THE EARLY DEVELOPMENT OF THE MARSUPIALIA. 19 



ovum which is alone concerned in the production of the 

 embryo and its fcetal membranes. 



We have but to recall the conclusion already reached that 

 the clear vacuolated zone at the upper pole of the ripe ovum 

 of Dasyurus consists of siirphis material, mainly in the form of 

 fluid of deutoplasmic nature Avhich has not been utilised in 

 the upbuilding of the formative cytoplasm, and the signifi- 

 cance of this remarkable and, so far as the Mammalian ovum 

 is concerned, absolutely unique occurrence becomes at once 

 manifest.^ We have to do here with an actual elimination of 

 surplus deutoplasmic material by the Marsupial ovum — a phe- 

 nomenon only paralleled elsewhere, so far as I. am aware, and 

 even then but distantly, by the curious temporary separation 

 of the so-called yolk-lobe which occurs during the cleavage 

 of the yolk-laden eggs of certain Molluscs (Nassa, Ilyanassa, 

 Modiolaria, Aplysia, Dentalium) and Annelids (Myzostoma, 

 Chgetopterus). In these forms cleavage of the ovum into the 

 first two blastomeres is accompanied by the separation of a 

 portion of the ovular substance in the form of a non-nucleated 

 mass or so-called yolk-lobe. This latter, which has been shown 

 to be connected wath the formation of determined organ- 

 anlagen, reunites Avitli one of the two blastomeres, and then 

 the same process of abstriction and reunion recurs at the 

 second cleavage.' We have here evidently a purely adaptive 

 phenomenon, the object of which no doubt is to permit of the 

 total cleavage of the yolk-laden ovum on what are presumably 

 the old ancestral lines, and I believe a comparable explanation 

 will be found applicable to the elimination of surplus yolk- 

 material by the Marsupial ovum. 



As regards the significance of the occurrence of the deuto- 

 plasmic zone in the ovum of Dasyurus, holding the views that 

 T do as to the phylogeny of the Marsupialia (viz. that the 

 Metatheria and Eutheria are the divergent branches of a 



1 Yide "Adcleudum'' (p. 121), in wliicli reference is made to the dis- 

 covery by Prof. Van der Striclit of the elimination of deutoplasm in 

 the ovum of Vesperugo. 



2 Cf. Korschelt n. Heider, ' Lehrbnch d. vergi. Entwicklvmgs- 

 geschichte,' Lief. 3, p. 107, 1909. 



