186 



EDWARD MCCRADY, JR. 



On the average such an animal would be between 70 and 80 

 days of age. The latter figure represents the usual time of 

 weaning. I have found that the early pouch young cannot 

 control their own body temperature. They are 'incubated' 

 within the marsupium. The control of body temperature 

 seems to begin between the sixtieth and seventieth days. One 

 cannot help but wonder whether the reptilian lung and the 

 poikilothermal condition are correlated. As long as he does 

 not have to maintain a high rate of metabolism to provide his 



con,tii 



Fig. 58 Functional respiratory epithelium (from Bremer, '04). A and B 

 from a 12.5-mm. opossum. C from a lizard. 



own source of heat, the relatively inefficient oxidation ac- 

 complished through a visible epithelium may be as adequate 

 for a mammal as it is well known to be for reptiles and am- 

 phibians. In the adult opossum, which does control his own 

 body temperature, an epithelial lining is no more demonstrable 

 than it is in any other adult mammal. A careful investiga- 

 tion of the temporal relation between the change in metabo- 

 lism and the change in alveolar lining should be made (see 

 remarks in the Alveolar Epithelium Conference reported by 

 Macklin, '36). 



