52 CARL GOTTFRIED HARTMAN 



c. Formation of entoderm in the opossum 



The conclusions which follow are based upon four blastocysts 

 of No. 40 and three of No. 43 and corroborative evidence is fur- 

 nished by a study of the somewhat transitional stages of eggs 

 in batch No. 144. From a study of these blastocysts the con- 

 clusion is inevitable that entoderm formation in the opossum 

 proceeds essentially as in Dasyurus, Macropus and Parameles 

 as described by Hill ('10), namely by a prohferation from special- 

 ized cells of the unilaminar blastocyst. 



An inspection of text figure 6 and figures 27 to 38 will show 

 that at this stage the formative or 'embryonic' area of the 

 blastocyst is sharply marked off from the extra embryonic region. 

 The cells of the former vary from cubical to somewhat elongated, 

 those of the latter are greatly attenuated. In cross-section 

 through the center of the embryonic disc this is seen to occupy 

 from one-fifth to one-third of the circumference of the section, 

 but on account of the great thinness of the non-formative cells, 

 the volume of this region is greater than that of the non-formative. 



At different points in the embryonic portion of the blastocyst 

 wall certain cells appear which stain more darkly than cells sur- 

 rounding them. These show a tendency to push out from their 

 place in the wall in quite the same manner as described by Hill 

 for similar cells in sections of blastocysts of Dasyurus. Inas- 

 much, furthermore, as these cells are seen frequently to divide 

 and give off dark-staining entodermal cells on to the inner surface 

 of the embryonic disc, we may conclude that we have here the 

 counterparts of Hill's 'entoderm mother cells.' 



This differentiation of entoderm mother cells into primitive 

 entoderm does not proceed as fast as the multiplication of all 

 of the cells in the embryonic disc. For scarcely a section can 

 be taken through this region without cutting through one or 

 more cells in mitosis. As a result, the area becomes at first 

 densely studded with nuclei (fig. 30) and later the cells may be 

 crowded in a flattened mass three cells deep (fig. 36). Sections 

 of such blastocysts are almost exact copies of the corresponding 

 stage figured by Hill ('10) of blastocysts of Macropus. The 



