VIII.] THE HYPOBLAST AND MESOBLAST. 273 



stituting the parenchyma of the liver, developed, as we 

 have seen, from the hypoblast cylinders given off around 

 the primary hepatic diverticula. 



Homologous, probably with the hepatic cells, and 

 equally of hypoblastic origin, are the more spheroidal 

 c secreting cells ' of the pancreas and other glands. The 

 epithelium of the salivary glands, though these so exactly 

 resemble the pancreas, is of epiblastic origin, inasmuch 

 as the cavity of the mouth (p. 119) is entirely lined by 

 epiblast. 



The hypoblast lines the allantois, and the notochord 

 also is an hypoblastic product. 



From the mesoblast are formed all the remaining 

 parts of the body. The muscles, the bones, the connec- 

 tive tissue and the vessels, both arteries, veins, capillaries 

 and lymphatics, with their appropriate epithelium, are 

 entirely formed from the mesoblast. 



The generative and urinary organs are also de- 

 rived from the mesoblast. It is worthy of notice that 

 their epithelium, though resembling the hypoblastic 

 epithelium of the alimentary canal, is undoubtedly 

 mesoblastic. 



From the mesoblast lastly are derived all the mus- 

 cular, connective and vascular elements, as well of the 

 alimentary canal and its appendages as of the skin and 

 the tegumentary organs. Just as it is only the epider- 

 mic moiety of the latter which is derived from the 

 epiblast, so it is only the epithelium of the former 

 which comes from the hypoblast. 



The important events then which characterize the 

 fifth day are : 



1. The growth of the allantois. 

 F. & B. 18 



