348 



HUMAN UTERUS AND FCETAL APPENDAGES. 



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 con- 

 tinue until at full term, as shown by figure 204, the branches are long, slender, 

 and less closely set as well as less subdivided than at early stages. They have 

 nodular projections like branches arrested at the beginning of their development. 

 There are numerous spots upon the surfaces of the villi. Microscopic examina- 

 tion 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 of placentas of different ages the villi offer 

 characteristic differences; for the younger the stage, 

 the fewer the total number of branches and the larger 

 their average size. The older the placenta, the more 

 numerous and smaller are the branches as they appear 

 in sections (Fig. 190). 



Injected Villi. — The arteries and veins of the cho- 

 rionic membrane enter the villi. After a short course 

 in the main stalk of a villus, the vessels give rise to 

 many branchlets, and gradually the character of the 

 circulation changes, until in the smallest villous twigs 

 there are capillaries only (Fig. 205). The capillaries 

 are remarkable for their large size, and on this account 

 have been interpreted as arteries and veins by some of 

 the older writers. Their caliber is often sufficient for 

 the passage of from two to six blood-corpuscles 

 abreast. They are very variable in diameter, and 

 also peculiar in exhibiting sudden constrictions and 

 dilatations. In the short knob-like branches there is 

 often only a single capillary loop, but as the branch 

 becomes larger the number of loops increases and they 

 form anastomoses. In branches large enough to serve 

 as a stem, some one or two of the vessels may be enlarged. In the branches 

 large enough to admit of it, there are two (or sometimes only one) longi- 

 tudinal central vessels, the artery and vein of a superficial network of capillaries 

 (Fig. 206). The formation of loops and the large size of the capillaries are not 

 especially characteristic of the villi, but of the foetal blood-vessels in general. 



The histology of the villi is described in the section on the placenta in situ, 

 page 333- 



Fig. 206. — Portion of a 

 Small Injected Villous 

 Stem from a Placenta 

 OF ABOUT Five Months. 

 X 105 diams. 



