THE EARLY DEVELOPMENT OF THE MARSUPIALIA. 87 



At the moment of entering the oviduct it has a diameter of 

 3'5-4 mil). (2*5-3 mm. according to Caldwell), and is therefore 

 small relatively to that of a reptile of the same size as the 

 adult Monotreme, but large relatively to those of other 

 mammals, being about twelve times larger than that of 

 Dasyurus, and about eighteen times larger than that of the 

 rabbit. 



Cleavage is meroblastic. The first two cleavage planes are 

 at right angles to each other, as in the Marsupial, and divide 

 the germinal disc into four approximately equal-sized cells 

 (Semon, Taf. ix, fig. 30). Each of these then becomes sub- 

 divided by a meridional furrow into two, so that an 8-celled 

 stage is produced, the blastomeres being arranged symmetri- 

 cally, or almost symmetrically, on either side of a median line, 

 perhaps corresponding to the primary furrow (Wilson and Hill, 

 p. 37, text-figs. 1 and 2). Imagine the yolk removed and the 

 blastomeres arranged radially, and we have at once the open 

 ring-shaped 8-celled stage of Dasyurus. The details of the 

 succeeding: cleavagfes are unknown. Semon has described a 

 stage of about twenty-four cells (Semon, Taf.ix,fig.31),mwhich 

 the latter formed a one-layered circular plate with no evidence 

 of bilateral symmetry, and this is succeeded by a stage also 

 figured by Semon (figs. 32 and 33, cf. also Wilson and Hill, 

 PL 2, fig. 2), in which the blastoderm has become several 

 cells thick, though it has not yet increased in surface extent. 

 It is bi-convex lens-shaped in section, its lower surface being 

 sharply limited from the underlying white yolk. No nuclei 

 are recognisable in the latter, either in this or any subsequent 

 stage, nor is there ever any trace of a syncytial germ-wall, 

 features in which the Monotreme egg differs from the 

 Sauropsidan. 



The next available stage, represented by an egg of Oruitho- 

 rhynchus, described by Wilson and Hill ("07, p. 38, PL 2, fig. 

 4), and by an egg of Echidna, described by Semon ('94, p. 69, 

 figs. 22 and 33), is separated by a considerable gap from the 

 preceding, and most unfortunately so, since it belongs to the 

 period of commencing formation of the germ-layers. The 



