SPERMOPHILUS TRIDECEMLINEATUS MITCH. 427 



ducts opening upon the intermediate portion become flattened, increasing the size 

 of the chamber still further. 



B. Blastocyst.— The blastocyst is rapidly increasing in size because of the multi- 

 plication of cells in the trophoblast and it changes from an almost spherical to a more 

 and more oval shape. The outer border does not yet reach the walls of the fixation- 

 chamber. Rauber's layer may be still complete or in process of disintegration. The 

 germinal area is becoming slightly oval and rapidly increases in size. The unequal 

 growth of the trophoblast walls causes the germinal area to shift gradually from its 

 place nearly opposite the fixation-pole, so that sections through the centre of the 

 fixation-mass may not cut the germinal area at all (PL XXXI, Figs. 32-34). The 

 fixation-mass increases greatly in volume; its nuclei are larger and more numerous. 

 The mass still presents a convex surface to the vascular connective tissue, while its 

 thin edges have penetrated in all directions between the epithelium and the con- 

 nective tissue. Later from the convex surface of this mass are developed root-like 

 processes which extend in all directions into the connective-tissue stroma (PL XXXI, 

 Figs. 36-38). These roots branch and subdivide again and again. The adjacent 

 connective-tissue cells become flattened and later form sheaths around the larger 

 roots. Accompanying these changes there is a hollowing of the inner surface of the 

 fixation-mass until a distinct cup is formed. The thin walls of this cup contain the 

 nuclei, which do not extend into the roots (PL XXX, Fig. 6; PL XXXI, Fig. 36). 

 The protoplasm of the cup and roots now becomes distinctly fibrillated, resembling 

 a mass of delicate fibrin threads (PL XXXI, Figs. 36-38). From this time the fixa- 

 tion-mass begins to degenerate. 



4. Separation of the Fixation-mass. — A. Uterus. — The external swellings of the 

 uterus are now very pronounced; internally the fixation-chamber shows a large 

 cavity with smooth walls, except where it is deeply pitted by the cup of the fixation- 

 mass just described. From the margins of this cup the walls of the chamber are 

 lined with a continuous layer of epithelium whose cells have undergone no change. 

 The enlargement of the chamber has involved nearly all of the intermediate portion 

 of the uterine lumen, leaving only a narrow strip between the now enormously dilated 

 fixation-chamber and the still slightly altered placental chamber. During these 

 changes the enlarging blastocyst comes into contact with the epithelium fining the 

 fixation-chamber, resulting in histolysis of the cells of the epithelium. By this means 

 the blastocyst is now brought into contact with the connective-tissue walls of what 

 will be called the decidual cavity. The remnant of the intermediate portion next to 

 the placental chamber becomes dilated and merged into the walls of the decidual 

 cavity, the placental chamber becomes expanded, the trophoblast cells surrounding 



