DAVENPORT: PROCESSES CONCERNED IN ONTOGENY. 189 
the papille 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. 
Fic. 21. 
In Figure 19 a general thinning of the epidermis is seen in passing from 
stage a to stage B. 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 17 
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. 
III. Next, 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 Trochopore stages of the annelid Eupomavus 
uncinatus, seen in sagittal section. Drawn to same scale. Outlines copied from 
Hatschek, '86, Arb. Zool. Inst. Wien, VI. See also K. & H.. Fig. 118. 
