DEVELOPMENT OF THE OPOSSUM 45 



the 2-celled egg, as previously described. Their position within 

 the cavity of the blastocyst is due to the migration of the cells 

 to the albumen layer some time after the 4-celled stage is passed. 



Besides these inclusions, the blastocyst cavity usually con- 

 tains entire cells as well as cell fragments bearing chromatin 

 matter (text fig. 5). Selenka also described and figured an 

 included cell in his 42 and 68-celled stages and mentions an 

 included cell in his much damaged 20-celled egg. Such cells 

 are found in the cavities of thirty-nine out of forty-four blasto- 

 cysts in my possession. 



The cellular material mentioned in the preceding section comes 

 to lie in the cavity of the blastocyst in two ways : first, by failure 

 of cells in about the 16-celled stage to reach the periphery and 

 second by proliferation from the blastocyst wall. The first 

 method seems highly probable, a priori, and is strongly sug- 

 gested by the presence of an included cell in the 16-celled egg, 

 specimen 85 (12), cited above. The model made of this egg 

 shows that the included cell had little chance of finding its 

 place in the blastocyst wall. The included cells of the 50-celled 

 stages are, moreover, in some cases fragmentary and show signs 

 of disintegration. 



The second method suggested is apparent from an inspection 

 of certain blastocysts shown in text figure 5 {F, I, L). There 

 are four types of cells in these blastocysts: 1) the ordinary flat 

 to cubical cells definitely located in the blastocyst wall ; 2) one 

 or more much larger cells, often columnar in shape, also situated 

 in the blastocyst wall and frequently darkly stained; these cells 

 sometimes jut out far into the cleavage cavity (text fig. 5 K); 

 3) included cells connected with the cells of the blastocyst, 

 clearly cases of proHferations from the cells of class 2; these 

 proliferated cells in several cases make a complete bridge across 

 a sector of the blastocyst (text fig. 5 F); 4) ordinary included 

 cells floating in the cavity singly or connected with other cells 

 as if just divided. Since the majority of these early blastocysts 

 are absolutely normal, the included cells doubtless have no 

 special significance. A study of blastocysts in the next stage 

 indicates that the included cells do not take part in the formation 



