DEVELOPMENT OF THE OVUM. 



93 



part by the decidua serotina. Villi containing vessels grow out 

 from the chorion into the decidua. 



line of union 



^5^— prechorion 



embryo 

 amnion 



somatop/eure 

 coelom 



splanchnopleure 



Fig. 72.— Showing the folds of the somatopleure uniting over the embryo and becom- 

 ing demarcated into Amnion and Prechorion. 



Origin of Ova and Spermatozoa. — The cells which give rise 

 to the ova and spermatozoa, according to sex, are derived from the 

 mesothelial lining of the coelom. They appear with the cleavage 



neural canal 



aorta 



Wolff an duct. 

 Wolffian body 

 Mullerian duct 



genital ridge 



Fig. 73. — Diagrammatic section of the abdominal region of the coelom, showing the 

 position of the Genital Ridges from which the Ovary or Testicle is formed. 



of the mesoblast and formation of the coelom. Soon after the 

 coelom is formed (beginning of third week) a ridge — the genital 

 ridge — is seen on the roof of the coelom, situated on each side of 

 the attachment of the mesentery of the gut (Fig. 73). The ridge 

 lies over the intermediate cell mass (Fig. 85 ; p. 1 11), and is covered 



