BREEDING HABITS OF THE OPOSSUM HARTMAN. 357 



yolk mass affords another particular in which the egg harkens back 

 to its yolk-bearing Sauropsidan ancestors. To attain the vesicle 

 stage requires about 40 hours of development. 



CELL-LINEAGE. 



Recurring a moment to the 16-celled stage it may interest the 

 reader to know that Prof. Hill, in his study of the Brazilian opossum, 

 D. aurita, showed that the 16 cells are divided into 8 slightly larger 

 and 8 smaller cells. Doubtless one group develops into the embryo ; 

 the other into accessory structures of use only in fetal life (tropho- 

 blast, chorion). Professor Hill thus corroborates the present writer's 

 theory that the embryo arises from only one of the first two cells 

 of the 2-celled stage, the other cell developing into the accessory and 

 temporary structures. 



FORMATION OF ENTODERM AND GROWTH OF VESICLE. 



At the 50-celled stage the entoderm begins to differentiate. Certain 

 cells in the upper half of the vesicle swell and drop below the level 

 of their fellows into the lumen of the vesicle (fig. 33, pi. 8). Here 

 they multiply; and as the vesicle grows they spread out under the 

 outer laj^er to form the entoderm. The vesicle grows and becomes 

 progressively more thin walled (figs. 34 and 35, pi. 8). Soon the 

 entire vesicle is lined with the thin flat entoderm (fig. 37, pi. 8). At 

 this stage, then, the vesicle is a double wall. It is now 0.8 mm. in 

 diameter and 4^ days of development have passed. There is still 

 some albumen left at the pole opposite the embryonic area (fig. 37. 

 pi. 8). This area occupies only about one-sixth of the surface but it 

 is the thickest and densest portion of the wall. 



FORMATION OF MESODERM. 



This " bilaminar " or two-layered vesicle continues to grow for 

 another day without much change. Soon after the beginning of 

 the sixth day cells may again be seen dropping out of the superficial 

 layer of the embryonic disk to take their place betAveen the two layers. 

 These cells are the first mesodermal cells. They appear in a definite 

 group toward the margin of the disk (m, figs. 39 and 40, pi. 9) . Soon 

 the primitive streak is clearly laid down and the embryo may be 

 said to begin to take shape. Nearly half of the embryonic life is 

 passed — in six days the embryo will be born a " chylopoietic, warm- 

 blooded, oxygenating, innervating, free-willing" mammal. (Meigs.) 



IMPLANTATION. 



At first the eggs are usually found toward the cervix uteri some- 

 scattered amonf 

 101257—23 24 



what scattered among the delicate folds of the soft mucous membrane 



