Compensatory Regulation. 79 



because there is a very definite line of cleavage along which the 

 break always takes place, and similar materials may be assumed 

 to exist at the regenerating surface at the time of the operation in 

 all individuals of a set of experiments. If, therefore, the body 

 is cut in two at various levels and the branchiae are thrown off at 

 the "breaking joint" any differences in the regeneration of the 

 branchiae and opercula may be considered as due to differences 

 in the posterior body operations. 



In the first lot of Apomatus the branchiae were removed in the 

 manner mentioned but there was no operation on the body. In 

 the second lot the body was cut between the third and fourth 

 thoracic segments in addition to the removal of the branchiae, and 

 in a third lot the thoracic cut was made between the first and 

 second segments. The last two operations in a great number of 

 the cases naturally caused the death of the animals, but in general 

 a very interesting result was obtained. In the cases where the 

 body was cut in two in the thorax as mentioned the opercular 

 differentiation appeared much earlier than in those in which the 

 body was intact. 



When the body is uninjured except for the removal of the bran- 

 chial circlets the branchial buds to the number of eight or nine on 

 each side appear simultaneously or nearly so, although there is a 

 slight gradation in size from dorsal to ventral edge of the branchial 

 ridge from the beginning. The few remaining buds to be 

 developed are added from the ventral edge. These bud-like 

 processes increase rapidly in length and soon appear as long 

 slender filiform processes which usually take on the secondary 

 pinnules before the appearance of the first traces of opercular 

 differentiation. The opercular differentiation then appears as a 

 vesicular enlargement in the next to the dorsal branchia on each 

 side. 



In the two-segment and four-segment thoracic pieces, left by the 

 thoracic cuts in lots two and three, the development does not follow 

 this course. The opercular bud is from the start very evidently 

 different from the others, being larger and more spherical than the 

 branchial buds proper. This is well shown in the figures. 

 (Figs. 25 and 26.) The branchiae in this case, however, are also 

 thicker and shorter than the corresponding ones where" the body 

 remains intact. It is very evident that while in the one case 

 where the body is intact the operculum passes through a distinct 



