wtili liu'ji other diiriiiii: the lirst vear of life, but their nnicin Willi ihr ImmIv of the vertebra takes place betwtHjn the thinl .Old rii.ditli vciirs. At a much later periinl five accessory canU-.ia ul' H-.-i Ural ion are added to each vertebra. Two of these til I.. Oil to the body an<l jrive rise to two annular ]>lates of i.i.iu . I In- epiphyses, one for tlu' upper or cephalic surface and .•III lix ihr opposite t)r eautlal snrt:u*e. The remaining three I . (Oi I ■ briMiin rr>p<M*tively t<) the spinous process and the two ii.iii.»viii:ii» pi'orusses. The i*piphyses do not acrpi ire osseous Mil. Ill wiih ilu' vrrtrbra projuT until al)out the twenty-fifth

hi I transverse process of a e<'rvieal vertebra, enI III. I i.'i.oniii, and <'onsistini: of an aiiteriorand a j>osterior 
• iimrr than th«' tran>vcr>r process ])roper, since 

-iiir:i I port i(>iM> t hi' rudiment of a cervical rib. I '.II 1.1 ihi (Mitr (it' the fii>ion ot' this nidimentarv rib with I J, I. Ill ^.1 . pitMi-s, the vertebral artery, which passes i , . ,1 (lu III i' -iMTtimidctl by tlu» two processes, and thus .1, . h.ii I \ ii .d luiii-vcrM* processes di tier from those of ,1, I t, III III. f III the possession of a foramen.' I I. aUi- III. I ill*' rtxlH, bein^ strikintrly modified eervic^al

I I 111. 1 .1.1.1. li '.uiir iiiitlii>rili«'s. :iN Miiiot, that tht* >k)ium]()08 I ti.. ..«., I » . tiul iliiii thi' arti'iy j^rows through the ossifying vertebrae, require special mention. The atlas contains less and the axis more than an ordinary vertebra, since that which corresponds to the body of the atlas never unites with it but fuses witli the body of the axis to constitute its odontoid process.

The atlas presents two centers of ossification for its neural arches — the so-called posterior arch — just as other vertebra do. Unlike other vertebrae, these centers do not unite with the body but become joined to each other on the ventral side of the position of the cliorda by a piece of cartilage which results from the chondrification of the hypochordal brace, referred to on page 376. This forms the cartilaginous ventral or anterior arch of the atlas, which, in the first year of life, develops a center of ossification. The arch acquires bony union with the lateral parts between the fifth and sixth years.

The axis or epistropheus develops from the usual centers of ossification and from an additional one for its odontoid process. Bony union of the odontoid process with the proper bodv of the axis occurs in the seventh year. The odontoid process, in common with every other vertebral body, is traversed in the cartilaginous stage by the notochord.

The transverse processes of the lumbar vertebrse, like those in the cervical region, include not only the transverse process proi)er but also the rudiment of a rib.

The sacral vertebra each present the usual ossific centers. Inasmuch as they become articulated firmly with the pelvic bones and undergo fusion to form a single adult bone, the sacrum, their form is much modified during the course of development. The transverse processes of each side coalesce to form the lateral mass of the sacrum. Each transverse process consists, as in the cervical and the lumbar vertebrae, of the transverse process proper and a rudimentary rib, the center of ossification for the latter being quite distinct during early stages of development. The intervertebral disks of the sacral vertebrae begin to ossify in the eighteenth year, the process being completed in the twenty-fifth year.

Tlio coccygeal vertebrse are quite rudimentary. Each one is ossified from a single piece of cartilage, and usually from hut a single (H'uter of ossification. Occasionally the first piece of tli<.' coccyx <levelops from two ossific centers, the pnx?ess l)eginning at hiiili. Ossification begins in the second vert<*l)ra lu'twcen the lifth and the tenth years ; in the third, shortly l)elon' puberty ; in the fourth, scnm after puberty. The lower three pieces fuse into one before middle life, and this unites with the first, and the latter with the sacrum, at variable periods thereafter.

The Development of the Ribs and Sternum

Reference has been made in the preceding pages to the liganieiita iulcriiuiscularia as strips or l)ands of embryonal conn<'('live tissue lying between adjacent muscle segments, which have ()rigiii;i(<'d, in conuunn with the sheath of the chorda, iVoiii tlx' c(>lls of the sclerotomes. TIh^ ligamenta intermnscularia become invaded by the costal j)roeesses of the primitiv<' v<'rt<'bral bows, th(^ costal process, which is the ventral division ol* the tip of tlw i)ow, growing ventnul and j)enetratiiig th(^ substance of the ligament to constitute a curved rod of connective tissue, the forerunner of the future rib. Thus (lu'rc are form<'d coiniective-tissne rej)resentatives of the ril)s, ench of which is enibedded in the looser connective tissue nl' th<' corresponding intei'nniscular ligiunent. It is b\ the development of* cartilage within these curved rods of conden-ed moenchvme, the membranous ribs, that the cartiliiginous ribs are jn'odnced. 'Vhv pi*ocess of chondrification commences in the >econd month, but does not involve the proximal <'n(U of the ribs, the tissue heie becoming ligamentous and servinir to bind to<reth<'r the ribs and the vertebi.e. Ivibs are formed throughout the entire extent of the Vertebral <M>lumn, except in the coccygeal region, but while in tlu' lower vertebrates the entire series goes on to mature de\.lt»|>meut, in mammals, including man, their growth is arrc-ted in the cervical, hunbar, and sacral regions. In the case ot' man and mammals only the thoracic ribs persist and iM'conh' adult structures.

As the distal (ventral) extremities of the ribs advance toward the ventral median line, the tips of the first five, six, or seven each exhibit an enlargement. These broadened ends soon coalesce, thus forming on either side of the median line a continuous strip of cartilage, the anlages of the sternum. The other ribs remain free at their ends. The sternum is therefore produced from two lateral halves, a circumstance that explains some of its anomalies, as for example, cleft sternum, which is a condition due to arrested development or deficiency of union.

The ossification of the ribs begins in the second month of fetal life and from a single center for each. The process does not involve the entire rib, a portion near the distal extremity remaining cartilaginous and becoming the adult costal cartilage. Accessory centers of ossification for the head and tubercle appear between the eighth and fourteenth years of life.

The ossification of the sternum proceeds from numerous centers. There is one for the manubrium and from six to tw^elve for the gladiolus. The ensiform acquires a center of ossification in the early years of life, but for the most part remains cartilaginous.

Although, as stated above, the ribs of adult human anatomy are limited to the thoracic region, their rudimentary representatives are found throughout the other regions of the vertebral column. In the cervical, lumbar, and vsacral regions each rudimentary rib becomes blended with the transverse proceas of the corresponding vertebra to form the transverse process of human anatomy. It is from the persistence of the seventh rudimentary cervical rib and its failure to fuse with the corresponding transverse process that the anomaly of a free cervipal rib results ; while the presence of a thirteenth or lumbar rib, as occasionally met with, is due to the unusual development of the first lumbar rudimentary rib.

The Development of the Head Skeleton

Just as the wkck'tdii of tlif trunk consists of a doraally sihiatcd bony casir tor tlit- protection of the spinal con! aud a wrics of vcntrul nr licnisil arclics (or the protection of the orgiins of circiilutioii iiiiil respiration; so does the head skeleton comprise n Iiony eiise for the accommodation of the lirain with smaller ac(.^;ssory osseous coni[>artment3 for the orgiuis of special sense, as the orbits and the nasal chamlwrs; and also a ventrally situated ai>i>aratus which constitutex both a receptacle for the oral anil the pbarii'ngeal jxirts of the digestive system and a nicebanlsm for the mastication of


rounding' th<> h<'adsimilar l.> that i.l" ■ till' ventral parts, -.v stni<'tTircs,<-oiistiliit from the nie.-ioilcnn

iirt. the cranial capsule, or brain-case, is exli'iit IVnm the c-i.iinective tis-^iic sai-id of tlic cliorila. its .>riL'in thus being r spiml chmm. On the other hand, he jaws and the liyojd bone and related ^' till- <(>-ealle<l viBceral skeleton, develop tissue of the visceial aicbes. As in the of ih.' trnrik skeleton, the eraniimi is first outlined in branous tis^n<' re>nliiii,n I'rom the dilliTcnliatioii of the embryonal connective tissue which ensheaths the head-enJ of the chorda, and also of the connective tissue of the visceral arches, this differentiation producing the membranotis primordial cranium. The metamorphosis of the membranous cranium into cartilage brings about the cartilaginous stage of the cranium, while the replacement of the cartilage by bone is the final step in the process.

