DAVENPORT: PROCESSES CONCERNED IX ONTOGENY. 



189 



the papillae which precede the formation of feathers, and in the ingrowing 



follicles which form hairs. 



5. A general thinning of the walls of the organs is found to accompany 

 the development of many Invertebrates. Figure 21 illustrates this fact. 



Fig. 21. 



In Figure 19 a general thinning of the epidermis is seen in passing from 

 stage a to stage /?. A good example of local thinning is seen in the 

 formation of the ependyma of the roof of the brain and medulla. 



The processes of thickening and thinning may go on side by side, as in 

 the formation of the lens of the vertebrate eve (Figure 22)" where the 

 outer and inner layers of the lens are at an early stage of nearlv equal 

 thickness, but where, in the later stages, the outer layer becomes thin, 

 and the inner layer enormously thick. 



IIP. Xext, we must consider the processes by which a single con- 

 tinuous layer experiences an interruption in its continuity. This may 

 take place (6) by a complete atrophy of a part of the membrane, thus 

 forming a hole or rift in it, or (7) by a separating off of a certain piece 

 from the membrane. 



Fig. 21. A, younger, and B, older Troehopore stages of the annelid Eupomatus 

 uneinatus, seen in sagittal section. Drawn to same seale. Outlines copied from 

 Hatseliek, '86, Arb. Zool. Inst. Wien, VI. See also K. & H., Fig. 118. 



