REPRODUCTION. 



659 



characteristics of both ancestral cells. From this parent cell the 

 new being develops through successive division, multiplication, and 

 differentiation of cells. 



The Fixation of the Ovum. If the ovum is to develop into a 

 new being it is essential that it be retained within the cavity of the 

 uterus. This is accomplished by the development of specialized 

 structures on the surface of the uterine mucosa and on the surface 

 of the ovum. With the fertilization of the ovum, the mucous mem- 

 brane of the uterus takes on an increased growth ; it becomes hyper- 

 trophied and vascular, and develops small elevations known as villi. 

 Inasmuch as this membrane is detached and discharged at the birth 

 of the fetus, it is known as the decidua vera. With the fertilization 



FIG. 324. IMPREGNATED UTERUS, WITH 

 FOLDS or DECIDUA GROWING UP 

 AROUND THE EGG. The narrow 

 opening, where the folds approach 

 each other, is seen over the most 

 prominent portion of the egg. 

 (Dalton.) 



FIG. 325. IMPREGNATED UTERUS ; 

 showing the connection between the 

 villosities of the chorion and the 

 decidual membranes. (Dalton.} 



of the ovum, the zona pellucida or radiata also develops villosities, 

 and as it passes from the Fallopian tube into the uterus the villi 

 interdigitate, and its further progress is retarded. (Figs. 324 and 

 325.) In a short time a portion of the decidua vera grows up 

 on all sides and encloses the ovum. Its retention is thus secured. 

 That portion of the decidua which grows around the ovum 

 is termed the decidua reflexa; while the portion to which the 

 ovum attaches itself is termed the decidua serotina, and is of interest 

 for the reason that it becomes the seat of development of the placenta, 

 the organ by which the fetus is nourished. As development advances 

 the decidua reflexa also increases in size and extent, and about the 

 end of the fourth month comes into contact with the decidua vera, 

 with which it ultimately fuses. 



