544 



Scientific Proceedings, Royal Bublin Society. 



When this process has been described in forms other than the 

 rabbit, we shall be in a position to see how far the development 

 of this rodent agrees with or differs from that of other mammals. 



The extension of cells of the blastodermic vesicle over the em- 

 bryonic area is probably to be accounted for in most cases by the 

 sinking of the latter into the cavity of the former. I have pre- 

 viously alluded to the various views which are held concerning 

 their fate. These Deckenzellen are really a portion of the blasto- 

 dermic vesicle, that is, of the yolk-sac, and they form the first adhe- 

 sion beftoeen the ovum and the parent. 



It has recently been demonstrated that in marsupials the yolk- 

 sac forms (at all events at first) the only adhesive surface of the 

 embryo. Osborn (15) states that in the Yirginian opossum 

 (Didelphys virginiana), and in other unnamed forms from Australia, 



Fig. 6. Fig- 7. 



]Jiagram of the foetal membranes of the Virgi- Diagram of the foetal membranes of the 



nian opossum (D. virginiana) with two villi native bear (Phasoolarctos cinereiis), 



greatly enlarged. The processes of the cells also showing the amasboid processes 



of the villi have been drawn too large and of the (epiblastic) cells of the subzunal 



too numerous ; they doubtless correspond to membrane. After Caldwell, 

 the pseudopodia described by Caldwell. — 

 After Osborii, slightly modified. 



,il. allantois ; am. amnion ; amb. amoeboid processes of subzonal membrane ; hy. hypoblast of 

 yolk-sac ; f. t. sinus terminalis ; 5. 2.. subzonal membrane; y. s. yolk-sac. The black line 

 in both figures indicates the extension of the vascular system. 



that portion of the subzonal membrane which is in contact with 

 the yolk-sac, chorion of the yolk-sac, gives rise to very simple vas- 

 cular villi, by means of which the embryo is slightly attached to the 

 uterine wall (fig. 6). Caldwell (7) found that in Phascolarctos cinereus 



