SPERMOPHILUS TRIDECEMLINEATUS MITCH. 435 



EXPLANATION OF PLATES XXX-XXXI. 



All the figures are photomicrographs from sections of the uterus of Spermophilus tridecemlineatus and were 

 made by the writer with a Zeiss photomicrographic stand, optical bench, and equipment, a Thompson 90° direct- 

 current, self -feeding arc lamp and a 11 X14 enlarging and reducing camera; the whole was supported by a swinging 

 table to get rid of vibrations. For the lower magnification a Zeiss micro-planar and for the higher a Zeiss 4-mm. 

 apochromatic objective without eyepiece were used. The sections were stained in toto with paracarmine and 

 • counterstained with picric acid. A color screen consisting of a cell containing Zettnow's fluid was used, as were 

 several varieties of Seed's plates. Figures 1-8, 11, and 15 are magnified about 16 diameters; all others about 185 

 diameters. 



PLATE XXX- 

 Figs. 1 to 4. Four consecutive transverse sections of the uterus showing the ventral half of its lumen. Beginning 



dilation is shown in the fixation-chamber in which there is a young blastocyst just becoming attached- 



The few glands in the region of the fixation-chamber and the dilated gland-ducts in the intermediate 



portion are well shown. Compare also Figures 6 to 8, and 11. 

 Fig. 5. Ventral portion of the fixation-chamber shortly before the attachment of the blastocyst. The ventral 



groove can be seen and the blastocyst shows distinctly the trophoblast, the inner cell-mass, and the 



entoderm. 

 Fig. 6. Ventral portion of the uterine lumen with a lateral implantation at the completion of the fixation process. 



Note the roots of the fixation-mass extending into the stroma and the deep cupping of its inner wall. 



Owing to the plane of section the enlargement of the fixation-chamber and extension of the blastocyst 



walls are not shown. 

 Figs. 7, 8. Sections from two uteri showing the more usual implantation in the floor of the fixation-chamber. 

 Figs. 9, 10. Consecutive sections showing the fixation-mass, which in Figure 9 is just perforating the epithelium. 

 Fig. 11, Transverse section across the whole uterus at the beginning of pregnancy, showing the characteristic form 



of the uterus. The size and relations of the placental chamber, the intermediate portion, and the fixa- 

 tion-chamber are well shown. 

 Figs. 12, 13. Consecutive sections of the fixation-chamber, containing a younger blastocyst than that in Figure 5. 

 Fig. 14. Section of a blastocyst at the beginning of attachment. 

 Fig. 15. Longitudinal horizontal section of the uterus at the level of the fixation-chamber and containing a free 



blastocyst. Note the lateral folds of mucous membrane projecting into the lumen. 

 Fig. 16. Section of a blastocyst slightly older than that shown in Figure 14. Note the cupping of the epithelium 



at the point of contact with the fixation-mass. 

 Figs. 17, 18. Two sections from a blastocyst at a stage slightly older than that shown in Figure 16. The blastocyst, 



which shows perforation, has attached to it the disintegrating zona. 

 Fig. 19. The fixation-mass nearly through the epithelium. Note the intimate relations of the capillaries to the 



mass. 

 Fig. 20. Fixation-mass just perforating epithelium. Compare with Figures 9 and 18. 

 Figs. 21, 22. Consecutive sections through the blastocyst in the perforation stage. Figure 21 shows clearly the 



differentiation of the trophoblast into Rauber's layer and at the opposite pole the fixation-mass. 

 Fig. 23. A blastocyst at a stage older than that shown in Figure 19. Note the increase in size of blastocyst and 

 the beginning of the extension of the fixation-mass under the epithelium. A bit of the disintegrating 

 zona is still attached to the blastocyst. 



PLATE XXXI. 



Figs. 24 to 28. Consecutive sections through a blastocyst which shows the fixation-mass breaking through the zona 



to perforate the epithelium. 

 Fig. 29. A blastocyst at the beginning of the perforation stage similar to the one shown in Figure 16. 

 Figs. 30, 31. Consecutive sections of the blastocyst at a stage similar to the one shown in Figure 23. Note the 



burrowing of the fixation-mass, and the zona attached to one side of the blastocyst. 



