354 HUMAN UTERUS AND FCETAL APPENDAGES. 



form a complex network of which the filaments and meshes are extremely varia- 

 ble in size. The nuclei are oval, granular, and do not always have accumulations 

 of protoplasm about them forming main cell-bodies. (Compare description of 

 first stage of the mesenchyma, page 65.) By the end of the third month the 

 cells have assumed nearly their definite form (Fig. 214). Their protoplasm is 

 increased in amount and forms a large body around each nucleus. The network 

 has become simpler and coarser, the meshes bigger, and the filaments fewer and 

 thicker. In the matrix are numerous connective-tissue fibrillae, not yet disposed 

 in bundles. In older cords there is an obvious increase in the number of fi- 

 brillae and they form wavy bundles. In the cord of yet older stages the matrix 

 also contains mucin which may be stained by alum hematoxylin. In such cords 

 so stained the blotch of color appears in the intercellular spaces. 







-^ .- - 



Fig. 215. — Ectoderm of an Umbilical Cord of a Human Embryo of Three Months. 

 Ec, Ectoderm, mes. Mesoderm, c. Mesenchymal cell, a, Outer layer of ectoderm, b. Inner layer of ecto- 

 derm. X 545 diams. 



The ectoderm is at first a single layer of cells, as it is also over the body of the 

 embryo, and as it remains permanently over the amnion. At three months we 

 find the ectoderm to be two-layered, corresponding to the second stage of the 

 epidermis of the embryo. In still older stages there is slight increase in the 

 number of layers of the ectoderm, but it never passes much beyond the stage of 

 the embryonic epidermis at the fourth month. Figure 215 is from a cord at 

 three months. The outer layer, a, of ectodermal cells is granular and stains 

 much more darkly than the inner layer, h, in which also cell bundles are more 

 distinct. 



The Structure of the Human Yolk=sac. 



The human yolk-sac may be preserved in Zenker's or Tellyesnicky's fluid, 



