FIFTH AND LATER WEEKS. By the middle of the fifth week the embryo is nine milli meters in length. The am niotic sac is now so much en larged that it is everywhere in contact with the chorion. A true umbiliCal cord of some length has developed from the abdominal pedicle attaching the embryo to the chorion as did the pedicle. The head is as large as the rest of the body, due to the rapid growth of the brain. The three pri mary divisions of the brain are more marked and bent upon each other, and the cere bral hemispheres have begun to grow. The spinal .cord is a thick-walled tube. The va rious cranial and spinal nerves now extend some distance into the body, and the motor nerves reach she premusele masses of the head and shoulder. The gill clefts and arches are undergoing marked changes. From the first arch the up per and lower jaws are de veloping. From the first cleft the external auditory canal and ear-drum are form ing, and from the pharyngeal portion of the cleft the Eustachian tube and middle ear; while the small bones of the ear arise from the dor sal portion of the first gill-arch. From the second arch is forming the hyoid apparatus, and the remaining arches and clefts are beginning to disappear. The arm and leg are enla-vged and show two segments. The skeletal system consists of condensed mesenchymal tissue.
By the time the embryo is five weeks old it has attained a length of 11 millimeters. The head is still very large and bent at right angles to the main axis of the body, and the fea tures of the face are forming. The general body musculature is well advanced and has most of its nerve-supply. The ribs and the muscles of the thorax and abdomen have grown some dis 'ance into the thin membrane which in early stage. constitutes the ventral body-wall. The limbs are much enlarged, and contain a skeletal core of cartilage surrounded by the developing muscles. Only the fourth pair of the aortic arches imw persists entire. The others are modi fied in various ways to form the larger arteries connected with the aortic arch. The arm keeps about two weeks in advance cf the leg in its in ternal differentiation. The vertebral column and ribs are formed partially in cartilage. The liver has been increasing rapidly in size, and in this embryo, as in the preceding stage. it forms with the heart most of the large projecting abdo men and thorax. The allantois forms a long narrow tube extending from the hindgut into the umbilical cord as far as the ehorion; at a later stage the umbilical portion becomes obliterated. while the portion near 111e hindgut enlarges into the bladder.
In an embryo seven weeks old and 20 milli meters long the early fretal features are fairly well marked. The head has now become nearly erect. The fusion of the maxillary and nasal proc is proceeding rapidly to form the upper jaW, and the nasal pits have appnlached nearer to the median line. The arms and legs are much elongated. show the three segments seen in adults, and lingers and toes are to be distin guished. Every muscle of the body can now be recognized and has also its nerve-supply. Most
of the skeletal elements are present in cartilage. but portions of the skull are never so repre sented. The ribs have extended nearly to the mid ventral line, and the thoracic and abdominal nmseles have pushed farther out. By the end of the second month the per Id lney is fairly well formed and the NVoltliatt body. which so far in the life of the embryo has performed the func tion of an I•xerctury organ, begins to lose its im portan•e. All but the middle portion, which later forms the sexual gland. atrophies. Its duct in the male tomes the as deferens, in the female degenerates: while in the female the Altillerian ducts form the Fallopian tubes and uterus, and in the male I I hp end of the second month, therefore, most of the organs found in the adult are formed. and the main processes henceforth until birth are the growth and shifting about of these organs. The third month establishes the human form, al. though the head unduly preponderates. The In ihs acquire definite shape, and imperfect nails ,.re Pre emit. The external sexual organ.; become differentiated into male or female. During the fourth month short loiirs without pigment appear on the s•.ilp and rails of tl.e Lolly. Tliv . mid lir .111• ( The ii 0111M,, Mid t int, tines which wa re in the cord now lie entirely within the abdominal it y. The head forms one fourth of the body.
ideal inn of n any of the lames Is w111 1111I•r From this lin the is called 'foetus,' and its development and characteristics are described under FlETUS.
BIBLIOGRAPHY. Wilhelm His, Anatomic menschBibliography. Wilhelm His, Anatomic mensch- lichcr Emb•yoncn (Leipzig. 1880-85). There are many other valuable articles by His. F. P. Mali, "A Human Embryo Twenty-Six Days Old," in Journal of Morphology (Boston, 1S91) ; "De velopment of the Human Ccelom," in Journal of Morphology (Boston, See also other valuable papers by Mall. Hubert Peters, Ueber die Einbettung des mcnschlichen Eies and dos friiheste bisher bekanntc ,.nenschliehe Placenta tions stadium, (Leipzig and Wein, 1S99) ; Graf.
V. -'pee, "Nene Beobachtungen fiber sehr frfihe Entwickelungsstufen des menschlichen Eies," in Archie fiir Anatornie nod Physiologie (Leipzig, 1896). Eternod, "Premiers stades de la, circula tion sanguine dans Feeuf et l'embryons humains," in Anatomischer Anz'igcr (Jena. 1899) : Bardeen and Lewis, "Development of the Limbs, Body wall, and Back in Man," in American Journal of Anatomy, vol. i. (Baltimore. 1901) ; F. Keibel, "air Entwickelungsgeschichte der menschlichen nrogenitalapzsarates." in Archie fiir Anatomic und. Physiologic (Leipzig. 1896) : Dr. J. Kolb mann, Lchrbuch, der Entwickclungsgcschichte des Menschen (Jena, 1898) ; Charles S. Minot, Human Embryology (New York, 1892) ; George W. l'iersol, article "Embryology." in An Ameri can Test-Book of Obstetrics (Philadelphia, 1895) ; f. Whitridge Williams, Obstetrics (New York, 190•) ; J. Mayfair MeMurrich, The Derelopmcnt of the Human Body (Philadelphia, 1902).