THE EMBRYOLOGY OF THE OPOSSUM 181 



when 1 had called in two witnesses. Thirteen days and 2 hours 

 after an observed copulation a young opossum had been found 

 in the pouch (its actual birth not having been observed). I 

 removed the foetus from the nipple and placed it on the 

 mother's abdomen. The hair was dry and the foetus stuck 

 to it so that although he struggled vigorously he was unable 

 to make progress in any direction. Wetting my finger 1 

 moistened all the mother's hair in that region with saliva, and 

 also moistened the foetus' skin. The latter 's movements 

 quickly became effective. In less than 3 minutes it had 

 clambered up the inclined plane of its mother's abdomen and 

 disappeared into the pouch. Three minutes later I open the 

 pouch and found it safely attached to another nipple not the 

 one it had first appropriated. 



The significance of this experiment seems to be that though 

 the mother does not pick up the young, her licking the hair of 

 the abdomen seems to be an essential service. 



The negative geotropism. Hartman ('20, pp. 255 and 256) 

 pointed out that, "If the skin be tilted, the embryos can be 

 made to travel upward and even away from the pouch, for 

 they are negatively geotropic." Langworthy ('28, p. 214) 

 noted that the vestibular mechanism does not begin to func- 

 tion until about 41 days after birth. So it is evident that if 

 the young does orient itself away from gravity, it does so 

 without the assistance of any vestibular mechanism. 



Both of these observations have been confirmed and an ex- 

 planation suggested as follows (Larsell, McCrady and Zim- 

 mermann, '35, pp. 106 and 107) : 



The effect of gravity on the position of the body of the pouch 

 young, before vestibular reflexes appear, is obvious when the 

 young are placed upon a rough towel or a hairy arm. The 

 forefeet are well enough developed at birth to make possible 

 grasping of hairy or similarly rough surfaces, but the hind 

 feet do not have this capacity. When the surface upon which 

 the young is placed is tilted at the proper angle, the body, held 

 in position only by the grasping action of the forefeet, swings 

 in obedience to the pull of gravity, the outspread hindlegs 

 serving only to brace it and prevent it from rolling. In this 

 manner the head is always directed upward. 



