THE STRUCTURE OF THE HUMAN YOLK-SAC. 



Its growth and differentiation are rapid. During the second month it consists 

 merely of numerous cells (Fig. 257) imbedded in a clear substance. The cells 

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

 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 89.) By the end of the third month the cells 

 have assumed nearly their definite form (Fig. 258). 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. 



FIG. 259. 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 ectoderm. 



X 545 diams. 



In older cords there is an obvious increase in the number of fibrillae 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. 



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 259 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, b, 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, 

 stained in toto with alum .cochineal, imbedded in paraffin, and cut in transverse 

 sections. Yolk-sacs of the second month are preferable for study. 



