500 



THE PHYSIOLOGY OF REPRODUCTION 



already heen traced up to the stage when they consisted of shnple 

 stalks of niesoblast with a double ectodermal co\'eriug. In the 

 core are de\-eloped capillary ^'essels whicli are continuous witli 

 the vessels of the " Haftstiel," and later with those of the 

 umbilical cord. After the excavation of the necrotic zone, some 

 of the stalks reacli the decidual surface and attach the ovum to 



Fig, 154:. —Median longitudinal section of an embryo of 2 mm. (von Spee). 

 (From (^)uiiii's A'liatniiu/, Longmans.) 



i\. Villus; '■./■., core of villus; mes., me.soderm ; c.';., connecting .stalk; p.x., 

 pvimitive streak ; nl/., allantois ; //.s., yolk-sac ; JJjit., entoderm ; ves., 

 ve.ssels ; /'., heart ; n.//., notochordal plate ; «., amnion. 



it. At first the attached ends of these primary villi are plasmodial, 

 Ijut later the cytolilast proliferates and forms thick rounded masses, 

 the " Zellsaulen," over whicli the sync}'tiuni disappears. This 

 fonns the permanent attachment lietween the \'illi and the decidual 

 surface. The spaces lietween the stalks foi-m the primary inter- 

 villous spncr, whicli is thus entirel}' in the plasmodiblast. The 

 primaiy \'illi form Ijuds (.)f their thi-ee layers which develop into 

 secondary villi. Of these some may also liecome attached to the 

 deciihia, while others hang- free in the intervillous space. By a 

 similar process other \\\\\ are also de\'elope<l, till the whole system 



