SPERMOPHILUS TRIDECEMLINEATUS MITCH.^ 429 



day after impregnation. During the fifth day there is a dilation of the ventral por- 

 tion of the lumen accompanied with a flattening of the lining epithelium. During 

 the sixth day the epithelium of the decidual chamber disintegrates; that in the extreme 

 ventral portion persisting for a time as a clump of cells (Epithelreste). In the region 

 of the embryonal pole the uterine lumen closes by the union of its epithelium and 

 the fusion of the connective-tissue walls above the region of the "Trager"or ectopla- 

 centarconus. The remainder of the epithelium surrounding the blastocyst disappears, 

 and the blastocyst is contained in a decidual chamber derived from the ventral por- 

 tion of the uterine lumen. The trophoblast comes into close contact with the con- 

 nective-tissue stroma. Selehka and Duval figure enlarged cells of the trophoblast 

 as extending out and attaching the blastocyst to the uterine epithelium. Burckhard 

 does not show this. Sobotta's figures in his preliminary paper are somewhat sche- 

 matic, but his Figure 12 suggests this. His forthcoming monograph will probably 

 determine this point. 



The Lagomorphi are represented only by the rabbit, the subject of scores of 

 important investigations. Here we may particularize only those of Van Beneden 

 ('80), Kolliker ('82), Minot ('89), Masius ('89), and Duval ('89-'91). 



In this species the uterine lumen shows six longitudinal folds. Minot ('89) terms 

 the mesometrial pair the placental folds or lobes; the lateral pair, one on either side 

 the periplacental folds; and the antimesometrial pair, the obplacental folds. The 

 ovum, entering the uterus about the end of the third day, rapidly increases in size, 

 and during a period of about four days it remains free and unattached. In the uterine 

 cavity the blastocyst increases in size from about 0.9 millimetre on entrance to 4.5 

 or 5 millimetres by 3.5 to 4 millimetres at the end of the seventh day. Fixation takes 

 place during the eighth day by the attachment of a horseshoe-shaped area of the 

 trophoblast, external to the germinal area, to the two placental folds of the uterus. 

 The germinal area is opposite the space between these folds. 



This space and the placental folds correspond to the cavity and borders of what 

 I have called the placental chamber in Spermophilus, in which the true placenta is 

 developed much as in the rabbit. The periplacental and obplacental folds of the 

 rabbit correspond closely to the intermediate portion in Spermophilus, and the 

 space between the obplacental folds in the rabbit to the fixation-chamber of Sper- 

 mophilus. In the rabbit the uterine epithelium becomes lost from the surface of 

 the peri- and obplacental folds, and the blastocyst finds itself in a connective-tissue 

 decidual chamber occupying the whole uterine lumen. The distal portions of the 

 glands persist in the obplacental portion and give rise to a regenerating epithelium. 



Our knowledge of the Hystricomorphi is also limited to one species, the guinea- 



