ORGANIC EVOLUTION 



209 



the thick fibrous dermis should be seen overlaid by the epi- 

 dermis of several cell layers, and invaded by the large 

 mucus glands that depend from and open through the epi- 

 dermis. The parts of kidney and spinal cord may perhaps 

 be identified by comparison with figures 131(7 and 1176, re- 

 spectively. 



FIG. 134. Diagrams of types of epithelium, a, ciliated epithelium of the 

 pharynx; b, isolated cells of the same; c, a gastric gland from the stomach 

 wall; m, its mouth; n, the contact layer at the surface: o, the middle layer 

 o* mucus-secreting cells ; p, the gland cells of the deepest part (pepsinogen 

 secreting) ; d, a single villus from the wall of the intestine ; q. goblet cells 

 (mucus-secreting) ; r, a group of replacement cells (center of cell increase); 

 e, a single goblet cell; f, a bit of the wall of the lung showing the thin 

 respiratory epithelium (stippled); s, s, capillaries containing blood 

 corpuscles. 



The record of this study may consist in a few drawings and 

 a larger number of diagrams of the things studied. 



Study 29. The early development of an amphibian. 



Materials needed: Egg masses of various stages of 

 development, preserved in two per cent, formalin; wash 

 out formalin before using. 



