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 tlie organ at about this stage 

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

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



I I 



c.o: ^B 



Ki<;. 101. Illustrating the development of the cement-organ of Polypterus. B represents 

 p;irt of a transverse section, the other figures portions of horizontal sections. 



A and B, sta^e 'JO ; C, stage 23 ; D, stage 24 ; E, stage 2C>. r.o, cement-organ. 

 The darker tune indicates ectoderm. 



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

 set ins to dip down as a dee]) 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. 



