SPERMOPHILUS : TRIDECEMLINEATUS; MITCH. 423 



into the uterus, corresponds to the fundus of the human uterus. It is to some portion 

 of the walls of this chamber that the blastocyst becomes attached. This will be 

 referred to as the fixation-chamber (Nebenkammer, Fleischmann; Obplacental region, 

 Minot). 



The remainder of the lumen connecting these two chambers, the third part, 

 will be called the intermediate portion (Schlussspalt, Fleischmann; Periplacental 

 region, Minot). 



The uterine epithelium forms an unbroken layer of simple columnar cells, for 

 the most part ciliated, which rests on a well-marked basement membrane. The 

 glands are tubular with ducts opening upon the free surface; the convoluted tubes 

 extend radially through the connective-tissue layer almost to the circular muscle- 

 coat, and the distal portions are twisted into rather close clumps or knots. The 

 cellular lining of the ducts is continuous with the uterine epithelium, the cells becom- 

 ing cubical in the gland. The lumen of the gland-tube shows in section a coagulum 

 of secretion which stains with eosin. In the non-pregnant uterus the glands are 

 quite uniformly distributed; but with the beginning of pregnancy the ducts become 

 dilated and prominent in the placental chamber and intermediate portion, and almost 

 disappear in the region of the fixation-chamber, being obliterated by the rapidly 

 multiplying cells of the connective tissue. Cell-division does not occur in the epithe- 

 lium of the uterine lumen and gland-ducts; but mitoses may be noticed in the cells 

 at the distal ends of the glands. The ovum shortly after leaving the oviduct sinks 

 into the fixation-chamber. From this region its path towards the vagina is a tor- 

 tuous one, due to the many lateral projections of" the mucous membrane. Figure 

 15 (PI. XXX) shows a free ovum in a longitudinal horizontal section of the fixation- 

 chamber. D'Erchia (:01, Tav. 1, Fig. 2) has shown a similar condition in the uterus 

 of the mouse. 



B. BLASTOCYST AND ZONA PELLUCIDA. The ovum in most cases enters the 

 uterine cavity still surrounded by the zona pellucida, which may disappear very soon 

 or persist until after the fixation of the ovum to the uterine wall (PI. XXXI, Figs. 24- 

 31). Before disintegration it is, in section, a well-marked band, rather thick, homo- 

 geneous and brittle, and is closely adherent to the trophoblast layer. As it disin- 

 tegrates the outer portion softens and disappears first, the inner portion persisting 

 for a time as a thin rather horny membrane (PI. XXX, Figs. 17, 18, 23). Sobotta 

 (:01) describes the zona in the mouse as becoming lost while the ovum is still in the 

 oviduct. Spec ('83) describes and figures the cells at the fixation-pole in the ovum 

 of the guinea-pig as extending protoplasmic processes through the zona and suggests 

 that these processes may aid in the attachment of the ovum to the uterine wall. Spee 