Bones that develop from centers of ossification in previously formed masses of cartilage are styled primordial bones, while those that are pixxiuced independently of cartilage,, either in the skin covering the membranous cranium, or in the mucous membrane lining indentations in its walls, arc known as coveiing or dermal bones. The development of bone is therefore said to be either endochondral or membranous. For the most part, the bones of the base of the skull are of endochondral formation, while those of the vault are develoj)ed in membrane. The membranous or dermal bones are similar in point of origin to the exoskeleton — placoid and ganoid scales — of certain fishes.

The Membranous Cranium. — The membranous braincase is differentiated from the mesenchymal tissue which ensheaths the anterior or head-end of the chorda. As previously stated, the anterior end of the chonla is at a point ventrad to the mid-brain vesicle, in the angle formed by the latter with the fore-brain, at a position corresponding with that of the i)ituitary body (Fig. 178). The skeletogenous sheath of the chorda, in this situation as elsewhere, results from the multiplication of the cells of the sclerotomes, since this region of the body undergoes segmentation in common with the trunk. The number of bead-segments is uncertain. According to recent investigations upon shark embryos, there are at least nine primitive segments formed in the headregion.

The skeletogenous sheath of the chorda spreads out dorsad to cover the brain- vesicles. From the terminal point of the chorda, beneath the inter-brain, the sheath advances anteriorly to invest the fore-brain, which latter at this stage is bent over vcntrcul. From the part investing the fore-brain, a protiibcnint mass, the nasofrontal process, extends toward the j)riinitivo inoutli-cavity, constituting the anterior or upper h()iin<lary of the hitter. Meanwhile the mesencbsrmatic tissue of the visceral arches — that is, that part of the mesodcrnii(^ tissue of these structures wliich does not form muscular tissue — is un<lergoing similar transformation into menibninous tissue. The first visceral arch divides into an anterior or upp(T part, the maxillary process, and a posterior or hnvvv mass, the mandibular arch, these being the membninous jaw arches. The four jaw arches, with the nasofrontal process, form the boundaries of the primitive mouthcavity, the mandibular arches of the two sides having united in the median line to form its lower border, and the maxillary arches having fused with the lateral nasal and the nasofrontal j)n>cesses to c( institute its upper boundar}'.

"^rhe membranous primordial cranium, then, consists of a cnniplete connective-tissue investment for the brain-vesicles, of tlui nieiiibi-anous jaw arches, and of the hyoid and the branchial a relics, and presents in its walls the indications of the cavities for speeial-sens<» organs in the shape of the surface iuvauinations which constitute rtv-^pectivcly the otic vesicle, the Icns-vesieh', and the nasid pits.

The Cartilag^inous Cranium. — By the further differentiation of the memi)ranous cranium the cartilaginous stage is attainecl. The development of cartilage begins in the second month. \Vhih» thc^ membmnous cranium furnishes a coni]>let(* <*a])snle for the brain, tJK* cartilaginous brain-case is deficient, sinc(^ the process of ehondrification <loes not affect the i-egions of the future parietal and frontal bones. This is true at lea>t <>f man and the high<*r vertebrates. In those case^ where the >i<elet«)n rcMuains })ermanently cartilaginous, as in selachians (sliarks, <log-tish, ct<\), the entire brain-case partici])ates in the chondritying ])rocess. As the skull ext<'n(ls verv nnich farther forward than the end of the chorda — wliich latter terminat<*s at the j)osition of the future sella turcica — the regions of the |)rimitive skidl are designated respectively chordai and prechordal (Kolliker), or vertebral and everid>ral (Gegenbauer), according as they fall behind or in front of the end of the chorda.

The formation of cartilage begins in the region corresponding to the base of the future skull. On each side of the end of the chorda a mass or bar of cartilage is formed, extending forwand and backward, this pair of parallel bars being designated the parachordal GartUagflB(Fig. 179,1). Farther forward,

Fig. 1T9.— Kirtt tundameat of the cartilaginous primordial c Wlcderebelm) ; 1. J^ntStage! Cchordai jpE, parachordftl cartilage TV. Batbke'a trabecule cranll ; PR, passage for the bypophysla S, A, O naeal pit apcic veiiiclc, otncysl. 2. Second Stage: C, chorda; B, basilar plate TV trabeculie cranll, which have become united In front to eonrtilute the nagal Beptum (S) and the ethmoid plat«; a.AF, proccsBusnf the ethmoid plate enclosing the naial otsan ; (H, foramina olfactorla for (he passage of the olfactory ni^nca FF poilorhltal process; SK. nasal pit; A, 0. optic and lahyrtnihlne veskle*

in the prechordal r^Ion, another pair of cartilaginous masnea is producetl, known as the trabetmla cituiU. The latter are not straight bars, but have somewhat the form of a pair of calipers. In a short time the cranial tni)>eculfe unite with each other, but not throughout their entire extent, an aperture being left at the position of the pituitary body. It is through this aperture that the oropharyngeal diverticuluni, which forms the anterior lobe of the pituitary body, projects to come into rolatioa %vith the diverticulum from the inter-brain, which pnxliices the posterior lol)e. At a later period ossification fM-curs lien*, as elsewhere in the Ikiso of the skull, thus eoniph'tely isolating the pituitary IkkIv from the wall of the j)harynx. The jKiraehonhil eartilages also fuse with each other an<l with the eraniti! tral)ecula?, the four pieces now foniiinji: <»»H^ mass. The j»n)oess of ehomlrification extends to iither parts of the memhranous enuiiuni so as to produce a eartilagiiKnis hrain-ease, just as, in the case of the vertebral column, the «lorsal extrusion of cartilage-formation gives rise to a case (»r canal for the sj»inal cord. As before stated, however, the chondrifyiug j)nx'ess does not affect the entire niembnuious cranium in the higher vertebnites, chondrificatiou oceurring around the ]»osition of the foramen magnum and in the lateral walls of the cranial capside, while parts of tli(? vault remain membranous. The anterior extremities of \\n\ unite(l cranial trahecuhe become so modified in form as to constitute the plate of the ethmoid and the nasal capsule for the lo<lg('ment of th<» olfactory epithelium. In each lateral n'<rion the cartilaginous ear capsule is differentiated.

Meanwhile the cartilaginous visceral skeleton is developing from the memhranous .-truetures of the visceral arches. As in the ease of the hniin-eapsule, the ehondrifying process does not involve all \yav{< of the membranous visceral skeleton, ]>arts of the latter heing replaced later by dermal or c<^vering l)one — that i<, bones that develop in membrane without having been ])revion>ly mapped out in cartilage.

In tlu; first visceral arch, the formation of cartilage occurs only in the mandil)nlar jMU'tion, the maxillary pnx'ess contiimintr memhranou*^. The eartilairc <>f the mandibular arch a])p<'ars in the form of a eurved bar running ventrodorsally. '^rhi> bar divides into a smaller j)roximal or dorstd piece, the palato(juadratum of comparative luiatomy, and a longer distal or ventral s(^gment, Meckel's cartilage, "^rhe ])alato-quadratum sul)s<'(|nently divides into two ])arts, the cartilaginous anlages resj)ectiv<'ly ol* tin- ])aIato-j)teryg<)id plate and the incus. MeckeFs cartilage like\vi>e undergoes division, there being se])arate<l from the chief mass a small ju'oximal st»gment called the arti<*ulare, which is the forerunner of the future malleus. Thus th(» cartilaginous bar of the mandibular arch has to do with the formation of certain of the ossicles of the middle ear as well as, to a limited extent, with the development of the mandible.

In the second visceral or anterior hyoid arch, chondrification also occurs, but not throughout its entire extent. A bar of cartilage, the hyoid bar or Beichert's cartilage, is produced in this arch and undergoes division into three segments, of which the proximal or dorsal is the forerunner of the future stapes of the middle ear, while the other two pieces represent respectively the styloid process and the lesser horn of the hyoid bone. The tissue intervening between the position of the styloid process and the lesser hyoid cornu does not chondrify in man but remains membranous and becomes the stylohyoid ligament (see Fig. 185).

