THE EMBRYOLOGY OF THE OPOSSUM 151 



specimens of stage 32, which is not enough, as the shifting 

 seems to take place quite rapidly. 



Accordingly I can only say that if the shifting is in general 

 similar to that which occurs in other animals (and I have no 

 reason to think it is not) then the situation at the end of 

 stage 32 is as follows : The thyroid has migrated caudally until 

 it is just ventral to the ultimobranchials (fifth pair). The 

 third pair, including both dorsal and ventral components, has 

 migrated most caudad of all so that it lies posterior to the 

 aortic arches and just anterior to the pericardium. The 

 fourth pair lies between the thyroid and the third pair. The 

 only peculiar detail here is that the dorsal (parathyroid) ele- 

 ments of the third and fourth complexes migrate caudally with 

 the ventral (thymus) components instead of remaining near 

 the thyroid. 



Changes in the digestive tract. The greater curvature of 

 the stomach (original dorsal border) is bulging very promi- 

 nently to the left. The yolk stalk has pinched off leaving a 

 temporary hollow diverticulum at the level of the union of 

 right and left omphalomesenteric veins (fig. 47, reconstruc- 

 tion). A new outgrowth from the intestine, the caecal 

 diverticulum, can be seen where the umbilical vein curves to 

 run out into the allantois. This diverticulum was noticed, but 

 not identified, by Heuser ('19). 



Miscellaneous details. The otocyst has acquired its first 

 semicircular canal. A nasolachrymal chord has grown down 

 from the anterior corner of the eye toward the nasal epi- 

 thelium, but has not yet reached it. The turbinals are de- 

 veloping. 



The mesenchymatous anlagen of all cartilaginous bones 

 except some of the caudal vertebrae are present. The ver- 

 tebral anlagen become less and less differentiated in caudal 

 progression, until in the tail itself there are still somites in 

 early stages of differentiation. There are about sixteen of 

 these in the tail. Chondrification is beginning in all cartilage 

 bones except the periotics and some of the caudal vertebrae. 

 No membrane bones except the clavicle are present. 



