112 FRANK HELVESTINE, JR. 



only a morphological change, but also a chemical change. His 

 illustrations representing a migration of mitochondria into and 

 through the cuticle or distal cell-border to form cilia are far 

 from convincing. The fact that the niitochondria lie between 

 the nucleus and the distal cell-border in ciliated epithelium holds 

 some significance for this author. I might suggest that this is 

 the natural place to look for mitochondria in ciliated cells, or any 

 other epithelium of the columnar type, as normally these cells 

 show a marked polarity, and with the nucleus situated well to 

 the base of the cell, the only position left for the main mass of 

 mitochondria to occupy is between the nucleus and the distal 

 border of the cell. Moreover, the analogy between the segrega- 

 tion of mitochondria about the idiozome of spermatocytes and 

 the basal bodies of ciliated cells is very suggestive as regards 

 the homology between centrosomes and basal bodies. These 

 facts render Saguchi's claim of a mitochondrial origin of cilia 

 dubious. 



SimiMARY 



1. Basal, lateral, laterofrontal, and frontal epithelium can be 

 distinguished in the gill filaments of Cyclas. The cells of the 

 lateral, laterofrontal, and frontal epithelium are ciliated and are 

 derived successively from the non-ciliated cells at the base of the 

 filaments. 



2. Mitosis may occur in the non-ciliated basal epithelium. 

 The ciliated cells of the lateral, and especially of the latero- 

 frontal epithelium divide exclusively by amitosis. Mitosis 

 occurs in the ciliated epithelium of the intestine, but the cells 

 dividing by this method do not possess cilia and cannot therefore 

 be classified as ciliated cells. 



3. The parent ciliated cell of the laterofrontal epithelium 

 divides by amitosis, thus producing a group of narrow, cylindric 

 transitional cells with four basal granules and four cilia each. 

 These transitional cells, by a partition of the basal granules and 

 the splitting of the cilia, form cells with tufts of long cilia which 

 renew worn-out cells of the frontal epithelium. Worn-out cells 

 of the frontal epithelium disintegrate, passing through a stage of 

 karyorrhexis, and are resorbed. 



