Development of the Fifth Day 273 



of the fore-gut. As they continually increase 

 in size, they gradually force the horizontal sep- 

 tum down on each side of the heart, until 

 the pericardial cavity is reduced to very nar- 

 row limits (Fig. 84, / c). 



These spaces in which the lungs lie are the 

 pleural cavities, and in the bird they remain 

 continuous, at their posterior ends, with the 

 general body-cavity. 



Histological differentiation. — It is at this time 

 that histological differentiation may be said to 

 commence, though from the earliest hours of 

 embryonic life the cells of the different germ- 

 layers were more or less dissimilar. 



It may be defined as 



" a process by which the structure of the cells is mod- 

 ified, so that cells become dissimilar in structure by 

 acquiring an organization which adapts them to spe- 

 cial functions. The cells which arise during the seg- 

 mentation of the ovum differ but slightly from one 

 another. As development progresses we find the cells 

 change, some in one way, some in another, so that 

 many kinds of cells are produced, but of each kind we 

 find a large number of cells. Each kind of cell may be 

 said, roughly speaking, to form a tissue for itself. Cells 

 of each tissue offer visible peculiarities by which they 

 may be readily distinguished from one another under the 

 microscope. It thus appears that the production of 

 tissues is the main result of differentiation, so that this 



