304 



CLEAVAGE (SEGMENTATION) AND BLASTULATION 



figs. 144G, H; 145J-L.) This condition is found in the monkey, 

 human, pig, rabbit, etc. 



(2) On the other hand, in certain marsupials, such as the American opos- 

 sum, Didelphys virginiana, and the BraziUan opossum, Didelphys 

 aurita, the inner cell mass is much less prominent during earlier stages 

 of the blastocyst. In these species it is indicated merely by a thickened 

 aggregation of cells at one pole of the blastocyst (fig. 147E-G). 



(3) In the marsupial or native cat of Australia, Dasyurus viverrinus, cleav- 

 age results in an early blastocyst in the form of a hollow sphere of 

 rounded cells. As the blastocyst expands, the cells increase in number 

 and become flattened to form a thin layer of cells apposed against the 

 shell membrane without an apparent inner cell mass or embryonic 

 knob (fig. 148A-C). 



A conspicuous feature of cleavage and early blastocyst formation in the 

 marsupials should be emphasized. For in this group, the early blastomeres 

 apparently use the framework of the zona pellucida as a support upon which 

 they arrange themselves. As a result, the blastocoelic space of the blastocyst 



Fig. 148. Early blastular conditions of the marsupial cat of Australia, Dasyurus 

 viverrinus. (After Hill, '10.) (A) Early blastula. (B) External view of blastocyst, 0.6 

 mm. in diameter. The cells are becoming flattened and finally reach the condition shown 

 in (C). (C) Section of wall of blastocyst. 2.4 mm. in diameter. 



