Day 14: The Skeleton

Developing Skeletons

Compare these two images. Both are stained with alizarin red to show the calcification of their bones. The image on the right is a 4 day old rat. The one on the left is a 40 day old cat fetus. Notice how much more calcification has taken place in the 4 day old rat. The primary centers of ossification extend much farther from the centers of its long bones than those of the cat. Also notice that primary centers of ossification have appeared in the phalanges (toes) of the rat; they are not yet present in the cat fetus. Could the structure indicated by the black arrow (near the rat's elbow) be a secondary ossification site of the humerus?

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