18 J. P. HILL. 



animal pole of tlie egg-, the opposite pole in relation to which 

 the formative cytoplasm is situated being the lower or 

 vegetative. The deutoplasmic cytoplasm thus lies in the 

 upper hemisphere, whilst the formative cytoplasm occupies 

 the lower. If Yan der Stricht's determination of the poles of 

 the ovum of Vesperugo be accepted, then we must conclude 

 that the poles of the Dasynrus ovum are exactly reversed as 

 compared with those of the bat's egg. In this connection it 

 may be recalled that Lams and Doorme ['07] have demon- 

 strated the occurrence in Cavia of an actual reversal of the 

 original polarity of the ovum, prior to the beginning of 

 cleavage. These facts may well give us pause before Ave 

 proceed to attach other than a purely secondary significance 

 to the exact location of the formative and deutoplasmic con- 

 stituents in the Metatherian and Eutherian ovum. But 

 besides this apparent difference in the location of the deuto- 

 plasmic constituents of the ova of Dasyurus and Yesperugo, 

 there exists yet another which concerns the fate of these con- 

 stituents in the respective eggs. In Yesperugo, Yan der 

 Stricht shows that the " deutoplasm " remains an integral 

 part of the egg, and retains its polar distribution in the 

 blastomeres up to at least the 4-celled stage. ^ In Dasyurus, 

 on the other hand, the fate of the deutoplasmic mass is a very 

 different, and, indeed, a very remarkable one. It does not 

 remain an integral part of the segmenting egg as in Yesperugo, 

 but prior to the completion of the first cleavage furrow it 

 becomes bodily separated off, apparently by a process of 

 abstriction, from the formative cytoplasm as a clear rounded 

 mass which takes no further direct part in the developmental 

 pi'ocesses. As soon as its elimination is effected, the remainder 

 of the cytoplasmic body of the ovum, formed of the formative 

 cytoplasm alone, divides into the first two equal-sized blasto- 

 meres, tlie first cleavage plane being coincident with the polar 

 diameter and at right angles to the plane of separation of the 

 deutoplasmic mass, or "yolk-body " as we may term it (PI. 2, 

 figs. 14-16, 19, y.h.), so that it is this formative zone of the 

 ' Vide, however, "Addendmn" (p. 121). 



