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Show 1889.] THE BODY-CAVITY IN LIZARDS, ETC. 463 the chick are fairly similar, and (p. 214) giving the resume from which a quotation has already been made (p. 459). III. (e). On the Development of the two Parts of the Post-hepatic Septum of the Bird. W e have now to refer to the developmental changes grouped under heads B & C (p. 456). First, with regard to the development of the " oblique abdominal septum." Ravn (op. cit, p. 140) calls by the name of " vena-cava-falte " or " plica venae cavae " the ridge that occurs in Mammals as a backward continuation of the embryonic lung-substance, because through it the vena cava inferior passes on its way from the kidneys to the liver and heart. But it will be seen on reference to the Plates, figs. 5 & 6, which together with fig. 7 show three sections of a chick of 7th day \ that there is a horizontal ridge of the same nature on the left side also (y, y), where of course there is no vena cava inferior. Later stages in the development of this septum are shown in the series of transverse sections of embryo of 10th day (figs. 20, 21, 22, y, y), and in the transverse sections of 12th day (figs. 27 & 28, y, y). It will be seen that the growth of the abdominal air-sacs has considerable influence on the development of this septum, into which they extend from its outer anterior border. W e may say in fact that though a starting-point is supplied by the ridge (y) which is visible as a backward continuation of the pulmonary mesoblast in the earliest stages, it is only when the abdominal air-sacs have attained some degree of development (as at about the time of the completion of the avian diaphragm, 9th to 10th days), that a true septum reaching from the lateral walls to the median vertical mesentery (cf. fig. 22, y, y) is apparent and begins to extend posteriorly and obliquely ventralwards. This oblique abdominal septum, or anterior and dorsal portion of the post-hepatic septum, is shown in its adult relations in figures 45 and 46, y, y (transverse sections). The ventral part of the latter is formed by the laterally extended ventral ligament of the gizzard and hinder part of liver, which goes by the names of gastrohepatic ligament and the great omentum. W e see this membrane in the longitudinal-vertical section of 10th day (taken to the right of the middle line, wherefore the gizzard does not appear) (fig. 18, /3) extending from the ventral body-wall to the posterior side of the liver. It is shown also in figs. 13, 17, 19, 20, 26, 27, 28, and also in 29, /3, /3. Further, I would call attention to fig. 16, which is a longitudinal-horizontal section through the more dorsal part of the gall-bladder (b.g) and the antero-ventral corner of the lung (pul.) of a chick of the 12th day. On the right 1 Owing to the curvature of the embryo at that date, these are partly horizontal-longitudinal and partly transverse. They are very similar to figures 581 and 582 in Duval's Atlas ; but the latter fail to show the presence of the ridge y on the left side. |
