SEX DETERMINATION IN DINOPHILUS GYROCILIATUS. 343 
coccus, Foraminifera, and, perhaps most important of all, 
various Naviculse and other bottom species of Diatoms. 
When supplied with these in abundance, however, it does not 
always increase and become numerous. There seem to be 
some other conditions that I have been unable to discover to 
account for this increase in numbers besides that of food. 
In the large tank in the laboratory, its numbers remain very 
much the same throughout the different seasons of the year. 
Text-pig. 3. 
Transverse section of large female in the region of the ovary, 
showing several female eggs. Limiting membrane of ovary 
shown in black, fe. pr. Female pronucleus, g. Gut. Im. m. 
Limiting membrane of ovary, og. c. Oogonial cells, oct. f. 
Female egg. m. pr. Male pronucleus. 
At times it almost disappears, but always reappears again 
shortly afterwards, in somewhat increased numbers. This 
tank is connected with the general water circulation through- 
out the laboratory, but the worms have not spread to the other 
tanks. If a white porcelain plate or dish is placed for some 
hours on the side or bottom of this tank all the Dinophilus 
in the tank will soon collect on it, and this affords a ready 
means of obtaining them in numbers. In this manner almost 
every individual in the tank can sometimes be withdrawn. 
They never show a tendency to collect on the glass side of the 
