DEVELOPMENT OF THE OPOSSUM 29 



The distribution of eliminated yolk is as would be expected, 

 a priori, if one accept the interpretation of the 2-celled stage 

 advanced above. If there were any doubt as to these views on 

 the expulsion of the yolk in the 2-celled stage on account of the 

 meager available material on which the conclusions were based, 

 the explanation presented is fully justified by a study of the 

 abundant material secured at the end of the second cleavage. 

 The four blastomeres in all cases are almost completely sur- 

 rounded by precipitated material, the 'yolk.' Rarely do the 

 blastomeres touch each other and still more rarely are they in 

 contact with the albumen layer or with the zona, in case the 

 latter still persists. 



The structure of the 'yolk' in the eggs of specimens Nos. 81, 

 83, and 54 is peculiar to these eggs (figs. 11 to 18) and was not 

 met in any others, not even in the blastocysts furnished by No. 

 83, in addition to the 4-celled stages. It is a more or less homo- 

 geneous mixture of discharged cytoplasm, fat (yolk) and coagu- 

 lum (protein of the egg sap) — all of these mixed together and 

 precipitated by the fixing fluids into fiocculent masses (fig. 18). 

 There is often a more or less even distribution of small yolk 

 granules (fig. 17) or fat vacuoles, but they are not as prominent 

 as in other eggs. In other stages the eliminated material is 

 usually massed in lumps, in which the black yolk globules or 

 clear vacuoles are plainly seen; and in many eggs the coagulum 

 appears as a thin precipitate separate from the other substances. 

 Of the 4-celled eggs only egg No. 52 (3) d shows the usual appear- 

 ance of the yolk; perhaps because of a different treatment of the 

 preparation. 



In less than half of the specimens at this stage one polar body 

 was seen; in a few both polar bodies were found. The zona 

 pellucida was observed in only two or three eggs. The blasto- 

 meres themselves have no cell membranes, excepting again No. 

 52 (3) d. The cells are usually well marked but there is no 

 sharp, clear-cut line or membrane (figs. 17 and 18). 



The blastomeres of the different eggs vary considerably in 

 size, measuring from 0.043 to 0.056 mm. But this variation is 

 not due to differences in size of eggs, that is, of volume enclosed 



