THE EMBRYOLOGY OF THE OPOSSUM 45 



blastocyst and the reduction of the albumen. Both of these 

 processes are more conspicuous in the events of the fifth day, 

 but their first stages can be seen in vesicles approximately 

 4 days old. 



Among these vesicles, which are from 0.20 to 0.34 mm. in 

 diameter, the endoderm mother-cells have undergone further 

 differentiation. The spherical endoderm mother-cells, which 

 were free in the blastocoel, soon come into contact with the 

 external layer of protoderm. They then flatten out upon it 

 just as the spherical blastomeres in early cleavage stages 

 flatten out upon the zona pellucida, and doubtless as a result 

 of the same reaction stereotropism. When more or less flat 

 and in contact with one another or with the protoderm they 

 are called primitive endoderm cells (stages 11 and 12 and 

 fig. 14). These cells multiply mitotically and soon accumlate 

 to a thickness of two or three cells immediately under the 

 protoderm, which at the same time thickens throughout the 

 region that they touch. 



The medullary plate. The most interesting and unexpected 

 fact that I have come across in my study of the embryology 

 of the opossum is that the dense plate of cuboidal superficial 

 cells thus formed is a medullary plate (McCrady, '37). In 

 the light of the embryology of all the lower vertebrates this 

 is exactly what one should expect to find ; but in the opossum 

 this structure was observed 50 years ago by Selenka (1887) 

 and has since then been seen in many other marsupials, and 

 has been studied by many investigators, all of whom regarded 

 it as an 'embryonic area.' In other words, it has always been 

 supposed that all the embryonic ectoderm (epidermis as well 

 as nervous system) is derived from this plate, and that the 

 ectoderm outside this region gives rise to nothing but extra- 

 embryonic membranes. As a matter of fact, none of the 

 epidermis is derived from this plate, and the extra-embry- 

 onic ectoderm is not in any way demarcated from that which 

 will produce body epidermis. The previous interpretation 

 was based upon analogy with the supposed separation of em- 

 byronic and non-embryonic or trophoblastic cells in the 



