368 HUMAN UTERUS AND FETAL APPENDAGES. 



time the villi alter in shape (Fig. 244), becoming more and more filamentous. By 

 the fourth month only a few tapering threads with very few branches remain. 

 The villi disappear almost completely from the chorion laeve, except near the edge 

 of the placenta. The villi of the chorion frondosum or placental region, on the 

 contrary, make an enormous growth. At first they are short, thick-set bodies of 

 irregular shape, as shown in figure 245. At twelve weeks their form is ex- 

 tremely characteristic (Fig c 246). The main stem gives off numerous branches 

 at more or less acute angles, and these again other branches, until at last the 

 terminal twigs are reached. The branches are extremely irregular and variable, 

 though in general club-shaped and constricted at the base. The branches may 

 be bigger than the trunk which bears them, or of any less size. In older stages 





FIG. 244. ABORTING VILLUS FROM THE HUMAN FIG. 245. FRAGMENT OF THE CHORION OF FIGURE 



CHORION L^VE OF THE SECOND MONTH. 84, HIGHLY MAGNIFIED. 



EC, Ectoderm. Mes, Mesoderm. Vi, Villus formed 

 wholly by ectoderm. 



there is a progressive change. During the fifth month we find the irregularity 

 of shape, though still very marked, decidedly less exaggerated (Fig. 247). The 

 branches tend to come off at more nearly right angles. One finds very numerous 

 free ends, as of course only a small portion of the branches touch the decidual 

 surface. The branches, too, are less out of proportion to the stems, less constricted 

 at their bases, -less awkward in form. The gradual changes continue until at full 

 term, as shown by figure 248, the branches are long, slender, and less closely 

 set as well as less subdivided than at early stages. They have nodular projec- 

 tions like branches arrested at the beginning of their development. There are 

 numerous spots upon the surfaces of the villi. Microscopic examination shows that 

 these spots are proliferation islands, as we may call them, or little thickenings 

 of the ectoderm with crowded nuclei. Not all the villi, ^however, have changed to 

 the slender form, for some still preserve the earlier, clumsier shapes. In sections 



