TYPES OF CHORDATE BLASTULAE 



363 



FORMATIVE 

 CELLS 



Fig. 176. Early development of blastoderm of the opossum. (Modified from Hartman, 

 '16.) (A) Blastocyst wall composed of one layer of cells from which entoderm ceils 

 are migrating inward. (B-D) Later development of the formative portion of the blasto- 

 derm. Two layers of cells are present in the formative area, viz., an upper epiblast layer 

 and a lower hypoblast. Trophoblast cells are shown at the margins of the epiblast and 

 hypoblast layers. 



Both hypoblast and epiblast are connected peripherally with the periblast 

 tissue. 



5. Formation of the Late Mammalian Blastocyst (Blastula) 



a. Prototherian Mammal, Echidna 



In Echidna, according to Flynn and Hill ('39, '42), a blastoderm some- 

 what comparable to that of reptiles and birds is produced. An early primary 

 blastular condition is first established, consisting of a mass of central cells 

 with specialized vitellocytes at its margin (fig. I75A). A little later, an ex- 

 tension of this blastoderm occurs, and a definite primary blastocoelic space 

 is formed below the blastoderm (fig. 175B). During this transformation, 

 small, deeper lying cells (shown in black, fig. 175B) move up to the surface 

 and become associated with the thinning blastoderm which essentially becomes 

 a single layer of cells (fig. 175C). The marginal vitellocytes in the meantime 

 fuse to form a germ-wall syncytium. This state of development may be re- 

 garded as the fully developed primary blastula. A little later, this primary 

 condition becomes converted into a two-layered, secondary blastula, as shown 

 in figure 175D by the secondary multiplication and migration inward of the 

 small cells to form a lower layer or hypoblast. The latter process may be 