In the third visceral arch, or the posterior hyoid arch, a rod of cartilage develops which represents the greater cornu of the future hyoid bone. Ventral to this, there is formed a median unpaired piece of cartilage, the copula, belonging to the arches of the two sides, which later develops into the body of the os hyoides.

To summarize, the head skeleton in the cartilaginous stage of development presents an imperfect cartilaginous brain-case, capsules for the organs of smell, sight, and hearing, and a cartilaginous visceral skeleton, the several parts of which map out the lower jaw, the hyoid bone, the styloid process, and the ossicles of the middle ear.

The Osseous Stage. — The bony condition of the head skeleton is brought about in part by the development of bone from centers of ossification in the cartilages described above, and in part by the growth of covering or dermal bones in the integument covering those areas which are deficient in cartilage ; in other words, by both endochondral and membranous ossification. It may be stated in general terms that the bones of the baseand of the sidesof the skull, including the auditory ossicles, the ethmoid, and the inferior turbinated bone, are produced by ossification in cartilage and are hence called primoi'diaJ bones; and that the bones of the vault of the cranium, and for the most part of the face, result from the membranous method of

nssi Rent ion, unci :irp tlicrcforo stylod (Icrmal or covering bones. Smio of tii(! iiulivkliial bones, however, are partly of carlila}rinouij and partly of nicnibraiioiia origin, the several {Mrtioiis reniuining iwrnmncntly distinct in certain lower vertebrates, but in niEin nuiting so iutiniately witli (iich otlitT a» to present no trai-e tif their previously separate comlition.

The occipital bone consists of two genetically distinct jKirts,

the )iUiK>rior or intetpatietal -por tion, which \n a dermal bone,

and the occipital bone proper,

rijiin. The OBsiflcation of the latter

(Hie on each side of the fommen

mitions, one in frtmt of the foramen

ihi' liii^ilar process, and one (losterior to that fli)Ort«re for

III' lalmliir imnioii nfilic !»iiie ni>t belonging to the iuter >ial Minium. ( Wiliciiiinn lnjiiii^ in these centers early

K' ihird li'ial ni«n[h and pr«<-<rds at ^yv]\ rate that at tlic

ol' bii'ih ihi- lioin' n>n-^i-.ts of fmir bony jwns which are 

tliii) hiyers of cartilage.

tiiaiii separate tlntingboiit

ijrir-ls, iTsjteetively, the ex supra-occipital (Fig. 181).

ihennion with it of the in riial bone that ossifies from

! wifli the .-iUpra-oiK^iintal

iilli ot'fi'tal life. Consisting at panilcil liv caililagc, the oceip 1 be<-onii-s a ,-ii,._de by the end i.f the thin! or fourth

ir by thi- buiiy uiiiuii c.fllie sciKinitt' se-rmeiits.'

J'lic temporal bone is made up of thi-ee genetically distinct

' In s.iiiir- m^,-' \}\i- union of llir iii(i'rii:irifl;il willi llii' sii]irH-<>n-ipitnl is iiniili'iiv Ihi' ailiill Ihiiu' tluii iiiTwiitin;! Iwii iniusvcrau linsures which s, iiui: rriilii LMi'li laliTjl :iii^lv, tiiwunl tlio lULiliun line.

parts, the smuunoBal or BqaamosTgomatic, the petrosal or petrom&stoid or periotic, and the tympanic. At the time of birth these three elements of the bone are still separate from eacli other, the tympanic being an incomplete ring, and the petro

mastoid being still without a mastoid process. The petromastoid is the only part of the temporal bone that is outlined in cartilage, the squamozygomatic and the tympanic being represented in the eartilaginous stage of the cranium by mem bran OU3 tissue.

The. sqnunosygomatic (Fig. 182) is ossified in previously

Fig. l^J.— S«iuamozy«<>mjitic 1^7) and tyin|»aiiic ('), of t«'iu|M>ral IxtUL' at birth.

formed niemhrano from a single center of ossification, which ai>j)oar.s in the lower part of this segment at about the seventh \v<»('k. The proeess of bone-formation extends in all directions from this center, but especially upward into the squamosa and outward and forwaril into the zygoma. The periotic or petromastoid results from the ossification of the cartilaginous ear-oaj>sule, which latter constitutes a part of the cartilaginous portion of the early cranium. It should be remembered that the essential part of the orgiui of hearing, the internal ear, is differentiated from a small pouch of epithelium, the otic vesicle, wliieh is produced by an infolding or iiivairination of the surface* ectoderm, and that it is the cartilat^inniis tissue cuclosiuir the otic vesicle and its outgrowths, the semicircular canals and the cochlea, that constitutes the cartilai^iuous car-capsulc.

The (Ksitication of (he ])criotic is usually descrilx?d as pro(•('(Mlinir iVom three ceuter<. The first of these, the opisthotic, inake< its apix-araiiec iu the hitter part of the fifth month on the outer wall ot' th<' ea]>sule, at a poiut corresj)onding to the ])o>i{iou of the |U"ouiout<»ry, \\ heuee the formati<m of bone -preads iu such uiauuer a> to ]>ro(lu(M' that part of the petros:i which is below the iuterual auditory eanal. A secoml center, the pro-otic, aj>]>eais a little later over the superior seniiein'ular eaual aud <::ive< rise to that ])art of the ])etrosa above the iutiMMial auditory uieatus, aud also to the inner and up|)er part of the uia>toidea. The third nucleus, the epiotic, arises iu the ueiiihborhood «»f the ])o-iterior semicircular eanal. ( )>sifieatiou proeee(N rapidly, the three parts speedily uniting to t'orui oue boue, the [xriotic or petroiuastoi<l. The |)etrous portion of the ju'riotic is the ruon' important and the more constant. I'he luastoid is of variable size in different animals, and in the human sp<'cies, at birth, it is fiat and devoid of the ma>toi<l pnx'ess w hich is so conspicuous in the mature condition of the skull. The mastoid process develops during the first two years of life, but its air-cells do not appear until near the age of puberty.

The pars tyznpamctis, or the tsrmpanic (Fig. 182), whicli constitutes the bony part of the wall of the external auditory meatus, is ossified in membrane from a single center of ossification. This center appears in the third fetal month in the lower part of the membranous wall of the external canal, from which point the pnxiess of bone-formation extends upward on either side so as to form an incomplete bony ring, open above. This tympanic ring is situated external to both the ear capsule and the ossicles of the middle ear and gives attachment to the periphery of the tympanic membrane. The further growth of the tympanic ring being in the outwanl direction, it becomes a curved plate or imperfect cylinder of bone which constitutes the bony wall of the external auditory canal. At birth, the pars tympanicus still has the form of the incomplete ring, its further development taking place during the first few years of life. The extremities of the ring unite with the squamozygomatic before birth. The tympanic unites also with the petrosa except in a region adjacent to the proximal end of Meckel's cartilage, where an aperture is left which is the petrotympanic or Glaserian fissure. Since upon the part of Meckel's cartilage which is thus enclosed bv the union of the two bones is formed the long process of the malleus, the presence of this process in the Glaserian fissure is accounted for.

The stybid process of the temporal bone belongs to the visceral-arch skeleton. It ossifies in two parts in small masses of cartilage that belong to the anterior hyoid arch. One, the tympanohyal, gives rise to the base of the process (Fig. 186); it begins to ossify before birth and soon unites with the temporal. The other segment, the stylohyal, undergoes ossification later and joins with the tympanohyal only after adult age is reached. Sometimes it remains separate throughout life.

The sphenoid bone is for the most part ossified in cartilage. The body of the bone is represented in the fetus by two separate parts, the posterior body, or basisphenoid, or post

.spli<'noi<l (Fijr. 1S.3, hs\, wliicli incliulos all that part of the IhmIv ni* th<» niatiiro bono whicli is jx)stcrior to the olivary rinlnrnco and to whirh belong the greater wings (alisphenoitls): and an anterior body or presphenoid (;>^), situattnl in front of the olivary ominonoe, to which belong th|? lesser wings (orbitos|)h('noids). The ossification of the basisphenoid {)roeeeds from two centers placed side by side, which

Ki«.. l.s:;. - Splu'noiil Imhu\ fiflh or sixth A tnl month: seen from above: p«. pre>I))h-iiui«1 itr iiiiti-riur Ixuiy, \\ itti h'sscr wiii}:-^; (i{(, greater wings ; 6<, baffisphenoid

or |»i»'ti'i lur I'luly.

i|»jMar in the eii
:htli w(M'k. 1\vo months later two secniid.-irv (M-nh rs M])pear for the lateral parts of the b(Kly.

