CLEAVAGE. 



49 



differentiation into cells (inner cell mass), which are destined to form the 

 embryo proper, and cells (outer cells covering layer) which are to engage in 

 the development of certain accessory structures. 



Recent studies of opossum ova (Hartman) have shown that in this form the 

 morula stage is absent. During segmentation the blastomeres migrate periph- 

 erally and form a single layer of cells around a central cavity, although a few 

 cells usually remain free within the cavity. At about the 4o-celled stage the 

 majority of the cells forming the wall of the hollow structure (blastocyst) begin 





1 



FIG. 33. Four stages in the development of the bat. van Beneden. 



a, Section of morula; b, section of later stage of morula, showing differentiation of outer layer of 

 cells; c, section of still later stage, showing vacuolization of central cells; d, section showing outer 

 layer (trophoderm) and inner cell mass. 



to diminish in thickness, while a few at one point increase in thickness. The 

 latter proliferate to form a little mass which probably corresponds to the inner 

 cell mass described for the bat. The layer of thin cells forming the major 

 portion of the wall of the blastocyst may be considered as comparable with the 

 covering layer in the bat. The cells in each region are probably lineal descend- 

 ants of one or the other of the two primary blastomeres, although the latter 

 exhibit no distinguishing features; one blastomere gives rise to embryonic 

 structures proper and the other to extraembryonic or accessory structures. 



