500 THE PHYSIOLOGY OF REPRODUCTION 



already been traced up to the stage when they consisted of simple 

 stalks of mesoblast with a double ectodermal covering. In the 

 core are developed capillary vessels which are continuous with 

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

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

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



FIG. 154. Median longitudinal section of an embryo of 2 mm. (von Spec). 

 (From (Jtwn'n's Anatomy, Longmans.) 



v., Villus ; '.*., core of villus ; mes., mesoderm ; '..*., connecting stalk ; p.*., 

 primitive streak : "//., allantois ; u.s., yolk-sac ; Ent., entoderm ; iv*., 

 vessels ; h., heart ; ./>., notochordal plate ; ., amnion. 



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

 but later the cytoblast proliferates and forms thick rounded masses, 

 the " Zellsiiulen," over which the syncytium disappears. This 

 forms the permanent attachment between the villi and the decidual 

 surface. The spaces between the stalks form the primary inter- 

 villous spate, which is thus entirely in the plasmodihlast. The 

 primary villi form buds of their three layers which develop into 

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

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

 similar process other villi are also develop!, till the whole system 



