REPRODUCTION 



ment, and which acts as a most efficient protection against 

 external violence. 



2. Attachment to the Mother. 



The ovuin gets enclosed in the uterine mucous membrane, 

 which regenerates round it as the decidua reflexa after men- 

 struation (Fig. 160, D.K). 



Almost as soon as the ovum is embedded in the maternal 

 mucous membrane, it 

 becomes surrounded 

 by a nucleated mass 

 of protoplasm the 

 trophoblast, formed of 

 the cells of the ecto- 

 derm, and this probably 

 transfers nourishment 

 from the mother to 

 the ovum. At the 

 end of about a fort- 

 night, the mesoblast 

 of the embryo extends 

 out in a number of 

 finger - like processes 

 into the trophoblast, 

 and soon afterwards 

 blood vessels shoot 

 into these, and the 

 chorionic villi are 

 formed (Fig. 161). 

 The precise origin of the first blood vessels in these is 

 not known, but ultimately they are derived from the 

 allantoic arteries which pass out from near . the posterior 

 end of the hind gut. As the villi grow, the blood vessels 

 in the maternal mucosa are dilating, and the capillaries 

 form large sinuses or blood spaces. Into these the 

 chorionic villi pass, and thus the loops of foetal vessels 

 hang free in the maternal blood, and an exchange of material 

 is possible between the mother and foetus. The placenta 

 thus formed acts as the fcetal lung, giving the embryo the 

 necessary oxygen and getting rid of the waste carbon dioxide. 



27 



FIG. 160. Longitudinal Section through the 

 human uterus and ovum at the fifth week of 

 pregnancy. D.S., Decidua serotina, which 

 will become the placenta; D.R., Decidua 

 reflexa ; D. V. , The uterine mucous membrane 

 called the Decidua vera. 



