110 FRANK HELVESTINE, JR. 



frontal epithelium viewed at right angles, I have identified as a 

 transitional cell arising by amitotic division from the larger 

 parent cell of the laterofrontal epithelium. 



Contrary to the conclusion of Saguchi that mitosis is the ex- 

 clusive mode of division in ciliated epithelium of invertebrates, 

 I find that ciliated cells in the gill filaments of the fresh-water 

 mussel, Cyclas, divide only by amitosis. As to mitosis in ciliated 

 epithelium, I find that the cells undergoing mitotic division 

 possess no cilia. Saguchi in his description also states that before 

 undergoing mitosis the cell loses its cilia. As these cells possess 

 no cilia during division, it cannot properly be said that ciliated 

 cells divide by mitosis. Saguchi confirms Jordan's findings in 

 vertebrates and concludes with him that amitosis is the exclusive 

 method of proliferation of ciliated epithelium in these forms. 

 Since Saguchi admits that the cells in which he saw mitosis in 

 invertebrates possessed no cilia, and since I have found this to 

 be the case also in my material, and further that ciliated cells 

 of invertebrates do divide by amitosis, the conclusion seems 

 justified that ciliated cells where they prohferate as such do so 

 exclusively by amitotic division both in vertebrates and in 

 invertebrates. 



From the above it follows that the proximate factor deter- 

 mining whether a cell of ciliated epithelium is going to prolifer- 

 ate by mitosis or by amitosis is the absence or presence of cilia. 

 The question at once arises as to why cells possessing cilia should 

 always divide by amitosis. Is the cause a structural one or 

 a functional one? Jordan suggests that amitosis is due to a lack 

 of a centrosome in these cells, while Saguchi reaches the con- 

 clusion that amitosis in these cells is ''due essentially to the 

 degree of differentiation of the cell-plasm," which latter may be 

 classed as a functional cause. 



Saguchi claims to be able to demonstrate the presence of a 

 centrosome in ciliated cells. His illustrations and descriptions 

 do not unequivocally bear out this assertion. The difficulties 

 attending the identification of such a minute body as the centro- 

 some from among a large mass of mitochondria render such an 

 undertaking practically impossible. In my preparations no 



