370 



HUMAN UTERUS AND FETAL APPENDAGES. 



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. 234). 



Injected Villi. The arteries and veins of the chorionic 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. 

 249). 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 con- 

 strictions 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 the branches large enough 

 to admit of it, there are two (or sometimes only one) 

 longitudinal central vessels, the artery and vein of a 

 superficial network of capillaries (Fig. 250). The forma- 

 tion of loops and the large size of the capillaries are not 

 especially characteristic of the villi, but of the fetal blood- 

 vessels in general. 



The histology of the villi is described in the section 

 on the placenta in situ, page 356. 



FIG. 250. PORTION OF A 

 SMALL INJECTED VILLOUS 

 STEM FROM A PLACENTA OF 



The Structure of the Amnion. 



The structure of the amnion may be studied in sec- 



ABOTJT FIVE MONTHS, x tions, such as will be obtained by the student in con- 

 nection with the sections of the chicken and pig em- 

 bryos. These preparations will show the early stages. When the amnion is 

 first formed, it consists of two layers of cells, both very thin, and with somewhat 

 widely separated nuclei in each layer. Sometimes the nuclei lie in small groups. 

 Between the two layers is a distinct space. The layer facing the embryo is a 

 continuation of the embryonic ectoderm, and is more regular and better defined 

 than the second or mesodermal layer, which is less regular and sends at in- 

 tervals protoplasmic processes across the space between the two layers to attach 

 themselves to the ectoderm. 



Human Amnion at Two Months. A section is shown in figure 251. The 

 ectoderm, EC, is still very thin, but where the nuclei are placed the layer is a little 

 thicker. The. mesoderm, on the other hand, has become quite thick, and is 