'rih- presphenoid likewise develops fnjni two centers, which nre appMniit in ilie ninth week. The union of the |)ir-plu'n<)id with tlir basisplieiioid occurs in the seventh or eiL'lith nionili. Maeli greater wing develops from a sinprle eenlci* of o»ili<:ition, uliieli is ]>resent in the eighth week. The pmrrssof o>sifieaiion >j)read> from this center to produce imi (nilv the LTi'eatrr wini: but also the external pterygoid j)lal('. i'lie ureatrr win^s remain separate from the body until <nnie tiinedurinsr tli(» lir>t vear after birth. P^aeh lesser wing <»s>ities from a center that appears about the ninth wrcL. 11ie les<er >vinij:s unite with the j)rcsj)henoid in the >i\ih fetal montii.

The internal pterygoid plate dilfers from the other parts of the .vpheiioiify in cartilage but in membrane. It is stated. howev(»r, that its hamular process tir-^t hreomes eartilaiLrinons bet'ore it ossities. It is, therefore, a run rin(/ hum'. \\< center or <'enters of os>itication aj>pear in the fourth month in the connective tissue in the lateral walls of the oro])harvnireal t-avitv. In manv animals this plat<' ac(|uires n«» connecti(»n with the external pterygoid plate, but remains throughout life a distinct l>one, the ptervgoiil. In man it fuses witli the external plate in the fifth month.

The presphenoid with its attaclied leaser wings, and the basisphenoid, to which are united the greater wings and the pterygoid plates, remain permanently separate bones in some animals. In man, as noted above, the two parts of the body of the bone unite shortly before birth, although the greater wings remain separate until some ranutha after that event.

The etlunoid bone and the Inferior turbinate are formed in cartilage, resulting from the ossification of the jKiHterior portion of the cartilaginous nasal capsule (Fig. 184, m). The

vtlli Ih^ nntl ruTltj at (hcplMea dmtEnntri by n 't K. »rt1lnge»t ibe iiikwI Hptum ; n, (urbliml cartilage ; J. omnn nf Jacobsnn : J', the iilaoe wbeie II apclW into tlia nasal raviij- ; gf, palatal proeeia; of, maiillarj pruccu; iJ. dental rtdgc


latter represents the anterior extension of the cartilaginous trabecule cranii so mmlified as to constitute a rc<»ptacle for the olfactory epithelium. The anterior part of this capsule remains cartilaginous throughout life as the septal and lateral cartilages of the nose. By the ossification of the posterior part of the nasal capsule the ethmoid and the inferior turbinate bones are produced. Ossification, beginning in the fifth month, involves the lower and the middle turbinals and a psirt of the lateral masses. The ixwificatlon of the superior turbinal, of the vertical plate, of the crista galli, and of the

rcintiiiiinjL^ jxirts <»f tho lateral masse? is efiected after birth. Tlic Imiiiv iiiiinii <»f till' lateral masses with the median plate i> <'oinplct(*<l U'twii'ii the fifth ami seventh years.

Tlif frontal bone is a ot evening or dermal l)one, being ossitird ill iiK'inhran*.' tVom two centers of ossificatioUy one for earh iatt-nil half. These centers are situateil above the <)rl>ital arrhrs and are tir>t a]»|mrent in the seventh week. At hirtli, tin* two halves of the bone are still se|)aratey their niiiuii not nr<Mirriiig until during the first year of life. Sonieiinic> till* union fails to take platv, the ciimlitiou of the per>i>t<Mit frontal or metopic suture being known lus metopism. M<toj»i-in is ronsid(*ral)ly more common in European skulls than in tlm.-t' «»f lower tyjH'.

TIk- parietal bone i< also o.-sificnl in membrane. It develops from tuo mhlci wliirli soon c<»alesce. Their position eonx?-jMnid- to that of tile future parietal eminence.

TIh.' bones of the face are for the most jxirt dermal l>ones. < )f tin-**, tin* nji|K'r and the lower maxilhe and the palate Imiiu- IxJoiiLT to the vi-ccral-arch skeleton. The others devrlnp ill tin- iiHinhranous wall <>f the cranial eapside.

Tlw nasal jithI lacrimal bones ossify each from a single (•(•lit*!-, uhirli a|»|M':ir- in the ci^dith week.

The malar i- o--ifn d in nicniKrane from three nuclei, the ppKM-- iMMiniiiiiir in tlic eighth week.

riic palate bone is iorinr<l in ninrous membrane fn>m a -iiw^lc cciitf r whirli i- .-itnal<_'(l at the jniiction of the vertical and tlic horizontal pl.itc^.

riic vomer d<'V('l(»|>s from two center- of ossification which a|>|H':ir at the hack |)art of the cartilaLrinotis nasid septum. Ivi'h <'eiiirr Liive- rise to a laniina ol' hone, the two laminae

rMdually iinitiiiLr with each other from behind forwanl, and

eiiil>r:uinL'" Ix'tween them anteriorly the septal cartilage.

The vomer and the palate bone are examples of the formation of JM)iie in iniieoiH in<'inl>rane. The centers of ossification lir-t aj)peMr in the eighth week in each ca-e.

'i'he skeleton of the visceral arches includes the upi>er and lower niaxilhe, the liyoi<l Ixnu* with a ])art of the styloid |)rocess, tlu? ear ossicles, and the j)alate bones. The ]>alate hone- have heeii referred to above. These hones of the visceral-arch skeleton are partly primordial and partly membranous.

The snperior maxilla comprises two parts, the superior maxilla proper and the intermaxillary bone. While these intimately unite in man, in some animals, as the dog, they are permanently distinct, the intermaxillary lK)ne constituting the important and conspicuous premaxilla of the dog. The superior maxilla ossifies in membrane — within the membranous maxillary process of the first visceral arch — from an uncertain number of centers. It seems probable that there are five nuclei of origin, one for the palate process, one for the malar or external part of the bone, one for the portion internal to the infra-orbital foramen and a part of the nasal wall (orbitonasal center), one for the part of the bone between the frontal process and the canine tooth, and one for the premaxilla. The formation of the antmm begins in the fourth month by the development of a recess or fossa on the inner or nasal wall of the bone.

The palate process is formed by the growth, on the inner aspect of the bone, of a shelf-like projection which advances toward the median line until it meets and unites with its fellow of the opposite side (Fig. 172j. The horizontal plate of the palate bone develops similarly and very shortly after, and thus is produced the hard palate, which separates the nasal chambers from the mouth. The two halves of the hard palate unite first in fnmt, their union being completed by the twelfth week. If union is incomplete, the anomaly of deft-palate results. The intermaxillary segment begins its development in the seventh or eighth week upon that part of the nasofrontal process which lies between the nasal apertures. In the fifth month the intermaxillaries fuse with the maxillse, the line of union being indicated by a suture which is apparent upon the oral surface of the palate processes. The intermaxillaries contain the germs of the four incisor teeth. As previously mentioned, deficiency of union between the maxilla and the intermaxillarv results in the deformity of hare-lip. Obviously, the hiatus in hare-lip will be found to be not me<lian, but lateral, corresponding to the position of the line of normal union.

Tliti lower jaw or mandible is intimatcl}' associated ia its (K^vclopriiuiit witli that of the malleus and incus of the middle (•(ir. Inasmuch as thoric thrt-e Inmes are dilfcrentiatcd from ihi! it:irti[:i;riiioiis anil inumliniiioiis visceral skeleton of the first vi.-wrrtil arch it is di'sir.ibk< to consider their develop IIHlll toj^tiuT.

As dfscrilied aliove, the inenihmiiotis jaw-arches form the hiliral and Idwit Iiniiiidtirics of the month-cavity, the first vis<i'ral arch dividing into the nuixillary process and the iiuiiidihiihir :m-li. Thenr apjH'iirs in the mandibular arch a bar of cartilage which abuts liy its jmiximal extremity upon th Iter wall of the au<litory labyrinth. This cartilaginous

" r'ii;)il<'t'ii n-i-Lka nld with the 1til. 1'lii- liiniT Jaw lomewhU 'li i.'iiti'iiil:> III till' niHlkiii. The lyuitaiiicu!! iKvikible: Aa.nwlfiirlilnip-. .Vt: uk, biiiijr lower

>. iia* ,,:,h,l,,

orliim, Meckel's cartilage (Fifr.

