18 



Structure of Wall. — The Entoderm. 



The wall consists of three layers: entoderm, mesenchyme, and 



mesothelium (coelomic epithelium). The thickness of the wall varies 



considerably for different regions. The wall is thicliest at the distal 



pole and considerably thicker over one half of the surface. The 



Meso- 

 thelium 



Opening 



of 

 Tubule 



into 

 Cavity 



Tubule 



Blood Vessel 



Fig. 3. Photomicrograph of a section through the wall of the umbilical vesicle of 

 a 9.2 mm human embryo, showing an entodermal tubule opening into the cavity of the 

 vesicle. The epithelium lining the tubule is continuous with that lining the vesicle. 

 The entodermal cells of the tubule have a vacuolated cytoplasm and contain proximally 

 large mucinous masses (ergastoplasm, Braxca) and distally various cell-inclusions 

 (secretory granules, Branca). In the lower portion of the section is shown a tubule 

 lined with flattened cells. This is the more usual character of the epithelium lining 

 the tubules of the vesicle of the 13 mm embryo. The tubules are filled with an 

 amorphous coagulum. Magnified 640 diam. Reduced ^/^ in reproduction. Made by 

 Dr. Leopold Jaches, Cornell University Medical College, New York City. 



