450 SOME GENERAL PROBLEMS OF DEVELOPMENT 



Fig. 234. — Diagrammatic section through human uterus and embryo at the 

 seventh or eighth week of pregnancy. The blood vessels of the embryo extend 

 out to the placenta, but the circulation of the embryo and that of the mother 

 are distinct (c/. Fig. 237). The technical designations of parts by the author 

 of the figure, as given below, are not explained, but the student can see from 

 the figure how the embryo is placed in the uterus and how its blood vessels 

 extend out to the placenta where they form a capillary system that is indepen- 

 dent of, although closely associated with, that of the mother. 



al, allantois; am, amnion; c, openings of the oviducts (Fallopian tubes) into the uterine 

 cavity; c', cervix or outer opening of uterus filled with mucous plug (the reference letters 

 c, c, c' are placed in the cavity of the uterus) ; ch, chorion with vascular villi growing into 

 the maternal tissue, decidua capsularis and decidua basalis; dr, decidua capsularis; ds, 

 decidua basalis or basal plate; dv, decidua vera; i, embryo; u, umbilical cord; y, yolk-sac; 

 v', yolk stalk. (From Kellicott, " Chordate Development," copyright, 1913, by Henry 

 Holt and Co., reprinted by permission.) 



