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THE POPULAR SCIENCE MONTHLY. 



3.1 millimetres; from the ear to the tail, 8.0 millimetres ; the tail, 3.2 

 millimetres. Thus the total lenojth of the smallest was 14.3 milli- 



metres, or about one-half an inch. 

 These smaller ones resemble the hip- 

 popotamus 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 em- 

 bryos 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, 



Figs. 5, 6. Front and Side Views op Smallest Embryo Opossum (D. Virginiana). Enlarged ; 

 entire length when straightened out, one-half inch. 



and its cicatrix marks the embryo as it did in Prof. Owen's kangaroo, 

 where it wi'ongly 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 begin- 

 nings 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 be- 

 comes 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 out- 

 grow 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 



