THE PLACENTA. 



439 



surface of the uterus, nor those of the outermost layer, next to 

 the circular muscles of the uterine wall. 



Fig. 169. — A transverse section across the uterus and the contained blasto- 

 dermic vesicle of a Eabbit at the end of the ninth day. Cf. Figs. 145 and 

 146, in which the blastodermic vesicle and embryo of this age are shown 

 in surface view and in sagittal section. (In part after Duval.) x 8. 



AW, side fold of the amnion. C, mesoblast of the upper wall of the blastodermic 

 vesicle, beyond the embryonal area. E, epiblast of the blastodermic vesicle ; the upper 

 reference is t) the thickened epiblast of the placental area. GU, uterine glands. G"W, 

 modified uterine glands of placental region. H, hypoblast of blastodermic vesicle. MI, 

 outer or longitudinal muscles of the wall of the uterus. MR inner or circular muscles of 

 the wall of the uterus. MM, mesometrinm. NG, neural groove of embryo, in trans- 

 verse section. PR, median cleft between the placental lobes of the uterus. SI, sinus 

 terminalis. TJC, capillaries of placental region, with thickened perivascular walls. 

 UL, giant cells. UV, blood-vessels of uterus. YS, yolk-sac, or cavity of blastodermic 

 vesicle. 



During the tenth day, the ectoplacental epithelium increases 

 greatly in thickness, and becomes excavated by irregular chan- 

 nels or lacunse, which according to Duval open into the maternal 



