THE EMBRYOLOGY OF THE OPOSSUM 143 



coronary appendage of the liver (also called the dorsal pillar 

 of the diaphragm). The part that these folds play in sepa- 

 rating the pleural cavity from the peritoneal cavity will be 

 described in connection with stage 34. 



The ureteric bud. About 48 hours before birth the first 

 rudiment of the ureter is just forming as a very small bud 

 from the wolffian duct near the place where it empties into 

 the cloaca (fig. 45, I and F). Even at this early stage the 

 mesenchyme immediately around the bud shows a definite 

 condensation, the metanephric blastema. This mesenchyme is 

 part of the same nephrogenic mass in which the mesonephric 

 tubules are formed. At first (16174) it is situated right in 

 the posterior end of the mesonephros. Later (16173) it 

 migrates medially and dorsally out of it. 



The primitive choanae. The disappearance of the naso- 

 oral groove noted in connection with the external distinctive 

 features of stage 31, is caused by the overgrowth and fusion 

 of its edges, converting the groove into a tube connecting the 

 roof of the mouth with the surface of the snout (fig. 45, re- 

 construction and section B). This involves the fusion of the 

 median and lateral nasal processes and the maxillary process. 

 This direct conversion of the oral end of the naso-oral groove 

 into the primitive choana is characteristic of the Sauropsida, 

 but is not supposed to occur in mammals. 



The motor nerves of the eye. With the appearance in 

 stage 31 of the third, fourth, and sixth nerves the list of 

 cranial nerves is complete. The oculomotor develops rapidly, 

 and can be followed all the way to the eye, but the trochlearis 

 and the abducens are extremely rudimentary at this time. 



Miscellaneous details. The stomach is noticeably enlarged. 

 Rathke's pouch has pinched off from the oral cavity. Sympa- 

 thetic trunks have formed in the thoracic region. The cerebral 

 hemispheres are enlarged. Periotic mesenchyme has con- 

 densed slightly as the first stage in the formation of the 

 otic capsule. The allantois is as big as the head and hangs 

 free in a pocket of the yolk sac. It is probably already func- 

 tioning as an urinary reservoir, for the wolffian duct became 



