202 EDWARD MCCEADY, JE. 



premolar. With this one exception the milk dentition is abortive 

 that is to say, the other deciduous teeth do not erupt, but remain as 

 functionless vestiges until a comparatively late stage. The second 

 tooth to erupt belongs to the permanent set. It is the second premolar 

 and is present at 75 days. Between the seventy-fifth and eighty-fifth 

 days the last four incisors, the canine, and the first premolar, 

 erupt. By the ninety-fifth day the first molars and the first incisors 

 are present, and only the last three molars and the third premolar 

 remain to be cut. 



With teeth ready for solid food, the young are ready to be weaned. 

 This usually occurs between the eightieth and the ninetieth days. 

 At this time the animal has developed his adult auditory range about 

 eight octaves (McCrady, Wever and Bray, '36). Curiously enough, 

 the notes he hears best from this time on (about 7000 c.p.s.) are not 

 at the center of his range, but are decidedly nearer the high end. 

 And this fact is probably responsible for the fact that he can be 

 startled more violently by rustling leaves or hay than he can by 

 talking or even loud shouting. The sounds made by the rustling of 

 leaves are composites of short wave lengths, and are much louder 

 to the opossum's ear than they are to ours. 



The sound which the young makes when separated from the mother 

 is also a composite of short wave lengths and is probably very loud 

 to the opossum 's ear. 



. Very soon after the mother has weaned a litter and ceased to 

 lactate, she goes into another oestrus. I have seen a female copulate 

 on the second day after she was separated from her litter. She 

 produced a new litter 13 days later. 



Some experimental techniques. It is hoped that one of the uses 

 of this description of normal stages of opossum development will be 

 to serve as a background for various experimental studies. In this 

 connection it may be of interest to mention some of the experiments 

 which have been performed on embryos and pouch young in this 

 laboratory to indicate the type of procedure employed and the suita- 

 bility of the animal for operative work. 



The only experiment with embryos so far is the preliminary study 

 of the behavior of specimens of stages 33 and 34 already described on 

 page 182. The advantageous feature of the opossum embryo for such 

 studies is that it has no placenta, and correspondingly, can be re- 

 moved from the maternal uterus without the ligation or section of 

 any part of its circulatory system. 



On specimens of stage 35 two operations have been performed. The 

 lens vesicle of the eye has been removed, and the limbs have been 

 amputated. In neither of these cases was any anaesthetic employed 

 on the pouch young. If the mother was not too excited she also was 



