VIII.] HISTOLOGICAL DIFFERENTIATION. 269 



to the body wall, which forms the anterior mediastinum 

 of human anatomy. 



By these changes the pericardial cavity is converted 

 into a closed bag, completely surrounded at its sides by 

 the two lateral halves of the body cavity, which were 

 primitively placed dorsally to it. These two sections of 

 the body cavity, which in the chick remain in free 

 communication with the undivided peritoneal cavity 

 behind, may, from the fact of their containing the 

 lungs, be called the pleural cavities. 



Histological diflferentiation. The fifth day may also 

 bo taken as marking the epoch at which histological 

 differentiation first becomes distinctly established and 

 begins to make great progress. 



It is of course true that long before this date, even 

 from the earliest hours, the cells in each of the three 

 fundamental layers have ceased to be everywhere alike. 

 Nevertheless the changes undergone by the several cells 

 have been few and slight. The cells of epiblastic origin, 

 both those going to form the epidermis and those in- 

 cluded in the neural involution, are up to this time 

 simple more or less columnar cells ; they may be seen 

 here elongated, there oval, and in another spot spheroi- 

 dal ; here closely packed, with scanty protoplasm, there 

 scattered, with each nucleus well surrounded by cell- 

 substance ; but wherever they are found they may still 

 be recognized as cells of a distinctly epithelial character. 

 So also with the cells of hypoblastic origin, whether 

 simply lining the alimentary canal or taking part in the 

 formation of the compound glands. Even in the meso- 

 blast, which undergoes far more changes than either of 

 the other layers, not only increasing more rapidly in 



