Development of the Fifth Day 275 



" The actual process of differentiation shows itself 

 both in the protoplasm and in the nucleus of the cell. 

 The changes in the former are the more conspicuous, 

 and therefore the better known. The changes in the 

 nucleus have still to be adequately studied." ' 



According to Minot there are two types of 

 differentiation, both starting from the same 

 point- — -the undifferentiated embryonic cell. 

 In the first type, as the cells are proliferated 

 certain ones are differentiated into new forms, 

 while the rest of them remain undifferentiated 

 and retain the power of proliferation. The 

 epidermis is an illustration of this type. Some 

 of the cells which are formed by the multipli- 

 cation of its lower layer pass towards the sur- 

 face and differentiate into horny cells ; while 

 other cells remain at the base of the epidermis 

 and continue to multiply. In the second type 

 of differentiation, all the cells become at once 

 differentiated, and lose partly or completely 

 their power of multiplication. This type is 

 illustrated by the central nervous system. The 

 growth of the brain, after the earlier stages, 

 takes place by the growth of the individual 

 cells, rather than by the increase in the num- 

 ber of the cells. After their formation, the 



' Minot. 



