THE EMBRYOLOGY OF THE OPOSSUM 49 



During the rapid increase in diameter which accompanies 

 the migration of the endoderm the walls of the vesicle become 

 markedly thinner. This thinning affects all portions of the 

 vesicle but in the medullary plate it seems less conspicuous. 

 The migration of so much endodenn from beneath the medul- 

 lary plate, however, reduces that region to a thickness of only 

 two cell layers again (i.e., medullary plate and endoderm). 

 The medullary plate remains distinctly thicker than any other 

 portion of the vesicle. 



The end of the fifth day is not characterized by any very 

 distinctive feature. The vesicle has enlarged to about 

 0.50 mm., and has not yet reached the shell membrane. The 

 vegetal pole is at the center of the albumen. The endoderm 

 extends to the general neighborhood of the equator of the 

 vesicle. 



Stage 14 



The bilawinar Hastocyst. The end of the sixth day is 

 marked both by the contact of the medullary plate with the 

 shell membrane, and by the completion of the endodermal 

 lining of the vesicle. The diameter of the vesicle at this time 

 is about 0.75 mm. (stage 14 and fig. 16). 



The cells of the different layers show the following dis- 

 tinguishing characteristics. The ectodermal cells outside the 

 medullary plate are intermediate in thickness; they stain 

 more lightly than any of the others; they contain practically 

 no yolk granules; their nuclei are far apart (on account of 

 the flatness of the cells) ; and their cytoplasm is rather easily 

 broken down by fixing agents. The medullary plate cells are 

 nearly columnar, so that their region is thicker than any 

 other part of the vesicle and their nuclei are very close 

 together ; their cytoplasm is denser and more yolk laden than 

 that of the rest of the ectoderm, and rather more basophilic. 

 The endodermal cells are thinner than any others; their nuclei 

 are larger in polar view than those of the ectoderm, and they 

 stain very darkly. 



