297 



proximately equal calibre throughout their extent, or may end in 

 expanded bulbs (Fig. 6). I find no evidence of tubules blind at both 

 ends as described by Meyer. In my specimen all the tubes can ulti- 

 mately be traced to their openings into the cavity of the yolk sac. 

 They are simple or branched ; the branching is dichotomous and at 

 this stage of development could not be definitely traced beyond a se- 

 cond division. The glands may extend throughout the entire width 

 of the middle zone, in places pressing upon the mesothelial layer so 

 as to greatly attenuate its cells. Their course through the mesodermal 



Fig. 4. Transverse section of wall of yolk sac showing the typical arrangement 



of its three layers, a blood vessels ; m mesothelium ("zona esterna") ; E entoderm 



("zona interna"); J/ mesoderm ("zona media"). Stain, hematoxylin and Congo red; 

 thickness, 10 micra; photomicrograph, ^ 270. 



wall is rarely straight, so that a section shows numerous tubes cut in 

 transverse and oblique planes (Figs. 2, 6). The epithelium lining 

 these glands is similar to and continuous with that lining the cavity 

 of the yolk sac (Fig. 2). In places the glands are so closely packed 

 as to give somewhat the appearance of hepatic tissue (Fig. 6); but 

 the close morphological and physiological similarity which Graf Spee 

 beUeves he has discovered between yolk sac and liver is not apparent. 



