436 IMPLANTATION OF OVUM IN SPERMOPHILUS TRIDECEMLINEATUS MITCH. 



Figs. 32 to 34. Three sections at intervals out of some twenty-five into which the oval blastocyst was cut. The 

 germinal area is no longer opposite the fixation-mass; Rauber's layer has disappeared from the surface; 

 the fixation-mass is larger and about to develop roots. In Figure 33 the beginning of the cupping of the 

 inner side of the fixation-mass is seen. Note also that the adjacent stroma has become more vascular. 



Fig. 35. Section through the cup of the fixation-mass just before its separation. Note the thinned wall of the 

 fixation-mass, the diminished nuclei, and the disintegrating roots. The epithelium of the fixation-chamber 

 adjacent to the fixation-mass is beginning to undergo retrograde changes from contact with the blasto- 

 cyst. 



Figs. 36 to 38. Three sections at intervals from a series of thirty-six sections through a blastocyst at the completion 

 of the fixation process. All show details of the branching root-like processes of the fixation mass extend- 

 ing into the stroma. Figure 36 shows the roots and the cupping of the fixation-mass. Figure 37, taken 

 near the bottom of the cup, shows roots extending in all directions. Figure 38, still farther on in the 

 series, shows sections of the roots free in the stroma, whose cells in some places are seen to form sheaths 

 around the roots. 



