298 



One finds all gradations of these glands between those with very 

 narrow lumina and tall columnar entodermal cells, and those with 

 very wide lumina and flattened cells (Fig. 6). As determined by 

 serial sections the latter are invariably the expanded ends of some of 

 the former whose cells are similar to those lining the general cavity 

 of the yolk sac. Fig. 6, taken at a level of 350 micra from the 

 distal pole, shows several extreme types and a few gradations. Under 

 low magnification it is easy to confuse the crypts with the flattened 

 epithelium, with blood vessels; however, the cells are rtever so flat- 

 tened as to cause the nuclei to bulge, and the cell borders are usually 



distinct. Moreover, the blood vessels 

 invariably contain a few erythro- 



Fig. 5. 



Fiar. 6. 



Fig. 5. Transverse section of yolk sac at the proximal pole. E entoderm lining 

 the cavity of the sac ; 31 mesoderm ; a blood vessels containing erythroblasts ; e endo- 

 thelium of blood vessels; m mesothelium. Stain, hematoxylin and eosin; thickness, 

 10 micra; Camera lucida drawing, )x( 150. 



Fig. 6. Transvei-se section of wall of yolk sac showing cross section of glands (G?). 

 Lettering same as in previous figures. Camera lucida, ^ 140. 



blasts. Fig, 6 contains but a single blood vessel with a distinct wall 

 at a. Fig. 2, taken at a level of a little more than V2 mm from 

 the distal pole, shows the arrangement of the glands in this portion. 

 With the exception of a single one at a located below to the left, 

 the cavities are all those of crypts at difi'erent levels and at difi'erent 

 stages of expansion. 



