422 IMPLANTATION OF THE OVUM IN 



in development between the ovarial and vaginal segments of a given uterine horn 

 containing, for instance, five to six pregnancies. 



In the very early stages of development the entire uterus had to be sectioned, 

 the series examined under a microscope, and the portion of ribbon containing the 

 embryo mounted; more often the whole ribbon was mounted, examined, and only 

 the important slide retained. 



II. PREPLACENTAL HISTORY OF SPERMOPHILUS TRIDECEMLINEATUS. 



i. Period Prior to Fixation. — A. Uterus. — Spermophilus tridecernliheatus has 

 a two-horned uterus, each horn being suspended from the dorsal wall of the abdom- 

 inal cavity by a fold of peritoneum, which closely adheres to the outer muscular layer, 

 except dorsally where the two sides of the fold come together, forming the broad liga- 

 ment or mesometrium. This mesometrium contains the rich vascular and nervous 

 supply of the uterus supported by a loose connective tissue. Of the two muscular 

 tunics the external longitudinal one is prolonged into the mesometrium; the inner 

 layer, circular and slightly thicker, is separated from the outer one by a small amount 

 of vascular connective tissue; both coats are relatively slight and become greatly 

 thinned during pregnancy. The inner tunic envelops the connective-tissue stroma 

 of the mucosa, which forms a well-marked layer surrounding the uterine lumen. The 

 connective-tissue cells vary from round to spindle-shaped or branched, have large 

 round nuclei, and show in the non-pregnant animal a dense inner and looser outer 

 zone. The stroma is highly vascular, numerous blood and lymph capillaries extend- 

 ing in all directions and in close relation with the basement membrane and glands. 

 The lumen of the uterus has a characteristic shape and is peculiar in some 

 respects (PL XXX, Fig. 11. Compare Burckhard, :01, Taf. XXVI, Fig. 1, mouse; 

 v. Spee, : 01, p. 133, Fig. 1, guinea-pig). It consists of a long rather narrow T-shaped 

 slit extending dorsoventrally, in which it is necessary to distinguish three portions. 

 The first of these is the dorsal or mesometrial third and is flattened and extends hori- 

 zontally to the principal transverse axis of the lumen, forming the top of a capital T. 

 This constitutes the -placental chamber (Scheibenhohle, Fleischmann, '93; Placental 

 region, Minot, '89). 



At the antimesometrial or ventral end of the lumen is the second part, con- 

 sisting of an irregular cavity or pocket, which may frequently extend at an angle 

 to the long axis and whose floor at intervals shows a more or less well-marked groove 

 (PL XXX, Figs. 5, 11, 12). This cavity, which receives the ovum soon after its entrance 



