THE EAlilA' DKVElJJl'MEXT *JF THE MAliSUPlALIA. 17 



animal, tandis qu'il diminue an pole vegetatif, et le deuto- 

 plasme^ parseme d'un plus grand nombre de boules graisseuses, 

 constitue une masse spherique excentrique, voisine des deus 

 globules polaires" (Van dei- Striclit, '03, pp. 44-45). Ic is 

 evident, then, that the fertilised ovum of Vesperugo exhibits a 

 polarity comparable with tliat of the ripe ovarian ovum of 

 Dasyurus, and that the vitellogenetic processes in the ova of 

 these two widely separated forms proceed along lines almost 

 identical, at all events so far as their broad outlines are con- 

 cerned. In both we find during growth a progressive 

 vacuolisation of the egg-cytoplasm consequent on the elabora- 

 tion of a deutoplasmic fluid. In both, the "pseudo-alveolar" 

 condition so engendered is followed by one in which there 

 is recognisable a diiferentiation into a peripheral "forma- 

 tive" zone rich in deutoplasmic gi-anules, and a central 

 "deutoplasmic" zone rich in fluid yolk, and finally in both 

 there occurs a segregation of the granular '■formative" and 

 fluid yolk-constituents to opposite regions of the egg, with 

 resulting attainment of a definite polarity. In view of the 

 close general agreement in the vitellogenetic processes, and in 

 the constitution of the ova in Yesperugo and Dasyurus, it 

 might be expected that the poles would accurately correspond, 

 but such is not the case if Yan der Stricht's determination of 

 the poles in the ovum of Yesperugo is correct. In the latter, 

 according to Yan der Stricht, the deutoplasm is located at 

 that pole from which the polar bodies are given oif; at the 

 opposite pole the "plastic" vitellus accumulates, and close to 

 it the two pronuclei unite and the first cleavage spindle is 

 formed. Accordingly Yan der Stricht concludes that " le 

 premier pole correspond au pole vegetatif, le second au pole 

 animal des oeufs a deutoplasme polaire (0. Hertwig)." In 

 Dasyurus, on the other hand, I am perfectly convinced (and 

 adequate reason for my conviction will be forthcoming in the 

 course of my description of the processes of cleavage and 

 germ-layer formation) that the pole of the ripe ovum in 

 relation to the mass of deutoplasmic cytoplasm is not the 

 vegetative pole, but represents morphologically the upper or 



VOL. o6, PART 1. NKW SERIES. 2 



