DEVELOPMENT OF THE OPOSSUM 49 
places against the wall of the ovum and here undergo further 
division and further flattening until they come into mutual 
contact and thus complete the blastocyst wall, leaving the yolk 
within the cavity. Figure 19, plate 15, is a section through an 
ovum of 26 cells; figure 20 through one of 28 cells. In both 
cases there are gaps in the wall of the blastocyst, indicating 
that this is not yet complete. ‘The same is true of two eggs of 
30 and 32 cells, respectively, in which the gaps are fewer in 
number (fig. 3, pl. 4). In figure 10, plate 13, and figure 1, 
plate 14, are shown sister ova of 32 and 34 cells, respectively; 
their walls are practically continuous and the blastocyst may be 
considered complete. Occasionally more advanced blastocysts 
still have gaps in their walls, as, for example, the one shown in 
figure 6, plate 6, which has 46 cells. We may say, however, 
that, on the average, the blastocyst wall is completed when the 
32-celled stage is reached or soon thereafter. No polarity is 
evident in the egg, the cells being of uniform size and structure 
throughout. Not long after this the entoderm formation is 
initiated. 
Hence, in the opossum the blastocyst is completed at a much 
earlier stage than in Dasyurus, where the blastomeres of the 
16-celled egg are arranged in two superimposed rings at the 
equator of the egg. To form the blastocyst wall they must 
proliferate and migrate toward either pole, and the blastocyst 
is not completed until the gaps at the two poles are closed. In 
the opossum, on the contrary, to complete the blastocyst all 
that is necessary is the closing of the gaps between the cells 
which are early distributed more or less evenly at the periphery. 
The just completed blastocyst of Dasyurus contains more than 
three times the number of cells (90 to 130) than does the corre- 
sponding stage of the opossum, and it is three times as large. 
At this stage in the opossum, neither the ovum nor its en- 
velopes have increased perceptibly in size (pl. 12). The albu- 
men layer lies over the ovum as thickly as before, again in 
striking contrast with the condition in Dasyurus, in which the 
albumen layer is completely resorbed when the. egg has reached 
the 16-celled stage. The opossum blastocyst is completed about 
