THE STRUCTURE OF THE AMNION. 349 



The Structure of the Amnion. 



The structure of the amnion may be studied in sections, such as will be ob- 

 tained by the student in connection 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 more or less irregular and sends 

 at intervals protoplasmic processes across the space between the two layers 

 which attach themselves to the ectoderm. 



Human Amnion at Two Months. A section is shown in figure 207. 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 

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

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



EC 



Mes 



Msth 



FIG. 207. TRANSVERSE SECTION OF A HUMAN AMNION OF Two MONTHS. 

 Re, Ectoderm. Mes, Mesenchymal mesoderm. Msth, Mesothelium. X 2 5 diams. 



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

 in the mesenchyma are observable. No blood-vessels, lymphatics, or nerves 

 have been found. 



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

 tions 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. 208, ect} has 

 grown somewhat in thickness. Usually the cells are cuboidal (Fig. 208, 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. 208, B), and its nuclei, which are transversely elon- 

 gated, lie further apart. As regards the mesoderm, it will be noticed that there 

 is usually, perhaps always, a layer of nearly homogeneous basal substance or 

 matrix immediately underneath the ectoderm and remarkable for containing no 

 cells. Sometimes the remaining portion of the mesoderm is broken up so as to 



