THE EMBRYOLOGY OF THE OPOSSUM 71 



which in these forms is transitory, is in the opossum perma- 

 nent ; and, as Selenka has shown, it contributes the major part 

 of the definitive amnion. 



Differentiation in the medullary plate and the neural crest. 

 The medullary plate has elongated until it is about six and 

 one-half times the length of the primitive groove, which is 

 still clearly visible in its caudal tip (fig. 20, totomount and 

 section H) ; and in the brain region the anlagen of the three 

 primary divisions are now distinguishable. 



The forebrain consists only of its diencephalic portion, and 

 is the region under which the subcephalic fold extends in 

 figure 20 (totomount and I). It includes the above-mentioned 

 optic cups and infundibular groove, and it is broader than the 

 midbrain region next to be described. 



The midbrain is demarcated laterally by two very shallow 

 grooves best seen in sections (vide fig. 20, C). Anteriorly and 

 posteriorly it is demarcated by the flaring edges of the wider 

 fore- and hind-brains. Selenka was unable to see any definite 

 lateral border to the midbrain. This is another indication that 

 most of his information about this stage came from his study 

 of the whole embryo before sectioning, and that some of the 

 sections were either lost or injured. The groove which bounds 

 the midbrain laterally being shallow and being underlaid by 

 the thick opaque mass of mandibular mesenchyme is almost 

 impossible to see except in sections. 



The hind brain extends from the point where the gasserian 

 ganglion (v.i.) is proliferating, back to the somite region. 

 In its anterior third it includes at least three recognizable 

 neuromeres (fig. 20, 1). It will later show six. 



The cordal portion of the medullary plate is composed of 

 a narrower cervical region and a broader thoracic region. 

 The latter, which is posterior to the seventh somite, includes 

 Hensen's node and the primitive groove. 



The neural crest is proliferating most abundantly just 

 anterior and just posterior to the first gill pouch. These pro- 

 liferations are at the sites of the anlagen of the gasserian 

 and acoustico-facial ganglia, respectively (vide fig. 21), but 



