THE EMBRYOLOGY OF THE OPOSSUM 81 



The processes of formation of the branchial furrow, its 

 deepening and subsequent development of local outpocketings 

 show a definite cephalocaudal progression. This is best il- 

 lustrated in figure 31 in which the principal point to be noticed 

 at present is the striking similarity between the section 

 through the lung region in stage 25 and that through the 

 second branchial pouch in stage 24+. These two sections on 

 casual examination might be confused. The only important 

 differences are: first, that in the case of the lung anlagen 

 the endodermal pouch does not actually touch the ectoderm; 

 second, that the mesothelium in contact with the lateral side 

 of the lung pouch is markedly thicker than that touching the 

 branchial pouch; and third, that the location of the lung sec- 

 tion may be identified by the presence of the anterior end of 

 the allantoic vein immediately dorsal to and more or less in 

 contact with the vitelline vein. 



The endodermal constituent of the lung is thus a local out- 

 pocketing from a longitudinal groove, which outpocketing in 

 early stages is indentical with others from the same groove 

 which become gill pouches. The lung pouch is simply the 

 hindermost gill pouch. Further evidence for this interpreta- 

 tion will be discussed in chapter X when the rudiments here 

 described will be followed to a more familiar stage. 



Miscellaneous details. The origin of the branchial pouches 

 and lungs as secondary, metameric outpocketings from origi- 

 nally longitudinal, non-metameric grooves, is another illustra- 

 tion of the fact referred to above that all metamerism in the 

 vertebrates is secondary. 



The notochord is in process of separation from the endo- 

 derm. From about the posterior end of the limb ridge back 

 to Hensen's node it is a solid cord of cells quite free from 

 the endoderm. But anteriorly it may be seen in all stages of 

 pinching off. The originally flat plate wrinkles high in the 

 midline so that its sides approach each other and include a 

 narrow, vertical slit between them. When the sides unite 

 ventrally, the notochord pinches off from the endoderm and 

 sometimes shows a transitory notochordal canal; but where 

 formed at all, this canal is quickly effaced. 



