davenport: processes concerned in ontogeny. 189 



the papillfB which precede the formation of feathers, and in the ingrowino- 

 follicles which form hairs. 



5. A general thinning of the walls of the organs is foinid 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 p. 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 eye (Figure 22), where the 

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

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

 and the inner layer enormously thick. 



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

 tmuous 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, younprer, and B, older Trochopore stages of the annelid Eiipomatns 

 unoinatus, seen in sagittal section. Drawn to same scale. Outlines copied from 

 Hatschek, '86, Arb. Zool. Inst. Wien, VI. See also K. & II., Fig. 118. 



