Page:Popular Science Monthly Volume 7.djvu/670

650 noting, however, that the manus is already a little the wider and more prominent.

In Fig. 7 the manus is not only larger but has protruded so as to display the wrist and elbow regions, and a slight prominence upon the anterior border marks the position of the thumb (pollex). In these three specimens I have not as yet found the ear, but in 3 and 4 the eye is quite apparent.

In Fig. 6 the ear is a triangular flap, as in most early mammalian embryos.

The manus and pes have enlarged and present shaded portions corresponding with the thinner tissue between the finger and toes. In the former this is to become the web; in the latter it is wholly removed so as to leave the toes free. The muzzle is partly covered by the manus, but it is already somewhat pointed, as in the next figure.

In Fig. 9 the manus and pes of the left side are shown as if removed from the trunk, so as to expose the flat and prominent muzzle. The ear is a large flap, but still projects forward so as to cover the opening. The pollex has separated from the other digits, and the latter are elongated and bent downward. The pes is longer and the signs of subdivision more distinct. A web connected the limbs and the trunk as in the older specimens, but it was somewhat torn, so that the exact extent could not be determined, and I preferred to wait for a better specimen to show it.

The specimens above described were taken from a Brazilian species, Nyctinomus Brasiliensis (which is also found in this country). Figs. 10 and 11 were from the common "little brown bat" (Vespertllio subulatus). As might be expected, the increasing limbs are packed about the body more or less irregularly. But in Fig. 10 the limbs of the two sides are placed with almost exact symmetry so as to cover the face and the body. One eye is covered, the other peeps out over the index-finger. The ears are firmly held down by the thumbs, and one of the nostrils is partly hidden by the web. The lower part of the trunk and the tail are bent upward, and the knees are thrown outward so as to bring the great-toe (pr) upon the outer instead of the inner side. The whole suggests an effort upon the part of the embryo to not only occupy the least possible space, but also to screen itself from observation, and neither see, nor hear, nor smell.

In Fig. 11 is shown an older embryo, of natural size, outspread so as to display the characteristic features of bats; the greatly-elongated fingers; the separation of the thumb; and the extension of the web, with the reticulated arrangement of vessels and nerves upon it. This nervous expansion seems to enable bats to perceive the proximity of bodies by the change in the pressure of the air.