268 WILLIAM A. KEENER. 



nematocysts as lying in vacuoles" (Martin, 1908, p. 263). It had 

 been the experience of students in my classes to find discharged 

 nematocysts enclosed within a single cell (Fig. i). Such isolated 

 nematocysts were obtained by adding a drop of i per cent, 

 acetic acid to a slide containing specimens of living flatworms. 

 Some time after a cell of this kind had lain in the dilute acetic 

 acid a large vesicle appears at the pole from which the discharged 

 nematocyst must emerge (Fig. 2). Such distortion has fre- 

 quently been observed under similar conditions. 



So much for my observations upon the conduct and anatomy 

 of living specimens and upon temporary preparations. 



Before taking up the evidence afforded by permanent histologi- 

 cal preparations I must interpolate my hesitancy, at least, to 

 follow Martin when he says, "I have found very young Micro- 

 stoma in which nematocysts were already present, and at first 

 this presented a difficulty to this theory of the derivation of 

 nematocysts, but I do not believe this is insuperable when we 

 consider that nematocysts in the case of an animal which has 

 fed largely on Hydra can be found in almost any tissue of he 

 body. I have found them in the testes (Fig. 8), and although I 

 have not yet found them in an ovum, I believe that the yolk- 

 cells might readily carry them into the cocoon thus causing the 

 infection of the young forms" (Martin, 1908, p. 271). It does 

 not appeal to my judgment that nematocysts with cnidoblasts 

 should be able to "infect" the young forms through the yolk-cells. 

 Nematocysts within vacuoles, without attending cells to propo- 

 gate them, infecting young forms through yolk-cells seem to be 

 altogether improbable. 



When I consider this last statement of Martin and note his 

 oversight of the cells that in Microstoma do enclose some of the 

 nematocysts I am led to believe that the final word has not 

 been spoken concerning the nematocysts of Microstoma. 



Such was my attitude when during September, October, 

 November and early December, 1910, collections of Microstoma 

 were made with a view to studying their histology. 



Many specimens were fixed in chrom-aceto-formal; others 

 were fixed in saturated solution of corrosive sublimate to which 

 5 per cent, of glacial acetic acid had been added. Both of 



