90 EDWARD MCCRADY, JR. 



For instance, the pharynx develops gill pouches and lung 

 rudiments at an earlier stage than in any of these other 

 animals. That the lung begins its development early is not 

 surprising, for the lung must be put to use in the opossum at 

 an extremely early stage of general body development. The 

 chick begins to breathe during the eighteenth day of incuba- 

 tion, which corresponds to about the twenty-fourth day after 

 mating in the opossum; the rabbit begins to breathe during 

 the thirtieth day; but the opossum has been breathing since 

 the thirteenth day ! The opossum at this time corresponds in 

 general development to a chick of about the seventh day of 

 incubation, or a rabbit of about the fifteenth day of gestation. 



The reason for the precocity of the branchial pouches is 

 perhaps less obvious; but, if the theory that the lungs are 

 really modified gill pouches be correct, then part of the ex- 

 planation may be that the whole branchial apparatus begins 

 development early because the lungs, which are part of it, must 

 be used early. But, regardless of the origin of the lungs, 

 much of the essential mechanism of sucking and breathing, 

 e.g., the larynx, is derived from the branchial arches, which 

 must, accordingly, be precocious. 



Other unique features in the opossum are the precocious 

 development of the myelencephalon, the cervical cord, the 

 anterior somites (the opossum has about 13 somites before 

 the neural folds make contact anywhere), and the ear pri- 

 mordium. These features, I believe, are all associated with 

 the early birth of the marsupial. The myelencephalon is pre- 

 cocious because the nerves to the sucking mechanism (the 

 tongue, etc.), the lungs, and the stomach, must be used at an 

 early stage. The cervical cord must be rushed ahead to as- 

 sist in the sucking mechanism, to provide a brachial plexus 

 to control the forelimb (by means of which the opossum gets 

 into the pouch), and to provide a phrenic nerve to move the 

 diaphragm and make breathing possible. The anterior 

 somites must be developed at a rapid rate to provide muscu- 

 lature for the forelimb. 



