THE EMBRYOLOGY OF THE OPOSSUM 201 



This perfect parallelism between the development of the scrotal 

 anlagen and the lips of the pouch makes it impossible for the present 

 author to avoid the conclusion that the lips of the pouch represent 

 the labia majora, the homologues of the scrotum in higher mammals. 

 The situation of the labia majora in this case some distance anterior 

 to the clitoris is certainly no more surprising than the situation of 

 the scrotum at the same distance anterior to the penis. The fact that 

 this homology has apparently never been suggested before, though a 

 number of investigators have studied the development of the pouch 

 and the descent of the testes, suggests that there must be some obvious 

 and fundamental objection to it ; but I confess I have been unable to 

 think of any such objection. 



Nerve endings appear in the cochlear duct and in the vestibular 

 parts of the inner ear about 4 weeks after birth, but neither the 

 acoustic nor the vestibular apparatus is functional at this time. 

 Vestibular reflexes appear about 6 weeks and acoustic reflexes about 

 7 weeks after birth. The range of hearing is at first restricted and 

 sensitivity is poor. As sensitivity improves, the range of hearing 

 extends to higher and lower notes. The first notes heard are around 

 1500 c.p.s. which is about the center of the adult opossum 's range. 



The 50-day-old opossum is about the size of a mouse, and has a short 

 and sparse coat of fur. Sometime between then and the sixtieth day 

 it opens it eyes and mouth, and for the first time since birth is free to 

 crawl about on its mother's body. It cannot yet control its body 

 temperature, however, and does not leave its mother. Also it is not 

 yet weaned. It must continue to nurse for some 30 more days, and 

 it returns frequently to the pouch for this purpose. If separated 

 from its mother at this time, its body temperature soon goes down 

 approximately to room temperature, and it goes into a sort of coma 

 resembling hibernation. 



By the time it is 70 days of age the young can control its body 

 temperature, can hear five or six octaves, and can venture on slight 

 excursions away from its mother; but it still returns to the pouch 

 for food. By 80 days it has become the size of a large rat. The 

 mother's pouch has enlarged enormously to accommodate the growing 

 young, but it cannot hold a large litter of 80-day-old specimens. 

 Accordingly special arrangement has to be made for the last part 

 of the nursing period. This arrangement is that the mother's nipples 

 elongate to some 1 inches, which makes it possible for the young to 

 lie outside the pouch while they nurse. 



The first tooth to erupt is the deciduous third premolar, which 

 appears at about 60 days of age and has a curious molariform shape. 

 It functions as a molar until sometime between the ninetieth and the 

 three-hundredth days when it is shed and replaced by a permanent 



