THE EMBRYOLOGY OF THE OPOSSUM 47 



Stage 12 



The beginning of expansion of the reside. During the 

 formation of the endodcrm mother-cells and the primitive 

 endoderm (stages 10 to 12) the vesicle enlarges from 0.17 mm. 

 to 0.34 mm. This expansion, which is rather sudden, is 

 probably due for the most part to stretching under the influ- 

 ence of fluid accumulating in the blastocoel. It results in a 

 noticeable thinning out of all the cells of the vesicle, though 

 those of the medullary plate still appear thicker in sectional 

 view than any of the others. 



The eggs of Hartman's litter no. 194' were removed from 

 the uterus exactly 4 days after copulation, and form a good 

 type series for stage 12. 



VI. THE FIFTH AND SIXTH DAYS 



Stages 13 and 14. The definitive endoderm. The bilaminar 

 blastocyst. The uterine glands. 



Stage 13 



The definitive endoderm. In the early part of the fifth day 

 the primitive endodermal cells begin to migrate beyond the 

 bounds of the medullary plate, and as they do so they become 

 very much flatter and exhibit long pseudopod-like processes. 

 The migration is almost certainly accomplished by amoeboid 

 movement, though this cannot be established beyond question 

 with evidence from fixed material alone. These new, ex- 

 tremely flattened, migrating cells are the definitive endoderm. 



About the time the endoderm begins to migrate beyond 

 the medullary plate two changes in the gross aspect of the 

 vesicle become apparent: 1) it changes from a perfect sphere 

 to an ellipsoid with the shortest diameter in the direction 

 of the egg axis; and 2) it becomes eccentric in relation to the 

 egg shell and albumen (stage 13 and fig. 15). 



The explanation of the first of these two changes is not 

 clear, but it is fairly certain that the second, the eccentricity 

 of the vesicle, is due to the more rapid absorption of the 



