Ill 



CEMENT-ORGANS 



179 



head on each side as shown in Figs. 100, A, and 197, C, c.o. A longi- 

 tudinal section through the centre of the organ at about this stage 

 (stage 26, Fig. 101, E) shows that the organ is covered by the 

 ordinary 2-layered ectoderm. Round the lip of the opening at its 



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Fig. 101. — Illustrating the development of the cement-organ of Polypterus. B represents 

 part of a transverse section, the other figures portions of horizontal sections. 



A and B, stage 20 ; O, stage 23 ; D, stage 24 ; E, stage 26. c.o, cement-organ. 

 The darker tone indicates ectoderm. 



free end, the superficial layer of ectoderm stops, while the deep layer 

 seems to dip down as a deep involution to form the secretory epithelium 

 (c.o) which lines the cavity. All the appearances seem to point to 

 the secretory epithelium being ectodermal in its nature. How 

 deceptive these appearances are will be gathered from an inspection 

 of Fig. 101, A-E. 



