64 INTRODUCTION TO SEXUAL PHYSIOLOGY 



in juxtaposition. Tlie proliferation is greatest at one end, and 

 extending forward from this is a band known as the primitive 

 streak, running in the centre of which is the prifnitive groove. 

 Anterior to tlie primitive streak there arise two folds between 

 which is the medullary groove. The folds eventually unite, and 

 the groove becomes converted into the neural canal. Above the 

 primitive streak is the amnion, and below it is the yolk sac. 



The next process that occurs is the folding off of the embryo. 

 The cavity of the amnion becomes enlarged, and dipping down 

 over the head, tail, and sides of the embryo, the latter is con- 

 verted into a sort of tube, the inside of which is a part of the 



Amniotic cavity 

 Archenteron 



Chorion 



-Diagram showing the extension of the mesoderm. 

 (From Grcti/s Anatomy.) 



Fig. 41. — Diagram showing a very early stage in the development of 

 the human ovum. (From Graifs AiKdomi/.) 



original yolk sac folded off from the rest of the sac. The part 

 of the yolk sac contained within the embryo becomes the 

 alimentary canal, as already mentioned. The part of the yolk 

 sac outside the embryo — the umbilical vesicle — is connected with 

 the alimentary canal of the embryo by the vitelline duct. The 

 aUantois grows out from the posterior part of the embryonic 

 alimentary canal, pushing its w^ay along the mesoderm, but in 

 man this structure never undergoes any great development 

 or acquires much importance ; in reptiles and birds, on the other 

 hand, the aUantois extends right round the inside of the. chorion 

 and acts as a res])iratory organ. The part of the aUantois within 

 the embryo eventually becomes the bladder. 



With the growth of the amnion around the embryo the yolk 

 sac and vitelline duct are brought into contact with the body 

 stalk or abdominal pedicle, a portion of thickened mesoderm 



