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TEXT-BOOK OF EMBRYOLOGY. 



no such epithelium was found and the ovum had the appearance of having 

 penetrated the stroma by which it was surrounded (Fig. 493). Thus, for the 

 first two weeks of gestation, the ovum lies embedded in the stroma of the uterine 

 mucosa, giving so little surface indication of its presence that it is practically 

 impossible to locate it except by serial sections of the entire mucosa. After 

 two weeks the position of the ovum begins to be indicated by a slight prominence 

 of the mucous membrane, the summit of the prominence being marked by an 

 entrance plug consisting of coagulum, cast off cells and fibrin (Fig. 74). In 



Inner cell 



Uterine 

 epithelium 



Thickening of 

 trophoderm 



Thickening of L 



trophoderm 



Degenerating 

 uteri-- epithelium 



FIG. 492. Successive stages in the implantation of the ovum of Spermophilus citillus. Rejsek. 



a- Ovum (blastodermic vesicle) lying free in the uterine cavity, b, Later stage in which the 



syncytial knob (thickening of trophoderm) has penetrated the uterine epithelium as far as 



the basement membrane, c, Still later stage in which the trophoderm has penetrated the 



uterine stroma; the cells of the uterine epithelium at the point of entrance are degenerating. 



the Bryce-Teacher ovum no such entrance plug was found (Fig. 493). At 

 this stage the plug contains no glands or blood vessels. Later it becomes 

 organized and replaced by connective tissue. Whatever the mode of fixation 

 of the ovum to the uterus, there immediately result important changes in the 

 uterine mucosa which lead to the formation of the decidua. These changes are 

 both destructive and constructive. They are destructive in that the epithelial 

 covering of the ovum, the trophoderm, has some solvent action on the uterine 

 mucosa and breaks down the walls of the maternal blood vessels thus allowing 

 the blood to flow around the ovum (Fig. 493). They are constructive in that 

 they result in the formation of the decidua. 



