DEVELOPMENT OF THE OPOSSUM 56 



(figs. 29 and 32). All of the cells multiply faster than the ento- 

 dermal cells differentiate and as a result the cells become massed 

 and the embryonic area tends to thicken. The process is con- 

 tinued until the embryonic disc is about three cells deep (figs. 

 35 and 36) The innermost of these cells are clearly entodermal. 

 From the margin of the disc, entodermal cells may be seen to 

 m"grate out in an elTort to complete the lining of the blastocyst. 



If one compare sections through the embryonic area of the 

 opossum blastocysts which have just been described with sec- 

 tions through corresponding stages of Macropus andParameles, 

 as figured by Hill, the similarity is very striking. In (xW cases 

 the area tends to be much thicker than that of Dasyurus. This 

 agreement of the opossum with Macropus and Parameles is corre- 

 lated with the agreement in size of blastocysts as compared with 

 the blastocysts of Dasyurus at the time of entoderm formation; 

 for this process in Dasyurus is preceded by a period of extensive 

 growth. The bearing of this on the subject under discussion 

 will appear presently. 



Surface views of this stage of the opossum blastocyst were 

 not studied, since the technical difficulties of making surface 

 preparations are almost insurmountable on account of the small 

 size of the opossum blastocyst and the interference of the super- 

 imposed albumen. The corresponding stage in Dasyurus has 

 a diameter of 4.0 mm. (eight to ten times that of the opossum 

 blastocyst) and the albumen has long since disappeared (K and 

 J, text fig. 4). Inasmuch as surface views were not available, 

 the entoderm mother cells were not studied in toto, hence it 

 was not possible to determine whether or not these cells possess 

 pseudopodial prolongations and are capable of amoeboid move- 

 ments. Several cells seen in section, however, as pointed out in 

 detail above, strikingly resemble those shown by Hill in draw- 

 ings of sections through the embryonic area and interpreted by 

 him as primitive entoderm cells. These facts as they occur in 

 the opossum are presented as observed, and the interpretations 

 of the conclusions suggested are those which a careful study 

 of the material seems fully to justify. 



