THE STRUCTURE OF THE AM N ION. 



371 



readily seen to be separated into two parts, a thin mesothelial layer, Mstk, cover- 

 ing the surface of the amnion toward the chorion, and a mesenchymal layer, Mes, 

 which makes up the greater part of the membrane. Traces of fibrillar structure 

 in trie mesenchyma are observable. No blood-vessels, lymphatics, or nerves have" 

 been found. 



EC 



- : , Mes 





Msth 



FIG. 251. TRANSVERSE SECTION OF A HUMAN AMNION OF Two MONTHS. 

 EC, Ectoderm. Mes, Mesenchymal mesoderm. Msth, Mesothelium. X 250 diams. 



Human Amnion after the Fifth Month. This should be studied both in sections 

 and in surface views of the whole membrane, small pieces being mounted with 

 the ectodermal side up. The preparation may be stained with alum .hematoxylin 

 and eosin. Sections show that the ectoderm (Fig. 252, ec(] has grown somewhat 



FIG. 252. Two SECTIONS OF THE HUMAN AMNION. FIG. 253: SURFACE VIEW OF THE HUMAN AMNIOTIC 

 A, From an embryo of eight months; B, at term. EPITHELIUM OF THE FOURTH MONTH. 



ect, Ectoderm, mes, Mesoderm. a, Meso- pi, Protoplasm, pr, Intercellular processes, nu, 

 thelium. X 340 diams. Nucleus. X 1225 diams. 



in thickness. Usually the cells are cuboidal (Fig. 252, A), each with a rounded 

 top in which is situated the more or less nearly spherical nucleus. Sometimes, 

 however, the nuclei lie deeper down. Less frequently the epithelium is thin 

 (Fig. 252, B), and its nuclei, which are transversely elongated, lie farther apart. 

 As regards the mesoderm, it will be noticed that there is usually, perhaps always, 



