CHAPTER XII. 



THE DEVELOPMENT OF THE ORGANS IN MAMMALIA. 



In chap. X. we have described the early stages and 

 general development of the mammalian embryo. In 

 the present chapter we propose to examine the for- 

 mation of such mammalian organs as differ in their 

 development from those of the chick. This will not be 

 a work of any considerable extent, as in all essential 

 points the development of the organs in the two groups 

 is the same. They will be classified according to the 

 germinal layers from which they originate. 



The organs derived from the epiblast. 



Hairs are formed in solid processes of the deep 

 (Malpighian) layer of the epidermis, which project into 

 the subjacent dermis. The hair itself arises from a 

 comification of the cells of the axis of one of the above 

 processes ; and is invested by a sheath similarly formed 

 from the more superficial epidermic cells. A small 

 papilla of the dermis grows into the inner end of the 

 epidermic process when the hair is first formed. The 



