28 CARL GOTTFRIED HARTMAN 



possesses a potential, though concealed, polar differentiation into 

 formative and non-formative cells. This I believe is the true 

 interpretation. The subject will be pursued further below. 



b. Selenka's four-celled stage 



The normal 4-celled egg of the opossum as here described is 

 totally at variance with the single 4-celled specimen seen by 

 Selenka'^ as well as with the normal stage in Dasyurus. But I 

 am fully convinced that Selenka's only 4-celled specimen was 

 abnormal and in the process of disintegration. I have several 

 times seen such eggs consisting of four radially arranged 'blasto- 

 meres' or egg fragments. A striking case is an egg of batch No. 

 112, in which the four cells seem an almost perfect replica of 

 Selenka's specimen. At their upper ends the blsstomeres fit 

 together by their inner angular surfaces but their lower ends 

 are spread apart. The cells are crowded to one pole of the egg, 

 touching the zona with their upper ends, a condition to a cer- 

 tain extent true of the egg figured by Selenka. This egg was 

 studied in alcohol and xylol in toto and was then perfectly 

 oriented and imbedded in paraffin, and sectioned. The pre- 

 vious diagnosis of abnormality was sustained by a study of the 

 sections. The nuclei of the cells were very abnormally placed 

 and other signs of degeneration were apparent. All other eggs 

 of this batch No. 112 were in various stages of disruption. I 

 am perfectly convinced that Selenka's 4-celled stage is abnormal 

 and does not represent the normal condition in the opossum, 

 notwithstanding that it is comparable with the 4-celled egg of 

 Dasyurus. 



c. The distribution of yolk 



The blastomeres of the 4-celled eggs of the entire collection 

 are in the same stage of advancement; they have fully formed 

 nuclei and no case of mitosis occurs among them. It would seem, 

 therefore, that the blastomeres of the 2-celled stage divide prac- 

 tically at the same time. In all cases the relation of cells to 

 yolk is the same. 



^ Loc. cit., p. 113; figures 4 and 5, plate 17. 



