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primitive mesoblast. Tlie cells are spindle-shaped with long cytoplas- 

 mic processes, and large, oval nuclei. It possesses a vascular and a 

 glandular system, the former, in analogy with what is known to occur 

 in other forms, probably arising in situ from the blood islands; the 

 latter proceeding from the entoderm. According to Graf Spee, both 

 the glands and the blood islands arise earliest in the distal portion 

 of the sac and progress proxiraalward to the level of the pedicle; 

 also, at the time of theMisappearance of the blood islands and the 

 establishment of a vascular system, the gland system initiates its most 



active development. In the stage 



of development attained by this 

 embryo, the glands have invaded 

 every part of the sac with the ex- 

 ception of the extreme proximal 

 portion; only in this proximal 

 portion are typical blood vessels 

 found ; in the distal half these have 

 become capillaries. 



The yolk- stalk has a distinct 

 lumen which is completely lined 

 by polyhedral entodermal cells. At 

 places the lumen becomes almost 

 occluded, but an opening always 

 remains and is never less in dia- 

 meter than that of the nucleus 

 of the surrounding epithelial cells. 

 These cells are several layers deep, 

 their cytoplasm showing a reti- 

 culated structure, sometimes oc- 

 cupying only half the volume of 

 the cell, and in a few cases, being 

 confined to a mere rim at one 

 pole and containing a flattened 

 nucleus. In the mesoderm sur- 

 rounding the lumen at the level 

 shown in Fig. 3, are three blood 

 vessels partially filled] with nucleated red blood cells and differing 

 slightly in the thickness of the walls. Around two of these the endo- 

 thehal cells appear less flattened] and the mesenchyme more condensed. 

 The slightly thicker- walled vessels >re the vitelline arteries; the third 



Fig. 2, Transverse section of the 

 yolk sac near its distal pole showing 

 numerous glands. A blood vessel (a) is 

 seen just below the bend to the left of 

 the figure : Stain, hematoxylin and eosin ; 

 thicknees, 10 miera; photomicrograph, 

 X 45. 



