By Milan Klima
The improvement of the breast-shoulder equipment within the Marsupialia used to be inves tigated and in comparison with the stipulations in Monotremata and Placentalia. the implications have been completed via the research of fabric comprising altogether 109 histological serial sections of intrauterine embryos, neonates, and pouch younger from eleven marsupial species. also, fifty four skeletons of subadult and grownup marsupials from 25 species have been integrated for comparability. The embryonic states exhibit a robust similarity to the developmental level of the breast-shoulder gear within the monotremes. against this, the grownup breast-shoulder gear mostly corresponds to that during placentals. the next parts will be saw within the marsupial breast-shoulder equipment in the course of embryogenesis: scapula, metacoracoid, procoracoid, first rib, paired sternal parts, unpaired sternal point, and clavicle. the entire parts pointed out jointly shape a compact, non-stop arch in either the intrauterine embryos and the neonates. within the pouch younger, this arch is lowered relatively quickly after start, in order that a compact connection among the left and the precise 1/2 the physique now not exists. All that continues to be is a unfastened connection through the clavicle. The metacoracoid turns into the processus coracoideus scapulae. The procoracoid turns into the praeclavium. The unpaired sternal point fuses with the paired sternal point, producing the uniform manubrium sterni. the 1st rib takes its traditional place within the thorax. within the pouch younger, the breast shoulder equipment as a complete already exhibits the entire standard features that may be made up our minds in adults.
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Additional info for Early Development of the Shoulder Girdle and Sternum in Marsupials (Mammalia: Metatheria)
Ventral view (top) and transverse section (bottom) of the shoulder girdle and sternum. 25 mm (Table 2, no. 87). 5 mm (Table 2, no. 90). 0 mm (Table 2, no. 95) bryos, the scapular anlage consists of well-differentiated cartilaginous tissue. The metacoracoid and the procoracoid are histologically still in the stage of a chondroblastema. The anlage of the clavicle consists of poorly developed osteo bl~stema. The sternal bands of both halves of the body establish contact in the median plane and fuse.
Histologically, one can see a distinct boundary at this junction in the smaller embryos, whereas in the larger ones, a homocontinuous transition is already visible. The small procoracoid elements are at the front edge of the sternal bands, but do not fuse with them. In the median plane, between the front edges of both sternal bands, a massive unpaired structure, the pars chondralis interclaviculae, rests. It forms the unpaired sternal element, which starts to fuse with the sternal bands. This fusion process has progressed further in the 37 Fig.
26). The largest embryo, carrying the description "about to be born" in the original list of the material, shows a more compact connection of the breastshoulder apparatus than the younger embryo. The metacoracoid is very massive and establishes firm contact medially with the sternal band. The broad area of interconnection shows a homo continuous transition of both structures into one another. The cartilaginous tissue of the metacoracoid fuses with that of the sternal band without any visible borderline.
Early Development of the Shoulder Girdle and Sternum in Marsupials (Mammalia: Metatheria) by Milan Klima