THE EMBRYOLOGY OF THE OPOSSUM 197 



past. Mr. McClain's more detailed study may yet disclose 

 the connection between the hyoid arch and the anlage of the 

 stapes which I have been unable to find in the opossum. But 

 even if it does not, this connection has been found in so many 

 other mammals that the weight of the evidence will still favor 

 the homology of the stapes with part of the hyoid arch, and 

 with the medial end of the columella of lower vertebrates. 



Miscellaneous details. The nasal septum fuses completely 

 with the palate. Stenson's canals remain open. Labiodental 

 ledges are forming in upper and lower jaws. The naso- 

 lachrymal cord is still solid and still does not reach the nasal 

 epithelium. The vomer is represented by two cartilages as 

 in reptiles, rather than by one as in mammals. In the adult 

 opossum, however, there is a single, median vomer, as in other 

 mammals. This is strong evidence of the homology of the rep- 

 tilian and mammalian vomers, usually considered a moot 

 point. 



The interatrial septum still shows a few very small per- 

 forations. The ductus Bottali is constricting, but still patent. 

 The interventricular septum is incomplete in the bulbar 

 region. The twisting of the bulbus is complete, so that the 

 pulmonary trunk, which was originally caudal to the aorta, 

 has passed through three-quarters of a turn and now lies to 

 the right of the aorta. 



The tail has about 24 somites, which makes the total number 

 of segments formed 52. This is the final number. 



The medulla of the adrenal gland is in process of formation 

 from chromamn cells. 



The splenic epithelium separates from the underlying 

 mesenchyme, but there are no splenic sinuses and practically 

 no blood vessels. This situation is similar to that of a 15-mm. 



Pig- 



There is no corpus callosum between the cerebral hemi- 

 spheres, and none ever develops in the opossum. Johnston 

 ('13), however, by means of degeneration experiments has 

 shown that in the adult opossum certain fibers in the dorsal 

 commissure serve the same purpose and probably represent 

 the phylogenetic anlage of the corpus callosum. 