•.iNii.ial whifh is called,

- |>al:ito<|ii:iiIi-:itLnti. From the

Hie {i:<[iili>|ilery[ruiil process.

grows toward the roof of the mouth-cavity and becomes a separate segment. The piece of cartilage remaining, which represents the proximal end of the original bar, undergoes ossification, becoming the incus (Fig. 185, am). The posterior or proximal extremity of Meckel's cartilage, becoming a partly sej)arated cartilage, the articulare, ossifies to produce the mallens (Fig. 185, ha). Though the form of the malleus is recognizable, it is still in direct continuity with Meckel's cartilage. In the opposite direction it is articulated with the incus. As the tympanic ring develops, and the interval below, between tliis ring and the petrosa, is gradually narrowed to the petrotympanic or Glaserian fissure, the malleus comes to He within the tympanic cavity, being continuous, through the fissure, with Meckel's cartilage. Upon the separation of the malleus from the cartilage of Meckel, the long process of the malleus represents the former bond of union and therefore occupies, in the mature state, the Glaserian fiasure. The joint between the malleus and the incus represents the primitive vertebrate jaw articulation. In the shark, for example, the mandibular joint is between the two pieces into which the cartilaginous bar of the first visceral arch divides — that is, between the palatoquadratum and the representative of Meckel's cartilage, the mandibulare. In mammals, however, the malleus, as we have seen, loses its connection with the mandible, the joint between the latter and the skull, the temporomaxillary articulation, being secondarily acquired in a manner to be pointed out hereafter. While the malleus develops for the most part by ossification in cartilage, its long process develops in membrane as a small covering or dermal bone, the angulare.

The membranous lower jaw with its enclosed bar of cartilage becomes osseous, not by the ossification of the cartilage, but by the development of a casing of bone within the surrounding membrane. In other words, the lower jaw develops chiefly by the intramembranous method of boneformation. Several centers of ossification appear, and from these the process of bone production extends rapidly, forming, by the fourth month, a covering or dermal bone, the dentale (Fig. 185, uh)^ which is situated mainly on the outer side of Meckel's cartilage. A smaller plate appears on the inner side. Thus the cartilage comes to be surrounded by an irregular cylinder of bone. The cartilage of Meckel plays a comparatively unimportant part in the ossification of the lower jaw-bone and begins to degenerate in the sixth fetal month. Its distal extremity, however, undergoes ossification, thus aiding in the formation of a small part of the mandible near the symphysis; while a posterior segment, with the fibrous tissue encasing it, which extends from the temjx>ral bone to the inferior dental foramen, persists as the internal lateral ligament of the lower jaw. With these exceptions, Meckel's cartilage entirely disappears. The angle of the mandible and a small part of the ramus are also ossified in cartilage, which latter is developed independently of Meckel's cartilage. From the posterior part of the dentale the condyloid process develops and becomes articulated with the glenoid fossa of the temporal bone, thus establishing the temporomaxillary articulation. This joint, as previously stated, is a secondary one and replaces in mammals the primitive articulation between the mandibulare and the palatoquadratum of the lower vertebrates.

At birth, the two lateral halves of the inferior maxilla are united at the symphysis by fibrous tissue ; bony union occurs during the first or second year after birth.

To summarize, the inferior maxilla develops as a part of the visceral -arch skeleton and is chiefly a covering bone, since, with the exception of the angle, a portion of the ramus, and a small part near the symphysis, which are of cartilaginous origin, it is formed by the membnmous method of ossification. The two other products of the mandibular arch, the malleus and the incus, are ossified from cartilage, with the excej)tion of the processus gracilis of the malleus, which is of membranous origin.

The development of the hyoid bone, of the styloid process of the temporal bone, and of the stapes was referred to in considering the c4irtilaginous visceral-arch skeleton, but for the sake of clearness and completeness it may not be amiss to repeat, in this connection, Bome points previously mentione<!.

The membranons ulterior hyoid or second Tisceral arch, at a certain stage of development, presents, in ita interior, the dorsoventral cartilaginoiiii bar known as Keichert's cartilage. This is parallel with Meckel's cartilage, and, like it, is in contact by its dorsal or cranial end with the outer wall of the auditory labyrinth. A shorter bar of cari:ilage appears in the third visceral arch, which latter is known also as the posterior hsroid arch. Together, these two cartilaginous elements furnish the stapes of the middle ear and the hyoidean apparatBS, the latter consisting of tlic hyoid bone, the stylohyoid ligaments, and the styloid processes. In man the

llfOldeaii apparatuB and Inryni at dog.

hyoidean apparatus is somewhat nidlmontary, but in the dog and many other mammals it is present in its typical form (FIr. 18G). In such animals the stylohyoid ligament of human anatomy is roprescntrd by a b()ne, the epihyal, the hyoid bone being, therefore, connected with the skull by a series of small bones artieulatod with earh other. AH the elements of the hyoidean apparatus, ^ave the IwmIv and the greater cornna of the livotd liono, are produced bv Reichort's cartilage ; the hyoid Iwdy, known in comiKirutive anatomy as the basiliyal, .tihI the greater eornua, or the thyrohyals, ossify from the cartilage of the third arch, the cartilage for the body being a median unpaired segment known as the copula. Beicliert's cartilage undergoes division into five s^ments. The segment at the cranial end, upon ossification, becomes the stapes/ This ossicle, by the closing of the walls of the tympanic cavity, is isolated from the other segments. The second piece, the tsrmpanohyal, ossifies to form the base of the styloid process and ankyloses firmly with the temporal bone at the point of junction of the periotic portion of that bone with its tympanic plate. The third portion, the stylohyal, forms the lower part of the styloid process. It undergoes ossification later than the tympanohyal and does not acquire osseous union with it until the time of adult age. It sometimes remains separate throughout life. The fourth segment, the epihyal, does not even become cartilaginous in man, but remains fibrous, constituting the stylohyoid ligament. In most mammals it ossifies, to form a distinct bone, the epihyal. The ventral extremity of the cartilage of Roichert, the ceratohyal, produces the lesser cornu of the hvoid bone.


The upper and lower limbs articulate with the trunk through the modium respootivcly of the pectoral and pelvic jj^irdlcs, tho fornicr being constituted bv the scapula and the elaviele, and the latter by the ossa iiinoniiiiata. As in the ease of the axial skeleton, the hones of the limbs in their develo]nu(jnt ])ass sueeessively through a nienibnmous and a cartilaginous stage.

The general (h^velopnient of the upper and lower extremities is deseribed in a later section. As stated in that account, each linii)-bud is to be regarded as an outgrowth from, or as eorresj>ou(ling in posili(ni to, several primitive segments, the tissue composing the little bu(l-lik(» ])rocess subsequently differentiating into the muscular, cartilaginous, and connectivetissue elements (►f the member. The origin of each limb from more than one primitive segment has been established

^ Si'u ft)ot-n()te, i>:igt.' J 15).

chiefly by eml)ryological investigations upon the lower vertebrates, and is borne out by the fact that each extremity receives- its nerve-supply from a series of spinal nerves instead of from the nerve-trunk of any one segment.

The Development of the Pectoral and the Pelvic Girdles. — The pectoral or shoulder girdle consists in its earliest stage of a pair of curved bars of cartilage, each of which is made up of a dorsal limb occupying approximately the position of the future spine of the scapula and approaching but not touching the spinal column, and a ventral segment lying near the ventral surface of the trunk. At the angle of union of the dorsal and ventral parts is a shallow depression, an articular surface, which represents the point of articulation with the future humerus.

