THE EMBRYOLOGY OF THE OPOSSl'.M 163 



There is a well-developed septum spurium in the heart. 

 The conus is divided into pulmonary and aortic halves; and 

 the twisting of the conus has advanced until the pulmonary 

 half is now anterior. 



The posterior semicircular canal of the inner ear has been 

 formed. A diverticulum from the nasal epithelium into the 

 septum represents Jacobson's organ. Mihalkovios (1898) 

 commenting on Rose's (1893) description of this organ in 

 the opossum, points out that it is like that of rodents except 

 that it opens into Stenson's duct as in monotremes. Also 

 the fact that its duct arises not from the anterior end of the 

 organ but from the floor behind is a sauropsidan character. 



The metanephroi have migrated up to the lumbar region 

 but still consist only of the pelvis and condensed mesenchyme 

 surrounding it. 



Hanson ('20) observed the coracoid cartilage in an early 

 stage 33 opossum embryo (H.E.C. 924) extending all the way 

 from the scapula to the sternum without sutures. Later in the 

 same stage (16177) the middle portion of the coracoid dis- 

 appears and the sternal and scapular ends remain in carti- 

 laginous form. This is the condition when born. In later 

 stages the sternal portion fails to ossify and is resorbed. The 

 scapular portion becomes ossified and attached to the scapula 

 as a typical mammalian coracoid process. 



The membrane bones of the skull are laved down in stage 33 

 but not ossified. All cartilage bones show chondrification ex 

 cept the periotics. Ossification has begun in the clavicle and 

 in the mandible and maxilla. Precartilaginous mcscnrhyme is 

 recognizable around the trachea. 



The first definite anlage of the spleen can be recognized in 

 the thickening of the peritoneal epithelium of the 1ft side of 

 the dorsal mesogastrium. Cells are proliferating from this 

 point into the mesenchyme below. 



The dorsal and ventral pancreatic anlagen unite into a >iiml- 

 pancreas. The single duct is that derived from the ventral 

 portion. 



