. Le développement du poussin; une introduction à l'embryologie . dans le plan sagittal du corps, et thusne montre pas nécessairement la pleine étendue de l'un des éléments cartilagi-nous, mais seulement leurs relations générales. La tête du fémur est vue dans l'acétabulum, la large plaque de l'iliumau-dessus et les pubis et ischium comme tiges cartilagineuses de largeur almostequale en dessous, les pubis devant et l'ischiimi behindthe acétabuhmi. Dans ce stade, la ceinture de pehdc, dans cette et beaucoup d'autres espèces d'oiseaux, se compose de trois éléments distincts chaque côté dans des relations essentiellement reptiliennes. LA SK

• Contributeur: Reading Room 2020 / Alamy Banque D'Images
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. The development of the chick; an introduction to embryology . in the sagittal plane of the body, and thusdoes not necessarily show the full extent of any of the cartilagi-nous elements, but only their general relations. The head of thefemur is seen in the acetabulum, the broad plate of the iliumabove and the pubis and ischium as cartilaginous rods of almostequal width below, the pubis in front and the ischiimi behindthe acetabuhmi. In this stage the pehdc girdle, in this andmany other species of birds, consists of three separate elementson each side in essentially reptilian relations. THE SKELETOX 439 In the chick at a corresponding age the ihum is much moreextensive, and the ischium is united with it by cartilage- thepubis, however, has only a membranous connection with theilium (contra Johnson). In the course of development the distalends of the ischium and pubis rotate backwards until the twoelements come to lie substantially parallel to the ilium (Figs.242 and 249). The displacement of the ischium and pubis may //. u^ ^lx~^^~^i- /s.n. Is. -. Cr.N. oi.JV. Fig. 248. — Sagittal section of the right half of the bodyof Lams ridibundus, to show the composition of the pel-vic girdle; x 35. Length of the leg-bud of the embryo, 0.4 mm. (After Mehnert.)F., Femur, cr. N., Crural nerve. II., Ihum. I. s., Is-chium. Is. N., Ischial nerve, ob. N., Obturator nerve.P., Pubis. be associated wdth the upright gait of birds; it is fully establishedon the eighth day in the chick. The mode of ossification, whichis perichondral, is shown in Fig. 249. Later, the ilium obtains a very extensive pre- and post-acetabular union with the vertebrae. I have fomid no evidencein a complete series of preparations (potash) of attachment byribs arising as indei^endent ossifications. The ischium also fuses 440 THE DEVELOPMENT OF THE CHICK with the ventral posterior border of the iUum, and the pubis, except at its anterior and posterior ends, with the free border of the ischium. The spina iliaca, a

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