THE EAELY DEVELOPMEXT OF THE >[ARSrPIAL[A. 29 



tion of the volk-body is rapidly followed by the completion 

 of the division of the formative remainder of the ovum into 

 the first two blastomeres, the plane of division being- co- 

 incident with the polar diameter or egsr-axis and at rio^ht 

 angles to the plane of separation of the yolk-body PI. 2, ficr. 

 14). I obtained relatively little material between the stage 

 of the unsegmented ovum with two equal-sized pronuclei seen 

 in fig, 12 and the 2-celled stage (fig. I4j, both of which 

 are well represented in my material, so that it would appear 

 tliat the separation of the yolk-body and the division of the 

 formative remainder of the ovum are effected with considerable 

 rapidity. Fig. 13 shows, however, a section of an un- 

 segmented ovum in which the chromosomes of the metaphase 

 of the first cleavage figure are visible in the central region 

 of the formative cytoplasm, but situated, it is worthy of note, 

 nearer the future upper pole than the lower pole. The deuto- 

 plasmic zone (d.z.) still forms an integral part of the egg, and 

 there is no sign of commencing abstriction. I have also 

 sections of ova in a still more advanced stage of the first 

 cleavage, in which the daughter-nuclei have but recently been 

 constituted and are still quite minute, and the cleavage furrow 

 is well marked on the surface of the egg. In these ova the 

 yolk-body is already separated, so that we may conclude with 

 a fair degree of certainty that its elimination about coincides 

 with the first appearance of the cleavage furrow. 



Figs. 14—16 show the 2-celled stage, respectively in side, 

 lower polar, and end views. The blastomeres are of approxi- 

 mately equal size and otherwise quite similar. Selenka also 

 found the same to be the case in Didelphys, though in the 

 single specimen of the 2-celled stage he had for examination 

 (Taf. xvii, fig. 3) the blastomeres are displaced and somewhat 

 shrunken. Each blastomere has much the shape of a hemi- 

 sphere from which a wedge-shaped segment has been sliced 

 off, a form i-eadily accounted for when we ttike account of the 

 effect of the elimination of the deutoplasmic zone. After 

 that event, the formative remainder of the ovum has the 

 form of a sphere from which a somewhat bi-convex lens- 



