DEVELOPMENT OF THE OPOSSUM 31 



A study of this abundant material makes it certain that the 

 third and fourth cleavages take place in the opossum ovum in 

 a manner quite different from that shown by Hill to hold for the 

 egg of Dasyurus. In that form the fourth cleavage is for the 

 first time horizontal and divides the single ring of eight conical 

 cells into an upper ring of small, relatively yolk-free cells and a 

 lower ring of larger, yolk-rich and more darkly staining cells. 

 The former are said to be destined to produce the formative por- 

 tions of the blastocyst. Thus a single section of the 16-celled 

 egg may pass through all of the eight blastomeres of a given ring 

 (text fig. 4, E). This description does not hold in the slightest 

 degree for the opossum egg in the corresponding stage. 



The blastomeres of the 4-celled opossum egg having shifted 

 about at right angles as described above, one would expect the 

 planes of the third cleavage also to pass irregularly and at various 

 angles through the four blastomeres. This is the case. The 

 shifting of the blastomeres continues as cleavage progresses. 

 The cells migrate more and more to the periphery and spread 

 about irregularly into the form of a hollow sphere. When the 

 16-celled stage has been reached, or even before, all semblance 

 of orderliness seems to have vanished. 



In another respect the opossum egg of sixteen cells differs from 

 the corresponding stage of Dasyurus (compare E and F, text 

 fig. 4). In the latter species the albumen layer is entirely 

 absorbed and the progeny of the sixteen blastomeres, as these 

 multiply, come at once to lie against the shell. In the 16-celled 

 stage of the opossum, however, there has been as yet no per- 

 ceptible absorption of albumen, the layer of which is enormous 

 as compared with that in the egg of Dasyurus. The albumen 

 of the opossum egg is not exhausted until the bilaminar blasto- 

 cyst is some 1.4 mm. in diameter and the formation of the meso- 

 derm and the primitive streak is initiated (Selenka). 



2) Cleavage indeterminate. After the opossum egg has passed 

 the 4-celled stage the planes of cleavage of any two dividing 

 cells are never parallel; that is, the axes of the division spindles 

 in any given stage point different directions. Already in the 

 6- or 7-celled egg (text figs. 1 and 2) , the continued shifting may 



