152 Victor E. Emmel. 
Reference has already been made to the recent discussions of the 
direction of differentiation by Child (06, ’08), Holmes (’04, ’07), 
Zeleny (07), and Haseman (’07). Among these investigators Hase- 
man has studied the form most closely related to the lobster. From 
his observations upon the regenerating appendages of the crayfish, 
he concludes that differentiation in the chela proceeds from the tip 
toward the base, while the reverse is true for the walking legs. 
In view of the results of these investigators Zeleny has discussed 
three possibilities regarding the direction of differentiation in the 
development of an appendage. 
1. That “all the parts may appear at once”; (2) that “the progres- 
sion may be outwardly directed’, or (3) “it may be inwardly di- 
rected”. Zeleny points out that the first lacks evidence, that the 
second has been more especially favored by Pfliiger and Holmes in 
connection with certain general theories of development, and the third 
by Morgan, Driesch, and Child. From his own study Zeleny con- 
cludes that the second method may predominate in some animals, 
the third in others, and further that both modes may predominate 
at different times in the same animal, as appears to be the case in 
the antennule of Maneasellus macrourus. It is evident that the 
present data from the lobster fail to conform with any of the three 
modes of differentiation. But the evidence does support a fourth 
proposition, viz., that sequence of differentiation may be influenced 
or determined by the relative size and functional role of the regenerat- 
ing structures. Consequently, differentiation in the regenerating 
appendage is not necessarily either “centrifugal” nor “centripetal,” 
the new structures, on the contrary, differentiating rather according 
to size and functional relations, in a manner perhaps similar to the 
formation of organs in embryonic development,—a most striking 
illustration of which is furnished by Sutton’s (’83) rule for the 
differentiation of the epiphyses of the long bones in human osteology : 
“The centers of ossification appear earliest for those epiphyses which 
bear the largest relative proportion to the shafts of the bones to 
which they belong” (p. 480). 
IX. Summary. 
1. Two new structures are recorded in connection with the break- 
ing joint of the chela of lobsters. 
