Page:The New International Encyclopædia 1st ed. v. 07.djvu/46

* EMBRYOLOGY. 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 lorax and abdomen have grown some dis- tance into the thin membrane which in early constitutes the ventral body-wall. The are much enlarged, and contain a skeletal core of cartilage surrounded by the developing muscles. Only the fourth pair of the aortic arches new persists entire. The others an 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 rihs 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- !n. -n and thorax. The allantois forms a long narrow tube extending from the hindgut into the umbilical cord as tar a- the chorion; at a later the umbilical portion becomes obliterated, while the portion near the hindgut enlarges % into the bladder. In an embryo seven weeks old and 20 milli- meters Ion the early foetal features are fairly well marked. The head has now become nearly erect. The' fusion of the maxillary and nasal proc- esses is proceeding rapidly to form the upper jaw, ai il pit - ha ■ iched nearer to the median line. The arms anil legs are much elongated, show t!ie three tegment and lingers and toes an- to be distin- cle of 1 be body can now !»■ ized and l i 3 Qerve supply. Most of the skeletal elements are present in cartilage, hut portion- of tin- -lull an' never so repre The rihs have extended nearlj mid ventral line, and the thoracic and abdi - have pushed farther out. By the end of 'iid month the permanent kidney is fairly well formed and the Wolffian body, which 'it'- of tl mbryo ha- performed the fune i i orj organ, begins to lose its im- portance. ll hut ill.- mi. I. Mr portion, which exual gland, at rophies. It- duct e forms the ei ns, in 1 he female
 * while in Hi.- Miillerian

diet- form tin- fallopian tube, and uterus, and in I lie mal.' degi Hi i Lte By 1 1..' end of the econd month, therefore, most Of the organs found ill the adult are fori 1. and ■ in procee i forth unl il birth a re the growth ami shifting al i of the e organs. The third im I dishes 1 be human fori i the head inidiih The lire definite ha pe, and impel i. ■ i as il I ' - ii. il O] [B m it iate.l int.. ii Dm iiiL.' I he 1 ■ meat appea r on the -ealp am i he body. Tl v el ids, rl lip i .. a li a- ..p. " in the umbilical cord no's lie entirely within the abdominal rl h of i lie bodj . ol the bones i- well under i c tiled n 32 EMBRYOLOGY IN PLANTS. 'foetus,' and its development and characteristics are described under Fcetds. Bibliography. Wilhelm His, Anatomie mensch- Ucher Embryonen (Leipzig, 1880-85). There are manv other valuable articles by His. F. P. Mall, "A Human Embryo Twenty-Six Days Old," in Journal of Morphology (Bpston, 1891) ; "De- velopment of the Human Coelom," in Journal of Morphology (Boston, 1897). See also other valuable papers by Mall. Hubert Peters, Veber die Einbettung des menschlichen Eies und das friiheste bisher bekannte menschliche Placenta- turns stadium (Leipzig and 'Yein, 1899) ; Graf. V. Spec. "Xeue Beobachtungen fiber sehr friihe Entwickelungsstufen des menschlichen Eies," in Arcltiv fur Anatomie und Physiologie (Leipzig, 1896). Eternod, "Premiers stades de la circula- tion sanguine dans l'ceuf et 1'embryons humains," in Anatomischer Aneiger (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, "Zur Entwiekelungsgeschichte der menschlichen urogenitalapzsarates." in Archiv fiir Anal' und Physiologie (Leipzig, 1890): Dr. J. Kall- mann, Lrhrliurh '/< r Entirii-krlungsgcschiclitn des Venschen (Jena, 1898); Charles S. Minot, Human Embryology (New York, 1892); George W. Piersol, article "Embryology," in An Ameri- can Text-Book of Obstetrics (Philadelphia, 1S95) ; J. Whitridge Williams. Obstetrics (New York, 1902) ; J. Playfair McMurrich, The Development of th< Human Body (Philadelphia, 1902). EMBRYOLOGY IN PLANTS. The study thai .Lais with the earliest stages of plants. In general the embryo arises from the germination of a -p.ue. Iii ordinary usage, however, the term embryo is applied to the young plant which is developed within the seed. This embryo is the result of the germination of a fertilized egg. and ii is definitely limited by the fact that it passes into a period of rest protected by the seed coats. l Mosses. In mosses the fertilized egg rests in the female organ (archegonium), which is at the apex of a leafy stem. This fertilized egg begins to germinate at once, and its first wall, re- sulting in a two-celled embryo, is at right angles lo the long axis of the archegonium. The two cells thus formed proceed upon different lines oi development; the lower one produces a de- ciding tructuxe called the 'foot,' which pene- Irales into the tissue of the leafy stem and hors the embryo; the upper cell produces the i. which i- differentiated into stalk and cap sule. In this case, therefore, there is no strict oi at which (he structure maj be regarded as ceasing to he an embryo, the whole development 1 inuous. I I i il s. Among the ferns the archegonium in « hieh the fertilized i tcurs i- situated upon the under side of the -mall prostrate sexual plant (prothallium). In ordinary ea-es the fern embryo is formed in a omewhat different man- ner from that d for the mosses. The wall of the . in a two celled embryo, is parallel with the long axis of the Ilium, the t» lis thus formed facing ■ l.i Ilium Each of these cells immedia result ing in the idrant of 1 lie embryo Each "i i he e quadrant cell i dest ined to 'develop a definite body region I he i wo cells which face