Page:Popular Science Monthly Volume 8.djvu/168

156 3.1 millimetres; from the ear to the tail, 8.0 millimetres; the tail, 3.2 millimetres. Thus the total length of the smallest was 14.3 millimetres, or about one-half an inch. These smaller ones resemble the hippopotamus more than the opossum. Although found within the parent, they were, apparently, nearly ready to be born. A set of sixteen of these was taken from the uterus by Prof. Wilder. As the mother had but thirteen nipples, it is evident that improvidence would allow three embryos to perish. Sometimes as many as eighteen are brought forth, and often only twelve nipples exist. No attachment of the embryos to the uterine walls has been discovered, hence no true placenta is known. Still a kind of umbilicus is formed,



and its cicatrix marks the embryo as it did in Prof. Owen's kangaroo, where it wrongly led to the supposition that a placenta might have been attached. At birth, the hind-limbs appear as short stumps, with their flattened ends presenting slight marginal elevations, the beginnings of toes. These toes and legs gradually elongate. Soon each toe has one joint, and the inner toe becomes set off from the rest. Later, the two longer fingers show two joints, and, finally, the inner toe becomes a thumb with two joints, while each finger has three; and now the hind-foot closely resembles the hand of the higher quadrumana and man, while its fore-feet, much earlier developed, remain more animal-like, the great-toe being set off not so far from the others, but the fingers quite long. The hind-limbs are primarily much shorter than the front, but, developing faster, soon equal and afterward outgrow the others. The same is true of the young kangaroo, where the hind-limbs, shortest at first, finally become many times longer than those in front. Thus we see that what is smallest in the embryo