THE EMBRYOLOGY OF THE OPOSSUM 39 



the blastomeres lose contact with each other as a consequence 

 of the elimination of a considerable quantity of yolk (stage 4'). 

 This extruded yolk lies between them and around them, and 

 within the zona pellucida practically all the space not occupied 

 by the blastomeres is filled with yolk granules (fig. 12, A). 

 The blastomeres themselves have hardly become separated 

 from each other before some of them begin to flatten out 

 against the zona. Those which are not in contact with this 

 membrane are spherical. Those which touch it flatten out 

 against it. Even in the early cleavage stages (stages 5 and 6 

 and fig.12, B) this leads to some semblance of blastocyst 

 formation. By the 16-celled stage (stage 8) nearly all of the 

 blastomeres are somewhat flattened against the zona, though 

 the wall of the blastocyst is not yet complete and a few 

 spherical cells which are not yet in contact with the zona may 

 still be found. By the time there are thirty-two cells (stage 9 

 and fig. 12, D) all the gaps in the wall are usually filled, and 

 the unilaminar blastocyst is complete. 



This flattening out of individual cells against the zona pel- 

 lucida before there is any such thing as a blastocyst wall, and 

 when no morula stage has been formed, seems to be clear 

 evidence that the accumulation of fluid in spaces between 

 the cells of the morula by endosmosis is not the causative fac- 

 tor in blastocyst formation. It seems necessary to think of 

 stereotropism as a more likely factor. And in this light, per- 

 haps an important function of the zona is that of providing 

 the proper stimulus for the tropism which leads to blasto- 

 cyst formation. The yolk granules, of course, not exhibiting 

 any such tropism, are gradually crowded away from the 

 zona by the cells and thus passively come to occupy the cavity 

 of the blastocyst. 



So far the total volume of the egg proper has not increased. 

 The zona pellucida has become thinner until it serves only 

 to give a progressively diminishing distinctness to the inner 

 border of the albumen. The albumen layer, if it has changed 

 at all, is slightly thicker. The shell membrane, however, has 

 definitely increased in thickness, measuring now between 



