248 HUMAN EMBRYOLOGY AND MORPHOLOGY. 



body. Peyer's patches begin to appear in the intestine. Until 

 then the connective tissue of the body contains a jelly-like ground 

 substance corresponding to Wharton's jelly in the umbilical cord. 

 At the 5th month the mucoid substance in the connective tissue 

 begins to be absorbed (Berry Hart). 



Until the fifth month the marrow of bones is composed of 

 embryonic mesoblastic tissue (primary marrow). After the fifth 

 month this becomes invaded by lymphoid tissue (white marrow) 

 which in certain bones (vertebrae, etc.) becomes transformed 

 further into red marrow. (Hamar.) 



