38 CARL GOTTFRIED HARTMAN 



in den alteren Gastrulastadien ; man wird daher zu der Annahme 

 gedrangt, dass diese Dotterballen von den Furchungszellen ausge- 

 'stossen, bald aber wieder resobirt werden/ 



Selenka was thus of the opinion that the yolk is given off in the 

 8-celled stage and not earlier. 



It has been shown above that in the 4-celled stage of the 

 opossum egg the yolk practically surrounds the blastofneres. 

 In the next division the blastomeres exhibit a centrifugal tend- 

 ency and sooner or later come to lie closely appressed to the 

 albumen layer. Hence the eliminated yolk comes to occupy a 

 central position until absorbed. By what physiological activities 

 the blastomeres are forced against the albumen I am not pre- 

 pared to say. The fact is they do pass peripherally and occupy 

 a point of vantage where they may best absorb the nutriment 

 from the albumen and the included yolk as well. 



The distribution of yolk remaining within the blastomeres in 

 eggs from five to sixteen cells occurs as in preceding stages, 

 namely in a peripheral belt (figs. 19 and 23). The relative 

 quantity of yolk within and without the cells is again a variable 

 one. Where the blastomeres touch the albumen layer there is 

 often less yolk or even none at all, the yolk granules or vacuoles 

 being concentrated at the surface toward the cleavage cavity 

 (fig. 23). Preparation of eggs fixed in Bouin's fluid show large 

 vacuoles at the rounded free side of the cell. Sometimes these 

 vacuoles coalesce into irregular spaces as if the cells broke down 

 at this point under the action of fixatives, which recalls the 

 behavior of the delicate deutoplasmic portion of the unseg- 

 mented egg of Dasyurus.^ 



c. Absence of yolar differentiation 



Although there is a shifting of blastomeres in the egg of the 

 opossum after the second cleavage, the daughter cells of either 

 of the first two blastomeres are, no doubt, to be found in one or 

 the other hemisphere of the 16-celled stage (text figs. 1 to 3). In 

 models of certain eggs containing 6, 7, 8, 10, 12, and 16 cells 



^ Loc. cit., p. 115. 

 «Hill, p. 12. 



