THE EMBRYOLOGY OF THE OPOSSUM 89 



the roofing of the brain, and it is less advanced in the dif- 

 ferentiation of the head amnion, the pharynx, the somites and 

 the otic anlagen. 



General comments on comparative data. In the compara- 

 tive notes I have tried to select a representative type for each 

 of the principal groups of amniotes except the montremes, 

 for which last-mentioned the available data are insufficient. 

 The lizard was chosen to represent the cold-blooded Saurop- 

 sida because it is considered the least specialized living reptile 

 (Parker and Haswell, vol. 2, p. 338). The chick was chosen 

 to represent the warm-blooded Sauropsida because it is the 

 best known bird. The rabbit was selected for the placental 

 mammal, not only because it is fairly unspecialized, but also 

 because for the first 1(H days of gestation it has almost pre- 

 cisely the same developmental rate as the opossum. 



The existence of thei term Sauropsida implies that the rep- 

 tiles and birds have more in common with each other than 

 either has with the mammals. In general this is probably 

 true, but some conspicuous exceptions were listed in the com- 

 parative notes in this chapter. The vascular and urinogenital 

 systems appear in the lizard at a much later stage of develop- 

 ment than they do in either the marsupial or placental 

 mammal. In this case the other sauropsidan, the bird, accords 

 with the mammals, not with the lizard. The obvious implica- 

 tion of these facts is that the early differentiation of these 

 two systems is not a mammalian in contradistinction to a 

 sauropsidan feature, but is an homoiothermal in contradis- 

 tinction to a poikilothermal characteristic. Actually neither 

 the bird nor the opossum can control its body temperature at 

 birth, but both of them are 'incubated' until they can. This 

 parental attention enables them to maintain a constant 

 temperature and a high rate of metabolism. It seems very 

 likely that the high rate of metabolism in the warm blooded 

 animals makes necessary the early development of the circu- 

 latory and excretory mechanisms. 



There are some features in which the opossum is different 

 from all the other forms with which I have compared him. 



