THE EMBRYOLOGY OF THE OPOSSUM 57 



Stage 22 



The first somites and the parietal mesoderm. In specimen 

 17180 (see stage 22 in the second normal stage plate) in which 

 the medullary groove has become nearly twice as long as the 

 primitive groove, two slight condensations of the mesoderm 

 beneath the medullary plate on either side of the central 

 mesoderm-free strip represent the first somites. 



Selenka figured (somewhat diagrammatically) a similar 

 specimen (1887, Taf. XX) with three somites. In it he recog- 

 nized a longitudinal mesodermal thickening lateral to the 

 medullary plate, which he called the parietal mesoderm. In 

 the absence of a series of intermediate stages, however, he did 

 not realize that the ectodermal thickening which he now cor- 

 rectly calls the 'Medullarplatte' is the same ectodermal 

 thickening which in the bilaminar blastocyst he labeled 

 'Keimscheibe.' 



This ectodermal plate, which first appears in stage 11 

 (latter part of the fourth day), is continuously and conspicu- 

 ously recognizable from then on. Its visible distinction from 

 the rest of the vesicle in all the early stages seems to be due 

 principally to a greater density of its cytoplasm. In fixed 

 but unstained specimens this dense plate is more opaque than 

 the rest of the vesicle, so that it appears darker by trans- 

 mitted light and lighter by reflected light. In stained speci- 

 mens it is always more chromophilic not particularly baso- 

 philic or acidophilic, but more retentive than the rest of the 

 vesicle for almost all stains. 



I emphasize this difference in density of cytoplasm and con- 

 sequent staining reaction because the distinctive feature usu- 

 ally attributed to this plate (that of thickness) is rather in- 

 consistent. At the time when the vesicle is swelling rapidly 

 under the pressure of fluid accumulating in the blastocoel 

 (stages 11 and 12) the less dense portion of the vesicle be- 

 comes enormously stretched and flattened, whereas the denser 

 medullary plate remains comparatively thick and occupies a 

 smaller proportion of the surface. Later on, the so-called 



