THE EMBRYOLOGY OF THE OPOSSUM 187 



A few words about the gross anatomy of the lungs are in 

 order. The left lung in the possum is composed of only 

 one lobe, instead of two (fig. 56). The right lung, on the 

 other hand, has in addition to the usual three, an accessory 

 or infracardiac lobe the bronchus of which arises near the 

 hilum of the lung on a level with the first hyparterial bronchus, 

 and runs medially and caudally beneath the heart. Jazuta 

 ('32) observed that the right and left pleural cavities in the 

 opossum communicate with each other in the dorsomedial line. 

 This interpleural opening has been found in Didelphys, 

 Perameles, and various birds. This fact and the fact already 

 referred to, that the functional lung of the opossum at birth 

 is composed only of bronchi and bronchioles without alveoli, 

 Jazuta regards as strong evidence that the Marsupials repre- 

 sent a connecting link between the Eutheria and the Sau- 

 ropsida. 



I have not been able to find in any of my specimens, either 

 before or after birth, a left eparterial bronchus at the level 

 of or anterior to the left hyparterial. Bremer ( '04) described 

 such an eparterial bronchus in five newborn opossums, but 

 mentioned that it is always " smaller and slightly lower 

 placed, and the chambers supplied by it do not form the apex 

 of the lung." As in all my specimens the only such bronchus I 

 can find is well below the left hyparterial, I am forced to 

 suppose that what he saw was either an abnormality, or, more 

 likely, the first left dorsal bud, which occurs also in man and 

 in mammals generally, and is certainly of no phylogenetic 

 significance. There are many mammals, both high and low 

 in the phylogenetic series, in which a true left eparterial 

 bronchus has been described Bradypus, Phoca, Equus, 

 Auchenia, Elephas, Phocaena, Delphinus, and Cebus (to take 

 a list from Goodrich, '30, p. 610) and at best their phylo- 

 genetic significance is very uncertain. 



The intranarial epiglottis and the pumping of milk. As 

 the term implies, the epiglottis in the opossum projects into 

 the posterior nares or choanae (fig. 56). It is rather tubular 

 in form, and in this position fluid can pass around it on both 