The scapula is developed, except its coracoid process, from the dorsal part of the primitive shoulder-girdle. This soon acquires a form resembling that of the adult scapula with the infraspinous portion of the bone very much shortened. Ossification begins at the neck of the scapula about the eighth week, and in the third month extends into the spine. The ventral part of the cartilaginous shoulder-girdle extends almost to the median line of the chest-wall. It divides into two diverging bars, the lower one of which undergcx^s ossification in birds and in some other vertebrates to form the conspicuous coracoid bone. In mammals, however, it aborts and gives rise to a smaller element, the coracoid process of the scapula. At birth the human scapula is but partially ossified, the coracoid process, the acromion, the edges of the spine, the base, the inferior angle and margins of the glenoid cavity being cartilaginous. The coracoid process ossifies from a single center and acquires osseous union with the body c>f the bone at about the age of puberty. The acromion ossifi(s from two or three nuclei and joins the sj)ine between the twenty-second and twentyfifth years. Still other centers of ossification ai)pear from time; to time. Thus there is an accessory center for the base of the cf)racoid and the* adjacent part of the glenoid cavity, and one at the inferior angle of the hone, from which latter ossification extends along the vertebral border.

The clavicle does not develop from the primitive shouldergirdle, but is formed in membrane, for the most part, as a dermal bone. 1\^ ossification begins in the sixth or seventh week, before that of any other bone in the body. Subsequently, cartilaginous epiphyses are added, one at each end. It is by means of the epiphyses that the bone grows in length.

The cartilaginous pelvic girdle consists of a pair of cartilages, which are united with each other by their ventral extremities, and each of which, by its dorsal end, is articulated with the sacral region of the cartilaginous spinal column. At about the middle of each cartilage, on its outer surface, is a depression representing the future acetabular fossa. Anterior to the depression is a large ajjerture, the thyroid foramen, the upper and lower boundaries of which are respectively the pubic and ischiatic rwls or bars, which make up the ventral portion of the cartilage, while posterior to the fossa is the iliac segment, which has a somewhat irregular jilatc-likc form. Ossification Ix'^ins in the third month, ]>roccc(Iiii<2; from three centers, one for each of the tlire(? divisions of the innominate l)one. At the time of birth a large pro|)oriion of the orijxinal cartilage is still present, the os pnl>is, the ischium, and the ilium being separated from each other np to the age of puberty by strips of cartilag(». The ischium and the pubes unite first, and later acquire osseous union with the ilium. In addition to the three ])rimarv centers of ossification, other and secondary nuclei apj)ear at a later date in the crest of the ilium, the tuberosity of the ischium, and in i\w various spines and tubercles.

The skeleton of the free portions of each extremity, consisting at first of a continuous mass or rod of partially metamorphosed mesenchymal tissue, undergo(»s division into segments which represent the skeleton of the arm or of the thigh, of the forearm or of the leg, and of the hand or of the foot. This segmentation corresponds with that of the entire mass of the limb, both as to extent and order of appearance (see page 406). Nuclei of chondrification now appear, one in the center of each skeleton-piece, from which cartilage formation extends toward either end. The several cartilaginous elements thus pnKluced present approximately the respective forms of the future bones. The larger cartilages are present in the upj)er extremity in a six weeks' embryo, but not until somewhat later in the lower limb. All the bones of the extremities are of endochondral origin.

The long bones develop in a fairly uniform manner. The shaft or diaphysis ossifies from a single center, while the two epiphyses each present several centers. The centers for the diaphyses appear at about the eighth week, ossification proceeding at such rate that at birth only the ends of the long bones are cartilaginous. The centers for the epiphyses appear at various times after birth. Osseous union between the diaphysis and the epiphyses does not occur until the growth in length of the bone is completed. As the details conceniing the time of appearance and the number of these centers are to be found in the text-books of anatomy, they are omitted here.

Each bone of the carpus and of the tarsus ossifies from a single center, except the os calcis, which has two ossific nuclei. The bones of the carpus are entirely cartilaginous at birth, their ossification beginning in the first year with the appearance of a center in the scaphoid. The pisiform bone is the last of the series to ossify, its ossification beginning in the twelfth year.

The bones of the tarsus begin to ossify earlier than those of the carpus. The os calcis and the astragalus present osseous nuclei in the sixth or seventh ft^tal month, and the cuboid shortly before birth. With thos(^ excej^tions the tarsal bones undergo ossification between the first and the fourth years.

The metacarpal and the metatarsal bones and the phalanges present each a center of ossific^ation for the shaft and one epiphyseal center. In the case of the phalanges and of the metacarj)al bone of the thumb and of the great toe, the epiphyseal center is at the proximal extremity, while in the

n'inaininj: nu'tatarsiil anhaft begins in the eighth or ninth wwk of fetal life; <»f the epiphyses, n(»t until scvenil y«irs after hirtli. The development of the ungual or distal phalanges — of the hand, at least — is jH-euliar in that the ossification l)egins at the distal extnMuity, instead uf in the middle uf the shaft.

The Development of the Limbs

The linihs of vertehnites develop fi-om little bud-like i)r(KH'sses (Fig. iVl) liiat spring from two lateral longitudinal ridges, situattnl one on eaeli side of the body. Thes(» ridges are not exactly i^anillel with the nuMhau ]>hine of the body, but converge somewhat toward that jilane as they api)roaeh the «uidal end of the embryo. It results from this circumhiancc that the jMistcrior limbs are jilaeed chaser together than the anterior. In man, the limb-buds ai)pear soon after the thin! wivk. Kju'Ii bud contains a basis (»f j)rimitive eonnivtivc tissue oontributt»<l by several somites, as >vell as muscubr <tnietnrt\ whioh is th(? ofi«hoot from the muscle-plates 0-' J 1.^ numlKT of primitive si^gments.

  • Thr assumption of the origiu (►f each limb-bud from more

i «r^mitivc socmont is borne out by the nerve-sup])lv Th' fu]lv-l>"^*^' l"»l^' ^*"** extremity being innervated by ' ' ' ^ ■ •' <nir.-il nerves (compare page ;}GS). The eon.>a<m of the limM)ud i)ro(iuces the bony structures while the i»utgn)Wths from the mustrle-plates \r.y mnsriilntun*. Previous reference has been -.. t.^ ihe work of Banleeu and Lewis on v^ ihr limbs. Aci'ordiug to their findings,

T^y fVTcnd into the limb-buds, i)ut the limb .^ f«rtni the mestMichymal core of the bud.

""""'* -,,*; thf bnJ for the arm is at first ojiposite

i.\'/*!ol' the third ('crvical >cgm«'nt, ^. N.-^v* 10 its adult position ; and that the hud for the leg, at first attached at the region of the lower four lumhar and first sacral myotomes, extends to include the first lumbar and the second and third sacral segments, assuming later a more caudal position.

In the fifth week each limb-hud becomes divided, by a transverse groove, into two segments (Fig. 59, 12, 13), of which the distal part becomes the hand or foot, while the proximal j>ortion very soon afterward divides into the forearm and arm or leg and thigh. Even as early as the thirty-second day, the digitation of the limb-buds — in the case of the up]>er extremities — is indicated by four longitudinal parallel lines or grooves on the distal extremity of each (Fig. 59, 14). By the conversion of these grooves into clefls, the fingers appear, in the sixth week, as separate outgrowths. The development of the upper extremities precedes that of the lower by twelve or fourteen days, so that, when the fingers are present as distinct projections, the toes are just being marked off in the manner noted above for the fingers. The toes begin to separate, by the deepening of the intervening clefts, from the fiftieth to the fifty-third day. By the end of the eighth week, the fingers are perfectly formed, with the exception of the nails. The nails have their beginning in the seventh or eighth week, in little claw-like masses of epidermal cells, which are attached to the tips of the digits instead of to the dorsal surfaces. Subsequent transformations result in bringing the nail into its normal position on the dorsal surface of the distal phalanx. The nails are well formed by the fifth month, at which time the covering of modified e])idermal cells begins to disapi>ear. The extremity of the nail, however, does not break through so as to project beyond the finger-tip until the seventh month. A more complete account of the development of the nails will be found in connection with the origin of the skin (page 270).

