DEVELOPMENT OF THE OPOSSUM 35 



trary, the two superimposed rings maintain a position more or 

 less parallel with the equator, so that a given section through the 

 egg in the plane of the equator would pass through all of the 

 eight cells of either ring (text fig. 4: E). A section through a 

 16-celled egg of the opossum (figs. 22-24) never passes through 

 the centers of more than four or five cells. Such a section of the 

 16-celled opossum egg is not unlike a section through the 16- 

 celled stage of a Eutherian mammal (fig. 22) .^ The early arrange- 

 ment of the cells of the opossum egg into a hollow sphere makes 

 possible the early formation of the completed blastocyst. The 

 blastomeres in the eggs of Dasyurus, which are arranged in rings 

 near the equator, must proliferate toward their respective poles. 

 The blastocyst is completed when the cells have met and closed 

 the gap at either pole. In the opossum egg the blastocyst is 

 completed when the scattered gaps seen in the wall of the 16- 

 celled eggs have been filled by a further division and a flattening 

 of the cells. This process may be completed in this way in eggs 

 having as few as thirty cells. 



The blastocyst formation, however, is anticipated in the 

 8-celled stage or indeed even before this time. For in 5-, 6- and 



Fig. 4 Diagrams to show comparative size and structure of the eggs of 

 Dasyurus and Didelphys (X 75). A, C, E, G, I and K, eggs of Dasyurus (all 

 except K after Hill); B, D, F, H, J and L, eggs of Didelphys (original). ALB, 

 albumen; EMB.ECT, embryonic ectoderm; TR.ECT, trophoblastic ectoderm; 

 SM, Shell membrane; Y, yolk; ZP, zona pellucida. 



A and B, pronuclear stage; the ovum of the opossum is seen to be much the 

 smaller; the yolk mass (F) is seen at one pole of the egg of Dasyurus. By com- 

 paring C, a fresh 2-celled egg of Dasyurus, with D, a fresh 4-celled egg of Di- 

 delphys, one can readily note the relative size of entire egg and of ovum proper 

 and the relative thickness of albumen and shell membrane of the two species. 

 Note extruded yolk in C and D. Compare also E and F. E represents a section 

 taken through an entire 'ring' of the 16-celled Dasyurus egg; no such ringed 

 arrangement occurs in the opossum egg of 16 cells (F). G (surface view) and 

 H (section) are blastocysts of equal size, but in the former the cells lie against 

 the shell membrane, whereas in the latter the blastocyst itself is only a little 

 larger than the unsegmented ovum. J, opossum blastocyst at time of ento- 

 derm formation; K, arc of Dasyurus blastocyst in corresponding stage, after 

 long period of growth, which does not occur in Didelphys. /, section of blasto- 

 cyst of Dasyurus early in the period of growth, the period of 'no differentiation' 

 (Cf. 1 with G and J). L, section of complete bilaminar blastocyst of Didelphys. 



" Cf., for example, Assheton ('98), figure 6, plate 15. 



