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CARL GOTTFEIED HARTMAN 



the centers of the two pairs of blastomeres of any one egg. Some 

 of the eggs of this batch were studied in alcohol and, after stain- 

 ing with Delafield's, also in xylol. To facilitate the study of 

 eggs in toto, the shells were removed, when the arrangement of 

 the two blastomeres could be plainly made out. That the pairs 

 of blastomeres of these eggs lie at different levels is shown by the 

 fact that it is impossible to orient the eggs in such a way that a 

 single section may pass through all four of the cells. Usually 

 only two cells are found in any one section (figs. 11 to 16) ; some- 



A B C 



Fig. 1 Sketches made from wax models of 4- and 6-celled stages. A, No. 

 83 (2); B, No. 81 (6); C, No. 117 (4). In B, ZP, zona pellucida; ALB, albumen. 

 The space between blastomeres and zona in B is occupied by yolk and coagulum 

 (X 330). 



times a third cell may also be cut by the section (figs. 17 and 

 18). 



In the 6-celled and 7-celled stages (text figs. 1, C and 2, A) 

 the position of the pairs of daughter cells bears out the process 

 described above for the 4-celled stages. 



It is thus seen that if any polarity could be attributed to the 

 undivided opossum egg on account of the original position of 

 pronuclei at one side, or of other facts presented above, no 

 stretch of the imagination could find a trace of polarity in the 

 4-celled stage. The blastomeres appear to be identical and 

 shift their position as in the Eutheria. 



