122 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



indicated. The "30 days" columns take no cognizance of the stages of 

 division of the individuals occurring at the end of the "15 days " periods, 

 and for this reason the two columns of each culture are perhaps not 

 strictly comparable. The horizontal line of percentages is reckoned 

 upon the number of individuals existing at the beginning of the period 

 in question. 



It will be seen that during the first fifteen days solution " A " was pro- 

 ductive of the most rapid division, in some instances the second division 

 being accomplished ; but during the second period the worms scarcely 

 more than held their own. This is seemingly due to the prevalence of 

 bacteria. Culture " D " presents somewhat similar phenomena. In 

 " C " the falling off in the second period is probably caused by the dis- 

 appearance of food from the water and from the tissues of the body. 

 " B " is the control culture, and more nearly represents the ordinary rate 

 of reproduction under favorable conditions. These are winter cultures. 

 Propagation would doubtless be more rapid with fresh worms in summer 

 or fall. 



6. Methods. 



Dero does not present any special technical difficulties except that, in the 

 later stages of budding, separation of the zooids is likely to occur in the 

 process of killing, whereby control of the material is lost. I used as 

 killing and fixing reagents with about equal success (1) a saturated 

 aqueous solution of corrosive sublimate plus 1% acetic acid (hot), and 

 (2) a solution of hot picro-sulphuric acid (Kleinenberg's). Staining of 

 excellent quality was secured by Heidenhain's iron-hsematoxylin method. 

 Sections were made from 6 to 12 /a in thickness. 



7. Histological Features of the Budding Process. 



In the discussion of the role played by the different embryonic layers 

 in the production of the new organs made necessary by budding, I shall 

 first treat in a general way of the changes occurring in each la3'er, and 

 the organs which are wholly developed therefrom. Afterward, at the 

 risk of some repetition, I shall correlate the share of each layer in the 

 formation of the mouth, and such other structures as involve more than 

 one germinal layer. 



a. Ectoderm. , 



The first evidence of the bud-zone, as seen in optical sections of the living 

 worm, is a slight thickening of the ectodermal elements of the dermo- 



