364 



THE CHORDATE BLASTULA AND ITS SIGNIFICANCE 



regarded as a kind of polyinvagination. In this manner the secondary blastula 

 is formed. It is composed of two layers of cells, the epiblast above and the 

 hypoblast below with the secondary blastocoelic space insinuated between 

 these two layers. 



b. Metatherian Mammal, Didelphys 



The opossum, Didelphys virginiana, possesses a hollow blastocyst akin to 

 the eutherian variety. (See Hartman, '16, '19; McCrady, '38.) As observed 

 in the previous chapter, it is produced by a peculiar method. The early blasto- 

 meres do not adhere together to form a typical morula as in most other 

 forms; rather, they move outward and adhere to the zona pellucida and come 

 to line the inner aspect of this membrane. As cleavage continues, they even- 

 tually form a primary blastula with an enlarged blastocoel. 



Following this primary phase of development, one pole of the blastocyst 

 begins to show increased mitotic activity, and this polar area gradually thickens 

 (fig. 176A). At this time certain cells detach themselves from the thickened 

 polar area of the blastocyst and move inward into the blastocoel (fig. 176A, B) . 



INNER CELL MASS 



Fig. 177. Schematic drawings of early pig development. (A) Early developing blasto- 

 cyst. (B) Later blastocyst, showing two kinds of cells in the inner cell mass. (C) 

 Later blastocyst, showing disappearance of trophoblast cells overlying the inner cell mass. 

 (D) Later blastocyst. Two layers of formative cells are present as indicated with tropho- 

 blast tissue attached at the margins. 



