Compensatory Regulation. 



43 



//-/ 



In Fig. I OF, also 17 days after fertilization, we find a stage con- 

 siderably more advanced in which each of the middle branchiae has 

 already sent out three 



branches, which may / 



be labeled according 

 to age, II — I, II — 2 

 and II — 3. Of these it 

 will be seen that II — i 

 very early takes on 

 the character of one 

 of the main branchial 

 trunks, so that we thus 

 get a stage with four 

 pairs of branchiae. 

 II — 2 and II — 3 retain 

 the characters of pin- 

 nules of the Branchia 



II. Neither I nor III 

 has any branches at 

 this stage. 



Fig. iiA shows a 

 later stage taken from 

 the same dish at the 

 same time (17 days). 

 Here we still have 

 no branches of I and 



III. Branchia II has 

 five branches and the 

 first branch of L-II 

 (L-II — i) a branchlet 

 of itsown (L-II — I — i) 

 just appearing as a 

 very small knob. It 

 is very evident that 

 Branchia II is rapidly 

 outgrowing I and III 

 in strength. 



Fig. I IB (19 days old) gives a still older stage.^Branchia I on 

 each side has not increased much in size and is hardly larger than 



Fig. II. 

 Hydroides dianthus. Dorsal views. A — Age, 17 days (X47). 

 — Age, 19 days ( X 62). 



