DEVELOPMENT OF THE OPOSSUM 15 



In order to determine with accuracy the arrangement of the 

 blastomeres in 4- to 16- celled stages, wax models were made 

 after the method of Born. 



MATURATION AND FERTILIZATION 



a. The rnature ovarian ovum 



A detailed account of the ovarian egg is not now attempted. 

 It is deemed desirable, however, to determine two points: first, 

 the time at which the first polar body is given off, and second, 

 the distribution *of yolk in the ovarian egg. 



In the case of two females studied, ovulation had not yet 

 taken place three days after copulation. The Graafian follicles 

 were tensely filled with liquor folliculi and bulged out like great 

 glassy beads from the surface of the ovary. The sectioned 

 ovaries were found to contain ripe eggs (fig. 2), as was to be 

 expected; but younger and much smaller follicles taken from 

 other females also contained eggs which had given off the first 

 polar body. Hence, the first maturation takes place some days 

 before ovulation. More exact data cannot be given at this time. 



The ripe ovarian egg is broadly elliptical in form and measures 

 in section on the average 0.165 by 0.135 mm. It is surrounded 

 by a well-defined zona pellucida, within which the flattened 

 polar body is found. The polar body is given off at one pole of 

 the elongated egg or at the equator, or it may indeed be seen 

 at any other point on the margin of a section passing through 

 the long axis of the egg. The germinal vesicle of the egg has 

 not yet re-formed preparatory to the second maturation, for the 

 chromosomes are very short rods arranged in the form of an open 

 ring. They lie in the cytoplasm just beneath the surface of the 

 egg and usually to one side of the polar body (fig. 2). 



Aside from the position of the polar body there is no other 

 structural evidence of polarity in the ripe ovarian egg of the 

 opossum. Of the many eggs studied in sections none showed 

 any massing of yolk toward one pole such as is strikingly the 

 case in the egg of Dasyurus. This lack of polar concentration 

 of yolk in the mature egg of the opossum is the first important 



