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University of California Piihlications. [Zoology 



Fig. 19 presents two optical sections of the same fnistular 

 anlage from a larva with six proximal tentacles. The increase 

 in number and depth of stain and the decrease in size of the 

 endoderm cells, which were characteristic of the development of 



Fig. 20. Fig. 21. 



Fig. 20. Longitudinal section through a developing frustule. X 910. 

 Fig. 21. Section across stem, through base of developing frustule, show- 

 ing connection between frustular and enteric endoderm. X 910. 



the tentacles, are found here. The ectoderm cells above this 

 modified endoderm are more deeply stained, higher and more 