The Position of the Wmbs.— The paddle-like limbbuds at first project laterally almost at right angles with the axis of the trunk. At this time the future dorsal surface of eacrh limb looks toward the back of the fetal Ixxly (dorsad), the future flexor surface toward its anterior aspect (ventrad), while the first digits — the future thumb and great toe — and consequently the radius and tibia, occupy the side of the member that is directed headvvard or cephalad, the future little finger and fifth toe with the ulna and fibula looking caudad. As the limbs enlarge and differentiate into their respective segments, they apply themselves to the ventral surface of the body, this change in position being facilitated by the occurrence of the future elbow- and knee-flexions, which cause the flexor surfaces of the forearm and leg, respectively, to approach the corresponding surfaces of the upper arm and thigh. At abf)ut the same time, the distal segments, the hand and foot, become bent in the opposite direction, producing the condition of the limbs that is permanent in the Amphibia — that is, the condition in which the dorsal surface of the proximal segment of the limb faces in the same direction as the dorsal surface of the trunk, while the middle segment is flexed and the distal is extended. To establish the permanent condition of the human limbs, there occur an outward rotation of the arms and an inward rotation of the lower extremities, on their long axes. The thumb and nulius, therefore, instead of looking cephalad, are now directeil dorsad — with the forearm in the supine jxjsition and the arm outstretched — or laterad, away from the median plane of the body, if the arm hangs by the side in the anatomical j)osition. By the inward rotation of the lower limb, the great toe and the tibia come to lie toward the median plane of the body, cjiusing the extensor surface to look ventrad, the flexor surface, dorsad.



STAGE OF THE OVH«. FiiCT Weik. Second Week.


SegmenUtlon of fartlllied while pauine along ovl Great Increase In tl«. Cella of loner cell-maw rearranged lo form cnto cell* of Rauber.

eytlum (.--3 dar).

Ovnm Id ntema, embedded

C°ortSn'and il> Tilll (FlgJ 49). YapeularitalloQ of Fliorlau and ila villi.

Yalk-uc partly formed.


rf"yolk-wc. ""


Oral pit (12thMl«h day). «m-lmrt partly «.i-.n.ted from yHlk-BBc.



Ouiarml Cbuutan.

Body of embryo indicated.

VUcural arche. and clefia

NMo^fciltkr"™™ AllBiitoiCBtnlklFlgisTt. lHjllnctton betweuii chorion Jevc and rhorion fWn

Marked flexion of body (21irt to lOd day): sradual uncoiling after aSd day.

Vlacerararchea and jolk-aac

to flatten. CephBllo Bcxurea.


Heart wllb einitle caTlty present, aooo dlvldltig Into

VlaCBral-areh -veueli beglu to appear.

IMviilon of atrium begini.

DlceiUTa TlyaMm.

(5ut-lracl a strBlght lube connetted wlih yolk-«ae by a

Anal Plate.

Alimentary canal ptcocnla pharvni. euipbapu, atomach. and intwliue.

Fancrcaa bvguii.

l.lver-dlTPrileulum divides,

Blle-diicta acquire himlna.

brealiB down,


tral wflU of esophagus, arterward becoming a

Pulmonary anlage blftircall's, the two poaches being eonne-ted by « pedicle,^he prlraltlTC irwhea. with the pharynx.


Wolffian bodies reeoBnlsuble.


SeKmenlation of paraxial mesoikTm,

HeurBl canal : Its cells sho*

El obi oats and Kerm-cellH Fourth ventricle Indlcaled. Fore-bralQ mtd-brBln, and hind-brain -reniples. s-kiu illvidlns Into five vealelcs.

Auditory pll followed by ollc

nlfaclory plalM. Optic vL-hlclcflbi-frtn,


Vettmu 87item.

thicken. Yenlral roots of aplual

Anterior' lobe of hypopbrila


Smel&l Bsnae


Ollc vealcle with recenu

NHMlplu dl'itlnd. lJl4lc veaicic atalked and tmuBformetl IntooptlccDp,

MOBCular ByaCem.

Bkflleton Mid


iiicsodcrm. ft-Km.nlallon of paraxial

Somites or primitive acf


S.iniltcs or prlmlllre aeg Sk"ci(.'t.ien.ina ahcath of

phorrla. Llmb-hiidii Biijarent (aboot

2lst day).



Tabuulted Chronoloot of Development (Oontinued),


Fifth Week.

Sixth Week.

Body shows dorsal coDcavity in neck- NasofVontal, lateral nasal, and maxil


Globular and lateral nasal processes. Lacrimal groove. Third and fourth gill-clefts disappear in

sinus prscervicalis. Umbilical cord longer and more spiral. Umbilical vesicle begins to shrink. Length of fetus 1 cm. ({ inch). Larynx indicated.

lary procesnes unite. I'mbilical vesicle shrunken. Amnion Larger.

Primitive aorta divides into aorta and I)ulmonary artery.

The only corpuscular elements of the blood during the first month are the primitive nucleated red blood-cells.

Vitelline circulation atrophic and replaced by allantoic circulation.

Intestine shows flexures, notably the U-loop, inaugurating the distinction between large and small bowel.

Anal pit.

Right and left bronchi divide into three and two tubes respectively (5th to 7th week).

First indication of teeth in the form of the dental shelf.

Submaxillary gland indicated by epithelial outgrowth.

Duodenum well formed; caecum; rectum (end of week).

Larynx indicated as dilatation of proximal end of trachea.

Arytenoid oartiliiges indicated (though not cartilagiiiouM).

Thyroid and thymus bodies begun.

Genital ridges appear on wall of b<»dycavity and soon t>ecome the indifferent genital srlands.

Ducts of Mtiller apfiear.

(ienital tubercle, genital folds, and gen- ' ital ridge (external genitals). '

Epidermis present as two layers of cells.

CoIIh of oiiti>)-t>late proliferate and gradually hpreau out beneath epidermis.

Olfactory lobe begins.

Arcuate and choroidal Assures on mesial surfaces of fore-brain vesicles.

Cells of central canal of cord ciliated.

i Ridge-like thickening of roof of mid! brain.

Membranes of brain and cord indicated. I*inenl body U;gins. Dorsal roots of spinal ner\'es. Home tracts of spinal cr>nl indicated, and its lumen alters (Fig. 139).

Semicircular canals indicatcnl. Semicircular canals.

Eyes begin to move forward from side Concha of external ear. of head. ' Outer flbrous and middle vascular tu 1 nicR of eye. Eyelids

I Mandibles unite (a')th day).

, Meckel's cartilage.

I Limb-buds segment.

I Digitation indicated (32d day, for hand.

I^wer jaw begins to ossify.

Clavicle iHJgins to ossify.

Rilw l>egin to chondrify.

Bodies of vertebra: are cartilaginous.

Fingers as sefMirate outgrowths.



Tabulated Chronology of Development {Contiuued).

STAGE OF THE FETUS. Seventh Week. Eighth Week.

General Characten.

Fetal body and limbs well defined (Fig. 64).

Head less flexed.

No longer any trace of syncytium on dccidua vera.

Head more elevated (Fig. 65). Free tail begins to disappear. Subcutaneous lymph-vessels

present. Oils lining the coelom are

true endothelium.

VaBcular Bystem.

Interventricular septum of heart completed, tne heart now having four chambers.

Other corpuscular elements added to blood during second month.

Dlgeetlve System.

Transverse colon and descending colon indicated.

Parotid gland begins.

True endothelium lines the body-cavity.

Gall-bladder present (2d month).

Anlage of spleen recognisable (2d month).

Respiratory System.

Median and lateral lobes of thyroid unite.

lArynx begins to ehondrify. Formation of follicles of thymus.

Oenlto-nrlnary System.

Maximum development of Wolffian body.

Mailerian ducts unite with each other. Genital groove.

Bladder present as spindleshaped dilatation or allantois.

Suprarenal bodies recognizable.


Nails indicated by claw-lIkc masses of epithelium on dorsal surfaces of digits.

Corium indicated as a layer of spindle-cells beneath epidermis. Development of mammary glands began.

Nervous System.

Fore-brain vesicles increase in size disproportionately. Cerebellum indicated.

Sympathetic nerves discernible.

Special Sense Organs.

External nose definitely formed (Fig. 171).


Palpebral conjunctiya separates from cornea.

Muscular System.

Muscles begin to be recognizable, though not having as yet the characters of muscular tissue.

Ossific centers for vertebral arches and for vertebral l»odies; ossiflc renters for frontal bone and for sciuamosa.

Membranous primordial cranium begins to ehondrify.

Claw-like anlages of nails.

Skeleton and Limbs.

Ribs begin to ehondrify. Centers of ossification of bestsphenoid, of greater wings. of nasal and lacrimal bones, of malar, vomer, palate, neck of scapula, diaphysos of long bones and of metacar)>al bones. Fingers perfectly formed. Toes begin to seiMirate (53d day).

Tabulated Chronology of Development (Continued),

STAGE OP THE FETl'8. Ninth Week. Third Month.

