. Bulletin of the Museum of Comparative Zoology at Harvard College. Zoology; Zoology. 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 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
. Bulletin of the Museum of Comparative Zoology at Harvard College. Zoology; Zoology. 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 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. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Harvard University. Museum of Comparative Zoology. Cambridge, Mass. : The Museum
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