UMBILICAL CORD OF THE PIG 6 



an internal elastic membrane, but several muscle layers beyond 

 the tunica intima the elastic tissue forms complete fenestrated 

 membranes through many layers; toward the periphery of these 

 vessels the elastic tissue only occurs as scattered delicate fibrils. 

 The vein, on the contrary, contains a very robust internal elastic 

 membrane, while through the central half of the wall occur rel- 

 atively coarse scattered fibers. In the pig's cord neither arteries 

 nor vein contain an internal elastic membrane. The elastic 

 fibers are practically limited to the inner half of the wall, only 

 very delicate and widely scattered fibrils occurring peripherally. 

 In the arteries the elastic tissue forms membranes for from three 

 to five layers beyond the central two or three layers. Consider- 

 able variation occurs with regard both to the amount and the 

 disposition of both the smooth muscle and the elastic tissue 

 constituents of the wall of the umbilical vessels both in the pig's 

 cord and the human cord. 



ECTODERM 



The covering ectoderm constitutes a stratified epithelium of 

 generally four layers of cells (fig. 2) . In certain restricted areas 

 the epithelium only consists of three layers of cells, in others of 

 as many as eight layers of cells. This epithelium resembles the 

 transitional rather than the stratified squamous type. It is 

 comparable to the thicker portions of the epidermis of the three- 

 month human fetus, which consists of from four to six layers of 

 cells, including a superficial periderm. It differs from the ecto- 

 derm of the full-term human cord, which includes only two or 

 three layers of flattened keratized cells. It differs also from the 

 continuous abdominal ectoderm in that the latter includes about 

 eight layers of cells, all of which, except the basal cuboidal layer, 

 consist of greatly flattened cells. Small patches of partially 

 fused, keratized cells occur in five different regions of the section 

 here shown. The lowermost layer of the epithelium consists 

 of cuboidal cells; the outermost variously of thick, rectangular, 

 flattened or dome-shaped peridermal cells ; the intervening layers 

 include polyhedral and stout fusiform types. In those portions 



