Page:Popular Science Monthly Volume 27.djvu/213

Rh in early stages of existence, which either disappear, or remain as concealed and functionless organs, points to the present condition in the aberrant and specialized forms as being one derived from the more generalized type, in which the teeth were numerous and equal.



The Mystacocetes, or whalebone whales, are distinguished by entire absence of teeth, at all events after birth. But it is a remarkable fact, first demonstrated by Geoffroy Saint-Hilaire, and since amply confirmed by Cuvier, Eschricht, Julin, and others, that in the fetal state they have numerous minute calcified teeth lying in the dental groove of both upper and lower jaws. These attain their fullest development about the middle of fetal life, after which period they are absorbed, no trace of them remaining at the time of birth. Their structure and mode of development have been shown to be exactly those characteristic of ordinary mammalian teeth. It is not until after the disappearance of these teeth that the baleen, or whalebone, makes its appearance. This remarkable structure, though only a modification of a part existing in all mammals, is, in its specially developed condition as baleen, peculiar to one group of whales.

Baleen consists of a series of flattened, horny plates, several hundred in number, on each side of the palate, separated by a bare interval along the middle line. They are placed transversely to the long axis of the palate, with very short spaces between them. Each plate or blade is somewhat triangular in form, with the base attached to the palate, and the apex hanging downward. The outer edge of the blade is hard and smooth, but the inner edge and apex fray out into long,