164 C. M. CHILD 



In many series with whole animals I have found it desirable to 

 di\T[de Stage II into two stages, II a, and II b, II a including only 

 the earliest appearance of disintegration at any point and II b 

 the period when the eyes and cephalic ganglia have become 

 involved in disintegration, but other parts of the first zooid 

 have not yet been attacked. In* such series two important periods 

 instead of one are recorded, viz., the death of the head and the 

 death of the last part to remain alive. 



In my experiments by the direct method no attempt has been 

 made to determine the exact time of the entrance into a given 

 stage. That would of course be very difficult, but we may avoid 

 the difficulty simply by examining each lot at regular intervals. 

 At 20°C. half-hour intervals serve for KCN 0.001 m. and in most 

 cases for 4 per cent alcohol. With higher concentrations or higher 

 temperatures and sometimes with extremely small pieces shorter 

 time intervals are often desirable and with lower concentrations 

 and lower temperatures the time interval may be increased. 



But even with this method of procedure it is of course sometimes 

 doubtful whether a certain case should be recorded under one stage 

 or another. My general rule in cases of this sort is to record the 

 case under the earlier of the two stages in question: before the 

 next observation it has passed the critical point. 



In this manner then we can determine approximately the time 

 when disintegration begins in each individual or piece and in each 

 lot and we can also follow its course. The condition of each piece 

 in each lot is recorded at every period of observation, i.e., com- 

 monly every half-hour and a comparison of these records brings 

 out with much greater sharpness than a single record could the 

 essential differences of different lots. 



The record of a comparison between old and young worms is 

 given in table 1 by way of illustration. Lot 1 consists of ten phys- 

 iologically young worms 5 to 6 mm. in length. Lot 2 of old worms 

 18 to 20 mm. in length. 



'In this table the column 'Length of time' gives the length of 

 time in the reagent in hours and minutes at each observation, the 

 column headed 'Lots' gives the numbers of the different lots 

 composing the series and the headings I to V under the general 



