THE EMBRYOLOGY OF THE OPOSSUM 55 



from this fusion in a lateral direction, as well as from the 

 whole length of the primitive streak. The latter structure 

 has developed two folds along its edges near the posterior 

 tip so that it may now more appropriately be called the 

 primitive groove. The mesoderm occupies only a small oval 

 region around the primitive groove, and beyond this region 

 the medullary plate is still in contact with the endoderm. 

 Hartman's litter no. 338 represents this stage. 



Stage 19 



Elongation of the medullary plate. Shortly after the 

 primitive streak appears the posterior portion of the medul- 

 lary plate including it begins to grow in a caudal direction. 

 This process actually begins in stage 18, but it becomes con- 

 spicuous for the first time in stage 19. At this time the 

 whole primitive streak has become converted into a groove, 

 and its edges have begun to wrinkle as if they are growing too 

 fast for the surrounding cells to keep up with them. This 

 is particularly true at the posterior tip. The mesoderm now 

 extends anterior to Hensen's node, but there remains a 

 crescentic strip of the anterior portion of the medullary plate 

 which is still in contact with the endoderm. It is interesting 

 that at this time the mesoderm is continuous across the mid- 

 line in front of Hensen's node. 



Stage 20 



The mesodermal rim. Just before the mesoderm reaches 

 the anterior extremity of the medullary plate it begins to 

 extend beyond it posteriorly. The extreme edge of this 

 spreading sheet forms a dark irregularly circular line the 

 mesodermal rim along which the mesoderm is fused with 

 and possibly arising from the endoderm. In stage 20 this 

 line first becomes visible beyond the medullary plate in the 

 posterior region. In slightly later specimens (e.g., 17179) it 

 has migrated well beyond the medullary plate and com- 

 pletely surrounds it. 