Weight, 15 to 20 frrams ; length, 25 to 30 Weight (end of month), 4 ounces: length,

mm. (1 to Ij inchefi). 2) Inches.

Hard palate completed. - At first chorion Icve and chorion fron • Free tail has disapfteared. dosiim prt'oent : later, formation of

Differentiation or lymph-nodes begins placenta (see second frontispiece). (O. Schultze). (Uoaea divided. I

Pericardium indicated.

Placental system of vessels. Blood-vessels fienetrate spleen.

Anal canal formed by division of cloaca. Mouth-cavity divided fh)m nose (end of

(Anus opens at end of '2d month, ac-, cordiuK to Tourncux.)

month). Soft (Mlate completed dlth week). Papillie of tongue. Evagination for tonsil. Intestine begins to rect»de within abdomen (10th week). Rotation of stomach. Vermiform api>endix as a slender tube. Omental bursa, (tastric glands and glands and villi of intestine fairly well formed (lOth week I. Liver verj* large. Peritoneum has its adult histological characters.


External genitals begin to show distinctions of sex.

Ovary and testis distinguishable fVom each other.

Kidney has its characteristic features.

Urogenital sinus ac<iuires its own aperture by division of cloaca.

Union of ti-stis with canals of Wolftian

lM)dy conn>lete. Testes in false jx-lvis. Ovaries descenM. Prostate K*gun d'ith week).

r«>riuin projK»r present as distinct layer. | Nails not (juite iH»rfeetly fi>rmed. Hepinning Jif <ievelopiiient of hair as ' solid ingrowths of epithelium.

Corpus striatum in«licated. j Corpora quadrigemina represented by two elevations on mid-brain roof.

Cerebrum covers inter-brain. Fornix ' and corpus callosum iK'gun. Fissure ' of Sylvius. Calcarine fissure. Crura cerebri. Kestiftirm bodies. Pon.s.

! External ear indicated (Fig. Ciliary processes intlicated.


Eves nearly in normal iK>8ition. Eyelids begin to adhere to each other.

Centers of ossillcation of presphenoid. Ik'jrinningossiticationof occipital bone. " of les.«ier wings of sphenoid, and t)f of tympanic, of spine of scapula, of shafts of metatarsal bones. ossu innominata. '

Cnrtilacinovis arches of vertebrse close. Limbs have definite shape ; nails almost |)erfeetly formed.

Tabulated Chronology op Development (Coniinwd).

General CliaracterB.


Vasciilax System.

Digestive System.

Respiratory System.

Oenito-urlnary System.


Nervous System.

Special SenBo Organs.

STAGE OF THE FETUS. Fourth Month. Fifth Month.

Weight, 7j ounces: length, 5

inches. Head constitutes about oneI quarter of entire body.

Enamel and dentine of milkteeth. Germs of permanent teeth tl7th wk) : (for 1st molar, 16th wk). Muscularis (longitudinal and cirt>ular) of stomach and esophagus. Intestine entirely within abilomen. Acid cells of peptic glands. Malpighian I bodies of spleen. Anal ' membrane disappears.

! Cells of tracheal and bron' chial mucous membrane ciliated.

Weight. 1 lb. : length, 8 in. Active fetal movements begin. Two layers of decidua (Mialesce, obliterating the space between vera and reflexa. Lymphatic glands begin to appear.

Heart very large.

Salivary glands acquire lamina.

Villi of large intestine begin

to disai>piear. i Liver very large.

Meconium shows traces of bile (sometimes early in fourth month).

Sexual distinctions of external organs well marked. Closure of genital farrow. Scrotum. Prepuce. I*rostate well formed.

Pftpilln? of corium. Subcutaneous fat first appears. I^iiiugo or embryonal down on scalp and some other parts.

rnrieto-orcipital fissure.

Distinction between uterus

and vagina. Hymen begins.

j CorfHtra alhicantia. Tmnsvei

»rse fibers of p<m«.

Middle i>eduncle8 and chief fissures of cerebellum.

Spinal cord ends at end of riM'cyx.

Deposit of myelin on fihrrs of |Mist«Tior* roots, extending to liurdaeh and (ioll.


Panniculus adiposus. lanugo more abundant. Sel>aceous and sweat-glands iH'gin.

Fissure of Rolando. Body of fornix and corp. caliosum. I>on>:itudinal fibers in crura cerebri. Superior peduncles. A nterior pyramids of medulla. Chief transverse fissures of lateral lobes of cerelwllum. Deposit of myelin completed for tract of (roll and later of Burdach. and for short commissural fibers (Tourneux).

Kyolids and iiostriN closed. ("urtiljiKcof Kustnchiun tnlK?.

Orj^m of Corti indicated.


Difiirentiation of muscular


tissue of arms.

Skeleton and

Os»i<'(ms crntcr for inteninl

(>s*!ifl<«tion of stapes and petriisji. Opisthotic and prootic apjH-ar. Ossificati<m


ptrrvjroid |»ljite.

Antnnn of lliirhmore b^'cins.

<Ksin«'Mtioii of malleus and

iM'irins in middle and infe


rior turt)inals and lateral

Tnav»jr«i of t>thmoid. Internal pt<'ryiroi<l plate Aisea witlj •'Xternal. Intermaxillarifs fuse with maxilla. I^'us longer than arms.

Tabulated Chronology of Development (OonHwued),

STAGE OF THE FETUS. Sixth Month. Seventh Month.

Weight, 2 poands ; length, 12 inches. Vemix ca«eo«a begins to appear. Amnion reaches maximum size ; amniotic fluid of maximum quantity.

Weight, 3 pounds ; length, 14 inches. Sur»ce less wrinkled owing to increase of fiit.

Peyer*s patches. I Meconium in large intestine.

Trypsin in pancreatic secretion (fifth ■ Ascending colon partly formed, or sixth month). Csecum below right kidney.

Air-vesicles of lungs begin to appear.

Walls of uterus thicken.

Vernix caseosa begins to appear. Eyebrows and eyelashes begin.

Testes at internal rings or in inguinal canals.

Epithelial buds for sebaceous glands acquire lumina. Branching of cords of milk-glands. Eponychium of nails lost; nails said to break through, lanugo over entire body.

Collateral and calloso-marginal Assures.

Body of fornix and corpus callosum complete.

Hemispheres of cerebrum cover midbrain.

Cerebral convolutions more apparent.

Cori>ora nuadrigeraina.

Myelination of fibers of direct cerebellar

tracts. (Crossed pyramidal tracts not

until after birth.)

Lobule of ear more characteristic.

Lens-capsule begins to acquire trans- , mrcnry. Kyelids permanently open. , rupilliiry membrane atrophies.

DiflTorontintion of muscular tissue of , lower extremities.

i I^esser winps unite with presphcnoid. i Basisphenoid and presphenoid unite ' Mockel'H cartilHffo h<»cin« to rctrogrn<lo. I (7th or blh month). ' Ossific nuclei of os calcis and astragalus.

Tabulated Ciironoloot of Development (Conduded),

STAGE OF THE FETUS. Eighth Month. Ninth Month.

General Charftcters.

Weight, 4 to 5 pounds ; length,

16 Inches. Body more plump.

Weight, 6 to 7 pounds ; length,

20inches. Umbilicus almost exactly in

middle of body.

Vascular System.


Digestive System.

Ascending colon longer. Caicum below crest of Ilium.

Meconium dark greenish.

Respiratory System.

Oenito-orinary System.

Testes In inguinal canals.

Testes in scrotum. T^bia m^Jora in contact.


Vemix caseosa covers entire

body. Skin briehter color. Lunug(j begins to disappear. Nails project bi'yond hnj^er tii>a. Increase of subcutaneous


Lanugo almost entirely absent.

Galaetopherous ducts of milk-glands acquire lumina.

Nervous System.

Spinal cord ends at last lumbar vertebra.

Special Sense Organs.

Ossification of bony lamina spiralis and of modiolus.

Neuro-epithelial layer of retina completed; macula still absent.

Choroidal fissure closes.

Muscular System.

Skeleton and Limbs.

Ossification in lower epiphysis of femur, sometimes also in ui>i>er ei>iphy8e8 of tibia an(i humerus.

Tyinpanohyal begins to ossify.

Ossitlc nuclei for body and great horn of hyoid bone. 1